Resonance ionization mass spectrometry of thorium: determination of the autoionization level structure and a re-determination of the ionization potential

Pergamon Press Ltd

SpecrrochimicaAcm? Vol. 47B. No. 5, pp.633-613. 1992 Printed in Great Bntain.

TOPICS IN LASER SPECTROSCOPY Resonance ionization mass spectrometry of thorium: determination of the autoionization level structul’e and a re-determination of the ionization potential S. G. JOHNSON*, B. L. FEAREY and C. M. MILLER Isotope and Nuclear Chemistry Division, MS 5514

and N. S. NECTARY Chemical and Laser Sciences Division, MS 556.5 Los Alamos National Laboratory,

Los Alamos,

NM 87545,U.S.A. (Received 7 August 1991; accepted 9 November

1991)

Abstract-We report on the use of resonance ionization mass spectrometry (RIMS) for the analysis of thorium isotopes. The 23RUP% decay pair is useful for the dating of geologic samples in the 5 350000 year range, and details are needed on the resonance ionization spectrum of thorium in order to most efficiently effect ionization. This report describes the use of a two-color, two photon ionization process for thorium near the ionization threshold. A large number of autoionizing states are observed, a new value for the ionization potential, 50890 2 20 cm-’ (6.310 + 0.002 eV), is reported, and electric field effects are described.

ionization mass spectroscopy (RIMS) has found numerous applications in chemical [l] and isotopic [2, 31 analysis, surface analysis [4, 51, optical spectroscopy [6,7], and ionization potential measurements [8, 91. One of the inherent advantages of RIMS is the potential for high ionization efficiencies, relative to conventional ionization methods, thus allowing for potential improvements in minimum sample size, accuracy of measurement, and speed of analysis [ 10, 111. We are particularly interested in the isotopic analysis of thorium, primarily for applications in geochemistry and ge~hronology [12]. The measurement of uranium series disequilib~um is a well-established and valuable approach for geochronolo~cal studies: ~sequilibrium between 234’238Uand “I’h produced during magma melting and crystallization, or from redistribution by geothermal fluids, can be used to date samples younger than 35OOOOyears. In addition, the 230Th/232Th ratio can be used as a geochemical tracer to follow the evolution of magmatic or aqueous source regions. Uranium-thorium dates were previously determined by standard radiochemical methods (alpha spectrometry) [13, 141. This method is inherently slow, and requires relatively large amounts of material (as much as 10 g). Both the relatively low concentrations of U and Th and the long half-lives of 232Th and 23VTh contribute significantly to these limitations. Further, since large samples are typically needed, significant questions can arise concerning potential secondary alterations to the sample. More recently, mass spectrometric methods have been developed for the determination of uranium and thorium isotope ratios, with significantly improved precision, and reduced sample size [15-171. Further improvements are still possible: the ionization effkiency for thermal ionization mass spectrometry is = 1O-5-1O-3, thus affecting both the minimum sample size and the maximum isotope ratio measurable by this method. RESONANCE

* Permanent address: Analytical Chemistry Group, Argonne West National Laboratory, 83703, U.S.A. t Author to whom correspondence should be addressed. 633

Idaho Falls, ID

434

S. G. JOHNSON

et al.

Fig. I. Schematic of the experimental apparatus, where M are mirrors, and EM is a channel electron multiplier.

RIMS ionization efficiencies can be substantially higher than those for thermal ionization. With cw laser excitation [18], overall efficiencies as high as 10-3-10-2 (ions detected/atoms evaporated) have been reported [12]. For pulsed laser ionization, overall efficiencies are typically much smaller (=10P5), although saturation (100% ionization in the focal volume during the laser pulse) has been reported [19]. RIMS involves a multi-step excitation process, taking advantage of intermediate electronic resonances in the species of interest. The efficiency of the ionization process will depend to a great extent on the cross-sections for optical excitation and ionization (18, 201. In order to realize the greatest possible ionization efficiency, it is often necessary to utilize the autoionizing structure above the ionization thr&hold, since such features may have cross sections 2-3 orders of magnitude larger than for continuum photoionization [21]_ In this work, we describe the use of multi-step pulsed RIMS to characterize the autoionizing structure of thorium close to the ionization potential, and the lifetimes and field dependence of several states immediately adjacent to the ionization limit. In addition, we have re-determined the ionization potential of thorium by observing the energy onset for photoionization under both low electric field and field-free conditions.

