- Email: [email protected]
Application of resonance ionization mass spectrometry to the lanthanide elements in the wavelength region of 430–455 nm
Spectrochimica Acto. Vol.44B,No.2, pp. 147-153, Printed in Gnat Btitun
1989
0584-8547/89 s3.Go+.im Pergamon Press pk.
TOPICS IN LASER SPECTROSCOPY
Application of resonance ionization mass spectrometry to the lanthanide elements in the waveIength region of 430-455 nm J. P. YOUNG, D. L. DONOHUE and D. H. SMITH Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A. (Received 17 May 1988; in revised form 27 June 1988) Abstract-A study of all the Ianthanides except promethium has been carried out using resonance ionization mass spectrometry; the wavelength region for this investigation was 430-450 nm. A list of several wavelengths is given for each of these lanthanides where they can be determined without isobaric interference of other lanthanides. The optical routes for ionization were cataloged in terms of 1+ 1, 1 + 1+ 1, and 2 + 1 photon pathways where possible, and the known or partially known routes are presented. Possible pathways for the unknown optical routes are evaluated.
RESONANCEionization mass spectrometry (RIMS) is a technique that utilizes the resonant absorption of two or more photons by a free atom to transfer an electron through energy levels of the atom, ultimately creating an ion pair.* The cation thus formed can then be mass analyzed. Various modifications of the RIMS technique have been described in a number of papers ([l-5] and references contained therein). Because of the optical selectivity of the ionization process, RIMS is capable of reducing or eliminating isobaric interference that is often present in conventional mass spectrometric methods [l]. Due to the nature of the f-transition elements and their possible modes of formation, isobaric interferences are prevalent in the mass spectrometric determination of the lanthanides and actinides. Earlier studies of a few of the lanthanides and actinides revealed wavelengths at which they can be ionized by this spectral technique [2,6,7]. In this paper we report a study of the RIMS spectra of all the lanthanides except promethium (which was unavailable) in the spectral region of 430-455 nm. The study was undertaken for several reasons. We wished to generate a catalog of analytically useful RIMS wavelengths that could be used for mass analysis of the lanthanides. We wished to gain knowledge concerning possible optical routes that would be useful for ionizing these elements. Lastly, we wished to evaluate our ability to assign energy levels involved in these optical routes in preparing for future studies of the heavier actinides where RIMS studies will yield new and useful energy level information. EXPERIMENTAL The apparatus used for this study is somewhat different from that described before [6] in that a modified time-of-flight mass spectrometer was used for mass analysis. Laser
system
The spectral data were obtained with an NRG-DL-0.03 dye laser pumped by a Lumonics Model TE-262-2 nitrogen laser. The dye was C-440 at a concentration of 5 E-3 Molar in ethanol. This system produced S-10 ns pulses at a rate of l-5 Hz with a power of approximately 200-300 PJ per pulse and a
*Editor’s Note. See also the recently published review article: Analytical applications of resonance ionization mass spectrometry (RIMS) by J. D. FASET~and J. C. TRAVIS.Spectrochim. Acra 43B, 1409 (1988).
147
148
J. P.
YOUNG
et al.
bandwidth of 0.03 nm. The wavelength drive consisted of a d.c. motor connected to the micrometer adjustment for the dye laser grating.
