resonance ionization mass spectrometry data service

f ctrodrintin Afm Vol. 4sB. &ted in Great B&itt.

No. 9, pp. 11394203.1993

TOPICS IN LASER SPECTROSCOPY

A resonance ionization spectroscopy/resonauce ionization mass spectrometry data service. IV-Data sheets for Be, In, Li, K, Rb, Ag, Ti and V and an update of the data sheet for Ni E. B. SALOMAN Electron and Optical Physics Division, Na$;$;tit;te,oi Standards and Technology, Gaithersburg, 3 . . . (Received 26 Janwry 1993; accepted 5 Februury 1993) Abstract-A data service has been established at the National institute of Standards and Technology to

provide the necessary information to apply the techniques of resonance ionization spectroscopy (RIS) and resonance ionization mass spectrometry (RIMS) to routine use in analytical chemistry. This service collects and calculates the relevant atomic data, chooses appropriate resonance ionization schemes, and indicates pertinent operating details of successful RIMS studies. The first group of data sheets was published pre~o~ly covering the elements As, B, Cd, C, Ge, Au, Fe, Pb, Si and Zn. Tbe second group published covered the elements Al, Ca, Cs, Cr, Co, Cu, Kr, Mg, Hg and Ni. The third group published covered the elements Sb, Bi, P, Na and Sn. The fourth group of data sheets is presented here. It covers the elements Be, In, Li, K, Rb, Ag, Ti and V. It also provides an update of the previously published data sheet for Ni. Reprints of RWRIMS work are solicited so that those efforts may be included in future data sheets.

1. INTROBU~HON THE TECHNIQUES of

resonance ionization spectroscopy (RIS) and resonance ionization spectroscopy followed by mass spectrometry (RIMS) have been demonstrated in several state-of-the-art laser laboratories to be analytical techniques of extremely high sensitivity and selectivity that are applicable, in principle, to nearly all atoms El]_Thus RIS and RIMS can form the basis for ultrasensitive eiemental analysis applicable to problems in non-destructive testing, monitoring environmental pollutants, performing trace impurity analysis, measuring radioactive contamination, and many other areas. To meet their potential, these techniques have to be made available to analytical chemists and other scientists who do not have the most comprehensive knowledge of atomic structure and laser physics. The task is made more difficult for these scientists because much of the information needed to apply RWRIMS exists in scattered data bases. Many key data are not currently available at all (especially excited state photoionization cross sections). The chemists require the RIS schemes and atomic data that, will allow the techniques to be applied without sophisticated calculations and a major literature search in advance for each element to be measured. It is the objective of this data service to organize the available info~ation, supplement it with calculations (where gaps exist), and provide formatted data and application sheets to permit the routine use of RISRIMS in elemental analysis. The tirst group of data sheets published [2] covered the elements As, B, Cd, C, Ge, Au, Fe, Pb, Si and Zn. The second group of data sheets published [3] covered the elements Al, Ca, Cs, Cr, Co, Cu, Kr, Mg, Hg and Ni. The third group of data sheets published [4] covered the elements Sb, Bi, P, Na and Sn. The fourth group of data sheets is included here. It covers the elements Be, In, Li, K, Rb, Ag, Ti and V. It also provides some updated information of the previously published data sheet for Ni. Other data sheets will be published periodically. These sheets list the element, its stable isotopes, isotope shifts and hypefine structure, RIS schemes, atomic energy 1139

1140

E. B.

SALOMAN

levels, lifetimes, oscillator strengths, laser-excitation schemes, atom sources, estimates of laser power requirements, and references. Also included are the results of calculations of excited state photoionization cross sections by Hartree-Fock techniques involving several atomic configurations. Reprints, preprints, and other forms of information about successful RIS/RIMS work are solicited for inclusion in future new data sheets and in updates of the sheets presented here.

2. EXPLANATION OF DATA SHEETS The data sheets begin with the name, symbol, and atomic number, Z, of the element. Then the ground state atomic configuration is specified, followed by the energy of the first ionization energy with respect to the ground state. A conversion factor [5] of 1 eV = 8065.5410(24) cm-’ is used. Next comes an introductory paragraph, which includes RIS schemes that have been experimentally observed and additional schemes when needed for this element. Schemes that involve either one- or twophoton excitations may be cited. Two-photon excitation schemes need relatively high laser powers for saturation and often require a mass-selective detection system to avoid interferences from unwanted non-resonant or near-resonant ionization of concomitant species in the sample. A number of two-photon excitation schemes are included where the most obvious one-photon resonance transition wavelength is very difficult to generate with present day commercial teclmology or where a single-color scheme may be employed instead of a two- or three-color scheme for convenience and/or economy. The presence of intermediate levels, which may enhance particular two-photon schemes, is noted. Two-photon schemes are valuable when high-resolution Doppler-free techniques are required for additional isotopic selectivity [6]. Information about the sensitivity of demonstrated schemes is provided when that information is available. Other pertinent information and possible applications are included. A Grotrian diagram is included, illustrating the energy levels and transitions used in the RIS schemes. Standard atomic notation is used. In some cases so many levels are utilized in RIS schemes that groups of levels must be shown schematically. In these cases their energy is indicated as a range rather than providing the individual energies of each level. Next, we include information specific to each type of RIS scheme covered by the data sheet. The laser wavelengths in air are listed for the transitions in the RIS scheme (except for wavelengths less than 200 nm for which vacuum wavelengths are given), as well as the energies and abbreviated spectroscopic notation for the states involved. “Low. Level” stands for the lower level of a transition, El is its energy, “Res. Level” stands for the upper or resonance level of a transition, E, is its energy, A is the air wavelength (or vacuum wavelength if A c 200 nm) of the transition, and fabs is its absorption oscillator strength. Then we provide some parameters important to the energetics of the RIS process. Wherever possible, existing atomic data are used. However, in many cases these parameters must be calculated. These calculations are made using the Hartree-Fock codes of COWAN [7] with approximate relativistic corrections. In most cases the calculations involve several configurations for each state. For some elements the Hartree-Fock calculation produces relative configuration energies that disagree with the known level energies. In these cases the relative configuration energies are adjusted to the spectroscopic data before calculating the atomic data. The data sheet will note when this procedure is used. Lifetimes, 7, of the excited states involved in the schemes are given. Also provided, when a single photon transition is involved, is an estimate of the power required to saturate the first step of the RIS process. This estimate is for an idealized situation where all power is delivered within the spectral range of the absorption line for free, thermal atoms. It is obtained using the formalism of LETOKHOVet al. [8] where the cross section of the absorption is given by:

RIWRIMS Data Service

X2Ati

==2a

1141

(1)

where A is the wavelength of the transition, Ati is the Einstein transition coeflicient between the initial state and the resonant state (calculated with the Hartree-Fock code if necessary), and ho is the transition linewidth. For purposes of this estimate, in the case of solid samples, this linewidth is taken to be the Doppler width for a free atom at the boiling point. The saturation power is given by: P=& where h is Planck’s constant, c is the speed of light, and T is the excited level’s relaxation time (which is taken to be the excited state lifetime for this estimate). This estimate must be adjusted by the user to fit the actual experimental parameters used (e.g. pressure effects). For discrete transitions other than the first step, the cross section is estimated with Eqn (1) while for photoionization, in most cases, the cross section is calculated with Cowan’s code. Then an estimate of the laser energy needed to saturate the transition or the photoionization is obtained by using the condition No 2 1, where N is the number of photons in the laser pulse per unit area within the absorption bandwidth delivered within the lifetime of the excited state that absorbs the photon. This leads to a saturation energy (in mJ/cm2): E,, = l/(5.03 X 10-b ax) if u is in units of lo-l8 cm2 and A is in nm. Actual bandwidth (for discrete transitions) and laser pulse length must be considered as well as geometric factors to obtain a value of the required laser energy for a specific experimental arrangement. Also to be noted is that in the calculation of the photoionization cross sections, no account is taken of continuum resonances that may be present unless specifically stated. If present, these resonances can greatly increase the photoionization cross section, thereby reducing the required laser energy. We estimate an uncertainty of a factor of two or three in the calculation of the photoionization cross sections even in the absence of resonances. We follow this with some data on the isotopes of the element. For the stable isotopes, we list the isotope, the natural abundance (in parentheses), and the nuclear spin, I. For some radioactive isotopes of interest, we list the isotope, its nuclear spin, and its lifetime, 7. Where known for the transitions used in the RIS schemes, we provide the isotope shift and the hyperfine structure (hfs) interaction constants A (dipole interaction constant) and B (quadrupole interaction constant). The hfs splitting, E, may be calculated from the center of gravity of the level of the isotope by [9]: E=AC/2+

BDM

where

c = F(F+l)

- 1(1+1) - &Z+1)

and D = (3/2)C(C+ 1) - 2Z(Z+ l).Z(J+ 1) Z(2Z- l)J(W- 1) where F is the total angular momentum, both electronic and nuclear, Z is the nuclear spin, and .Z is the total electronic angular momentum. There is no hfs for an isotope if Z = 0. The quadrupole term is non zero only if Z 2 1. No state will show hfs splitting if .Z = 0 and the quadrupole term of the hfs will not be observable unless .Z 2 1.

1142

E. B. SALOMAN

The laser scheme section suggests possible means of generating the laser wavelengths required by the RIS schemes. In most cases where an experimental scheme is known, the laser used is cited. For proposed schemes, Nd:YAG is often proposed as the pump laser because of the availability of its harmonics for photoionization from excited states. Pumping with other lasers or flashlamps may often be equivalent to the given source (but note that the pump affects the wavelength range over which a given dye will operate). High repetition rate pump sources or cw lasers can improve sample utilization efficiency when used with continuous atomization sources. The atom source section suggests how a sample may be atomized for measurement. RIS and atomic data references are given. Information about any errors in this or the previous data sheets is solicited for inclusion with future data sheet sets.

Acknowledgements-I wish to thank G. S. HURST,J. E. PARKS,J. C. Ta~vts, and T. B. LUCATORTO for their assistance in the design of the data sheets. The NIST Atomic Energy Levels and Atomic Transition Probabilities Data Centers and especially A. MUSGROVE and J. F~.JHR provided much assistance to this work. This project is supported in part by the U.S. Department of Energy Office of Health and Environmental Research under contract DE-AI0586ER-60447.

REFERENCES [l] See, for example, G. S. Hurst and M. G. Payne, Specrrochim. Acta 43B, 715 (1988) and the references contained therein. [2] E. B. Saloman, Specrrochim. Acta 45B, 37 (1990). [3] E. B. Saloman, Spectrochim. Acta 46B, 319 (1991). [4] E. B. Saloman, Spectrochim. Acta 47B, 517 (1992). [5] E. R. Cohen and B. N. Taylor, Rev. Mod. Phys. 59, 1121 (1987). [6] C. W. Clark, J. D. Fassett, T. B. Lucatorto and L. J. Moore, Enhancement of the isotopic abundance sensitivity of mass spectrometry by Doppler-free resonance ionization, in Resonance Ionization Spectroscopy 1984, Eds G. S. Hurst and M. G. Payne, p. 107. The Institute of Physics, Bristol (1984). [7] R. D. Cowan, The Theory of Atomic Structure and Spectra. University of California Press, Berkeley (1981). [8] V. S. Letokhov, V. I. Mishin and A. A. Puretzky, Prog. Quantum Electron. 5, 139 (1977). [9] G. H. Fuller, J. Phys. Chem. Ref. Data 5, 835 (1976).

RISIRIMS

1143

Data Service

Data sheet for RWRIMS schemes Beryllium 2=4

Be

Ground State [l] First Ionization Energy [2]

75 192.07 cm-l

ls?P 1s = 9.322632 e;

RIS has been applied to the analysis of beryllium by a few groups. Two types of RIS schemes have been demonstrated and another type has been proposed for greater sensitivity and selectivity. CLARK et al. [A], WEN et al. [B], and FASSE~Tet al. [C] utilized schemes in which the Be atoms were excited from their ground state by a two-photon transition to one of the 2nd lD2 or 2sns IS0 levels, which was then photoionized by a third photon of the same color (a so-called 20~ + o1 process). These schemes have the simplicity of a single-color process and are amenable to Doppler-free techniques but suffer from the relative inefficiency of two-photon transitions and the lower selectivity of a high-laser-power single resonance process. A variation on this scheme was used by TRAVISet al. [D]. In it a two-photon transition from the ground state to the 2&s ‘S, was utilized and this excited state was photoionized by radiation from an infrared laser (a so-called 20, + o2 process). This scheme may also be used with Doppler-free techniques but has the disadvantages mentioned for the previous schemes. DOWNEY et al. [E, F] applied a scheme in which the Be atoms were excited from their ground state to the zF2p ‘fi resonant level by the first photon and then photoionized by a second photon of the same color (a so-called o1 + o1 process). They studied the depth profile of Be impurities in AlGaAs, GaAs, Al and Si with a detection limit of 1 ppm using analog detection. This scheme also has the simplicity of a single-color process but also suffers from the loss of selectivity due to having only a single resonance while requiring sufficient power to photoionize the resonant level. Therefore, two three-color schemes are proposed. Both have first steps that excite the atom from its ground state to the zS2p 1R resonant level. Then the scheme suggested by LETOKHOV[G] uses a second laser to excite the atom from this level to the 2F3d lD2 level, which may then be photoionized by a third laser. The other scheme uses the second laser to excite the atom from the 2F2p le level to the 2r4d lD2 level, which may be photoionized by IR from a Nd:YAG laser. These are so-called o1 + o2 + o3 processes. Analysis of trace quantities of Be is important since Be is a toxic material used in many applications. The isotope loBe may be used as a tracer of processes on a short geological time scale, such as monitoring the recent history of solar activity [HI. However, one must be aware of isotopic fractionation effects between Be isotopes [I] in such measurements. A Grotrian diagram,of the RIS schemes in beryllium is shown in Fig. 1 [2]. RZS schemes of type (wl + q) [E, F] One-color two-photon process consisting of a resonance step followed by photoionization with a photon of the same color as the initial step. X1 = 234.9 nm. Data for RIS schemes of type (w, + CO,)[2, 31

Lifetime and photoionization cross section from excited state [2, 4, 51 In tabular form are listed the laser wavelength, Al, to the resonant level, the resonant level, its energy, E,, the lifetime, T, of the resonant level, an estimate of the laser power, Psat, required to saturate the resonant transition, the photoionization

1144

E. B. SALOMAN

cross section of the resonant level, u, for radiation of wavelength X1, and an estimate of the energy, E,,,, delivered during the lifetime of the resonant state required to saturate the photoionization.

4 (nm) 234.8810

Flea. Level

(CL)---(Aa)

282p ’ P,’

42566.36

RZS schemes of type (ol + o2 +q)

2.0

530

24

35

or (ol + w2 + w2) or (ol + w2 + o3) [G]

Three-color, three-photon process consisting of two resonance steps followed by photoionization. X1 = 234.9 nm, A2 = 381.3 nm or 457.3 nm, A3 = X1 or A2 or 532 nm or 1064 nm. Data for RIS schemes

of type (q

Lifetimes and photoionization

+ o2 + wl) or (q + 0~ + 02) or (01 + O, + 03) 1% 3,61

cross section from excited states [2-6]

Listed in tabular form for the first step of these schemes are the laser wavelength, X1, to the resonant level, the resonant level, its energy, E,, its lifetime, 7, and an estimate of the laser power, P,,,, required to saturate the resonant transition.