A schematic of the apparatus is shown in Fig. 1. A XeCl excimer laser (Lambda Physik EMG 101) is used to pump two dye lasers (Lambda Physik FL 2000 and 2002) (221. A variety of dye solutions were used, including Stilbene 420 in ethanol, BBQ in ethanoutoluene and Polyphenol1 in ethylene glycol. Output energies were typically l-3 mJ in 15 ns pulses. Spectral width for the FL 2002 was c: 0.3 cm-‘, while for the FL 2000, the linewidth was ~0.6 cm-‘. For experiments in which two-color ionization was used, the pulse energy for resonant excitation was limited (50 d 5 Repulse-< 100 &J) by attenuating the pump beam to the amplifier stage of this laser. This yielded good pointing stability and excellent wavelength stability while allowing E pulse to be varied over a wide range. The dye laser beams were overlapped in space and counter-propagated through the source region of a lab-built 0.4 m time-of-flight mass spectrometer (TOFMS). The dye laser beam used for resonant excitation was -3 mm in diameter, while the beam used to excite autoion~zing states was < 2 mm in diameter, as determined with a scanning pinhole and phot~~ode [23]. Both beams were linearly polarized (Z 97%), parallel to the axis of the flight tube. The overlapping beams were located about 4 mm from the sample source.

RIMS of thorium: autoionization and ionization

635

Table 1. Thorium transitions applicable to RIMS Wavelength (vacuum nm) 395.38795 392.62049 384.07845 383.18605 382.94709 380.41547

Wavenumbers (cm-‘)

Lower State

J”

Upper State

J

25291.6155 25 469.8883 26 036.3476 26 096.9837 26 113.2683 26 287.0486

3865 3687 0 0 0 0

1 2 2 2 2 2

29 157 29 157 26 036 26 096 26 113 26 287

1 3 3 3 2 1

Measurements with a fast photodiode indicated that the beams were temporally overlapped to the extent, measurable with our electronics, of = 2 ns. Photoionization occurred in the region between two extractor grids of the TOFMS, separated by 1 cm. The extraction field could be varied from -10 to -200 V/cm, and could be applied continuously or pulsed at a variable delay after the photoexcitation process. The extraction region was followed by a focusing element, normally set at -200 V, and a drift tube operated at -1 kV. The TOFMS chamber [24] was routinely pumped to pressures I 1 X 10m7 torr, and was operated at pressures zz 5 x lo-’ torr. Detection was by a channel electron multiplier, followed by l-50 times impedance matching amplification. Signal processing was with a gated integrator set to accept pulses from ions in the mass range 230-235 au. Samples were prepared by applying =l pg of thorium, as ThOZ in a nitric acid/hydrofluoric acid solution, to standard rhenium filaments, 0.001 in x 0.03 in x 0.25 in. In some cases, these samples were overcoated with a graphite slurry before drying. After evacuation of the TOFMS, the samples were gradually (~30 min) heated to 1800°C. Laser excitation of all samples initially yielded ThO+ ions as well as Th+ ions. After 30-60 min of operation, only Th+ ions were observed. Samples normally exhibited a useful lifetime of 3-5 hr.

