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Anal. Chem. 1988, 6 0 , 1786-1791
(6) Morrall, S. W. Ph.D. Dlssertation, Colorado State University, Sept 1984. (7) Caravajal, G. S.;Leyden, D. E.: Maciel, G. E. “Solid State NMR Studies of Aminopropylsiiane Modified Silica,” in Si/anes, Surfaces and Inter faces: Leyden, D. E., Ed.; Gordon and Breach Science Publishers: New York, 1986; p. 383. (8) Chiang. C.; Liu, N.; Koenig, J. L. J . Colloid Interface Sci. 1982, 8 6 , 26. (9) Sudholter, E. J. R.; Huis. R.; Hays, G. R.; Alma, N. C. M. J. Colloid Interface Sci. 1985, 103, 554. (IO) Shinoda, S.;Saito, Y. J. CoNoid Interface Sci. 1985, 103, 554. (11) Schaefer, J.; Stejskal, E. 0. Topics in Carbon- 13 NMR Spectroscopy; Levy, G. C., Ed.; Wiley: New York, 1979; Vol. 3, p 284. (12) Stejskal, E. 0.; Schaefer, J. J. Magn. Reson. 1975, 18, 560. (13) Frye, J. S.;Maciel, G. E. J. Magn. Reson. 1982, 4 8 , 125. (14) Tegenfeklt, J.; Haeberlen, U. J. Magn. Reson. 1979, 36, 453. (15) Maciel. G. E.; Sindorf, D. W. J. Am. Chem. Soc. 1980, 102, 7606. (16)Maciel, G. E.; Sindorf, D. W.; Bartuska, V. J. J. Chromatogr. 1981, 205. 438. (17) Sindorf. D. W.; Maciel, G. E. J. fhys. Chem. 1982, 8 6 , 5208. (18) Sindorf, D. W.; Maciel, G. E. J . Am. Chem. SOC. 1983, 105, 3767. (19) Sindorf, D. W.; Maciel, G. E. J . fhys. Chem. 1983, 87, 5516. (20) Mehring, M. Principles of High ResolutionNMR in Solids; Springer-Verlag: New York, 1983; p 153. (21) Pleuddemann. E. P. ”Chemistry of Silane Coupling Agents”, Sily/ated Surfaces: Leyden, D. L., Collins, W. T., Eds.; Gordon and Breach Science Publishers: New York, 1980: p 31. (22) Marsmann, H. NMR: Basic Princ. frog. 1981. 17, 65.
(23) Coleman, 0. NMR of Newly Accessible Nuclei: Laszlo, P., Ed.; Academic: New York, 1983; Vol. 2. (24) Zeegers-Hayshens. T. Spectrochim. Acta 1965, 21, 221. (25) Huyshens, P. Ind. Chim. Selge 9985, 30, 801. (26) Batchelor, J. G. J. Magn. Reson. 1977, 2 8 , 123. (27) Sarneski, J. E.; Suprenant, H. L.; Molen, F. K.: Reilley, C. N. Anal. Chem. 1975, 47, 2116. (28) Sullivan, M. J.; Maciel, G. E. Anal. Chem. 1982, 54, 1615. (29) Hair, M. L.; Hertyl, W. J. fhys. Chem. 1970, 74, 91. (30) Marshall, K.; Ridgeweil, G. L.; Rochester, C. H.; Simpson, J. Chem Ind. (London) 1974, 775. (31) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. SOC. 1983, 105, 1848. (32) Maciel, G. E.;Zelgler, R. C.; Tan, R. K. “NMR Studies of C,,-Derivatized Silica Systems,” in Silanes, Surfaces and Interfaces; Leyden, D. E., Ed.; Gordon and Breach Science Publishers: New York. 1986: p 413
RECEIVED for review December 16, 1987. Accepted April 4, 1988. The authors gratefully acknowledge project support from National Science Foundation Grants CHE-8210014, CHE-8306518, CHE-8513247, and CHE-8610151 and assistance of the Colorado State University Regional NMR Center, funded by National Science Foundation Grants No. CHE8208821 and CHE-8616437.