The following table lists the wavelengths, A2, of the second step of these schemes to one of the 2s3d or 2s4d lD2 levels, an estimate of the energy needed to saturate the second step, E,,,, in A2 within the absorption bandwidth while the first step is being pumped, the second resonant level, its energy, E,, its lifetime, T, the photoionizing wavelength, A3, the photoionization cross section, u, of the resonant level for radiation of wavelength X3, and an estimate of the laser energy requirement, EF’t, in A3 for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

93E-l

6442831

1 11.1 1 E;

1

RISiRIMS Data Service

1145

192

crii’

2&s 2sSd 2s4d 2sSd

2s2

Fig. 1. Grotrian diagram of resonance ionization spectroscopy schemes in beryllium.

[A-D] One-color or two-color process consisting of a two-photon transition to the resonant level followed by photoionization with either a third photon of same color as used for the initial step or an infrared photon. AI between 280 and 311 nm, AZ= AI or 1064 nm.

RZS schemes of type (20, + wl) and (20~ + q)

Data for RIS schemes of type (20, + wl) and (2w, + 02) [2]

Lgetimes and photoionization

cross sections from excited states {2, 4, 51

The cross section for the two-photon transition at 290.7 nm was calculated [7] to be 5.2 x 10-m cm*s and .that at 310.3 nm to be 6.6 X lo+’ cm*s. The following table lists the wavelength, Al, the resonant level, its lifetime, T, the Wavelength used for photoionixation, A,, the cross section, o, for photoionization of the resonant level by radiation of wavelength AZ(which is equal to AI for the one-color process), and an estimate of the laser energy requirement, Esat, in this final wavelength for photoionixation efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

1146

E. B. SALOMAN

* Reference [5] calculates a very small value for this cross section. However, Refs [A] and [C] report strong RIS signals for this scheme.

Isotope data [9]

Stable Isotopes: 9Be (100%) Z = 3/2. Unstable Isotopes: ‘Be I = 312, r = 53.28 d; loBe I = 0, 7 = 1.52 X 106 y. Isotope shiftslhfs [lo, 11, B]

The isotope shift of the separation resonant level for one- and two-photon in the following table.

Resonant

The

Level

2s2p

lP,”

2s3d

‘D,

2s4d

‘Da

2s5d

‘Da

2s6d

‘D,

I I I I I I

between the ground state and the indicated transitions in loBe with respect to 9Be is given

Isotope

Shift

(10m3 cm-l)

428 468.7 440.3 444.0 443.3

structure constant for the 2s3d ‘D2 level in 9Be is The hfs of the 2.s4d lD2 and 2s5d lD2 levels in 9Be has been to be less than 5 x 10m3 cm- l. The ‘So states and all states in loBe have no

dipole

hypertine

A = 0.13 x 10e3 cm-l.

reported hfs.

Laser schemes

For 234.9 nm, excimer pumped frequency doubled (using BBO) LD466 dye laser or Nd:YAG pumped frequency doubled (using BBO) LD466 dye laser; for 280-291 nm, Nd:YAG pumped frequency doubled Rhodamine 590 dye laser [A]; for 310.3 nm, Nd:YAG pumped frequency doubled DCM dye laser [A]; for 381.3 nm, Nd:YAG pumped Exalite 384 dye laser; for 457.3 nm, Nd:YAG pumped Coumarin 460 dye laser; for 532 nm, second harmonic of Nd:YAG; for 1064 nm, Nd:YAG direct IR. Atom reservoirs and sources

Atoms of beryllium have been prepared for RIS studies by several methods: evaporation from a Re, W or Ta filament on which a slurry of Be or BeCl, and

RJSIRJMS Data Service

1147

graphite was dried [B-D]; and sputtering of a solid sample using argon or xenon ions P, Fl. RIS references C. W. Clark, J. D. Fassett, T. B. Lucatorto, L. J. Moore and W. W. Smith, J. Opt. Sot. Am. B 2, 891 (1985). J. Wen, J. C. Travis, T. B. Lucatorto, B. C. Johnson and C. W. Clark, Phys. Rev. A. 37, 4207 (1988). J. D. Fassett, K. G. W. Inn and R. L. Watters Jr, Development of the NBS beryllium isotopic standard reference material, in Resonance Ionization Spectroscopy 1988, Eds T. B. Lucatorto and J. E. Parks, p. 379. The Institute of Physics, Bristol (1989). J. C. Travis, T. B. Lucatorto, J. Wen, J. D. Fassett and C. W. Clark, Doppler-free resonance ionization mass spectrometry of beryllium, in Topical Meeting on Laser Applications to Chemical Analysis, Technical Digest 1987, Vol. 5, p. 81. Optical Society of America, Washington, D.C. (1987). S. W. Downey, R. F. Kopf, E. F. Schubert and J. M. Kuo, Appl. Opt. 29, 4938 (1990). S. W. Downey and A. B. Emerson, Anal. Chem. 63, 916 (1991). V. S. Letokhov, Laser Photoionization Spectroscopy. Academic Press, Orlando (1987). B. E. Lehmann, Resonance ionization spectroscopy: Applications in isotope geophysics, in Analytical Laser Spectroscopy, Eds. S. Martelhtcci and A. N. Chester, p. 203. Plenum, New York (1985). T. Nakajima and P. Lambropoulos, Theory of two-photon-resonant three-photon ionization by broadband radiation applied to the determination of isotopic abundances, in Resonance Ionization Spectroscopy 1990, Eds J. E. Parks and N. Omenetto, p. 59. The Institute of Physics, Bristol (1991).

Data references [l] C. E. Moore, Atomic Energy Levels, National Standard Reference Data Series (US. National Bureau Standards) NSRDS-NBS 35, Vol. 1 (1971). [2] L. Johansson, Ark. Fys. 23, 119, (1%2). [3] J. R. Fuhr and W. L. Wiese, Atomic transition probabilities, in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 10-128. CRC Press, Boca Raton, Florida (19%). [4] Compilation in R. Moccia and P. Spizzo, J. Phys. B 18, 3537 (1985). [5] J. A. Tully, M. J. Seaton and K. A. Berrington, J. Phys. B 23, 3811 (1990) and private communications. [6] T. N. Chang, Phys. Rev. A 39, 4946 (1989). [7] R. Moccia and P. Spizzo, J. Phys. B 18, 3555 (1985). [8] T. N. Chang and X. Tang, J. Quant. Spectrosc. Radiat. Transfer 43, 207 (1990). [9) N. E. Holden, Table of the isotopes (revised 1990) in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 11-33. CRC Press, Boca Raton, Florida (1990). [lo] C. W. Clark, Astrophys. /. 285, 322 (1984). [ll] 0. Poulsen, T. Andersen and N. J. Skouboe, J. Phys. B. 8, 1393 (1975).

RIWRIMS Data Service

1149

Data sheet for RWRIMS schemes Indium In Ground State [l] First Ionization Energy [A]

z = 49 ls22s22p63s23p63d’04s24p64d105s25p

2Py,2

46670.107 cm-’ = 5.786358 eV

A number of groups have applied RIS to the analysis of indium. Most applied schemes [B-N] show the In atoms being excited from one of the two levels of the ground state configuration by the first photon to a resonant level. The resonant level was then photoionized by either a second photon of the same color as the first (a socalled o1 + o1 process) or by a photon of another color (a so-called w1 + w2 process). The w1 + o1 process has the simplicity of a single-color process requiring only one laser, but has the reduced selectivity of a single resonance step in which the laser power must be sufficient to drive the relatively less efficient photoionization step. The o1 + o2 process allows for separate optimization of excitation and photoionization steps. In some schemes [C, E, F, H, M] this process is achieved using a single laser. The excitation step is driven by frequency doubled radiation while non-doubled radiation is used to drive the photoionization step. In other schemes of this type, the photoionization step is carried out using pump laser radiation [I, J, L]. PAPASet al. [H] using an w1 + o2 process and ion sputter atomization (SIRIS) measured the dopant concentration of In on a Si surface with a reported detection limit of 9 parts per trillion (pptr). CHEKALINer al. [I] using an wl + o2 process and flame atomization report detecting In in Sn at levels of 0.1-l ppb. ARLINGHAUSet al. [L] using another scheme of type w1 + w2 and SIRIS atomization, report carrying out a depth profile of SiOzInP samples and report a detection limit of 0.3 ppb for In in Si. A second type of RIS scheme was applied by BETEROVef al. [J, 0] and ZILLIACUSef al. [PI. In these schemes the In atoms are excited from one of the two levels of the ground state configuration to the 6s 2Sln resonant level from which they are excited to a high Rydberg level that can be ionized with very high efficiency by an electric field (a socalled o1 + U$ process). Due to the energy of the atomic levels, if the In atoms start in the upper level of the ground state configuration and an appropriate electric field Stark shifts the levels, then for the 31p 2p0 Rydberg levels, wl = o2 [O]. This type of scheme is highly selective owing to its two resonance steps and quite efficient due to the electric field ionization. With such a scheme, a detection limit of 0.01 ppb has been reported for In in Ge [O]. Yet another type of scheme was applied by MIRZA and DULEY [Q] and by D~NSZELMANNand NEIJZEN [A]. In these schemes the In atoms are excited from one of their ground state configuration levels directly to a high Rydberg level by means of a two-photon transition. The Rydberg level is then ionized with high efficiency by an electric field (a so-called 20$ process). With the proper choice of levels (5p 2P$,2 +-i 31p “P”) the two-photon transition can be resonantly enhanced by the presence of the 6s 2S1,2 level almost half-way between the two levels [A]. These schemes have the simplicity of a single-color process while being amenable to Doppler free techniques. A number of studies using the SIRIS technique have been applied to study the presence of In in Si [B, R, S] and report linearity of the In RIS signal for In concentrations of 8 ppb to 7.8 ppm and detection limits of 0.14 ppb. Calculations of oscillator strengths and photoionization cross sections for this data sheet are made using the Hartree-Fock code with relativistic corrections of COWAN [2] with empirical adjustment of the calculated Hartree-Fock energy of the 5s5p2 configuration to fit the observed spectrum [3] of In. A Grotrian diagram of the RIS schemes in indium is shown in Fig. 2 [3-5, A]. RZS schemes of type (wl + q)

and (q + w2) [B-N] One- or two-color two-photon process consisting of a resonance step followed by photoionization with either a photon of the same color as the initial step or with a

1150

E. B. SALOMAN 300

256.0 271.0

or

337 or 410.2 nm

oror

41862 41836

cm-’ cm’

39098 39049

cm-i cm’

32918 32892

cm’ cm’

2213

0

cn?

cni’

Fig. 2. Grotrian diagram of resonance ionization spectroscopy schemes in indium.

photon of another color. Al = 239, 252, 256, 271, 304, 326, 410 or 451 nm. A2 = AI or 2hI or 308, 337, 532 or 1064 nm.

Data for RIS schemes of type (or + 0,) and (or + 02) 13, 4, 6, 71 Low.

Level

Ree. 0.000

1

2212.598 0.000 271.0262

5P %,a0

303.9347

5P %,a

325.6078

I

2212.598 0.000 2212.598

0.000

I

2212.598

7d %,a

I

E, (cm-‘)

I

f,,

1 41836.443

1

0.009

7d 2%,a

41861.978

0.01

6d %,a

39048.576

0.079

6d %,a

39098.464

0.066

5d %,a

32892.230

0.36

I 32915.539

I

2212.598

451.1297

Level

I

0.31

5d %,a

32892.230

0.035

6s %,a

24372.956

0.141

6s %,a

I 24372.956

I

0.156

1151

RIURIMS Data Service

Lifetimes and photoionization cross sections from excited states [3, 4, 6, 8-101 The following table lists the wavelength, AI, the resonant level, its lifetime, T, an estimate of the laser power, Psat, required to saturate the resonant transition, the photoionizing wavelength, AZ, the cross section, u, for photoionization of the resonant level by radiation of wavelength A*, and an estimate of the laser energy requirement, E,,, in A2 for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

325.6078 1 5d 2Ds,21 7.5 1

325.8552

410.1751

451.1297

5d 2D,,2

6s 2S,,2

6s 2S,,2

7.0

7.1

7.1

35

220

34

13

326 532 652

1.8 14

308 337 410

0.47 0.72 1.6

300 337

0.47 0.72

28

Reference [F] reports the ion yield in their experimental set-up using the scheme 303.9 + 303.9 nm was similar to that for the scheme 303.9 + 607.9 nm but 33 times greater than for the scheme 410.2 + 410.2 nm. RI.!?schemes of type (ol + WE) [J, 0, P] Two-color, two-photon process consisting of a resonance step followed by excitation to a high-lying Rydberg state. The Rydberg state is ionized with very high efficiency by a suitably timed electric field pulse or by a constant electric field. AI = 410.2 or 451.1 nm. Ap = 451.2 or 456.3 nm.

E. B. SALOMAN Data for FUS schemes of type co1 + @)

[l, 3,571

Lifetimes and ionization cross section from excited levels [5, 6, 81 Listed in tabular form for the first step of these schemes are the laser wavelength, X1, to the resonant level, the resonant level, its energy, E,, its lifetime, T, an estimate of the laser power, Psat, required to saturate the resonant transition, and for the second step the laser wavelength, AZ, the Rydberg level, an estimate of its radiative lifetime rn, and an estimate of the energy, E,,,, delivered during the lifetime of the resonant state required to saturate the transition to the Rydberg level. Note that the actual lifetime of the Rydberg levels will depend on experimental conditions such as collision rates, electric and magnetic fields, blackbody radiation, etc. An electric field pulse ionizes the Rydberg level with near 100% efficiency.

RISRIMS

Data Service

1153

RZS schemes of type @I.$) [A, Q]

One-color process consisting of a two-photon transition to a high-lying Rydberg level. The Rydberg level is ionized with high efficiency by a pulsed or constant electric field. X1 = 451.2 nm. Data for RI.9 schemes of type (2wp) [4, 51

Lifetimes and ionization cross section from excited states [6, A]

The presence of the 6s 2SIn level almost exactly half-way between the ground state and the 31~ 2P levels enhances the efficiency of the two-photon transitions. The radiation lifetime of the 31~ 2P Rydberg levels is of the order of 100 l~.sbut the actual lifetime will be a function of experimental conditions. Electric field ionization can be 100% efficient. Isotope data [ll]

Stable Isotopes: l131n (4.3%) Z = 9/2; llsIn (95.7%) Z = 9/2. Unstable Isotopes: “‘In Z = 9/2 T = 2.806 d. Isotope shiftslhfs [ 12-151

The isotope shift of the indicated transition of the indicated isotope of In is given in the following table relative to this transition in l151n.

Isotope shift (1O-3 cm-l)

410.2 nm

The hyperfine structure constants for the indicated levels of the indicated isotopes of In are given in the following table.

E. B. SALOMAN Hypertine structure interaction constants ( 10m3 cm-‘)

For the natural isotopic mixture of l131n and “‘In, the hyperfine structure constants for the indicated levels of In are given in the following table.