RESULTS AND DISCUSSION

Autoionizing levels The experiments reported here involve the use of two pulsed dye lasers for multiphoton excitation and ionization of thorium with ion detection via a mass spectrometer. The primary purpose of these experiments was to determine the autoionization state structure of thorium, in order to improve the ionization efficiency for isotopic analysis. Experiments performed using a cw dye laser and an Ar+ laser for ionization will be reported separately [25]. The first laser, tuned to frequency vl, provides a photon which is used to selectively populate an excited state by a transition that is allowed from the ground state [26]. A second photon provides the energy for subsequent excitation or ionization. The second step can be accomplished by a second laser frequency, v2, or by a second photon of the first laser frequency, vl, providing 2 x hvI is of sufficient energy for the ionization process. These separate ati distinct processes will be denoted in the following manner: for a process involving one photon of v1 and one photon of v2, (1 + 1’); and for a process involving two photons of a single laser, (1 + 1). Listings of the optical transitions of thorium are available [26-291 for states well below the ionization potential. We chose several optical transitions for vl which fulfilled the following criteria: (1) they were in a spectral region which was accessible to both cw and pulsed dye lasers; (2) they were allowed transitions from the ground state; and (3) they were at energies such that 2 x hul was greater than the reported ionization potential [30]. The major transitions utilized are listed in Table 1, and include some transitions from atomic states that are thermally populated at the filament temperatures used for these experiments (see Experimental Section and Ref. [26]). Figure 2(a) is a single laser (1 + 1) spectrum. The spectrum was obtained by scanning the wavelength of the dye laser such that two photons provided sufficient energy to ionize thorium. The peaks in the spectrum correspond to known one-photon transitions

S. G. JOHNSONet

636

al.

Wavelength (nm) (b)

1.0 0.8

P c 3

0.8

rj s. z

0.4

.e v) 0.2

0.0 380

302

384

386

Wavelength hz(nm) Fig. 2. (a) Single color RIMS s~c~rn of thorium showing several peaks due to resonance with excited states of the thorium atom (see text for details). The inset shows a simplified energy level scheme for two-photon RIMS of thorium. (b) Two laser RIMS spectrum of thorium showing both (1 + 1) and (1 + 1’) transitions (see text for details) where Y, = 26113 cm-’ (382.94 nm). The peaks marked with dashes are due to bound excited states of thorium, while the remaining features are due to autoionization levels.

of tho~um (see Table 1). These transitions are used in subsequent spectra as absolute wavelength markers and allow us to determine the wavelength of unknown transitions to an accuracy of +O.OZ nm. The linewidths of the peaks in Fig. 2(a) are to a significant extent determined by power broadening. No attempt was made to obtain more accurate linewidths of these features, since these peaks are primarily used for wavelength calibration. Figure Z(b) is a spectrum obtained using two lasers with one, vl, tuned to the 26 113 cm-r (382.94 nm) optical transition and attenuated so that the generation of ions by that laser is minimal, i.e. very low fluence. The second laser, at full power (==2 mJ), was then scanned over the same wavelength region as in Fig. 2(a). The peaks marked with a dash in Fig. 2(b) correspond to (1 + 1) single laser generated features (i.e. see Fig. 2(a)). The large number of additional lines in Fig. 2(b) correspond to autoionizing states that are at energies of h( v1 + Us). These states lie above the ionization potential (IP) in energy. The observation of autoionizing states above the IP is a well documented occurrence for other atomic species [31],[32]. Figure 3 is equivalent to Fig. 2(b), except that u1 is resonant with a different optical transition. A series of similar spectra were obtained using several intermediate states, indicated in Table 1. Analysis of these spectra yielded a large number of autoioni~ng levels which are listed in Table 2. Table 2 should be viewed as evidence of the large number of autoionizing states present in the continuum above the IP for thorium. However, this listing is certainly not exhaustive since autoionizing states higher in energy undoubtedly exist, and others may be accessible from different intermediate

RIMS of thorium: autoionization

380

and ionization

384 382 Wavelength hn(nm)

637

386

Fig. 3. Two color RIMS spectra similar to Fig. 2(b), but with v, = 26036 cm-’ (384.08 nm). The linewidths of individual features is instrument and laser power limited.