Pulsed Laser Resonance Ionization Mass Spectrometry for Elementally Selective Detection of Lead and Bismuth Mixtures B. L. Fearey and C. M. Miller Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
M. W. Rowe,‘ J. E. Anderson, and N. S. Nogar* Chemical and Laser Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Pulsed laser, single-color, (2 4- 1) multiphoton ionization is used to achieve elemental selectlvity wMle concurrently eliminatlng Isobaric interferences for lead (Pb) and bismuth (BI) mixtures detected via resonance ionization mass spectrometry. Experimental resuns are compared with theoretical calculations by using a slmple rate equation formalism. The following oscillator strengths were determined: Pb, 6p7p 3P0 6p7s 3P,0, f = 0.4 f 0.1; Bi, 6p2 (3P0)7p J = 6p27s 4P,,2, f = 0.07 f 0.02. I n addition, the Bi ionlzatlon cross section at 64412 cm-’ was estimated to be (5 f 2) X cm2. Relative efficlencles of the lonlzatlon processes for these elements and a comparison between pulsed and continuous laser excitation are dlscussed.
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Interest in measuring large isotope ratios for rare isotopes with small sample sizes continues to increase ( I , 2). Resonance ionization mass spectrometry (RIMS) overcomes the problem of isobaric interferences frequently encountered in standard thermal ionization mass spectrometry ( I , 3). The particular driving force for the present studies is the need to measure large isotope ratios in bismuth (Bi), including specifically the neutron-deficient isotopes, the production of which is a measure of high energy neutron fluences. In standard thermal mass spectrometric analysis of Bi, lead (Pb) Current address: Chemistry Department, Texas A&M University, College Station, TX 77843.
isotopes are particularly ubiquitous and are the primary isobaric interferences, i.e. natural zos-20sPbobscure zffi-208Bi, The elementally selective technique of RIMS is a logical solution to this problem. In addition, there is a need to measure the small but finite quantity of the radioactive isotope 210Pbin lead ( 4 ) . Lead solder is used extensively in interconnections of computer chips, where the decay of minute amounts of zloPband its daughters (zlOBiand 210Po)can damage the integrated circuit. This problem may become acutely important for future generations of supercomputers, where smaller chips will lead to more intimate contact between the solder and the integrated circuit. Here, the sensitivity of RIMS may be used to advantage. In this work, one consideration was to limit experimental complexity, which suggested the use of a single dye laser for excitation and ionization. For the high ionization potentials of the elements in question, the number of ionization pathways available was limited. One alternative was to utilize a frequency-doubled dye laser operating in the ultraviolet region to give a ( I + 1) (photon to resonance plus photon to ionize) resonance ionization process (3). A simpler and largely overlooked second alternative is the use of a visible dye laser in a (2 + 1)process (5). The latter was chosen for the present case.
EXPERIMENTAL SECTION The basic experiment consisted of a pulsed dye laser, tuned t o resonance with an atomic transition and focused into a quadrupole mass spectrometer. The particular species then is
0003-2700/88/0360-1786$01.50/06 1988 American Chemical Society
ANALYTICAL CHEMISTRY.
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VOL. 60, NO. 17, SEPTEMBER 1, 1988.
war
Table I. Literature, Experimental, and Calculated Parameters for Power-Dependent Studies
Exclmer Laser
calm parameters two-photon transitiona two-photon energy,
Ph
Bi
-
-
6 ~ 3P0 7 ~6p23Po 6p2 ?Po)7p J = 3/20 6p3 4S3,,0 44401 42941
"10
enhancing state/ energy, em-"
6p7s 3P,o/35287
6p27s 'P,/,/32588
f,. f..
0.1996 0.4 f O.ld
0.141'
2.4 x 5.0 X
6.4 X (5 + 2) x 10-'8d
ionization cross section, cm2
Filar
1
0.07 1.1'
10454
* 0.026
laser parameters
exptl values
beam diameter, pm line width (fwhm),em-' pulse duration (fwhm), ns laser wavelength, nm
425 1.5 14 450.44
465.76
"Reference8. bAverage of values from ref 11-16. cReferenee 17. dDetermined in this work. *Reference 19, see text for discussion. ,Reference 18. connan, Cur,.", Power S U ~ ~ l , ~
Figure
1. RIMS
experimental diagram.