Hype&e

structure interaction constants (10e3 cm-*)

Laser schemes

For 239.0 nm, Nd:YAG or XeCl pumped frequency doubled Coumarin 480 dye laser; for 252.1 and 256.0 nm, Nd:YAG or XeCl pumped frequency doubled Coumarin 500 dye laser; for 271.0 nm, Nd:YAG or XeCl pumped frequency doubled Coumarin 54OA dye laser; for 303.9 nm, Nd:YAG pumped frequency doubled Rhodamine 640 dye laser [G]; for 308 nm, XeCl laser [J]; for 325.6 and 325.9 nm, Nd:YAG pumped frequency doubled .DCM dye laser; for 337 nm, nitrogen laser; for 410.2 nm, Nd:YAG or XeCl pumped DPS dye laser; for 451-457 nm, Nd:YAG or XeCl pumped Coumarin 460 dye laser; for 532 nm, second harmonic of Nd:YAG; for 607.9 nm, Nd:YAG pumped Rhodamine 640 dye laser; for 651.2 nm, Nd:YAG pumped DCM dye laser; for 1064 nm, Nd:YAG direct IR. Atom reservoirs and sources

Atoms of indium have been prepared for RIS studies by several methods: evaporation from a crucibl‘e [D] or a graphite furnace [P] or carbon reduction on a rhenium filament [G]; an atomic beam [J, 01; flame atomization in an acetylene/air rod flame [I]; and by ion beam sputtering (SIRIS) [B, C, E, F, H, K-M, R, S]. RIS references A. Donszelmann and J. H. M. Neijzen, Acta Phys. Pol. A63, 201 (1983). N. Winograd, J. P. Baxter and F. M. Kimock, C/rem. Phys. Lett. 88, 581 (1982). F. M. Kimock, J. P. Baxter and N. Winograd, Surf. Sci. 124, LA1 (1983). M. L. Muchnik, Yu. V. Orlov, G. D. Parshin, E. Ya, Chemyak, V. S. Letokhov and V. I. Mishin, Sov. J. Quanium Electron. 13, 1515 (1983). [E] N. Winograd, Ion beam studies of surfaces by multiphoton resonance ionization of sputtered neutrals, in Resonance Ionization Spectroscopy 1984, Eds G. S. Hurst and M. G. Payne p. 161. The Institute of Physics, Bristol (1984). [F] F. M. Kimock, J. P. Baxter, D. L. Pappas, P. H. Kobrin and N. Winograd, And. Chem. 56, 2782 (1984). [A] [B] [C] [D]

RIS/RIMS Data Service

1155

[G] L. J. Moore, J. D. Fassett and J. C. Travis, Anal. Chem. 56, 2770 (1984). [H] D. L. Pappas, D. M. Hrubowchak, M. H. Ervin and N. Winograd, Science 243, 64 (1989). [I] N. Chekalin, M. Marunkov and I. Vlasov, Analytical applications of RIS in flames, in Resonance Ionization Spectroscopy 1988, Eds, T. B. Lucatorto and J. E. Parks, p. 175. The Institute of Physics, Bristol (1989). [J] I. M. Beterov and V. L. Kurochkin, Space charge effects under nonlinear ionization of indium in a two-frequency field, in Resonance Ionization Spectroscopy 1988, Eds T. B. Lucatorto and J. E. Parks, p. 371. The Institute of Physics, Bristol (1989). [K] S. W. Downey, A. B. Emerson, R. F. Kopf and J. M. Kuo, Surf. Interface Anal. 15, 781 (1990). [L] H. F. Arlinghaus, M. T. Spaar and N. Thonnard, J. Vat. Sci. Technot. A 8, 2318 (1990). [M] M. Andersson, H. Fallgren and A. Rosen, A time of flight mass spectrometer for the analysis of solids, gases and liquids, in Resonance Ionization Spectroscopy 1990, Eds J. E. Parks and N. Omenetto, p. 459. The Institute of Physics, Bristol (1991). [N] T. Gilbert, Applications de la spectromttrie de masse par ionisation laser rtsonnante a l’etude des processus multiphotoniques, des interactions laser-matCriaux et a l’analyse de traces dans les materiaux. These, Universite d’OrlCans (1991, unpublished). [0] I. M. Beterov, V. L. Kurochkin and I. G. Yudelevich, 1. Appl. Spectrosc. 42, 11 (1985). [P] R. Zilliacus, J. Likonen and I. Auterinen, Precision and accuracy of RIS-analysis, in Resonance Ionization Spectroscopy 1990, Eds J. E. Parks and N. Omenetto, p. 279. The Institute of Physics, Bristol (1991). [Q] M. Y. Mirza and W. W. Duley, Proc. R. Sot. London, Ser. A 364, 255 (1978). [R] J. E. Parks, H. W. Schmitt, G. S. Hurst and W. M. Fairbank Jr, Ultrasensitive elemental analysis of solids by sputter initiated resonance ionization spectroscopy, in Laser-Based Ultrasensitive Spectroscopy and Detection V, Ed. R. A. Keller, p. 32. SPIE-The International Society for Optical Engineering, Bellingham (1983). Proc. Sot. Photo-Opt. Instrum. Eng. 426, 32 (1983). [S] H. F. Arlinghaus, M. T. Spaar, N. Thonnard, A. W. McMahon and K. B. Jacobson, Proc. Sot. Photo-Opt. Instrum. Eng. 1435, 26 (1991).

Data references [l] C. E. Moore, Atomic Energy Levels, National Standard Reference Data Series (U.S. National Bureau Standards) NSRDS-NBS 35, Vol. 3 (1971). [2] R. D. Cowan, The Theory of Atomic Structure and Spectra. University of California Press, Berkeley (1981). [3] S. George, G. Guppy and J. Verges, J. Opt. Sot. Am. B 7, 249 (1990). [4] I. Johansson and U. Litz6n, Ark. Pys. 34, 573 (1967). [5] J. H. M. Neijzen and A. Diinszelmann, Physica lllC, 127 (1981) and extrapolations on the data from this paper. [6] Calculated using the Hartree-Fock code with relativistic corrections of Ref. [2] with adjusted relative configuration energies. [7] J. R. Fuhr and W. L. Wiese, Atomic transition probabilities, in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 10-128. CRC Press, Boca Raton, Florida (1990). [8] K. B. Blagoev, Ya. F. Verolainen, V. N. Gorshkov and V. A. Komarovskii, Opt. Spectrosc. (USSR) 59, 566 (1985). [9] G. Jiinsson, H. Lundberg and S. Svanberg, Phys. Rev. A 27, 2930 (1983). [lo] M. A. Zaki Ewiss and C. Snoek, J. Phys. B 16, L153 (1983). [ll] N. E. Holden, Table of the isotopes (revised 1990), in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 11-33. CRC Press, Boca Raton, Florida (1990). [12] J. Eberz, U. Dinger, G. Huber, H. Lochmann, R. Menges, R. Neugart, R. Kirchner, 0. Klepper, T. Ktihl, D. Marx, G. Ulm and K. Wendt, Nucf. Phys. A464, 9 (1987). [13] R. Menges, G. Huber, G. Ulm and T. Ktihl, Z. Phys. A 320, 575 (1985). [14] G. H. Fuller, J. Phys. Chem. Ref. Data 5, 835 (1976). [15] C.-J. Lorenzen, K. Niemax and K.-H. Weber, Opt. Commun. 52, 178 (1984).

1157

RISiRIMS Data Service

Data sheet for .RIS/RIMS schemes Lithium Li Ground State [l] First Ionization Energy [2]

43487.150 cm-l

z=3 W2.r 2&,2 = 5.391721 eV

RIS has been applied to the analysis of lithium by many groups. Four types of RIS schemes have been demonstrated. In the simplest schemes, the lithium atoms are excited by the first photon from their ground state to one of the 2 2P or 3 2P resonant levels, which is then photoionized by another photon of the same color, as demonstrated by FASSETTet al. [A], or another color, as demonstrated by KARLOV et al. [B], ARISAWA et al. [C] and proposed by KRAMERet al. [D] (so-called o1 + o1 or or + o2 processes). The single-color scheme has the simplicity of a single-color process. All have the lower selectivity of a single resonance step but the two-color schemes allow for separate optimization of resonance and ionization steps. HERGENR~DERand NIEMAX[E] demonstrated a second type of scheme in which the lithium atoms were excited from their ground state to a 2 2P resonant level by the first photon. Then this level was excited to the 3 2S level by the second photon and that level was excited to an it 2P (n 2 15) level by the third photon. This high Rydberg level was subsequently ionized by an electric field (a so-called o1 + o2 + OF process). One of these schemes will be covered in this data sheet. These schemes have the high selectivity of three resonant steps combined with the high efficiency of electric field ionization. However, it is experimentally complex, requiring three tuned lasers and an electric field. KRAMER et al. [D] demonstrated a third type of scheme that is capable of single-atom detection. In their scheme the lithium atoms were excited from their ground state to a 2 2P resonant level, which was then excited to a 3 2D level by a second photon. This level was then photoionized by a third photon of the same color as one of the first two photons (so-called w1 + o2 + w1 or w1 + w2 + w2 processes). A related scheme was used by HERGENR~DERand NIEMAX[El. They excited the lithium atoms from their ground state to a 2 2P resonant level with the first photon. This level was then excited to the 3 2S level by a second photon and this level was then photoionized by a third photon of another color (a so-called w1 + w2 + w3 process). Another scheme of this type is suggested by laser enhanced ionization work [F]. The lithium atoms are again excited from their ground state to a 2 2P resonant level by the first photon, then excited to a 4 2D level by the second photon, from which they can be photoionized by IR from a Nd:YAG pump laser. This type of scheme can be quite sensitive and has the selectivity of two resonance steps. A fourth type of scheme was demonstrated by LORENZENand NIEMAX[G] in a thermionic diode and by BLAZEWICZet al. [HI. In these schemes the lithium atoms are excited by a two-photon transition from their ground state to a level of the same parity as the ground state and this state is photoionized by a third photon from the same laser (a so-called 2wr + w1 process). The two-photon resonant levels used were the 4 2S, 3 2D, and 4 2D levels. These schemes have the simplicity of a single-color process but have reduced selectivity owing to the single resonance requiring relatively high laser power. They are amenable to Doppler-free techniques for isotopically selective excitation. RIS has been applied in Li to the study of the photodissociation cross section of LiI [I], the reaction of Li with O2 in the presence of noble gasses [J], the fluctuations in atomic and molecular systems [K], to the measurement of the diffusion coefficient of Li in argon [L], and to materials analysis [Ml. A Grotrian diagram of the RIS schemes in lithium is shown in Fig. 3 [l, 21. RZS schemes of type (wl + wl) or (wl + w2) [A-D]

One- or two-color, two-photon process consisting of a resonance step followed by photoionization with ~a photon of the same wavelength as the initial step or by a

1158

E. B.

SALOMAN

r/43467 cni' 43376

cm-'

4 h,z* 5/z / 36623 cri' 35012 cn? 31263

c&i'

30925

c&i'

27206 cni' \ 3 zD,,z,5/z

14904 cm'

' ocni'

Fig. 3. Grotrian diagram of resonance ionization spectroscopy

photon of another wavelength. or 337.1 nm or 646.5 nm.

schemes in

X1 = 323.3 nm or 670.8 nm. X2 = 266 nm or 323.3 nm

Data for RIS schemes of type (wl + q) or (0, + 02) [I, 3] A, (M

Low.

E, (cm-l)

Level

Res.

Level

E, (cm-‘)

f tin

323.2657

2

2%,2

0.00

3

2h,2'

30925.38

3.60-3

323.2657

2

2%,2

0.00

3

2pI,2'

30925.38

1.8E-3

670.7756

2

2s,,2

0.00

2 2p2,2'

14904.00

0.502

670.7909

I

2 2%,2

I

0.00

I

I

14903.66

I

0.251

Lifetimes and photoionization cross section from excited states [l, 4, 51 In tabular form are listed the laser wavelength for the first step, X1, to the resonant level, the resonant level, its lifetime, T, an estimate of the laser power, P,,,, required to saturate the resonant transition, the wavelength used for photoionization, AZ, the photoionization cross section, u, of the resonant level for radiation of wavelength AZ, and an estimate of the energy, Esat, delivered during the lifetime of the resonant state required to saturate the photoionization.

RWRIMS

* Reference

Data Service

[B] reports a measurement of 10 f 3

x

lo-‘* cm2 for this cross section.

RZS schemes of type (q

+ o2 + o$) [E] Three-color, three-photon process consisting of two resonance steps followed by excitation to a high-lying Rydberg state. The Rydberg state is ionized with very high efficiency by a suitably timed electric field pulse or by a constant electric field. X1 = 670.8 nm. X2 = 812.6 nm. A? = 618.2 nm.

Data for RIS schemes of type (ol + o2 + OF) [l, 3-61 Low.

Level

E,

(cm-‘)

670.7756 812.6446 618.1744

2 %,z 2 zp3,2° 3 %,z

0.00 14904.00 27206.12

670.7756 812.6446 610.1744

2 2 3

14904.00 27206.12

670.7909 812.6221 618.1744

2 2 3

670.7909 812.6221 618.1744

2 *s,,* 2 *P,,z” 3 %,2

*s,,* 2p3,2’ *s,,* *s,,* =~I,*’ *s,,*

0.00

0.00

14903.66 27206.12 0.00

14903.66 27206.12

Res.

Level

2 =p3,2* 3 %,z 32 2P,,2’ 2

2P3,2e

3 *s,,* 32 *P,,,’ 2

*~l,za

3 *%,r 32 *P,,,’ 2

*~I,*’

3 *s,,* 32 *P,,,’

E,

(cm-‘)

f &S

14904.00 27206.12 43378.31

0.502 0.113 3.8E-6

14904 .oo 27206.12 43378.31

0.502 0.113 7.63-6

14903.66 27206.12 43370.31

0.251 0.113 3.83-6

14903.66 27206.12 43378.31

0.251 0.113 7.63-6

Lifetimes and ionization cross section from excited levels [l, 4, 6, 71

Listed in tabular form for the first step of these schemes are the laser wavelength, X1, to the resonant level, the resonant level, its energy, E,, its lifetime, T, and an estimate of the laser power, Psat, required to saturate the resonant transition.

E. B. SALOMAN

1160

The following table lists the wavelengths, A2, of the second step of these schemes to the 3 2S level, an estimate of the energy needed to saturate the second step, Es,,, in A2 within the absorption bandwidth while the first step is being pumped, the second resonant level, its lifetime, 7, the wavelength of the transition to the Rydberg level, X3, the lifetime of the Rydberg level, rn, and an estimate of the laser energy requirement, EEt, in A3 delivered during the lifetime of the resonant state required to saturate the transition to the Rydberg state. Electric field ionization can be 100% efficient.

oftype (wl + w2 + wl) or (ol + w2 + w2) or (wl + w2 + 03) [D, E] Two- or three-color, three-photon process consisting of two resonance steps followed by photoionization. AI = 670.8 nm, A2 = 460.3, 610.4 or 812.6 nm, A3 = AI or A2 or 532 or 1064 nm.

RZS schemes

Data for RIS schemes of type (ol + o2 + OJ

or (ol + 02 + 02) or (WI+ 02 + ~3) [I, 3,4,61

RN/RIMS Data Service

1161

Lifetimes and photoionization

cross section from excited states [l, 3-71 Listed in tabular form for the first step of these schemes are the laser wavelength, X1, to the resonant level, the resonant level, its energy, E,, its lifetime, T, and an estimate of the laser power, Pa, required to saturate the resonant transition.