Mass spectrometer The time-of-flight (TOF) mass spectrometer was a Bendix Model CVC-2000 adapted for laser ionization. The modifications to the ion source included installation of an optical window to admit the laser beam. The mechanical and electrical modifications to the source are shown in Fig. 1. A bracket was added to the ion lens system to hold a single filament assembly for generating the atomic population. A grounded stainless steel grid was added to the lens system to admit atoms to the ionization region. The electrical biasing arrangement is shown in the figure. Under normal TOF conditions, the ions are formed by a pulsed electron beam and are then extracted and further accelerated by pulses 1 and 2 (Fig. 1) which are applied to successive grids. The final acceleration to 3000 V is accomplished by d.c. voltages applied to the last two grids. Modifications for laser work included replacement of the pulsed electron beam by a 5-10 ns laser pulse. The atomization filament (Re or Ta) was biased by O-20 V negative to suppress thermally produced ions. The two extraction grids were no longer pulsed but were held at variable negative potentials between 0 and 300 V supplied by a battery. The voltages applied to the last two grids were unchanged. The detection system was the normal continuous dynode electron multiplier connected to a preamplifier. The ion signal was sent to a boxcar integrator (Stanford Research Systems Model SR-250, Palo Alto, CA). The boxcar was triggered by the synchronization pulse from the pump laser. A variable delay of the boxcar window allowed selection of the particular time-of-flight (mass) to be measured. The output of the boxcar could be read on a digital voltameter or plotted vs time on an x-y recorder. The raw ion signal was also displayed on a storage oscilloscope (Tektronix Model 7414). Wavelength measurement was accomplished with a 0.25-m spectrograph equipped with a photodiode array detector and calibrated against a mercury vapor lamp. The wavelengths of all significant spectral features were measured in a static (non-scanning) mode.
ORNL-DWG 65-9635
NORMAL TOF ION SOURCE ELECTRON TRAP
FILAMENT
I\ I\ ir -4.5kV
-Lr PULSE I
-2.7
hV
PULSE 2
RIMS MODE
SAMPLE FILAMENI
I
4 0 TO -300V
BEAM
t
-4. 5h V
+
-2 ‘.l hV
1
Fig. 1. Modifications to the TOF sample source for RIMS use.
Applications of RIMS to lanthanide elements
149
RESULTS ANDDISCUSSION RIMS spectra in the wavelength region of 430-455nm were observed for all the lanthanides except promethium. A partial list of wavelengths is given in Table 1; using these wavelengths any of these lanthanides can be selectively ionized in the presence of the others.
Table 1. Selected major RIMS wavelengths for the lanthanides in the range of 430-455nm First transition
Energy Number of isotopes
1 (nm)*
cm-’
La
434.84 438.11 441.40’
22 990 22819 22646
Ce
432.48 436.40’ 436.90 439.60
23 116 22 908 22 882 22 141
Pr’
433.45 439.24 447.42
23 064 22760 22 344
430.60 438.22’ 442.27 444.51
23217 22813 22604 22487
433.02 435.63 436.58’ 441.96
23 087 22 949 22 899 22 620
EU’
444.89 446.04 446.15
22 468 22413 22 311
Cd
432.72 447.38 447.62’
23 103 22 346 22 334
Tb’
433.43 437.43 442.28
23065 22 854 22 553
DY
433.45 440.41 445.45’ 453.21
23064 22 700 22 443 22 059
435.25 436.02 443.79 448.70
22 969 22 928 22 527 22 280
440.23 442.48:
22 709 22 593
435.98 438.77 448.14
22931 22 784 22 308
Yb
441.04’ 446.71
22 667 22 397
LU
451.90*
22 125
Nd
Sm
Ho’
Er Tm’
1
1
Init. st.
Final st.
314 514 314 514
‘F, IFI ‘F, ‘F,
-
51*
3H5 2F’h T,2
*Wavelengths are accurate only to the bandwidth of the laser system, 0.03 nm. ‘No isobaric interference to this element. ‘Wavelength that removes isobaric interference of other lanthanides.
-
150
J. P.
YOUNG
et al.