Table 2 lists the resonant optical transition (v~), as well as the relative intensity of each line, and an assignment of total angular momentum, J, to 45 of the autoionizing levels. Assignment of J values to the other levels was uncertain due to the limited number of intermediate states of differing J values in the spectral region of interest. Further information on state identification may be inferred from selection rules applying to transitions from the autoionizing states to the ground state of the ion [33]. Such selection rules were proposed previously to account for an unusually narrow autoionizing linewidth (0.07 cm-‘) in gadolinium, indicating a rather long lifetime of -0.5 ns, i.e. a predominantly forbidden transition from the autoionizing level to the ground state ion [33]. Autoionizing states may consist of an atom with an inner shell electron that is promoted, or may correspond to production of an excited state ion. The typical measured width of an autoionizing line in our work was -3 cm-’ (see Fig. 4) indicating a lifetime of 3 ps. The linewidths of the lasers used in this determination were < 0.6 cm-‘; the contribution of these lasers to the measured autoionizing linewidth was small. Reported lifetimes for autoionizing states for other species range from 0.5 ns to 1 ps [32, 331. This typically represents the time for an electron to undergo a non-radiative transition from an inner shell to a less energetic orbital. Electron correlation then results in ejection of a valence electron. In the present experiment, we noted some variation in the linewidth of the autoionizing lines (widths from -1-4 cm-l were observed). This variation is due to varying overlap of initial and final state wave functions (in this case the highly excited atom and the ion), as well as the selection rules noted above. The greater the overlap of wave functions, the shorter the lifetime of the autoionizing state, i.e. the transition becomes more allowed. Figure 5 features a plot of the intensity of a typical autoionizing line vs the pulse energy of the ionizing laser. The intermediate resonance used was 26113 cm-l (382.94 nm). The plot displays the typical linear behavior at low pulse energies with evidence of saturation at higher energies. The method of AMBARTSUMYAN ec al. [34] can be applied to obtain the ionization cross section of the autoionizing level. This involves determining the intercept of two lines, one drawn through the linear, low fluence, portion of the plot, and the second drawn through the saturated segment of the plot. Using the fluence, @int, at the intercept, one obtains the autoionizing cross section [34] from @int(Tauto= 2 hvz. This rather simple expression is true only at the saturation point described above. Furthermore, it has been shown for rubidium [34] that the saturation point does not depend upon atom vapor densities or extraction fields. The cross section obtained in this case is 1.2 X lo-l5 cm2 which is relatively states.

S. G. JOHNSON et al.

638 Table

Energy (ctt-I), 50891* 50909d 50 925b.d 50935b.d 50 959b.d 51003d 51 008b.d 51027* 51029 5 1034b.d 5 10438.b 51054b.d 51063a.d 51075b.d 51099-i 51103a.b.d 51 116b.d 5 1 1238.b.d 51129 51 142b.d 51164a.b.d 51 171b.d 5 1 194b.d 51 208a.b 51 238a.b.d 51 250”.b 51 256d 51268b 51 2928.b.d 5 1305”.b,d 51 318a.b 51 329b.d 5 1336-b 51346” 51354-b 51 361d 51 3688.b.d 51393b 51 398”.b.d 51414a.b 51 421b.d 51430”.b,d 5145od 51 462”.b.d 51 486b.d 51507a.b 51 521a.b 51532” 515378.b 51542” 51567” 51576” 51619” 51656” 51662b 51 691b 51 752b 51 761a 51771b 51 811a 51 876d 51889 51902d 51914d 51924d 5 1937d 519436 51 976d 51978C.d * a b c d e

W = = = = =

J=(

)

2. Autoionizing states of thorium Relative Intensity*

s S W

M,W

vs,w

(2)

I;; I:{ (2) (2) (3)

W W W W W W SW WS SW WS W,MS M WXW W s.w S,S,M SW S,M

M,W

(2) (2)

$1

= weak, M = medium, 26287 cm-’ 26113 cm-t 26097 c I 26036~:‘~ 25467 cm-’