The following table lists the wavelengths, AZ, of the second step of these schemes to one of the 3 2S, 3 2D, or 4 2D levels, an estimate of the energy needed to saturate the second step, Esat, in A2 within the absorption bandwidth while the first step is being pumped, the second resonant level, its lifetime, T, the photoionizing wavelength, X3, the photoionization cross section of the resonant level for radiation of wavelength X3, and an estimate of the laser energy requirement, Erit, in A3 for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

RZS schemes of type (20~ + ol) [G, H]

One-color process consisting of a two-photon transition to the resonant level followed by photoionization with a third photon of same color as used for the initial step. AI = 545.9, 571.1, or 639.1 nm.

1162

E. B.

SALOMAN

Data for RIS schemes of type (20,

639.1466

1

2 2S,,2

0.00

+ ol) [l]

I

I

31283.08

Lifetimes and photoionization

cross section from excited states [l, 4, 5, 81 The presence of the 2 *P levels roughly half-way between the ground state and the 3 *D levels may enhance the efficiency of the two-photon transition for these levels. The following table lists the wavelength, Al, the resonant level, its lifetime, T, the cross section, cr, for photoionization of the resonant level by light of wavelength Al, and an estimate of the laser energy requirement, E,,,, in X1 for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

Res.

639.1466

3

Level

2%,2

14.6

7.9

39

Isotope data [9]

Stable Isotopes: 6Li (7.5%) Z = 1; ‘Li (92.5%) Z = 312. Unstable Isotopes: none with T > 1 s. Isotope shiftlhfs [l, 10-14, G]

The isotope shift of the indicated transition in ‘Li with respect to 6Li is given in the following table. The table gives the wavelength of the transition, A, the lower level, the upper level, and the transition isotope shift. Note that for two-photon transitions (S + S or S + D) this shift is twice that of each of the two photons involved in the transition.

RWRIMS Data Service

I

Lower Level

Upper

2 %,a

460.3

2 aP112.

3/z”

3,2*

480

4 =Ua,z.

5/2O

164

5,z”

513.4

l

2 2S1,a

4 =Ua,a,

571.1

*

2 2S1,a

4 2S1,z

610.4

2 2Pr,a,

618.2

3 2S,,a *

3

3/z”

I

2 =s,,,

812.6

I

2 2pl,2,

2D3,2,

3/z-

312’

92 219

2D3,2.

5,z”

444.1

I

2 2p,,2.

312’

351.3

I

3 2S,,a

3

Shi

488.9

32 aP,,a. 3,2*

2 =%,a

670.8

Isotope

(low3 cm-l)

3 2P,,a.

545.9

639.1

Transition

Level

I

323.3

1163

31

* Two photons of this wavelength are required for the transition. Each accounts for half of the isotope shift.

The hyperfine structure constants for the indicated levels of the indicated isotopes of Li are given in the following table.

Hyperfine structure interaction constants (10V3 cm-‘)

* The hfs is smaller than the Doppler width.

Laser schemes For 266 nm, fourth harmonic of Nd:YAG laser [Cl; for 323.3 nm, frequency doubled Nd:YAG pumped DCM dye laser [A]; for 337.1 nm, N2 laser [B]; for 460.3 nm, Nd:YAG pumped Coumarin 460 dye laser; for 532 nm, second harmonic of Nd:YAG laser; for 545.9 nm, argon ion pumped Rhodamine 560 dye laser [G] or Nd:YAG pumped Rhodamine 590 dye laser; for 571.1 nm, argon ion pumped Rhodamine 590 dye laser [G] or Nd:YAG pumped Rhodamine 590 dye laser; for 610.4 nm, Nd:YAG pumped Sulforhodamine 640 dye laser; for 618.2 nm, Nd:YAG pumped Rhodamine 640 dye laser; for 639.1 nm, argon ion pumped Rhodamine 640 dye laser [G] or Nd:YAG pumped DCM dye laser; for 646.6 nm, Nd:YAG pumped DCM dye laser; for 670.8 nm, Nd:YAG pumped Rhodamine 640 [B, C] or DCM dye laser; for 812.6 nm, GaAlAs diode laser or Nd:YAG pumped LDS 821 dye laser; and for 1064 nm, Nd:YAG direct IR. Atom reservoirs and sources Atoms of lithium have been prepared for RIS studies by several methods: laser dissociation of a vapor of LiI molecules [D, I-L]; thermal vaporization [N]; a thermally

1164

E. B.

SALOMAN

produced atomic beam [Cl; sputtering with a pulsed ion beam [Ml; and passing an aqueous solution of lithium salt through an air/acetylene burner into a sampling cell

WI. RIS references [A] J. D. Fassett, L. J. Moore, J. C. Travis and J. R. DeVoe, Science 230, 262 (1985) and J. D. Fassett, private communication. [B] N. V. Karlov, B. B. Krynetskii and 0. M. Stel’makh, Sov. J. Quanrum Electron. 7, 1305 (1977). [C] T. Arisawa, Y. Maruyama, Y. Suzuki and K. Shiba, Appl. Phys. B 28, 73 (1982). [D] S. D. Kramer, J. P. Young, G. S. Hurst and M. G. Payne, Opt. Commun. 30, 47 (1979). [E] R. Hergenriider and K. Niemax, Continuous wave field ionization laser spectroscopy, in Resonance Zonizution Spectroscopy 1990, Eds J. E. Parks and N. Omenetto, p. 133. The Institute of Physics, Bristol (1991). [F] Yu. Ya. Kuzyakov and N. B. Zorov, Crit. Rev. Anal. Chem. 20, 220 (1988). [G] C.-J. Lorenzen and K. Niemax, J. Phys. B 15, L139 (1982). [H] P. R. Blazewicz, W. B. Whitten and J. M. Ramsey, Anal. Chem. 61, 1010 (1989). [I] B. E. Lehmann, S. D. Kramer, S. L. Allman, G. S. Hurst and M. G. Payne, Chem. Pfryr. Lett. 71, 91 (1980). [J] S. D. Kramer, B. E. Lehmann, G. S. Hurst, M. G. Payne and J. P. Young, J. Chem. Phys. 76, 3614 (1982). [K] J. Iturbe, S. L. Ahman, G. S. Hurst and M. G. Payne, Chem. Phys. Lett. 93, 460 (1982). [L] J. P. Judish and R. K. Wunderlich, J. Phys. B. 20, 2317 (1987). [M] J. E. Parks, D. W. Beekman, H. W. Schmitt and E. H. Taytor, Nucl. Instrum. Methods BlO/ll, 280 (1985). [N] L. J. Moore, J. D. Fassett, J. C. Travis, T. B. Lucatorto and C. W. Clark, Proc. Sot. Photo-Opt. Instrum. Eng. 426, 18 (1983).

Data references [l] C. E. Moore, Atomic Energy Levels, National Standard Reference Data Series (U.S. National Bureau Standards) NSRDS-NBS 35, Vol. 1 (1971). [2] C. E. Moore, Ionization Potentials and Ionization Limits Derived from the Analysis of Optical Spectra, National Standard Reference Data Series (U.S. National Bureau Standards) NSRDS-NBS 34 (1970). [3] J. R. Fuhr and W. L. Wiese, Atomic transition probabilities, in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 10-128. CRC Press, Boca Raton, Florida (1990). [4] G. Peach, H. E. Saraph and M. J. Seaton, J. Phys. B 21, 3669 (1988) and private communication. [5] Calculated using the Hartree-Fock code with relativistic corrections of R. D. Cowan, The Theory of Atomic Structure and Spectra. University of California Press, Berkeley (1981). [6] G. A. Martin and W. L. Wiese, J. Phys. Chem. Ref. Data 5, 537 (1976). [7] C. E. Theodosiou, Phys. Rev. A 30, 2881 (1984). [8] W. Hansen, J. Phys. B 16, 933 (1983). [9] N. E. Holden, Table of the isotopes (revised 1990) in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 11-33. CRC Press, Boca Raton, Florida (1990). [lo] A. Beckmann, K. D. Biiklen and D. Elke, 2. Phys. 270, 173 (1974). [ll] G. H. Fuller, /. Phys. Chem. Ref. Data 5, 835 (1976). [12] H. Orth, H. Ackermann and E. W. Otten, Z. Phys. A 273, 221 (1975). [13] J. Kowalski, R. Neumann, H. Suhr, K. WinkIer and G. zu Putlitz, Z. Phys. A 287, 247 (1978). [14] R. H. Hughes, Phys. Rev. 99, 1837 (1955).

RIS/RIMS Data Service

1165

Data sheet for RWRIMS schemes Potassium K Ground State [l] First Ionization Energy [l]

Z = 19 ls22s22p63s23p64s 2S1,2 35009.8140 cm-l = 4.340665 eV

RIS has been applied to the analysis of potassium by many groups. Three types of RIS schemes have been demonstrated and another was proposed for greater sensitivity and selectivity. BEEKMANet al. [A], CAPELLEef al. [B], and HARRISONet al. [C] used schemes in which the K atoms were excited from their ground state by the first photon to one of the 5 2p0 resonant levels, which was then photoionized by a second photon from the same laser (a so-called w1 + wi process). These schemes have the simplicity of a single-color process but have the lower selectivity of a single resonance scheme in which the laser power must be sufficient to drive the photoionization step. CALCO~T et al. [D] used a variant of this scheme by exciting the resonant level as above but then photoionizing with the UV output from a XeCl laser (a so-called w1 + w2 process). In this case, the laser power driving the resonant and photoionization steps can be separately optimized. NIEMAX and PENDRILL [E, F, G], working with a thermionic diode, used schemes in which the K atoms were excited from their ground state by a two-photon transition to an n 2S or it 2O level and then photoionized by a third photon of the same color (a so-called 20, + w1 process). These schemes also have the simplicity of a single-color process and are amenable to Doppler-free techniques but have the disadvantages of the relative inefficiency of the two-photon transition and the lower selectivity of a single resonance process using high laser powers. KUDRYAVTSEV ef al. [H, I] carried out isotopically selective ionization of accelerated K atoms using a scheme in which the K atoms were excited from their ground state by the first photon to the 4 2p;/2 resonant level, which was then excited to one of the 21 2O levels by a second photon. This high-lying Rydberg level was then ionized with great efficiency by an electric field (a so-called w1 + wp process). This is a very efficient process with the selectivity of two resonant steps. LETOKHOV [J] proposed a scheme in which the atom would be excited from its ground state to the 4 2p3/2 level by the first photon. This level would be excited to the 6 2S1,2 level by the second photon and then photoionized by a third photon say from a Nd:YAG laser (a so-called w1 + w2 + w3 process). However a calculation [2] indicates that the photoionization cross section from the 6 2S, level is rather small, so this proposed scheme would be inefficient. As an alternative, a scheme of this type is proposed in which the K atom is excited from its ground state to one of the 4 2p0 resonant levels by the first photon. A second photon excites the atom from this level to one of the 4 ‘0 levels from which it is photoionized by an IR photon from a Nd:YAG laser. This scheme has the selectivity of two resonant steps and should be relatively efficient. Analysis of trace quantities of K is important in determining the purity of semiconductor materials and in determining the presence of the small fraction of naturally occurring slightly radioactive isotope “OK. A Grotrian diagram of the RIS schemes in potassium is shown in Fig. 4 [l]. RZS schemes of type (wl + wl) or (wl + w2) [A-D]

One- or two-color, two-photon process consisting of a resonance step followed by photoionization with a photon of the same wavelength as the initial step or by a photon of another wavelength. Ai = 404-405 nm or 766-770 nm. A2 = A1 or 308 nm.

E. 3. SALOMAN

1166 306

or

693-697

or nm

34754 34754

cm-

27396

cm’

cm-'

li 27397 m-L’ 24720 eni' 24701 cti'

3043 cm' 12985 c&l

Fig. 4. Grotrian diagram of resonance ionization spectroscopy schemes in potassium.

Data for RIS schemes of type (ol + 0%) or (ol + y)

[l, 31

Lifetimes and photoionization cross section from excited states [l-4] In tabular form are listed the laser wavelength for the first step, Al, to the resonant level, the resonant level, its lifetime, T, an estimate of the laser power, P,,, required to saturate the resonant transition, the wavelength used for photoionization, AZ, the photoionization cross section, u, of the resonant level for radiation of wavelength AZ, and an estimate of the energy, ES,,, delivered during the lifetime of the resonant state required to saturate the photoion~ation.

RISBUMS Data Service

1167

Res. Level

4

(nm) 404.4142

5 ’ Pano

134

98

308 404.4 532

2.4 3.5 4.4

270 140 86

404.7213

5 ‘P,,’

137

95

308 404.7 532

2.4 3.5 4.4

270 140 88

788.4911

4 2 PJ/2’

308

5.7

110

769.8974

25.8

1.3

I

RIS schemes of type (q

+ ~5) [H, I] Two-color, two-photon process consisting of a resonance step followed by excitation to a high-lying Rydberg state. The Rydberg state is ionized with very high efficiency by a suitably timed electric field pulse or by a constant electric field. A1 = 766.5 nm. A5 = 460.5 nm.

Data for RIS schemes of type (q + OF) [l-3]

Lifetimes and ionization cross section from excited levels [ 1, 3, 51

Listed in tabular form for the first step of these schemes are the laser wavelength, Ar, to the resonant level, the resonant level, its energy, E,, its lifetime, T, an estimate of the laser power, Psat, required to saturate the resonant transition, and for the second step the laser wavelength, AZ,the Rydberg level, its lifetime Tn, and an estimate of the energy, E,,,, delivered during the lifetime of the resonant state required to saturate the transition to the Rydberg state. Electric field ionization can be 100% efficient.

4 (nm)

I Eil I(e%)

768.5 1 42Pu2’ 785.5 I 4’P&

I 13043

25.8

1.3

460.4572

21 ‘h/2

3.5E4

7.6

25.8

1.3

460.4573

21 2Qn

3.5E4

1.3

RZS schemes of type (ol + o2 + q)

or (q

+ o2 + WJ or (q

+ o2 + w3)

Two- or three-color, three-photon process consisting of two resonance steps followed by photoionization. A1 = 766.5 or 769.9 nm, X2 = 693-697 nm, A3 = A1 or A2 or 1064 nm.

E. B. SALOMAN

1168 Data for RIS

schemes of type (0, + o, + q)

Lifetimes and photoionization

or (ol + m2 + ~2) or (q

+ 0~ + 03) [l-3]

cross section from excited states [l, 3, 61

Listed in tabular form for the first step of these schemes are the laser wavelength, AI, to the resonant level, the resonant level, its energy, E,, its lifetime, 7, and an estimate of the laser power, P,,,, required to saturate the resonant transition.

The following table lists the wavelengths, AZ, of the second step of these schemes to one of the 4 *II levels, an estimate of the energy needed to saturate the second step, ES,,, in A2 within the absorption bandwidth while the first step is being pumped, the second resonant level, its lifetime, T, the photoionizing wavelength, A3, the photoionization cross section of the resonant level for radiation of wavelength X3, and an estimate of the laser energy requirement, Ef$, in A3 for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

RIYRIMSData Service

1169

RZS schemes of type (20~ + ol) [E-G] One-color process consisting of a two-photon transition to the resonant level followed by photoionization with a third photon of same color as used for the initial step. Al = 729.8 nm.