In this table, certain wavelengths for each element are marked with a double dagger; application of the RIMS technique at these wavelengths will minimize or eliminate the isobaric interference of the other lanthanide elements. The first transition of the RIMS route is given where possible, based on published energy level data [S]. Specific information concerning all the wavelengths at which a RIMS signal was observed is not given because of the space required, but the information is available in both spectral and tabular form from the authors. A general summary of all of the RIMS transitions observed for each of these elements is given in Table 2. The number of states populated to at least 5%, based on a Boltzmann distribution at a temperature of 15Oo”C,is also given in this table. In identifying transitions, only these initial states were utilized, and only steps involving the absorption of one or two photons were considered. Based on the energy difference between the initial states and the ionization potentials, two or three such steps would be required to ionize these elements in the wavelength range studied. In Table 2 are shown both the transitions in which the first step can be identified and those that are presently unidentified based on the use of single color photons and published energy levels [S]. In assessing the optical routes, both visual inspection and the use of a computer program, ETRANS [9], were used to evaluate proper matches. From an inspection of this table, it can be seen that, for some of the elements, a large percentage of the first transitions in the optical process can be identified. However, in the case of europium, holmium, and thulium, a very high percentage is not identifiable based on a consideration of a reasonable population of initial states and the available published energy levels [8]. It is interesting to note that the above three elements have a paucity of low-lying initial states below 10 000 cm - l , as shown in Table 3. In the ground state parity, there are only one to three assigned states; in the opposite parity, only holmium has any known states. It has Table 2. RIMS transitions of lanthanides 430-455 nm Levels populated 5% La Ce Pr Nd Pm Sm Eu Gd Tb DY Ho Er Tm Yb Lu
5 6 3 3 4 6 1 5 5 2 1 1 1 1 2
Identified
Unidentified
14 24 29 32
2 0 14 0
25 0 9 18 8 2 8 4 4 4
1 33 0 3 I 33 1 13 0 1
Unidentified % 13 0 33 0 4 100 0 14 41 95 47 78 0 20
Table 3. States of selected lanthanides that lie below 10000 cm-’ Eu (cm-‘) Odd 0*
Even -
*Ground state.
Ho (cm-‘)
Tm (cm-‘)
Odd
Even
Odd
0* 5420 8605 9741
8379 8427 9147
0* 8771
Even
Applications of RIMS to lanthanide elements
151
been postulated in the case of europium [lo] that the RIMS-active transitions might result from a hybrid resonance effect [l 11. It should be emphasized, as it was in the original discussion [lo], that the hybrid resonance effect is only a speculative explanation of the ionization route for this element. For lanthanides, hybrid resonance effects have been positively identified in spectral work on ytterbium [12]. These effects are relatively weak, however, when compared to normal optical transition probabilities, and they are only observed at high particle densities such as those available in atom beams. In our experimental results, the unidentified transitions appear relatively intense. See, for example, the RIMS spectrum of holmium in Fig. 2. There is no gross difference in intensity between the two assignable holmium RIMS peaks at 435.1 and 445.9 nm and the 33 other unidentified peaks. A possible alternative explanation for these unknown routes is that the initial boundbound transition is a two-photon process. The assignments of such routes have been given in studies of ytterbium [12] and tantalum [13]. It is likely that such routes are available for europium and cannot be assigned because the required higher lying states ( > 44 000 cm - ’ ) have not been identified [8]. In a different spectral region, around 580 nm, several twophoton RIMS processes for europium were conclusively proven by two color laser experiments [ 14). The two-photon case could also be made for holmium, as no two-photon coupled states above 39 000 cm- 1 are listed [8]. Since the resonant photoionization process can operate by either simple or complex routes involving all addressable states, known or unknown, it might be expected that more experimental than predicted transitions would be observed. Further, with a complete understanding of these unidentified transitions, new energy level data can be obtained. The inability to assign most of the observed thulium transitions is not readily explainable. The data in Table 4 are reorganized in the possible simple RIMS routes that involve photons in a sequence of 1 + 1, 1 + 1 + 1,2+ 1, and unidentifiable. The ionization potentials, IP, are also shown in this table. In general, RIMS ionization routes for lanthanides with the lower IPs (below about 45OOOcm-‘) can be identified as involving two photons; this includes the lighter lanthanides and lutetium. The rest of the lanthanides have higher IPs and more photons are required for ionization. Our results for ytterbium are identical to those first reported by MIRZA and DULEY[12], and they have previously assigned these peaks. These workers have also assigned some hybrid resonance peaks, but they are located at higher energies than the spectral region of our study. The data for lutetium are interesting; there are four routes assignable to 1 + 1 processes and one unidentified process. A partial energy level diagram with these routes shown is given in Fig. 3, along with the wavelengths of the ionization peaks. The identifiable routes are straightforward; the peak at 449.87 nm has also been identified by NOGAR and KELLER [15]. For the unidentified ionization route at 451.33, the energy of two such photons is greater than the IP of Lu by some 539 cm-’
r
I
I
I
I
7
1
450
445
440
435
L 430
WAVELENGTH (nm)
Fig. 2. RIMS spectrum of Ho in the range of430-455 nm. (Two 1 + 1+ 1 photon routesdenoted wth
an asterisk. The remainder of the transitions are by unidentified routes.)