ionized through a multiphoton process, and the mass spectrometer isolates and detects the mass of interest. More experimental details are given below. Figure 1shows the experimentalconfiguration in detail. A XeCl excimer laser (308 nm, 12 Hz, -100 mJ/pulse; Lambda Physik Model EMG101, Acton, MA) pumped Coumarin 460 (Exciton Chemical Co., Inc., Dayton, OH) dye in either of two dye lasers (Lambda Physik Model FL2000 or FL2002, Acton, MA), yielding pulse energies of 2-4 mJ. The laser pulse duration was -14 ns, measured with a fast photodiode (EG&G Electro-Optics Model FND-100, Salem, MA) with approximate line widths of 1.5 and 0.3 em-' for each dye laser, respectively, as judged by a monitor etalon (Molectron, Inc., Model DL-030, Sunnyvale, CA). The laser beam then was propagated hy using mirron and lenses to the mass spectrometer, where it was focused with a 100 mm focal length lens to a diameter of -425 pm. Laser beam diameters were estimated via hum spots on photographicfh (6).The laser beam was aligned such that it was 2-4 mm above the sample filament at the source of the quadrupole mass spectrometer (QMS). For power-dependent studies, a variahle attenuator (NewportResearch Corp. Model 935-5, Fountain Valley, CA) was used to vary the dye laser power delivered to the QMS. An energy meter (Laser Precision Corp. Model Rj-71OO/Rjp-734, Utica, NY) measured the pulse energy as the laser beam entered the QMS. The quadrupole mass spectrometer (Extrel Corp. Model C50, Pittsburgh, PA) was modified for both optical input and sample filament insertion. The QMS, pumped hy either a turbomolecular pump (Leyhold-HeraeusTechnologies, Inc., Model 150, Enfield, CT) or an ion pump (Perkin-ElmerModel 222, Eden Prairie, MN), Torr pressure range. The normally operated in the 1 X resonance ionization atom source consisted of a resistively heated rhenium filament onto which was deposited the sample overplated with platinum (vide infra). The filament temperature (-1500 K) was controlled hy a constant current power supply (Kepco, Inc., Model SM12, Flushing, NY). In operation,the spectzometer first was optimized hy ionizing the species of interest via electron bombardment. The ionizer was then shut down and the laser ionization signal optimized for both laser wavelength and beam position. The ions were detected via a channel electron multiplier (Galileo Corp. of America Model 4830, Eastchester, NY) operating a t 2000 V. After pulse shaping, the multiplier signal was sent to a gated boxcar integrator (EG&G Princeton Applied Research Corp., Model 162/164, Princeon, NJ) triggered by the excimer laser for time synchronization. Typically, a 500-11s gate with an
effective time constant of -5 s was utilized, giving acceptable signal-to-noiseratios. The boxcar output was displayed on a x-y recorder (Hewlett-Packard Co. Model 7046A, Palo Alto, CA). For optical spectra or power-dependence studies, the QMS was adjusted to the mass of interest. Mass spectra were obtained hy mass scanning the QMS, typically 0.5 ( m / z ) / sfor optimal performance. A critical component required in the performance of these analyses was the co-plating technique involving platinum (Pt) (7).The sample materials used in these studies were reagent grade metal nitrates (J. T. Baker Chemical Co., Phillipshurg, NJ) dissolved in 0.1 M HC1 and then diluted to a concentration of 1 g/L. The Pt plating solution consisted of platinum dinitrite hydrogen sulfate (platinum DNS, Johnson Matthey, Inc., Chester, PA) dissolved in 0.25 M HCI to a concentration of 5 g/L Pt. Samples and Pt were mplated onto high-purity rhenium filamehts (0.001 in. X 0.03 in. X 0.25 in., Rhenium Alloys, Inc., Elyria, OH). Typically, 20 pg of each element (separately or mixed) and 50 pg of Pt were eo-plated from a 100-pLdrop of 1.5 M HCI at 2.7 V for 30 min. The above solutions were delivered onto the filament via precision pipets (Eppendorf, Division of Brinkman, Westbury, NY). Following the plating, the filament was thoroughly rinsed with high-purity water to remove any residual acid. (See ref 7 for additional details.) The Pt eo-plating method significantly slowed the sample evolution from the filament relative to unplated samples and greatly extended the time available for analysis. Samples could be used in excess of 2 h a t filament temperatures of -1500 K with no significant degradation of signal. Samples prepared hy simple evaporative deposition, on the other hand, lasted less than 30 min.