Data for RIS schemes of type (20~ + wl) [I]

Lifetimes and photoionization cross section from excited states [l, 2, 61 The following table lists the wavelength, Al, the resonant level, its lifetime, 7, the cross section, u, for photoionization of the resonant level by light of wavelength X1, and an estimate of the laser energy requirement, Z& in X1 for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

A, (ml

Res. Level

7

u

(ns)

(lo-'*cn+)

Esat(mJ/cm2)

729.7753

4 %,z

286

13

21

729.8038

4

291

13

21

2D5,2

Isotope data [7] Stable Isotopes: 39K (93.2581%) Z = 3/2; 41K (6.7302%) Z = 3/2. Naturally occurring radioactive 40K has an abundance of 0.0117%. Unstable Isotopes: 40K Z = 4 T = 1.26 x lo9 y; 42K Z = 2 T = 12.36 h; 43K Z = 3/2 T = 22.3 h. Isotope shiftslhfs [g-11, E, G] The isotope shift of the 769.9 nm line of the indicated isotope of K is given in the following table relative to this line in 39K.

Isotope Mass Number

39

40

41

42

43

Isotope Shift (10m3cm-')

0

4.189

7.848

11.73

15.31

The isotope shift of the 766.5 nm line of the indicated isotope of K is given in the following table relative to this line in 39K.

Isotope

Mass

Isotope

Shift

Number (10m3 cm”)

39

40

41

0

4.217

7.88

1170

The 11.7 x cm-l. in the

E. B.

isotope shift of the 693.6 nm 10V3 cm-l and those of the The isotope shift of the 460.5 following table relative to this

SALOMAN

line of 41K with respect to this line in 3vK is 696.4 nm and 696.5 nm lines are 11.6 x lop3 nm lines of the indicated isotope of K is given line in 3vK.

The isotope shift of each 729.8 nm photon in the two-photon transitions in 41K with respect to that in 3vK is 9.8 x 10e3 cm-i. The hyperfine structure constants for the indicated levels of the indicated isotopes of K are given in the following table.

Hyperfine structure interaction constants (10m3 cm-‘)

Laser schemes

For 308 nm, XeCl excimer laser [D]; for 404.4 or 404.7 nm, XeCl or XeF pumped DPS dye laser [B]; for 460.5 nm, XeCl pumped Coumarin 460 dye laser; for 532 nm, second harmonic of Nd:YAG laser; for 693-697 nm, Nd:YAG pumped LDS 698 dye laser; for 729.8 nm, Nd:YAG pumped LDS 750 dye laser; for 766.5 or 769.9 nm, XeCl pumped Oxazine 750 dye laser or Nd:YAG pumped LDS 765 dye laser; for 1064 nm, Nd:YAG direct IR. Atom reservoirs and sources

Atoms of potassium have been prepared for RIS studies by several methods: thermal evaporation [A, D]; laser vaporization of K on a solid surface [A]; laser dissociation of KI vapor [B]; a glow discharge source using 10% KC1 in a graphite electrode [Cl; a Knudsen cell [Cl; and an accelerated atomic beam [H, I]. RIS references

[Al D.-W. Beekman, T. A. Callcott, S. D. Kramer, E. T. Arakawa, G. S. Hurst and E. Nussbaum, ht.

M [Cl

J. Mass Spectrom. Ion Phys. 34, 89 (1980). G. A. Capelle, D. A. Jessup, H. M. Borella and L. A. Franks, Resonance

ionization spectroscopy measurements of the vapor pressure of several molecular species, in Resonance Ionization Spectroscopy 1984, Eds. G. S. Hurst and M. G. Payne, p. 77. The Institute of Physics, Bristol (1984). W. W. Harrison, P. J. Savickas, R. K. Marcus and K. R. Hess, Laser enhanced ionization in glow discharge mass spectrometry, in Analytical Spectroscopy, Ed. W. S. Lyon, p. 173. Elsevier, Amsterdam (1984).

RIS/RIMS Data Service

1171

[D] T. A. Callcott, D. W. Beekman and E. T. Arakawa, Detection of K and rare earth isotopes using a resonance ionization source and a TOF-mass spectrometer, in Resonance Ionization Spectroscopy 1984, Eds G. S. Hurst and M. G. Payne, p. 337. The Institute of Physics, Bristol (1984). [E] K. Niemax and L. R. Pendrill, J. Phys. B 13, L461 (1980) and J. Phys. B 14, 1371 (1981). [F] K. Niemax, Acta. Phys. Pol. A61, 517 (1982). [G] L. R. Pendrill and K. Niemaz, J. Phys. B 15, L147 (1982). [H] Yu. A. Kudryavtsev, V. S. Letokhov and V. V. Petnmin, JETP Lett. 42, 26 (1985). [I] Yu. A. Kudryavtsev, V. S. Lekokhov and V. V. Petnmin, Laser detection of very rare long-lived radioactive isotopes, in Applied Laser Spectroscopy, Eds W. Demtroder and M. Inguscio, p. 329. Plenum, New York (1990). [J] V. S. Letokhov, Laser Photoionization Spectroscopy. Academic Press, Orlando (1987).

Data references [l] J. Sugar and C. Corliss, J. Phys. Chem. Ref. Data 14, Suppl. 2 (1985). [2] Calculated using the HartreeFock code with relativistic corrections of R. D. Cowan, The Theory of Atomic Structure and Spectra. University of California Press, Berkeley (1981). [3] J. R. Fuhr and W. L. Wiese, Atomic transition probabilities, in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 10-128, CRC Press, Boca Raton, Florida (1990). [4] R. W. Berends, W. Kedzierski, J. B. Atkinson and L. Krause, Spectrochim. Acta 43B, 1069 (1988). [5] Extrapolated from data of Ref. [6]. [6] C. E. Theodosiou, Phys. Rev. A 30, 2881 (1984). [7] N. E. Holden, Table of the isotopes (revised 1990), in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 11-33. CRC Press, Boca Raton, Florida (1990). [8] F. Touchard, P. Guimbal, S. Btittgenbach, R. Klapisch, M. de Saint Simon, J. M. Serre, C. Thibault, H. T. Duong, P. Juncar, S. Liberman, J. Pinard and J. L. Vialle, Phys. Lett. 108B, 169 (1982). [9] N. Bendali, H. T. Duong and J. L. Vialle, J. Phys. B 14, 4231 (1981). [lo] G. H. Fuller, J. Phys. Chem. Ref. Data 5, 835 (1976). [ll] S. Horblck, A.-M. Pendrill, L. Pendrill and M. Pettersson, Z. Phys. A 318, 285 (1984).

REVRIMS

Data Service

1173

Data sheet for RWRIMS schemes Rubidium z = 37

Rb

ls22s22p63s23p63d104s24p65s2S1,2 33690.81 cm-l = 4.177130 eV

Ground State [l] First Ionization Energy [2] RIS schemes applicable

to the analysis of rubidium have been studied by many types of RIS schemes have been demonstrated. In the simplest type of scheme the rubidium atoms are excited by the first photon from their ground state to one of the IZ2P” (n = 5-7) resonant levels, which is then photoionized by either another photon of the same color (n 2 6) or a photon of another color (socalled o1 + or or wi + o2 processes). Schemes of this type have been demonstrated by AMBARTZUMIANef al. [A-C], KLYUCHAREVand SEPMAN[D], CAPELLEet al. [E] and TOWRIEet al. [F]. The single-color schemes have the simplicity of a single-color process that requires only one laser. All have the lower selectivity of a process with only one resonance step but the two-color schemes allow for separate optimization of resonance and ionization steps. For situations where more selectivity is required, some schemes involving two resonant steps are suggested. Here the Rb atoms are excited from their ground state to a 5 2P” level by the first photon and this level is then excited to one of the 6 2D or 7 2D levels by the second photon. That level is then photoionized by a photon with the color of one of the first two photons or by an IR photon from the pump laser (so-called w1 + w2 + o i or o1 + o2 + o2 or o1 + o2 + w3 processes). Schemes of this type have been reported recently by SHAW et al. [G]. Another type of scheme was demonstrated by SMYTHet al. [HI. In these schemes, the Rb atoms are excited from their ground state by a two-photon transition to an IZ2S or n 2D resonant level, which is then photoionized by a third photon of either the same color as those used in the two-photon transition or of a second color (so-called 2wr + o1 or 20~ + o2 processes). The 20~ + or schemes have the simplicity of single-color processes but both types of schemes have reduced selectivity due to having a single resonance step that requires relatively high powers to drive the two-photon transition. Several other schemes that involve excitation to high Rydberg levels with subsequent collisional or electric field ionization [H-M] will not be covered here, nor will an elegant scheme

groups. A number of different

cm

crc; cm cni’ cni’

23793 23715

Cd

cni’

12817 cm12579 &i’

Fig. 5. Grotrian

diagram of resonance

ionization

spectroscopy

schemes

in rubidium.

1174

E. B. SALOMAN

of BEKOV et al. [N], which involves excitation, radiative decay, and then excitation to a Rydberg level. RIS has been applied to Rb for the measurement of excited state photoionization cross sections [B-D] and to the measurement of the vapor pressure curve of RbI [El. A Grotrian diagram of the RIS schemes in rubidium is shown in Fig. 5 [l, 21. RZS schemes of type (wl + ox) or (q

+ 02) [A-F] One- or two-color, two-photon process consisting of a resonance step followed by photoionization with a photon of the same wavelength as the initial step or by a photon of another wavelength. hi = 358.7, 359.2, 420.2, 421.6, 780.0, or 794.8 nm. A2 = 347.1, 420.2, 421.6, 441.6, 532, 694, or 1064 nm. Data for RIS schemes of type (ol + q)

or (ol + OJ [l, 31

Lifetimes and photoionization

cross section from excited states [l, 3-51 In tabular form are listed the laser wavelength for the first step, hi, to the resonant level, the resonant level, its lifetime, 7, an estimate of the laser power, P,,,, required to saturate the resonant transition, the wavelength used for photoionization, Aa, the photoionization cross section, u, of the resonant level for radiation of wavelength AZ, and an estimate of the energy, ES,,, delivered during the lifetime of the resonant state required to saturate the photoionization.

(a) (b) (c) (d)

Reference [C] reports a measurement Reference [C] reports a measurement Reference [C] reports a measurement Reference [D] reports a measurement

of of of of

1.9 f 0.3 17 2 3 x 15 + 3 x 9.6 k 2.5

x 1O-*8 cmZ. lo-l8 cm*. 1O-18 cm*. x lo-l8cm*.

RWRIMS

RIS schemes of type (q

Data Service

1175

+ o2 + 01) or (ol + 02 + 02) or (01 + 02 + 03)

Two- or three-color, three-photon process consisting of two resonance steps followed by photoionization. AI = 780.0 or 794.8 nm, A2 = 565, 572, 621, or 630 nm, A3 = AI or A2 or 1064 nm.

Data for RIS schemes of type (q + w, + ol) or (01 + 02 + 02) or (01 + ~2 + 03) 11, %6,71

Lifetimes and photoionization

cross section from excited states [l, 3-71 Listed in tabular form for the first step of these schemes are the laser wavelength, AI, to the resonant level, the resonant level, its energy, E,, its lifetime, 7, and an estimate of the laser power, P,,,, required to saturate the resonant transition.

A,

cm

Res.

Level

E, (cm-‘)

7 (ns)

Prat

(W/cm2)

780.0259

5 2p3,20

12816.56

27.0

0.76

794.7597

5 2p1,20

12578.96

29.4

0.71

The following table lists the wavelengths, Al, of the second step of these schemes to one of the 6 *O, or 7 *O levels, an estimate of the energy needed to saturate the second step, ESat, in A2 within the absorption bandwidth while the first step is being pumped, the second resonant level, its lifetime, T, the photoionizing wavelength, AS, the photoionization cross section of the resonant level for radiation of wavelength AS, and an estimate of the laser energy requirement, E$ in A3 for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

1176

E. B. SALOMAN

RZS schemes of type (2q + ol) or (2wl + w-J [H] One- or two-color process consisting of a two-photon transition to the resonant level followed by photoionization with a third photon of either the same color as used for the initial step or another color. X1 = 660, 697, or 760 nm. AZ = AI or 1064 nm.

Data for RIS schemes of type (20, + 0,) or (20, + 02) [l]

A, (ml

1Low.

Level

I E, (cm-‘)

Res.

Level

I

E, (cm-‘)

660.2828

5

2s,,2

0.00

660.3157

5

2%,2

0.00

696.9291

=

2sl,2

0.00

6

2D5,2

28689.41

696.9840

5 2s,,2

0.00

6

2D2,2

28687.15

759.9159

5

0.00

7

2%,2

2%,2

I

26311.46

Lifetimes and photoionization cross section from excited states [l, 4, 51 The presence of the 5 2P” levels roughly half-way between the ground state and the 7 2S,, levels may enhance the efficiency of the two-photon transition for this level. The following table lists the wavelength, AI, the resonant level, its lifetime, T, the photoionixation wavelength, AZ,the cross section, cr, for photoionixation of the resonant level by light of wavelength A2, and an estimate of the laser energy requirement, Besot, in A2 for photoionixation efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

1177

RWRIMS DataServim

660.2828

7 %r

660.3157

7 %,a

696.9291

6 %t

320

660 1064

3.4 8.8

89 21

331

660 1064

3.4 a.0

a9 21

244

697 1064

6.7 15.8

43 12

Isotope data [8]

Stable Isotopes: =Rb (72.17%) Z = 5/2; 87Rb (27.83%) Z = 3/2. (Note that s’Rb decays with a half-life of 4.88 x W’y.) Unstable Isotopes: =Rb Z = 5/2 T = 86.2 d.; 84Rb Z = 2 T = 32.9 d.; 86Rb Z = 2 T = 18.63 d. Isotope shiftslhfs [%16] The isotope shift for the transition with the indicated wavelength, A, of the specifkd

isotope with respect to the position of that line in 87Rb is given in the following table.

*tib

stib

%b

The hyperfine structure constants for the indicated levels of the stable Rb isotopes are given in the following table.

E. B.

SALOMAN

Hyperfine structure constants (lo-3 cm-*)

The hyperfine structure constants for the indicated levels of the indicated unstable Rb isotopes are given in the following table.

Hyperfine structure interaction constants ( 10m3 cm-l)

* The uncertainty of this value exceeds its stated value.

Laser schemes For 347 nm, doubled ruby laser [A, B, C]; for 3%-360 nm, Nd:YAG pumped frequently doubled LDS 750 dye laser; for 420-422 nm, doubled ruby pumped POPOP dye laser [B, C], or flash-lamp pumped LD 423 or LD 425 dye laser [El, or N2 laser pumped POPOP dye laser [N], or XeCl or Nd:YAG pumped Stilbene 420 dye laser [F]; for 441.6 nm, Cd ion laser [D]; for 532 nm, second harmonic of Nd:YAG; for 564-573 nm, Nd:YAG pumped Rhodamine 590 dye laser; for 620-660 nm, Nd:YAG pumped DCM dye laser; for 694 nm, ruby laser [B, C]; for 697 nm, Nd:YAG pumped LDS 698 dye laser; for 760-780 nm, Nd:YAG pumped LDS 795 dye laser or diode

RISRIMS

Data Service

laser [G]; for 794.8 nm, ruby pumped DTTCI dye laser [A], or Nd:YAG LDS 798 dye laser; for 1064 nm, Nd:YAG direct IR.