152
J. P.
YOUNG et al.
Table 4. RIMS routes for lanthanides 430-455 nm Ionization potential (cm-i) La Ce Pr Nd Pm Sm Etl Gd Tb DY Ho Er Tm Yb LU
44981 44672 44070 44 562 44519 45 134 49 603 47 295 47 900 48 561 49 262 49 879 50441 43 162
Photon routes If1 13 23 28 28 20 0 0 2 3 0 4 0 0 4
1+1+1
2+1
Unknown 2 0 14 0
1
1 1 4 5 0 9 16 5 2 3 2 0 0
0 1 2 4 0
1 33 0 3 I 33 I 13 0 1
/ lr 1 b
44301 43762
/I
///
/
(
t
1994 0
‘P’s/2 24300 400 22222 2Do,,, sp 22125
%s,a %/s EVEN
ODD
Fig. 3. Partial energy level diagram of Lu showing RIMS ionization routes.
and might demonstrate the presence of an autoionizing level at 44 301 cm-’ because of the sharpness of the RIMS peak at that wavelength. An alternate explanation of this unidentified peak could be given that involves the effect of laser side-bands, as discussed by NOGARand KELLER [ 151. Although the unidentified band in our case might somehow be a
Applications of RIMS to lanthanide elements
153
spuriosity of this type, it seems unlikely for several reasons. If it were side-band related, we would expect four unidentified peaks in our data; further, the construction of our dye laser is not conducive to side-band lasing. CONCLUSIONS RIMS spectra of all the lanthanides except promethium have been measured in the wavelength region 430-455 nm. A list of wavelengths has been presented whereby each of these elements can be ionized and mass analyzed without isobaric interference from other lanthanides. A set of possible ionization routes has been developed where possible for various RIMS peaks observed. Complete data for all the spectra are available from the authors. Acknowledgement-Research sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc.
REFERENCES [l] D. W. Beekman, T. A. Callcott, S. D. Kramer, E. G. Arakawa, G. S. Hurst and E. Nussbaum, Int. J. Mass Spectrom. Ion Phys. 34, 89 (1980). [Z] D. L. Donohue, J. P. Young and D. H. Smith, Int. J. Mass Spectrom. Ion Phys. 43, 293 (1982). [3] N. Winograd, J. P. Baxter and F. M. Kimock, Chem. Phys. Left. 88, 581 (1986). [4] C. M. Miller and N. S. Nogar, Anal. Chem. 55,481 (1983). [S] J. Fassett, J. Travis. L. Moore and F. Lytle, Anal. Chem. 55, 765 (1983). [6] D. L. Donohue, D. H. Smith, J. P. Young, H. S. McKown and C. A. Pritchard, Anal. Chem. 56,379 (1984). [‘I] D. L. Donohue, J. P. Young and D. H. Smith, Appl. Spectrosc. 39.93 (1985). [S] W. C. Martin, R. Zalubas and L. Hagan, NSRDS-NBS 60, National Bureau of Standards, April (1978). [9] D. H. Smith, H. S. McKown, J. P. Young, R. W. Shaw and D. L. Donohue, Appl. Spectrosc., in press. [lo] J. P. Young, D. L. Donohue and D. H. Smith, Int. J. Mass Spectrom. Ion Phys. !Wi,307 (1984). [ll] C. B. Collins, S. M. Curry, B. W. Johnson, M. Y. Mirra, M. A. Chellehmalzadeh, J. A. Anderson, D. Popescu and I. Popescu, Phys. Rev. A, 14, 1662 (1976). [12] M. Y. Mirza and W. W. Duley, Opt. Comm. t8, 179 (1979). [13] N. S. Nogar, S. W. Downey and C. M. Miller, Anal. Chem. 57, 1144 (1985). [14] D. L. Donohue, unpublished results. [15] N. S. Nogar and R. A. Keller, Anal. Chem. 57, 2992 (1985).