M,W,VS vs,s W M W WSM W,M WS W.S W

WM

M W,S W

s,w,s s,w NW

W

W W W,M W W W

W&f W

i+ W W M M M M M M M W S vs vs vs VS vs S MS S = strong,

Energy (cm-‘), 51988’ 51993b.d 51 998b.c.d 52001” 52006b.C 52012b-’ 52 020b.d 52026b.c,d 52 037b.c.d 52042b 52 05fFd 52 056c” 52 062” 52 070b,d 52 07852085’,b.c 52094’.h 52 130b,“ 52 14SC.“ 52 1516.” 52 161c 52 167b 52 174d 52 177d 52 187’,C.d 52 198”.b.d 52205’.b,d 52213”.h.d 52219b.” 52 227a.b.c.d 52 232d 52239” 52 2428.h.d 52 246b.‘1 52257b.J 52 26652 27452 280”.d 52 290b.= 52 298* 52312”,b.d 52 317c,d 52331b.d 52 358* 52 365a.b.c 52 378b.d 52 420-’ 52 436b 52446” 52 458” 52 482”,’ 52 527” 52 536a 52 550a 52 647” 52871” 52 942” 54 46P 54 482’ 54 648’ 54 690= 54 972’ 55ow 55 138’ 55212’ 55 322’ 55 417’ 55 502’

I=(

(2)

)

Relative Intensity* W M,VS M,W,M vs W M,WN M M,MS WSS S VS,M WN M S,M M,W W WS S,M W vs,w W W vs M M,S,VS M,VS,S M,W,W M,W,VS W W,M,W M W W M.W W;M M W NW

SW

$1 (2)

VS = very strong

M S W W M W M M W W M M M M W M W M W W W W W W W W W W W

RIMS of thorium: autoionization

and ionization

639

0.8

a 364.86

385.00

386.14 Wavelength (nm)

385.29

385.43

Fig. 4. Two cotor RIMS spectra of two autoionization lines with the more prominent feature dispia~ng a 3 cm-’ linewidth, Y, = 26036 cm-‘.

0.8 0.6

1

2

3

10s (Energy f~df Fig. 5. Power dependence plot of RIMS signal intensity of an autoionizing line vs u2 power, with Y, power constant. An ionization cross section of 1.2 x 10-ls cm2 can be inferred from this plot (see text for details).

large compared to typical continuum type of transition [33]. By comparison level of ionization present in the valley value of 50-100 can be obtained for vs continuum).

ionization cross sections, but reasonable for this of the peak heights in Figs 2(b) and 3 with the between autoionization features, an approximate the ratio of the two cross sections (autoionizing

The ionization potential for tho~um has been reported by several groups [30, 35-371 with values ranging from 6.0 to 7.8 eV. HILDENBRAND and MURAD1351 reported an IP of 6.2 2 0.2 eV, obtained via electron impact experiments. SUGAR [30] derived a value of 6.08 + 0.12 eV using correlations and extrapoiations from several Rydberg states of different actinides. The uncertainties of their methods, 1~-2~ cm-‘, are rather large by laser spectroscopy standards. In our initial experiments, laser scans with total energies 6.0&6.2 eV produced few, if any, detectable ions. This suggests strongly that the IP does not lie in this range. We first observed ions at energies near 6.3 eV. Using only this coarse threshold measurement, we find the ionization potential of thorium to be 6.30 2 0.03 eV. The significant discrepancy between this value and previously published work suggests that additional evidence be presented for this claim.

640

S. G. JOHNSONet al.

50900

405

Total Energy @n-i’) \ 50700

407 409 Waveiength (nm)

411

Fig. 6. Two color RIMS spectra obtained with Y, = 26287 cm-* (380.41 nm) and variable extraction field strength: (a) -20 V/cm; (b) -40 V/cm; (c) -60 V/cm; and (d) - ,80 V/cm (constant application of extraction field strength).