1179

pumped

Atom reservoirs and sources

Atoms of rubidium have been prepared for RIS studies by several methods: heating Rb metal [A, C, G-M], N2 laser dissociation of RbI [El, laser or ion ablation [F], and using a Rb atomic beam [N]. RIS references [A] R. V. Ambartsumyan, V. N. Kalinin and V. S. Letokhov, JETP Lert. 13, 217 (1971). [B] R. V. Ambartzumyan, N. P. Furzikov, V. S. Letokhov and A. A. Puretsky, Appl. Phys. 9, 335 (1976). [C] R. V. Ambartsumyan, A. M. Apatin, V. S. Letokhov, A. A. Makarov, V. I. Mishin, A. A. Puretskii and N. P. Furzikov, Sov. Phys. JETP 43, 866 (1976). [D] A. N. Klyucharev and V. Yu. Sepman, Opt. Spectrosc. 38, 712 (1975). [E] G. A. Capelle, D. A. Jessup, H. M. Borella and L. A. Franks, Resonance ionization spectroscopy measurements of the vapor pressure of several molecular species, in Resonance Ionization Spectroscopy 1984, Eds. G. S. Hurst and M. G. Payne, p. 77. The Institute of Physics, Bristol (1984). [F] M. Towrie, S. L. T. Drysdale, R. Jennings, A. P. Land, K. W. D. Ledingham, P. T. McCombes, R. P. Singhal, M. H. C. Smyth and C. J. McLean, ht. J. Muss Spectrom. Ion Processes 96, 309 (1990). [G] R. W. Shaw, J. P. Young and J. M. Ramsey, Resonance ionization of rubidium using sequential diode laser-driven transitions, in Resonance Ionization Spectroscopy 1992, Eds C. M. Miller and J. E. Parks, p. 297. The Institute of Physics, Bristol (1992). [H] M. H. C. Smyth, S. L. T. Drysdale, R. Jennings, A. P. Land, K. W. D. Ledingham, R. P. Singhal, D. T. Stewart, M. Towrie and C. M. Houston, Molecular and collisional processes during three photon ionisation transitions in caesium and rubidium vapours, in Resonance Ionization Spectroscopy 1988, Eds T. B. Lucatorto and J. E. Parks, p. 73. The Institute of Physics, Bristol (1989). [I] C. B. Collins, S. M. Curry. B. W. Johnson, M. Y. Mirza, M. A. Chellehmalzadeh, J. A. Anderson, D. Popescu and I. Popescu, Phys. Rev. 14, 1662 (1976). [J] K. C. Harvey and B. P. Stoicheff, Phys. Rev. Lett. 38, 537 (1977). [K] K. Niemax, Appl. Phys. B 32, 59 (1983). [L] T. J. Whitaker and B. A. Bushaw, Chem. Phys. Lett. 79, 506 (1981). [M] J. M. Ramsey, W. B. Whitten, D. E. Goeringer and B. T. Buckley, Collisional and electric-field ionization of laser-prepared Rydberg states in an ion trap mass spectrometer, in Resonance Zonizotion Spectroscopy 1990, Eds J. E. Parks and N. Omenetto, p. 301. The Institute of Physics, Bristol (1991). [N] G. I. Bekov, V. S. Letokhov and V. I. Mishin, Opt. Commun. 23, 85 (1977).

Data references [l] C. E. Moore, Atomic Energy Levels, National Standard Reference Data Series (U.S. National Bureau Standards) NSRDS-NBS 35, Vol. 2 (1971). [2] C. E. Moore, Ionization Potentials and Ionization Limits Derived from the Analyses of Optical Spectra, National Standard Reference Data Series (U.S. National Bureau Standards) NSRDS-NBS 34 (1970). [3] J. R. Fuhr and W. L. Wiese, Atomic transition probabilities, in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 10-128. CRC Press, Boca Raton, Florida (1990). [4] Calculated using the Hartree-Fock code with relativistic corrections of R. D. Cowan, The Theory of Atomic Structure and Spectra University of California Press, Berkeley (1981). [5] C. E. Theodosiou, Phys. Rev. A 30, 2881 (1984) and compilation therein. [6] W. A. van Wijngaarden, K. Bonin, W. Happer, E. Miron, D. Schreiber and T. Arisawa, Phys. Rev. Lett. 56, 2024 (1986). [7] D. von der Goltz, W. Hansen and J. Richter, Phys. Ser. 30, 244 (1984). [8] N. E. Holden, Table of the isotopes (revised 1990), in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 11-33. CRC Press, Boca Raton, Florida (1990). [9] C. Thibault, F. Touchard, S. Biittgenbach, R. Klapisch, M. de Saint Simon, H. T. Duong, P. Jacquinot, P. Juncar, S. Liberman, P. Pillet, J. Pinard, J. L. Vialle, A. Pesnelle and G. Huber, Phys. Rev. C 23, 2720 (1981). [lo] P. Grundevik, M. Gustavsson, A. Rosen and S. Svanberg, 2. Phys. A 283, 127 (1977). [ll] H. M. Gibbs and G. C. Churchill, J. Opt. Sot. Am. 62, 1130 (1972). [12] G. H. Fuller, J. Phys. Gem. Ref. Data 5, 835 (1976). [13] D. Feiertag and G. zu Putlitz, Z. Phys. 261, 1 (1973). [14] W. A. van Wijngaarden, K. D. Bonin and W. Happer, Phys. Rev. A 33, 77 (1986). [15] W. A. van Wijngaarden and J. Sagle, J. Phys. B 24, 897 (1991). [16] F. Ackermann, I. Platz and G. zu Putlitz, Z. Phys. 260, 87 (1973).

RIYRIMS Data !kvice

1181

Data sheet for RIS/RIM!3 schemes Silver & Ground State [l] First Ionization Energy [2]

z = 47 ls22s22p63s23p63d’04s24p64d’05s=Sln 61106.56 cm-’ = 7.576251 eV

RIS has been applied to the analysis of silver by one group. HAVRILLAetal.[A, B] used a scheme in which the silver atoms were excited from their ground state to the 5p =P’& resonant level by the first photon. The atoms were then excited from this level to the 5d =Dsn level by a second photon and then photoionized by a third photon of the same color as the second photon (a so-called o1 + 02 + o2 process). This scheme has the selectivity of two resonance steps although the power of the second step must be sufficient to drive the photoionization. This limitation can be removed by using the second harmonic of the Nd:YAG pump laser radiation for the photoionization step (a so-called o1 + w2 + o3 process). References [A] and [B] used both resonance and non-resonance photoionization to study the depth profile of Ag in solid samples. Several other possible RIS schemes of the type o1 + 02 + o3 will be suggested. In addition, schemes are considered in which the atom is excited from its ground state to one of the 5p =P” levels by the first photon, then excited from this level to the 8s 2&n level by a second photon. The 8s =??I/2is then excited by a third photon to the 485~5~ =P& autoionizing level (a so-called w1 + o2 + WY process). This scheme has two narrow, highly selective resonance steps and a third relatively broad step (about 42 nm FWIIM) to the autoionizing level that calculations [3] indicate should be a rather efficient ionization mechanism. Also suggested is a scheme in which the silver atoms are excited from their ground state to an lld =D level by a twophoton transition and then photoionized by an IR photon from a Nd:YAG pump laser (a so-called 20~ + o2 process). The two-photon transition may be enhanced by the 5p =P” levels that are approximately half-way between the ground state and the

61107 59752 59751 54214 54203

om’

48764

cni’

40744

elii’

30473

cm’

29552

cn?

cm_; cm.

cni’ cn?

0 cm’

Fig. 6. Grotrian diagram of resonance ionization spectroscopy schemes in silver.

E. B. SALOMAN

1182

lld 2D levels. This scheme should be amenable to Doppler-free techniques but it has the selectivity limitations of only a single resonance step, which requires relatively high laser power. Calculations indicate that the one-color, single resonance processes involving the 6p 2P levels are relatively inefficient especially for the 206-207 nm wavelengths required. Ultrasensitive detection of Ag may be useful in locating commercial deposits of silver and in monitoring waste streams for the loss of this material. A Grotrian diagram of the RIS schemes in silver is shown in Fig. 6 [l, 2,

41. RZS schemes of type (wl + w2 + 02) or (ol + w2 + 6.~~)[A, B]

Two- or three-color, three-photon process consisting of two resonance steps followed by photoionization. Ai = 328.1 or 338.3 nm, X2 = 405-422 or 520-548 or 768.8 or 827.4 nm, X3 = A2 or 532 or 1064 nm.

Data for RIS schemes of type (q + o2 + CO*) or (q + o2 + 03) [l, 3,.5, 61

Lifetimes and photoionization

cross section from excited states [l, 3, 5-81 Listed in tabular form for the first step of these schemes are the laser wavelength, hi, to the resonant level, the resonant level, its energy, E,, its lifetime, 7, and an estimate of the laser power, Psat, required to saturate the resonant transition.

RWRIMS

1183

DataService

The following table lists the wavelengths, AZ, of the second step of these schemes to one of the 6s *S, 5d *D, or 6d *II levels, an estimate of the energy needed to saturate the second step, ES,,, in A2 within the absorption bandwidth while the first step is being pumped, the second resonant level, its lifetime, T, the photoionizing wavelength, h3, the photoionization cross section of the resonant level, u, for radiation of wavelength h3, and an estimate of the laser energy requirement, I?:$, in A3 for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

405.5472

l.lE-3

6d 2%z

26

405.5 1064

0.48 14

1000 14

421.0956

8.63-4

6d 2D,,2

31

421.1 1064

0.48 14

970 14

421.2814

5.23-3

6d 2Ds,,

26

421.3 1064

0.48 14

970 14

520.9068

l.SE-4

5d 2D,,2

11

520.9 532

3.6 3.9

110 95

546.5498

l.lE-4

5d 2Ds,,

13

532 546.5

3.9 4.3

95 86

547.1547

6.43-4

5d

2%,2

11

532 547.2

3.9 4.3

95 84

768.7766

1.2E-4

6s

2%,2

22

532

2.0

190

827.3515

5.53-5

6s 2S,,,

22

532

2.0

190

RIS schemes of type (ol + o2 + &) Three-color process consisting of two resonance steps followed by excitation to an autoionizing level. A1 = 328.1 or 338.3 nm. A2 = 398.2 or 384.1 nm. A3 = 602 nm.

DataforRIS schemes oftype(0,+ o2 + I&) [1,3-51 Low.

328.0679

398.1576 602 338.2889

384.0744 602

Level

5s 2%,z 5P 2%,2e 8s

%2

El

(cm-‘)

I

Res. Level

E, (cm-l)I

0.00 30472.71

30472.71

55581.29

72177

0.00 29552.05

29552.05

55581.29

fabs 0.4754

55581.29 7.81-3 0.039

0.2316 55581.29 7.93-3 72177 0.039

E. B. SALOMAN

1184

Lifetimes and ~~~ionizution rates from excited states [l, 3-5, 71 Listed in tabular form for the first step of these schemes are the laser wavelength, hi, to the resonant level, the resonant level, its energy, E,, its lifetime, T, and an estimate of the laser power, P,,, required to saturate the resonant transition.

The following table lists the wavelengths, h2, of the second step of these schemes to the 8s *S level, an estimate of the energy needed to saturate the second step, Esot, in AZwithin the absorption bandwidth while the first step is being pumped, the second resonant level, its lifetime, T, the wavelen~h, h3, to the autoioni~ng level, the effective ionization cross section of the resonant level for radiation of wavelength As, and an estimate of the laser energy requirement, E$,i, in hJ for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

384.0744

0.010

398.1576

4.9%3

8s

%,2

98

602

9.4

35

98

602

9.4

35

The autoionization rate for the 485~5~ “P%j12 level is calculated to be 2.2 x lOi s-l in agreement with the measured value [4]. Thus the autoionization rate is not expected to be a limiting factor in the efficiency of these RIS schemes. The full width at half max~um (~~ of the final step in these schemes is expected to be about 42 nm. RIS schemes of type (20~ + w2)

Two-color process consisting of a two-photon transition to the resonant level followed by photoionixation with a third photon of a different color than used for the initial step. hi = 334.6 nm, A2 = 1064 nm. (Calculations [3] indicate a small photoionization cross section for the resonant level by 334.6 nm radiation.) Data for RIS schemes of type (20, + 02) [I]

Lifetimes and photoionization cross section from excited states [l, 31

The presence of the 5p 2P levels roughly half-way between the ground state and the lid *D levels may enhance the efficiency of the two-photon transition for these

RHRIMS DataService

1185

levels. The following table lists the .wavelength, Al, the resonant level, its lifetime, T, the cross section, o, for photoionixation of the resonant level by light of wavelength 1064 nm, and an estimate of the laser energy requirement, E,,, in 1064 nm for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

334.6234 I

lld 2D,,z I

334.6254 I

lld *D,,*

320

I

0.94

I 200

Isotope data [9]

Stable Isotopes: l”‘Ag (51.839%) Z = l/2, l=‘Ag (48.161%) Z = l/2. Unstable Isotopes: losAg Z = l/2 T = 41.3 d., lOs”‘AgZ = 6 T = 1.3 x 102 y., llO”‘Ag I = 6 T = 249.8 d. Isotope shiftslhfs [5, lo-121 The isotopeshifts of the 328.1 and 338.3 nm lines of the indicated isotope of Ag is

given in the following table relative to these lines in l”‘Ag.

Isotope Mass Number Isotope Shift (lo+ cm-l)

107

10&a

109

1lOm

0

-1.2

-15.1

-17.3

The isotope shift of the indicated line in lwAg with respect to that line in lo7Ag is given in the following table.

Wavelength

(ma)

Isotope

Shift

405.5

2.4

421.1

2.6

520.9

1.8

546.5

1.8

547.2

1.7

768.0

3.5

827.4

3.5

(10m3 cm-‘)

1186

E. B.

SALOMAN

The hyperfine structure constants for the indicated levels of the indicated isotopes of Ag are given in the following table. Hyperfhre structure interaction constants (lo+

cm-l)

Laser schemes

For 32g-339 nm, Nd:YAG pumped frequency doubled DCM dye laser [A] or KrF pumped DMT dye laser; for 384.1 nm, Nd:YAG pumped Exalite 384 dye laser or XeCl pumped Exalite 376 dye laser; for 398.2 nm, Nd:YAG or XeCl pumped Exalite 398 dye laser; for 405.6 nm, Nd:YAG pumped DPS dye laser; for 421.1 or 421.3 nm, Nd:YAG pumped Stilbene 420 dye laser; for 532 nm, second harmonic of Nd:YAG laser; for 520-548 nm, Nd:YAG pumped Coumarin 485 dye laser [A]; for 602 nm, Nd:YAG pumped Rhodamine 640 dye laser; for 768.8 nm, Nd:YAG pumped LDS 765 dye laser; for 827.4 nm, Nd:YAG pumped LDS 821 dye laser; for 1064 nm, Nd:YAG direct IR. Atom reservoirs and sources

Atoms of silver have been prepared for RIS studies by pulsed ion beam sputtering of a solid sample (SIRIS) [A]. Evaporation from a hot filament and laser ablation should also be effective methods to prepare atoms of silver for RIS. RI.5 references [A] G. J. Havrilla, M. Nicolas, S. R. Bryan and J. G. Pruett, Comparison of resonant and nonresonant ionization in sputtered initiated laser ionization spectrometry, in Resonance Ionization Spectroscopy 1990, Eds J. E. Parks and N. Omenetto, p. 405. The Institute of Physics, Bristol (1991). [B] G. J. Havrilla, M. Nicholas, S. R. Bryan and J. G. Pruett, Proc. Sot: Photo-Opt. Instrum. Eng. 1435, 12 (1991).