1989
0584-8547/89 s3.Go+.im Pergamon Press pk.
TOPICS IN LASER SPECTROSCOPY
Application of resonance ionization mass spectrometry to the lanthanide elements in the waveIength region of 430-455 nm J. P. YOUNG, D. L. DONOHUE and D. H. SMITH Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A. (Received 17 May 1988; in revised form 27 June 1988) Abstract-A study of all the Ianthanides except promethium has been carried out using resonance ionization mass spectrometry; the wavelength region for this investigation was 430-450 nm. A list of several wavelengths is given for each of these lanthanides where they can be determined without isobaric interference of other lanthanides. The optical routes for ionization were cataloged in terms of 1+ 1, 1 + 1+ 1, and 2 + 1 photon pathways where possible, and the known or partially known routes are presented. Possible pathways for the unknown optical routes are evaluated.
RESONANCEionization mass spectrometry (RIMS) is a technique that utilizes the resonant absorption of two or more photons by a free atom to transfer an electron through energy levels of the atom, ultimately creating an ion pair.* The cation thus formed can then be mass analyzed. Various modifications of the RIMS technique have been described in a number of papers ([l-5] and references contained therein). Because of the optical selectivity of the ionization process, RIMS is capable of reducing or eliminating isobaric interference that is often present in conventional mass spectrometric methods [l]. Due to the nature of the f-transition elements and their possible modes of formation, isobaric interferences are prevalent in the mass spectrometric determination of the lanthanides and actinides. Earlier studies of a few of the lanthanides and actinides revealed wavelengths at which they can be ionized by this spectral technique [2,6,7]. In this paper we report a study of the RIMS spectra of all the lanthanides except promethium (which was unavailable) in the spectral region of 430-455 nm. The study was undertaken for several reasons. We wished to generate a catalog of analytically useful RIMS wavelengths that could be used for mass analysis of the lanthanides. We wished to gain knowledge concerning possible optical routes that would be useful for ionizing these elements. Lastly, we wished to evaluate our ability to assign energy levels involved in these optical routes in preparing for future studies of the heavier actinides where RIMS studies will yield new and useful energy level information. EXPERIMENTAL The apparatus used for this study is somewhat different from that described before [6] in that a modified time-of-flight mass spectrometer was used for mass analysis. Laser
system
The spectral data were obtained with an NRG-DL-0.03 dye laser pumped by a Lumonics Model TE-262-2 nitrogen laser. The dye was C-440 at a concentration of 5 E-3 Molar in ethanol. This system produced S-10 ns pulses at a rate of l-5 Hz with a power of approximately 200-300 PJ per pulse and a
*Editor’s Note. See also the recently published review article: Analytical applications of resonance ionization mass spectrometry (RIMS) by J. D. FASET~and J. C. TRAVIS.Spectrochim. Acra 43B, 1409 (1988).
147
148
J. P.
YOUNG
et al.
bandwidth of 0.03 nm. The wavelength drive consisted of a d.c. motor connected to the micrometer adjustment for the dye laser grating.
Mass spectrometer The time-of-flight (TOF) mass spectrometer was a Bendix Model CVC-2000 adapted for laser ionization. The modifications to the ion source included installation of an optical window to admit the laser beam. The mechanical and electrical modifications to the source are shown in Fig. 1. A bracket was added to the ion lens system to hold a single filament assembly for generating the atomic population. A grounded stainless steel grid was added to the lens system to admit atoms to the ionization region. The electrical biasing arrangement is shown in the figure. Under normal TOF conditions, the ions are formed by a pulsed electron beam and are then extracted and further accelerated by pulses 1 and 2 (Fig. 1) which are applied to successive grids. The final acceleration to 3000 V is accomplished by d.c. voltages applied to the last two grids. Modifications for laser work included replacement of the pulsed electron beam by a 5-10 ns laser pulse. The atomization filament (Re or Ta) was biased by O-20 V negative to suppress thermally produced ions. The two extraction grids were no longer pulsed but were held at variable negative potentials between 0 and 300 V supplied by a battery. The voltages applied to the last two grids were unchanged. The detection system was the normal continuous dynode electron multiplier connected to a preamplifier. The ion signal was sent to a boxcar integrator (Stanford Research Systems Model SR-250, Palo Alto, CA). The boxcar was triggered by the synchronization pulse from the pump laser. A variable delay of the boxcar window allowed selection of the particular time-of-flight (mass) to be measured. The output of the boxcar could be read on a digital voltameter or plotted vs time on an x-y recorder. The raw ion signal was also displayed on a storage oscilloscope (Tektronix Model 7414). Wavelength measurement was accomplished with a 0.25-m spectrograph equipped with a photodiode array detector and calibrated against a mercury vapor lamp. The wavelengths of all significant spectral features were measured in a static (non-scanning) mode.