RESULTS Figure 2 shows the energy level diagrams for the applicable transitions for both atomic species. The frequencies and transition states studied are shown in Table I. These particular transitions were utilized so that both elements of interest could he ionized with the output from a single, highefficiency laser dye. Figure 3 shows typical mass spectra obtained for naturally occurring Pb and Bi, where in each case the laser is tuned to the particular atom's transition. For Pb, the laser was tuned to the two-photon wavelength corresponding to the 6p7p 3P0 6p23P0transition at 44401 cm-' (8). For Bi, the 6p2 (sPo) 7p J = 3/2 6p3 4S3,2transition a t 42 941 cm-' wm utilized (8). Also observed (hut not shown) was the Bi two-photon transition at 41 125 cm-I (6p2 PP0) 7p J = ' / 2 6p3 'S3/$)
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
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* a) 7 0 1
w 14
0 6 5 - 1
t
N
,
,
,
205
,
,
210
MASS
39
Flgure 3. Mass spectra of (a) Pb and (b) Bi 2.
p
26
W
13
0
Flgure 2. Energy level diagrams for (a) Pb and (b) Bi. Note that the
arrows indicate applicable laser frequencies. (8),although with an order of magnitude smaller ion signal (see implications below). In the case of lead, all isotopic ahundances are in agreement with published values for common material within the statistics of OUT measurements (1-2% deviation). Note that the zorPh(1.4% abundance) peak is clearly distinguishahle from the base-line signal with a shot-to-shot variation of approximately 20%. This suggests the capability for isotopic ratio measurements in excess of 1OO:l with the present very simple apparatus. For Bi, an excellent signal-to-noise ratio also was attained for the sole naturally occurring isotope, mBi. For each case, no interference from the other element or other impurities were detected either mass spectrometrically or optically (i.e., via limited laser scanning), even in mixed samples. These results point out both the selectivity and the fidelity of the RIMS technique; that is, the selectivity for particular elements and the lack of interferences in measuring isotopic ratios. Figure 4 shows the ion signal dependence on the laser energy for both Ph and Bi. Two separate data sets are shown for Bi.
Due to slight differences in samples, filament temperatures, and signal amplification, the absolute signal magnitude varied from one data set to another. These data sets thus were scaled to give maximum overlap between data points. A similar single set of data is shown for Ph. The solid lines and relative ion yields derive from calculated fits to the data based on a rate equation model. Note that for Ph, the power curve shows near-saturation (Le., nearly total ionization of the atoms in the laser focal volume) with moderate laser power, while for Ri, only partial saturation is achieved. The model and the implications of these results will he discussed below.
DISCUSSION In the present set of experiments, the dependence of ion signal on laser energy can he measured with a typical accuracy of +lo%. Fitting a calculated ion yield curve to experimental data is particularly valuable in allowing the determination of certain critical spectroscopic parameters, including both oscillator strengths and ionization cross sections. These, in tum, are important in the final analysis of the applicability of the RIMS techniaue to various elements of interest. in this case, P h and Bi. To calculate the ionization intensity dependence on laser power, a rate equation model was utilized (9). With the de&vation and details omitted, the final form of the equation is
where
ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
SPECTRAL ENERGY DENSITY (J/mZ. sec
a) 0.0
-Q X(3) = P 2 P = k ( 1 ) + k(-1)
+ k(2) + k(0) + k(p)
20.0
10.0
30.0
1789
.