Data references [l] C. E. Moore, Atomic Energy Levels, National Standard Reference Data Series (U.S. National Bureau Standards) NSRDS-NBS 35, Vol. 3 (1971). [2] C. M. Brown and M. L. Ginter, J. Opt. Sot. Am. 67, 1323 (1977). [3] Calculated using the HartreeFock code with relativistic corrections of R. D. Cowan, The Theory of Atomic Structure and Spectra. University of California Press, Berkeley (1981) with empirical term energy corrections. [4] A. M. Cant& E. Jannitti, M. Mazzoni, M. Pettini and G. Tondello, Phys. Ser. 19, 283 (1979). [S] J. Carlsson, P. Jonsson and L. Sturesson, 2. Phys. D 16, 87 (1990). [6] J. R. Fuhr and W. L. Wiese, Atomic transition probabilities, in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 10-128. CRC Press, Boca Raton, Florida (1990). [7] J. Zhankui, P. Jonsson, J. Larsson and S. Svanberg, Z. Phys. D 17, 1 (1990). [8] G. L. Plekhotkina, Opt. Spectrosc. (USSR) 51, 106 (1981).

RISIRIMS Data Service

1187

[9] N. E. Holden, Table of the isotopes (revised 1990), in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 11-33. CRC Press, Boca Raton, Florida, (1990). [lo] W. Fischer, H. Htihnermann and Th. Meier, Z. Phys. A 274, 79 (1975). [ll] M. Elbel and W. Fischer, Z. Phys 166, 504 (1%2). [12] G. H. Fuller, J. Phys. Chem. Ref. Data 5, 835 (1976).

1189

RIWRIMS Data Service

Data sheet for RI!3/RIlW schemes Titanium Ti Ground State [l] First Ionization Energy [2, A]

z = 22 ls22s22p63s23p63d=4s2 3F2 55072.5 cm-’ = 6.82812 eV

Several groups have applied RIS to the analysis of titanium. Three different types of RIS schemes have been used. Using one type of scheme and thermal atomization, MOOREet al. [B] studied many RIS lines, and MAYOet al. [C] used a subset of these lines, optimized to avoid interferences with vanadium RIS lines. GOBERTet al. [ID, E], GIBERT [F], and GELIN et nl. [G] used ion sputter atomization (SIRIS) and two of the schemes studied in Ref. [B]. In this type of scheme, the Ti atoms are excited from a thermally populated level of the ground state configuration to one of the w 3F”, z ‘PO, or v 31;b odd parity resonant levels by the first photon. The resonant level is then photoionized by a second photon of the same color (a so-called w1 + o1 process). These schemes have the simplicity of a single-color process requiring only one laser but have the lower selectivity of a single resonance step in which the laser power must be sufficient to also drive the photoionization step. The use of other resonance levels with this type of scheme has been reported recently [HI. With this type of scheme, detection limits of 10 ppb were reported [E, G] for the analysis of Ti in steel. A second type of scheme is a modification of the first, which uses separate lasers of different colors for the excitation and the photoionization steps allowing separate optimization of the intensities of each step. The Ti atoms are excited from a level of the ground state configuration to a resonance level by the first photon and then photoionized from the resonant level by a second photon from a pump laser or one of its harmonics (a so-called o1 + o2 process). Schemes of this type were applied by PELLIN et al. [I] using SIRIS who reported the ability to measure 30 ppb Ti impurity in the surface atomic layer of silicon wafers. Schemes of this type using the z 3Do resonant levels were studied by MARUYAMAet al. [J] as laser isotope separation schemes. A third type of scheme was demonstrated by YOUNGet al. [K] and SPIEGEL et al. [L]. In their schemes, Ti atoms are excited from their ground state to the t 3B resonant level by the first photon. This level is then excited to either the e 3F2 or the g 3F2 level by the second photon. The atoms are then efficiently ionized by exciting them from these levels to an autoionizing level (a so-called w1 + o2 + wf’ process). These authors have investigated a number of such schemes and the two most efficient ones will be covered here. This type of scheme has the enhanced selectivity of two discrete resonance steps plus a broader autoionizing step. Using the most suitable scheme (via the e 3F2), Ref. [L] reports determining the relative isotopic abundance of Ti in a hibonite-rich inclusion in a meteorite while removing only 4 x lOlo atoms of Ti or 8 X 1012 total atoms from the sample. They also report observing isotopic fractionation effects of as much as 48% favoring the odd isotopes over the even. This problem was dealt with by using material of known isotopic composition as a standard. Other types of scheme are possible. Some, studied by SOHL et al. [A], involve two-step excitation of either autoionizing or high Rydberg levels (which can then be efficiently ionized with an electric field). These will not be covered here. PARKSet al. [M] applied SIRIS to determine the concentration of Ti in lithium niobate crystals as a function of depth, finding values between 0.18 and 35%. However the RIS schemes used were not reported. Calculations of lifetimes and photoionization cross sections for this data sheet are made using the Hartree-Fock code with relativistic corrections of COWAN [3] with empirical adjustment of the calculated Hartree-Fock energies to fit the observed spectrum of Ti [l]. A Grotrian diagram of the RIS schemes in titanium is shown in Fig. 7 [l, 2, A, L].

ll!Xl

E. B.

SALOMAN

RIS schemes of type (q

+ q) and (wl + 02) [A-G, I, J] One- or two-color two-photon process consisting of a resonance step followed by photoionization with either a photon of the same color as the initial step or with a photon of another color. A1 = 293-299 or 496-507 nm. AZ = hi or 308 or 266 nm.

Data for RIS schemes of type (q + ol) and (ol + 4)

Lifetimes and photoionization

[l, 4-61

cross sections from excited states [l, 6-9, B] The following table lists the wavelength, Xi, the resonant level, its lifetime, T, an estimate of the laser power, P,,,, required to saturate the resonant transition, the photoionizing wavelength, AZ, the cross section, u, for photoionization of the resonant level by radiation of wavelength AZ, an estimate of the laser energy requirement, ESat, in A2for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state, and the relative intensity of the RIS scheme, Zrel, obtained from the data reported in Ref. [B].

RWRIMS

294.8256

1 v 3F30 1 4.8

1

180

295.6133

1 v ‘F,’

1

180

295.6797

1 4.9

v ‘Fzo

4.8

Data Service

295

940

1

296 308

I

840

I 130

1.2

580

I

0.75 0.64

1000

0.80

296.7223

1 v 3Fso

4.8

1

1500

1 297

1

0.75

296.8243

1 w 3F30

1 48

1

240

1 297

1

10

296.9959

1z

1 -1.4E5

1 297

1

1

220

1 297

1

900

I

50

900

IlO

65

1 100

l

120

5.6

80

I

5

‘PI“

9.5

297.0384

w ‘F2” 1

51

298.1468

w 3F,o

45

2400

298

11

63

250

298.3314

w 3Fs0

48

150

298

10

65

80

298.5477

w 3Fs0

51

140

299

10

65

16

496.7298

z 3D3Q

190

266

8.6

87

499. JO98

z 3D2o

180

140

266

8.5

500.9646

2 3D30

190

250

266

8.6

87

501.4185

a ‘Dlo

168

11

266

8.5 +

88

503.9959

z ‘Da“

180

14

266

8.5 +

88

506.4654

z 3D30

190

13

266

8.6

87

10

65

l

+

1

60

88

ences [E, F] report a measured value of 30 X 10-i* cm* for this cross section while obtaining a *R value of 1.3 x 1O-18 cm2 by a quantum defect calculation. t Reference [J] reports a measured value of 74 X 1O-18 cm2 for this cross section.

RZSschemes of type (ml+ co2+ &I) [K, L] Three-color process consisting of two resonance steps followed by excitation to an autoionizing level. AI = 517.4 nm. A2 = 443.4 or 548.8 nm. A3 = 649.9 or 566.3’nm. Data for RIS schemes of type (CL++ o2 + C&I) [l, 4-6, K]

* This undesignated autoionizing level has a width of 2.4 cm-’ [K]. t This undesignated autoionizing level has a width of 0.6 cm-’ [K].

1192

E. B. SALOMAN 57255

.&I

55192

d

55073

cm'

41872

czi'

37539

cm'

33655 34205

- -1 cm

1993720127 crri' 19323 cm'

e

307 cm' 170 Cti_f 0 cm’

Fig. 7. Grotrian diagram of resonance ionization spectroscopy schemes in titanium.

Lifetimes and autoionization rates from excited states [l, 4-6, 8, 91 Listed in tabular form for the first step of these schemes are the laser wavelength, X1, to the resonant level, the resonant level, its energy, E,, its lifetime, T, and an estimate of the laser power, Psat, required to saturate the resonant transition.

The following table lists the wavelengths, AZ, of the second step of these schemes to either the g 3F2 or e 3F2 levels, an estimate of the energy needed to saturate the second step, ESat, in X2 within the absorption bandwidth while the first step is being pumped, the second resonant level, its lifetime, 7, and the wavelength, X3, to the autoionizing level.

1193

RISIRIMS Data Service

From the measured autoionization widths, the autoionization rate is not expected to be a limiting factor in the efficiency of these RIS schemes. The scheme that passes through the e 3F2 level was reported to be superior for discriminating against calcium

M* Isotope data [lo]

Stable Isotopes: 46Ti (8.0%) Z = 0, 47Ti (7.3%) Z = 5/2, 48Ti (73.8%) Z = 0, 49Ti (5.5%) Z = 7/2, 5oTi (5.4%) Z = 0. Unstable Isotopes: 44Ti Z = 0 T = 47 y., 45Ti Z = 712 T = 3.078 h. Zsotope shz@/hfs [ll, J] The isotope shift for the transition with the indicated wavelength, A, of the specified isotope with respect to the position of that line in &Ti is given in the following table.

Isotope shift (10m3 cm-l)

The hyperfine structure constants for the indicated isotopes are given in the following table.

levels of the stable odd Ti

Hyperfine structure interaction constants (10m3 cm-‘)

Laser schemes

For 266 nm, fourth harmonic of Nd:YAG laser [J]; for 293-299 nm, Nd:YAG pumped frequency doubled Rhodamine 610 dye laser [B, D, E]; for 308 nm, XeCl laser; for 443.4 nm, XeCl pumped Coumarin 440 dye laser; for 496507 nm, Nd:YAG pumped Coumarin 500 dye laser; for 517.4 or 548.8 nm, XeCl pumped Coumarin 500 dye laser; for 566.3 nm, XeCl pumped Coumarin 540A dye laser; for 649.9 nm, XeCl pumped DCM dye laser. Atom reservoirs and sources

Atoms of titanium have been prepared for RIS studies by several methods: heating samples with carbon on a rhenium filament [B, C]; using a Ti atomic beam [A]; vaporizing a sample with electron bombardment [J]; and using argon ion sputtering (SIRIS) [D-G, I, K-M].

1194

E. B.

SALOMAN

RI.9 references [A] J. E. Sohl, Y. Zhu and R. D. Knight, J. Opt. Sot. Am. B 7, 9 (1990). [B] L. J. Moore, J. D. Fassett and J. C. Travis, Anal. Chem. 56, 2770 (1984). [C] S. Mayo, J. D. Fassett, H. M. Kingston and R. J. Walker, Anal. Chem. 62, 240 (1990). [D] 0. Gobert, B. Dubreuil, P. Gelin, J. L. Debrun and R. L. Inglebert, Trace analysis using laser postionisation of sputtered neutral atoms-Preliminary results obtained with a Nd:YAG pumped dye laser and a quadrupole mass spectrometer, in Secondary Ion Mass Spectrometry SIMS VI, Eds A. Berminghoven, A. M. Huber and H. W. Werner, p. 845. Wiley, Chichester (1988). [E] 0. Gobert, T. Gibert, B. Dubreuil, P. Gelin and J. L. Debrun, J. Appl. Phys. 70, 7602 (1991). [F] T. Gibert, Appliciations de la spectrometrie de masse par ionisation laser resonnante a l’etude des processus multiphotoniques, des interactions laser-mattriaux et a l’analyse de traces dans les mattriaux, These, Universitd d’Orl6ans (1991, unpublished). [G] P. Gelin, J. L. Debrun, 0. Gobert, R. L. Inglebert and B. Dubreuil, Nucl. Instrum. Methods B40/ 41, 290 (1989).

[H] R. K. Wunderlich, G.J. Wasserburg, I. D. Hutchoen and G. A. Blake, Systematics of the odd-even effect in the resonance ionization of osmium and titanium, in Resonance Ionization Spectroscopy 1992, Eds C. M. Miller and J. E. Parks, p. 127. The Institute of Physics, Bristol (1992). [I] M. J. Pellin, C. E. Young, W. F. Calaway and D. M. Gruen, Nucl. Instrum. Methods B13, 653 (1986).

[J] Y. Maruyama, Y. Suzuki, T. Arisawa and K. Shiba, Appl. Phys. R. 44, 163 (1987). [K] C. E. Young, D. R. Spiegel, M. J. Pellin, W. F. Calaway, S. R. Coon, J. W. Burnett, D. M. Gruen, A. M. Davis and R. N. Clayton, Three-colour resonance ionization of sputtered Ti for isotopic analysis of meteoritic samples, in Resonance Ionization Spectroscopy 1990, Eds J. E. Parks and N. Omenetto, p. 435. The Institute of Physics, Bristol (1991). [L] D. R. Spiegel, W. F. Calaway, A. M. Davis, J. W. Burnett, M. J. Pellin, S. R. Coon, C. E. Young, R. N. Clayton and D. M. Gruen, Anal. Chem. 64, 469 (1992). [M] J. E. Parks, M. T. Spaar and P. J. Cressman, J. Cryst. Growth 89, 4 (1988).

Data references [l] J. Sugar and C. Corliss, J. Phys. Chem. Ref. Data 14, Suppl. 2 (1985). [2] R. H. Page and C. S. Gudeman, J. Opt. Sot. Am. B 7, 1761 (1990). [3] R. D. Cowan, The Theory of Atomic Structure and Spectra. University of California Press, Berkeley (1981). [4] G. A. Martin, J. R. Fuhr and W. L. Wiese, J. I’hys. Chem. Ref. Data 17, Suppl. 3 (1988) with a 14% increase of the relative oscillator strengths from results of Ref. [5]. [5] N. Grevesse, D. E. Blackwell and A. D. Petford, Astron. Astrophys. 208, 157 (1989). ’ [6] Calculated using the HartreeFock code with relativistic corrections of Ref. [3] with adjusted relative configuration energies. [7] S. Salih and J. E. Lawler, Astron. Astrophys. 239, 407 (1990). [8] J. E. Lawler, Astron. Astrophys. 252, 853 (1991). [9] R. M. Lowe and P. Hannaford, 2. Phys. D 21, 205 (1991). [lo] N. E. Holden, Table of the isotopes (revised 1990) in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 11-33. CRC Press, Boca Raton, Florida (1998). [ll] G. H. Fuller, J. I’hys. Chem. Ref. Data 5, 835 (1976).