ORNL-DWG 65-9635
NORMAL TOF ION SOURCE ELECTRON TRAP
FILAMENT
I\ I\ ir -4.5kV
-Lr PULSE I
-2.7
hV
PULSE 2
RIMS MODE
SAMPLE FILAMENI
I
4 0 TO -300V
BEAM
t
-4. 5h V
+
-2 ‘.l hV
1
Fig. 1. Modifications to the TOF sample source for RIMS use.
Applications of RIMS to lanthanide elements
149
RESULTS ANDDISCUSSION RIMS spectra in the wavelength region of 430-455nm were observed for all the lanthanides except promethium. A partial list of wavelengths is given in Table 1; using these wavelengths any of these lanthanides can be selectively ionized in the presence of the others.
Table 1. Selected major RIMS wavelengths for the lanthanides in the range of 430-455nm First transition
Energy Number of isotopes
1 (nm)*
cm-’
La
434.84 438.11 441.40’
22 990 22819 22646
Ce
432.48 436.40’ 436.90 439.60
23 116 22 908 22 882 22 141
Pr’
433.45 439.24 447.42
23 064 22760 22 344
430.60 438.22’ 442.27 444.51
23217 22813 22604 22487
433.02 435.63 436.58’ 441.96
23 087 22 949 22 899 22 620
EU’
444.89 446.04 446.15
22 468 22413 22 311
Cd
432.72 447.38 447.62’
23 103 22 346 22 334
Tb’
433.43 437.43 442.28
23065 22 854 22 553
DY
433.45 440.41 445.45’ 453.21
23064 22 700 22 443 22 059
435.25 436.02 443.79 448.70
22 969 22 928 22 527 22 280
440.23 442.48:
22 709 22 593
435.98 438.77 448.14
22931 22 784 22 308
Yb
441.04’ 446.71
22 667 22 397
LU
451.90*
22 125
Nd
Sm
Ho’
Er Tm’
1
1
Init. st.
Final st.
314 514 314 514
‘F, IFI ‘F, ‘F,
-
51*
3H5 2F’h T,2
*Wavelengths are accurate only to the bandwidth of the laser system, 0.03 nm. ‘No isobaric interference to this element. ‘Wavelength that removes isobaric interference of other lanthanides.
-
150
J. P.
YOUNG
et al.