In order to further refine this measurement, other influences, such as external fields and perturbing states, must be considered. Since high-lying Rydberg states may be very sensitive to external electric fields, and since our preliminary experiments were conducted in a constant electric field of -80 V/cm, we have further investigated the effect of fields on the observed threshold of ionization. Using the 26287 cm-l (380.41 nm) transition as an intermediate, and scanning the second laser with an extraction plate bias of -20 V, the spectrum shown in Fig. 6(a) was obtained. The three small peaks to lower energy relative to the dashed arrow are single laser (I + 1) features initiating from thermally excited atomic states (the atomic analog of vibrational hot bands). Scans to lower energies than shown here were relatively featureless except for occasional hot band single laser (1 + 1) peaks. Thus, an apparent ionization potential (6.30 ev) for thorium is indicated by the dashed arrow in Fig. 6(a). However, when the extractor bias was increased in successive steps to -80 V (Figs 6(b), (c), and (d)), the spectrum near the apparent IP was observed to changekdramatically. The peak lying just higher in energy than the dashed arrow in Fig. 6(a) appears to evolve into several features on its low energy side. In addition, the intensities of these new features grow with increasing extractor bias. Another similar set of scans were performed using the 26113 cm-’ (382.94 nm) transition as an intermediate yielded comparable results. In contrast, scans to higher energy were also obtained and the relative intensities of the autoionizing states were electric field invariant. This data suggests that the peaks, which vary rapidly with applied electric field, are Rydberg states lying close to the ionization potential. Further confirmation of the identity of these levels is provided by two variable extractor field experiments, performed with a pulsed extractor triggered by the excimer laser. First, the extractor bias was varied from -fO to -40 V, the limit of our pulsed extractor, with a nominal delay of 200 ns between laser and extractor pulses. With y1 tuned to 26287 cm-’ (380.41 nm), results similar to those in Fig. 6 were obtained. The second set of experiments consisted of varying the time delay between the laser

RIMS of thorium: autoionization

and ionization

641

Total Energy (en-i’) 100 1

400

5oBoo

5070

I

402 404 Wavelength (nm)

I

406

Fig. 7. Two color RIMS spectra obtained under field-free conditions, v, = 26113 cm-’ (382.94 nm), with an extraction field of -40 V/cm applied at various delay times after the laser pulses: (a) 200 ns delay; (b) 1 ps delay; (c) 3 (LSdelay; and (d) 8 ps delay.

pulses and a -40 V extractor pulse (1 l~s in duration). With u1 tuned to the 26 113 cm-’ (382.94 nm) resonance, the time delay was varied from 200 ns to 10 ps, with the most pertinent data shown in Fig. 7. The three peaks, marked with dashes in Fig. 7(a), are observed to diminish in intensity relative to the autoionizing states that are observed to higher energy in the spectra as the delay time is increased. A lifetime for the states responsible for these features can be estimated to be 2 6 l~,sbased on these spectra. Atoms diffusing out to the edge of the extractor’s electric field during the longer time delays may also reduce the intensity of some features at long delay times. It is because of this possibility that only a lower limit of the lifetime of the states in question can be inferred. Since Rydberg states, having finite radiative lifetimes, will be influenced by the size and temporal history of the applied electric field, while autoionizing levels should show minimal effects, the data presented here strongly support the identification of these states as Rydberg levels. From the data in Figs 6 and 7, the onset of photoionization can be determined with good accuracy. The solid arrows in Figs 6(a) and 7(a) represent the same total energy ]h(vi + v2)I, within experimental uncertainty. This energy is 50890 ? 20 cm-’ (6.310 f 0.002 eV). This threshold marks the onset of photoionization for thorium. At higher energies, autoionizing levels have been unambiguously observed, while at slightly lower energies, Rydberg states have been tentatively identified. The photoionization threshold should thus closely reflect the ionization potential. While not as satisfying as the observation of a well-defined Rydberg series leading to an ionization limit, the abrupt appearance of an ionization threshold is not without precedent. WORDEN and CONWAY [32], for example, show photoionization spectra for neodymium and neptunium which lack any observable Rydberg series and additionally appear very much like our spectra (Figs 6 and 7) in the vicinity of the ionization threshold. The photoionization spectrum for gadolinium presented by BEKOV et al. [31], shows Rydberg states undergoing field ionization but not in an easily identified series.