RWRIMS Data Service

1195

Data sheet for RIWRIMS schemes Vanadium V Ground State [l] First Ionization Energy [2]

Z = 23 ls22s22p63s23p63d34s2 4F3,2 54413 cm-l = 6.7463 eV

A number of groups have applied RIS to the analysis of vanadium. Two different types of RIS schemes have been used. FASSEIT et al. [A], MOORE et al. [B] and MAYO et al. [C] used schemes in which the vanadium atom is excited from either the ground state or a thermally populated level of the a 4F ground state configuration or a thermally populated level of the a 6D low-lying configuration to an odd parity level by a photon in the 283-300 nm range. The odd parity level is then photoionized by a second photon of the same color (a so-called o1 + o1 process). These schemes have the simplicity of a single-color process but have the lower selectivity of a single resonance scheme in which the laser power must be sufficient to drive the photoionization step. With this type of scheme, multielement analysis was demonstrated [B] and a measurement of V impurities in SIMOX utilizing isotope dilution and RIMS was carried out with an equipment sensitivity in the pg/g range [Cl. The use of the isotope dilution technique avoids the need for standards to determine possible isotope fractionation effects. A second type of scheme was reported by THONNARD et aE. [D, E], but no information was provided about the wavelengths or atomic levels used. In this scheme, the atom is excited from a thermally populated low-lying level to the first resonant level. The atom is then excited from this resonant level to a second resonant level, which is subsequently photoionized by IR radiation from a Nd:YAG pump laser (a so-called w1 + o2 + o3 process). This type of scheme has the greater selectivity of two resonant steps for which the laser intensity can be individually optimized, good sensitivity for properly chosen transitions, and the easy availability of IR photons for photoionization with reduced likelihood they will produce significant non-resonant multiphoton ionization. Calculations have been carried out to identify several possible schemes of this type. THONNARD et al. [D] and PARKS et al. [F] utilizing the sputter initiated RIS technique (SIRIS) demonstrated good linearity for the RIS signal as a function of vanadium concentration over a range of 105 down to a concentration of 6 ppm with samples utilizing a variety of matrices. The determination of small concentrations of V and V isotopes may be important for determining impurities in semiconductor materials [C] and in tracing the metabolism of vanadium in biological systems [G] (where RIMS can avoid isobaric interferences with Cr and Ti). Calculations of transition probabilities and photoionization cross sections are made using the Hartree-Fock code with relativistic corrections of COWAN [3] with empirical adjustments of the calculated Hartree-Fock energies to fit the observed spectrum of V [l]. A Grotrian diagram of the RIS schemes in vanadium is shown in Fig. 8 [l, 21. RZS schemes of type (to1 + q) [A-C] One-color two-photon process consisting of a resonance step followed by photoionization with a photon of the same color as the initial step. A1 = 283-300 nm.

E. B. SALOMAN Data for RIS schemes of type (ol + 0,)

[l, 41

285.9970

a ‘Fw

137.38

u %/20

35092.52

286.4361

‘a ‘F,,,

323.46

u ‘b/Z0

35225.01

292.3620

a ‘Fw

552.96

v ‘b/20

34747.12

292.6262

a ‘FUZ

323.46

1.6E-3

34486.75 0.082 9.5E-4 1.7E-4

a ‘Fulz

294.2388

a ‘Fw

0.00

294.3188

a ‘Fw

0.00

294.6527

a ‘Fw

137.38

294.9627

a ‘%z

137.38

295.4334

a ‘Fw

137.38

295.5799

a ‘F,,,

552.96

295.7326

a ‘F,,,

323.46

296.2777

a ‘FUZ

323.46

297.7541

a ‘FWZ

552.96

298.1740

a Va12

299.4009

I

a ‘Fs12

‘Fw“

552.96

294.2318

33976.07 w ‘DuzO

137.38

33966.83 34065.72 7.4&-4

34030.06

2%20

w ‘=‘wO

33976.07 7.73-3

34374.88 34127.92

w %,zO

34065.72

I

0.00

I

0.12

34529.84

I

w %/zO

34127.92

Y 2%20

33527.68

Y 2b20

I

33527.68

0.11

I

0.011

Lifetimes and photoionization cross sections from excited states [l, 4, B, C] The following table lists the wavelength, X1, the resonant level, its lifetime, T, an estimate of the laser power, P,,,, required to saturate the resonant transition, the cross section, cr, for photoionization of the resonant level by radiation of wavelength, X1, an estimate of the laser energy requirement, ES,,, in X1 for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state, and the relative intensities, Ire,, obtained from the data reported in Refs ]B, Cl.

RIS/RIMSDataService

I

Res.

283.5642

Level

r

(ns) w 2%20

I

Pl*t (W/cd)

I

30

1197

E**t 0 (lo-" cm') I (mJ/cmZ)I

Ir*l

2.4

300

89

0.53

1300

100

283.8057

200

284.8773

250

I

600

I

0.69

I

1000

I

25

u 4%,20 200

1

550

1

0.53

1

1300

1

71

u 4D7/20

130

I

650

1

0.35

I

2000

I

140

285.5222

u 'h,aO

300

I

250

I

0.92

I

760

I

20

285.9970

u 'D,,zO

250

1

350

1

0.69

I

1000

125

286.4361

u 'bZO

200

I

310

I

0.53

I

1300

I

292.3620

v 'ho

2.7 1

250

1

8.6

79

850

1

2.4

290

1

25

160

1

1.8

390

I

79

1500 I

1.4

500

I

50

7000

0.63

1100

1

16

284.9170

285.1754

I

292.6262

'b,zO

293.5876

'hQ

293.7680

2b,20

I 86

293.8661

'S3/2O

/ 100

I

9-4

6300

I I

294.2318

140 I120

1

1

2.1

1

2.4

1

1

50

1 200

320

1 320

280

1

294.3188

w

‘h12°

120

5.0

130

93

294.6527

w

‘%120

I30

1.2

580

100

1.4

480

250

0.10

7100

320

1.8

380

50

0.09

7500

40

294.9627 295.4334

w

295.5799

2D3,20

83

‘D3,20

120

‘b/2o

9.4

295.7326

w

4b/20

130

296.2777 I

w

kb,2°

I130

297.7541

w

4D7,20

298.1740

Y

2%,20

14

299.4009

Y

2F3,20

14

RZS schemes of type (toI +

1300

1300

1

0.08

130

1

8400

1

95

0.09

7400

100

120

1.2

540

36

770

0.77

860

32

02 + 03) Three-color, three-photon process consisting of two resonance steps followed by photoionization. X1 = 410-459 nm or 618-626 nm, A2 = 358-430 nm or 316-318 nm, X~=1064Ilnl.

E. B.

SALOMAN

Data for RIS schemes of type (q + CO* + WJ [l, 4, 51

A1064

nm

/////////////////I/,

///////544l3

&I 49709 49933

f’H. g’G

350 316

-

430 316

or

nm

v$‘,$’ n2F”. 0 &,

33527 3747% cnT’

y%”

16196 26605 410 618

~ 459 - 626

cm’

cm’

or nm

Fig. 8. Grotrian diagram of resonance ionization spectroscopy

schemes in vanadium.

RWRIMS

Data Service

1199

Lifetimes and photoionization cross section from excited states [l, 4-61 Listed in tabular form for the first step of these schemes are the laser wavelength, X1, to the resonant level, the resonant level, its energy, E,, its lifetime, T, and an estimate of the laser power, PSat, required to saturate the resonant transition.

618.9364

= %,a0

18372.39

870

6.8

620.7272

= %,ZO

18258.89

890

8.9

623.0798

= %,a0

18198.08

390

9.3

625.1827

= 6J&0

18302.26

390

12.3

The following table lists the wavelengths, A*, of the second step of these schemes to one of the g 6G or f W levels, an estimate of the energy needed to saturate the second step, ESat, in A*, within the absorption bandwidth while the first step is being pumped, the second resonant level, its energy, E,, its lifetime, T, the photoionization cross section, a, of the resonant level for radiation of wavelength 1064 nm, and an estimate of the laser energy requirement, Ef&, in 1064 nm for photoionization efficiency near unity from the excited state for a pulse shorter than the lifetime of the excited state.

1200

E. B. SALOMAN

49932.11

16

0.017 49789.oa 16 8 'GM I I I I 1 316.7679 1 6.33-3 1 g 'G,,, 1 49932.11 1 16 316.4543

I 1

15

12

15

12

15

1

12

I

317.0646

5.63-3

g =G,,n

49789.08

16

15

12

317.3410

0.025

f %,,,

49875.12

15

15

12

358.1708

1.1

f xl,2

49875.12

15

15

12

400.8614

0.17

g 'G,,z

49932.11

16

15

12

401.6492

0.16

8 'G,,z

49789.08

16

15

12

428.5615

0.32

g 'Gs,s

49932.11

16

15

12

429.3744

0.34

g 'G,,z

49789.08

16

15

12

Isotope data [7] Stable Isotopes: 5oV (0.250%) Z = 6; 51V (99.750%) Z = 712. Unstable Isotopes: 48V Z = 4 T = 15.98 d.; 49V Z = 7/2 T = 331 d. Isotope shiftslhfs [8, 91 No data have been found on isotope shifts for the relevant transitions in V, but they are expected to be relatively small. The hfs interaction constants for the indicated levels of the indicated isotopes of vanadium are given in the following table.

Hypefinestructure interaction constants

RKYRIMS Data Service

1201

Laser Schemes For 283-287 nm, Nd:YAG pumped frequency doubled Rhodamine 590 dye laser [B]; for 292-300 nm, Nd:YAG pumped frequency doubled Rhodamine 610 dye laser [A, B]; for 316-318 nm, Nd:YAG pumped frequency doubled DCM dye laser; for 358.2 nm, XeCl or Nd:YAG pumped TMQ dye laser; for 400-412 nm, Nd:YAG pumped DPS dye laser; for 400-458 nm, NZ pumped Stilbene 420 dye laser; for 405-458 nm, XeCl pumped Stilbene 420 dye laser; for 428-458 nm, Nd:YAG pumped Coumarin 440 dye laser; for 618-626 nm, Nd:YAG pumped Rhodamine 640 dye laser; for 1064 nm, Nd:YAG direct IR. Atom reservoirs and sources Atoms of vanadium have been prepared for RIS studies by several methods: evaporation of the oxide from a Re tllament with Hz reduction [A] (this method produces a relatively large VO ion signal, which was a large background on the V RIS signal); evaporation of V from a Re filament coated with a carbon film [B, C] (this method produced a clean RIS signal); and argon ion sputtering of a solid surface (SIRIS) [F]. L aser ablation should also be a viable method of atomization. RIS references J. 6. Fassett, J. C. Travis, L. J. Moore and F. E. Lytle, Anal. Chem. 55, 765 (1983). L. J. Moore, J. D. Fassett and J. C. Travis, Anal. Chem. 56, 2770 (1984). S. Mayo, J. D. Fassett, H. M. Kingston and R. J. Walker, Anal. Chem. 62,240 (1990). N. Thonnard, J. E. Parks, R. D. Willis, L. J. Moore and H. F. Arlinghaus, Surf. Interface Anal. 14, 751 (1989). N. Thonnard, H. F. Arlinghaus, R. D. Wii, E.H. Taylor, M. C. Wright, W. A. Davis, M. T. Spaar and L. J. Moore, The status of real-world analysis problem solving using RK, in Resonance Ionization Spectroscopy 1990, Eds J. E. Parks and N. Omenetto, p. 271. The Institute of Physics, Bristol (1991). J. E. Parks, H. W. Schmitt, G. S. Hurst and W. M. Fairbank Jr, Sputter initiated RIS (SIRIS) for analysis of semiconductor impurities, in Resonance Ionization Spectroscopy 1984, Eds G. S. Hurst and M. G. Payne, p. 167. The Institute of Physics, Bristol (1984). L. J. Moore, J. E. Parks, M. T. Spaar, D. W. Beekman, E. H. Taylor and V. Larch, J. Res. Nat. Bur. Stand. 93, 328 (1988).

;$I WI PI [El PI PI

Data references J. Sugar and C. Corliss, J. Phys. Chem. Ref. Data 14, Suppl. 2 (1985). R. H. Page and C. S. Gudeman, J. Opt. Sot. Am. B 7, 1761 (1990). R. D. Cowan, The Theory of Atomic Structure and Spectra. University of California Press, Berkeley (1981). Calculated using the Hartree-Fock code with relativistic corrections of Ref. [3]. G. A. Martin, J. R. Fuhr and W. L. Wiese, J. Phys. Chem. Ref. Dota 17, Suppl. 3 (1988). W. Whaling, P. Hannaford, R. M. Lowe, E. Bi6mont and N. Grevesse, Astron. Astrophys. 153, 109 (1985). N. E. Holden, Table of the isotopes (revised 1990), in CRC Handbook of Chemistry and Physics 71st Edn, Ed. D. R. Lide, p. 11-33. CRC Press, Boca Raton, Florida (1990). W. Ertmer, U. Johann and G. Me&l, Phys. Lett. 858, 319 (1979). W. J. Childs, Phys. Rev. 156, 71 (1%7).

RWRIMS Data Service

1203

Update for RIS/RIMS data sheet for nickel Ni Z = 28 GOBERT et al. [A] report a measurement of the photoionization cross section of the y 3D8 level of Ni in which a value of 7 X lo- l* cm* was obtained for 300.2482 nm radiation. This value agreed with the value of 8 x 10-l* cm* calculated by them using the quantum defect method. This value disagrees with the calculation made for the RIS/RIMS data sheet for Ni [B], in which the unadjusted results of the Hartree-Fock code [C] were utilized. This calculation produced only very limited configuration mixing for the y 3D; and other nearby odd parity levels. The result reported in Ref. [A] suggests that for this case the mixing is greater. To obtain a more realistic amount of configuration mixing, a se’mi-empirical calculation was carried out in which the difference in the average energies of the 3&’ and 384s configurations was adjusted so that the resulting calculated energies agreed with the spectroscopic data for the 3&‘4p and 3da4s4p levels [D]. The results of this calculation are given in the table that lists for the cited (wr + ol) schemes in the wavelength ranges 282-284 nm and 294-304 nm, the wavelength, X1, the initial level of the scheme, Low. Level, the resonance level, Res. Level, the photoionization cross section calculated for this resonance level in the semi-empirical calculation for an o1 + o1 scheme, u, and an estimate of the energy, E sat7 delivered during the lifetime of the resonant state required to saturate the photoionization.

Level

Low.

Res. Level

0 (lo-18cd)

E..t W/cm2)

282.1289

a 3D,

Y 'F,'

0.43

1600

283.4545

a 'F,

Y 'Da'

0.65

1100

294.3910

a JD,

Y =D,'

1.3

520

298.1644

a 'D,

Y 'D,'

2.5

260

298.4128

a 3F,

299.2590

a

'D,

JFz'

299.4450

a 'D,

z '6,'

300.2482

a 'D,

Y =D,'

1.9

340

300.3618

a 34

Y 'D,'

1.4

490

301.1998

a =D,

Y 'D,'

0.72

910

301.9140

a 3F,

3F3'

1.8

360

303.1864

a =F,

3F,0

1.0

650

a

3D,

I

Y =D,'

I

1.9 2.4 3.aE-2

I

350 270 1.7E4

1.8

References [A] 0. Gobert, T. Gibe& B. Dubreuil, P. Gelin and J. L. Debrun, J. Appl. Phys. 70, 7602 (1991). [B] E. B. Saloman, Spectrochim. Acfu 46B,319 (1991). [C] R. D. Cowan, The Theory of Atomic Structure and Spectra. University of California Press, Berkeley (1981). [D] J. Sugar and C. Coriiss, J. Phys. Chem. Ref. Data 14, Suppl.2 (1985).