In this table, certain wavelengths for each element are marked with a double dagger; application of the RIMS technique at these wavelengths will minimize or eliminate the isobaric interference of the other lanthanide elements. The first transition of the RIMS route is given where possible, based on published energy level data [S]. Specific information concerning all the wavelengths at which a RIMS signal was observed is not given because of the space required, but the information is available in both spectral and tabular form from the authors. A general summary of all of the RIMS transitions observed for each of these elements is given in Table 2. The number of states populated to at least 5%, based on a Boltzmann distribution at a temperature of 15Oo”C,is also given in this table. In identifying transitions, only these initial states were utilized, and only steps involving the absorption of one or two photons were considered. Based on the energy difference between the initial states and the ionization potentials, two or three such steps would be required to ionize these elements in the wavelength range studied. In Table 2 are shown both the transitions in which the first step can be identified and those that are presently unidentified based on the use of single color photons and published energy levels [S]. In assessing the optical routes, both visual inspection and the use of a computer program, ETRANS [9], were used to evaluate proper matches. From an inspection of this table, it can be seen that, for some of the elements, a large percentage of the first transitions in the optical process can be identified. However, in the case of europium, holmium, and thulium, a very high percentage is not identifiable based on a consideration of a reasonable population of initial states and the available published energy levels [8]. It is interesting to note that the above three elements have a paucity of low-lying initial states below 10 000 cm - l , as shown in Table 3. In the ground state parity, there are only one to three assigned states; in the opposite parity, only holmium has any known states. It has Table 2. RIMS transitions of lanthanides 430-455 nm Levels populated 5% La Ce Pr Nd Pm Sm Eu Gd Tb DY Ho Er Tm Yb Lu
5 6 3 3 4 6 1 5 5 2 1 1 1 1 2
Identified
Unidentified
14 24 29 32
2 0 14 0
25 0 9 18 8 2 8 4 4 4
1 33 0 3 I 33 1 13 0 1
Unidentified % 13 0 33 0 4 100 0 14 41 95 47 78 0 20
Table 3. States of selected lanthanides that lie below 10000 cm-’ Eu (cm-‘) Odd 0*
Even -
*Ground state.
Ho (cm-‘)
Tm (cm-‘)
Odd
Even
Odd
0* 5420 8605 9741
8379 8427 9147
0* 8771
Even
Applications of RIMS to lanthanide elements
151
been postulated in the case of europium [lo] that the RIMS-active transitions might result from a hybrid resonance effect [l 11. It should be emphasized, as it was in the original discussion [lo], that the hybrid resonance effect is only a speculative explanation of the ionization route for this element. For lanthanides, hybrid resonance effects have been positively identified in spectral work on ytterbium [12]. These effects are relatively weak, however, when compared to normal optical transition probabilities, and they are only observed at high particle densities such as those available in atom beams. In our experimental results, the unidentified transitions appear relatively intense. See, for example, the RIMS spectrum of holmium in Fig. 2. There is no gross difference in intensity between the two assignable holmium RIMS peaks at 435.1 and 445.9 nm and the 33 other unidentified peaks. A possible alternative explanation for these unknown routes is that the initial boundbound transition is a two-photon process. The assignments of such routes have been given in studies of ytterbium [12] and tantalum [13]. It is likely that such routes are available for europium and cannot be assigned because the required higher lying states ( > 44 000 cm - ’ ) have not been identified [8]. In a different spectral region, around 580 nm, several twophoton RIMS processes for europium were conclusively proven by two color laser experiments [ 14). The two-photon case could also be made for holmium, as no two-photon coupled states above 39 000 cm- 1 are listed [8]. Since the resonant photoionization process can operate by either simple or complex routes involving all addressable states, known or unknown, it might be expected that more experimental than predicted transitions would be observed. Further, with a complete understanding of these unidentified transitions, new energy level data can be obtained. The inability to assign most of the observed thulium transitions is not readily explainable. The data in Table 4 are reorganized in the possible simple RIMS routes that involve photons in a sequence of 1 + 1, 1 + 1 + 1,2+ 1, and unidentifiable. The ionization potentials, IP, are also shown in this table. In general, RIMS ionization routes for lanthanides with the lower IPs (below about 45OOOcm-‘) can be identified as involving two photons; this includes the lighter lanthanides and lutetium. The rest of the lanthanides have higher IPs and more photons are required for ionization. Our results for ytterbium are identical to those first reported by MIRZA and DULEY[12], and they have previously assigned these peaks. These workers have also assigned some hybrid resonance peaks, but they are located at higher energies than the spectral region of our study. The data for lutetium are interesting; there are four routes assignable to 1 + 1 processes and one unidentified process. A partial energy level diagram with these routes shown is given in Fig. 3, along with the wavelengths of the ionization peaks. The identifiable routes are straightforward; the peak at 449.87 nm has also been identified by NOGAR and KELLER [15]. For the unidentified ionization route at 451.33, the energy of two such photons is greater than the IP of Lu by some 539 cm-’
r
I
I
I
I
7
1
450
445
440
435
L 430
WAVELENGTH (nm)
Fig. 2. RIMS spectrum of Ho in the range of430-455 nm. (Two 1 + 1+ 1 photon routesdenoted wth
an asterisk. The remainder of the transitions are by unidentified routes.)