642

S. G. JOHNSON et al.

This measurement is the most precise, and we believe the most accurate, ionization potential dete~ination for tho~um to date, although it does lead to a si~ificant disagreement (0.23 eV) with the value obtained by SUGAR[30]. This discrepancy may result from procedural errors in our determination, or from the method Sugar applied to determine IPs for the actinide series. This method requires a knowledge of the energetics for the 5f6s7p state. This state exists, and its energy is known, for some of the actinides, but has not been directly observed for thorium. Rydberg states and field ionization effects

The prominent peaks at lower energy than the solid arrows in Figs 6(a) and 7(a), and higher in energy than the dashed arrow, represent Rydberg states of thorium. Their position, at lower energy than the IP, the fact that they are not identifiable as single laser hot band (1 + 1) peaks, and their strong interaction with applied fields, makes their assignment as Rydberg states firm. Their intensity is comparable to that of the autoionizing states. Although this may seem unusual, this observation is not without precedent [31]. Ionization of the Rydberg states (and hence their appearance in our ionization spectra) is effected by field ionization [33, 34, 38, 391, as has been reported previously for numerous other atomic species [39]. The degree to which field ionization occurs depends on the strength of the electric field as well as the characteristics of the states in question. It is important to point out that the Rydberg states that undergo ionization via tunneling are separate and distinct from the autoionization states present above the IP such as those listed in Table 2. It is possible to determine the critical electric field strength, EC, which causes a particular Rydberg state to ionize. E,‘s noted in the literature range from 2 V/cm [4O] to lo-15 kV/cm [38, 41, 42], depending upon the value of the effective principal quanta number 143, 441, II*, for the Rydberg electron. PAISNERet al. [45] reported EC values as low as 1OOV/cm for Rydberg levels with N* - 40-45 for uranium atoms. The chemical similarity of uranium and thorium makes the extrapolation of these results to thorium at lo-80 V/cm quite reasonable. The arguments above support the assignment of the dashed peaks in Figs 6 and 7 as Rydberg levels with lifetimes 2 6 ps. These peaks are most likely not individual Rydberg levels, but unresolved manifolds of states. The spacing of Rydberg states with II L 40 would be ~1 cm-“ and thus unresolvable with the experimental resolution utilized for Figs 6 and 7. The Rydberg states that lie -150-200 cm-l directly below the IP may be observed in our experiments by the process of field ionization. The existence of clearly identifiable Rydberg and autoion~ing states lends support to the identification of the ionization potential in thorium.

A spectroscopic survey of thorium was performed using RIMS. The autoionization level structure for thorium within -2OOO cm-l above the IP is reported. Typical lifetimes of a few picoseconds and ionization cross section of =10-15 cm2 were determined for the autoionization levels. The process of field ionization was observed with low electric field strengths, -100 V/cm. The ionization potential of thorium determined using two-color resonance ionization mass spectrometry is found to be 50890 + 20 cm-’ (6.310 + 0.002 eV). This is the most precise and accurate determination of this quantity reported to date. The info~ation obtained in this study will be useful for future analytical determinations of thorium for geochemical dating and other purposes. The autoionizing states described in this work can be important for analytical applications of RIMS, because of the potential for increased sensitivity. These states typically have ionization cross sections that are 2-3 orders of magnitude greater than the continuum above the ionization threshold [32, 331. Therefore, when using multi-photon processes to ionize a species, such as thorium, it would be very beneficial to tune the ionizing laser in resonance with an autoionizing state in order to increase the ionization efficiency.

RIMS of thorium: autoionization

and ionization

643

Ac~~o~Ze~ge~e~~-~is research was supported by the U.S. Department of Energy under contract W7405ENG-36, and the BES office of Geosciences. The authors would like to acknowledge the technical assistance of J. E. ANDERSONduring the course of these experiments.

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