152
J. P.
YOUNG et al.
Table 4. RIMS routes for lanthanides 430-455 nm Ionization potential (cm-i) La Ce Pr Nd Pm Sm Etl Gd Tb DY Ho Er Tm Yb LU
44981 44672 44070 44 562 44519 45 134 49 603 47 295 47 900 48 561 49 262 49 879 50441 43 162
Photon routes If1 13 23 28 28 20 0 0 2 3 0 4 0 0 4
1+1+1
2+1
Unknown 2 0 14 0
1
1 1 4 5 0 9 16 5 2 3 2 0 0
0 1 2 4 0
1 33 0 3 I 33 I 13 0 1
/ lr 1 b
44301 43762
/I
///
/
(
t
1994 0
‘P’s/2 24300 400 22222 2Do,,, sp 22125
%s,a %/s EVEN
ODD
Fig. 3. Partial energy level diagram of Lu showing RIMS ionization routes.
and might demonstrate the presence of an autoionizing level at 44 301 cm-’ because of the sharpness of the RIMS peak at that wavelength. An alternate explanation of this unidentified peak could be given that involves the effect of laser side-bands, as discussed by NOGARand KELLER [ 151. Although the unidentified band in our case might somehow be a
Applications of RIMS to lanthanide elements
153
spuriosity of this type, it seems unlikely for several reasons. If it were side-band related, we would expect four unidentified peaks in our data; further, the construction of our dye laser is not conducive to side-band lasing. CONCLUSIONS RIMS spectra of all the lanthanides except promethium have been measured in the wavelength region 430-455 nm. A list of wavelengths has been presented whereby each of these elements can be ionized and mass analyzed without isobaric interference from other lanthanides. A set of possible ionization routes has been developed where possible for various RIMS peaks observed. Complete data for all the spectra are available from the authors. Acknowledgement-Research sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc.
REFERENCES [l] D. W. Beekman, T. A. Callcott, S. D. Kramer, E. G. Arakawa, G. S. Hurst and E. Nussbaum, Int. J. Mass Spectrom. Ion Phys. 34, 89 (1980). [Z] D. L. Donohue, J. P. Young and D. H. Smith, Int. J. Mass Spectrom. Ion Phys. 43, 293 (1982). [3] N. Winograd, J. P. Baxter and F. M. Kimock, Chem. Phys. Left. 88, 581 (1986). [4] C. M. Miller and N. S. Nogar, Anal. Chem. 55,481 (1983). [S] J. Fassett, J. Travis. L. Moore and F. Lytle, Anal. Chem. 55, 765 (1983). [6] D. L. Donohue, D. H. Smith, J. P. Young, H. S. McKown and C. A. Pritchard, Anal. Chem. 56,379 (1984). [‘I] D. L. Donohue, J. P. Young and D. H. Smith, Appl. Spectrosc. 39.93 (1985). [S] W. C. Martin, R. Zalubas and L. Hagan, NSRDS-NBS 60, National Bureau of Standards, April (1978). [9] D. H. Smith, H. S. McKown, J. P. Young, R. W. Shaw and D. L. Donohue, Appl. Spectrosc., in press. [lo] J. P. Young, D. L. Donohue and D. H. Smith, Int. J. Mass Spectrom. Ion Phys. !Wi,307 (1984). [ll] C. B. Collins, S. M. Curry, B. W. Johnson, M. Y. Mirra, M. A. Chellehmalzadeh, J. A. Anderson, D. Popescu and I. Popescu, Phys. Rev. A, 14, 1662 (1976). [12] M. Y. Mirza and W. W. Duley, Opt. Comm. t8, 179 (1979). [13] N. S. Nogar, S. W. Downey and C. M. Miller, Anal. Chem. 57, 1144 (1985). [14] D. L. Donohue, unpublished results. [15] N. S. Nogar and R. A. Keller, Anal. Chem. 57, 2992 (1985).