Hz)
40.0
0 A
Y
>
(Id)
2
0
and
Q = (P2- 4 k ( l ) [ k ( 2 )+ k ( ~ ) ] ) ' / ~
w
2
(14
l-
a A
Symbols are defined in Table 11. In these calculations, the two-photon absorption process is treated as a reversible single step. The laser intensity was assumed to be constant in the interaction region both along and transverse to the laser propagation axis, i.e. a uniform spatial distribution with neither absorption nor focusing. In the present case, this amounts to ignoring the Gaussian transverse nature of the laser intensity, as no significant focusing occurs on the scale of the length of the sample filament. Also, the extraction and collection efficiency for laser-produced ions was assumed to be constant throughout the same region. These assumptions have been shown previously (5,9)to be good approximations under a wide variety of conditions. The experimental ion yields were scaled to the calculated values, such that the curve could be superimposed on the data points. In order to calculate the ion yield, it also was necessary to calculate the two-photon excitation rate. This, in turn, requires knowledged of the two-photon cross section. This can be calculated by using the formalism of Grynberg and Cagnac (10). The effective two-photon cross section, ueff,can be defined as
where k(1) is the two-photon transition rate and I is the laser intensity. In this formalism, two-photon absorption occurs through the presence of intermediate "enhancing" states, which provide oscillator strength a t the appropriate wavelengths. For the case where a single intermediate state is primarily responsible for the enhancement, the two-photon transition rate can be calculated (10) on the basis of the formula
m,
where symbols are again defined in Table 11. Note that eq 3 is summed over all of the initial magnetic sublevels, mg, since the laser line width, I?, is much larger than the hyperfine nplittings. This is true for both P b and Bi (vide infra). In the ideal case, one would prefer to calculate two-photon cross sections and ion yields entirely on the basis of preexisting data. Oscillator strengths for the ground state to the enhancing intermediate states are indeed availabile for both P b (11-16) and Bi (17). For Pb, one also has a measured ionization cross section ( l a ) ,but one does not have the enhancing state to resonant state oscillator strength. Thus, for Pb, the shape of the power dependence curve was used to determine the single remaining free variable (besides scaling), the enhancer to resonant state oscillator strength. The calculated power-dependence curve, shown in Figure 4a, is in excellent agreement with the experimental data. The falloff from linear energy dependence a t higher energies is clearly visible. The determined oscillator strength of the enhancer to resonant transition, based upon this fitting procedure, is given in Table I.
W
U
LASER ENERGY (mJ) SPECTRAL ENERGY DENSITY (J/m2. sec
Hz)
b) 0 . 6 - ,
9
Y >
1
,
I
,
1
I
,
0.4-
Z
0 W
Ea w
0
Figure 4. Power dependence data for (a) Pb and (b) Bi. The 0,A, and W represent independent data sets, while the solid tine indicates calculated fits to the data (see text). The vertical axis corresponds to absolute ionization efficiency in the laser focal volume. For the present laser system, the measured pulse energies correspond to the indicated spectral energy densities.
For Bi, a literature value for the enhancer to resonant state oscillator strength is available (19),but it is inconsistent with our observed power dependence. No details of this prior measurement were given, so i t is difficult to evaluate its accuracy. (Were the literature oscillator strength employed, the curve of Figure 4b would be 90% saturated with respect to ion yield a t a laser energy of less than 1.5 mJ.) Therefore, this oscillator strength was treated as a free parameter, as was the Bi ionization cross section. (Note that the 6p3 2P1/20 6p3 4S312transition a t 21 661 cm-l, which is near the twophoton resonance for Bi, has an oscillator strength of only 1 x lo-' (20) and thus does not contribute significantly to the two-photon cross section.) Although there are two free variables, the observed Bi power-dependence curve could still be uniquely matched by calculation, as each of the two free parameters induced different behavior in the calculated curves; i.e. the oscillator strength induced curvature at low energies, while the ionization cross section drove the rate of saturation. It should be noted that although the behaviors were separable, they were not totally decoupled. Values from the literature and determined parameters are contained in Table I; the calculated fits for the power dependences are shown in Figure
-
-
4.
-
In addition, a simple evaluation of the Bi 6p2 (3P0)7p J = 6p3 4S3120transition oscillator strength can be made. Previous measurements (21) have shown that the ionization cross sections from the 6p2 (3P0) 7p J = '/? and J = 3/2states are approximately equal. Thus, the order of magnitude lower 1/20
1790
ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
Table 11. Definition of Symbols
For Equation 1: relative population of atoms in ionization continuum k ( l ) , k ( - l ) , k ( 2 ) rates of two-photon absorption, stimulated emission, and ionization k(O) spontaneous emission rate from resonant to ground states k(p) rate of irreversible loss from resonant state T laser pulse duration n(c)
Y r To,
fi, c
g, e, r Age, Aer "Jge,
"Jer
Ali
f~?J,l,m,O.J,m) fer 9
For Equations 2 and 3; effective two-photon cross-section laser intensity energy line width of either the laser (fwhm) or the lifetime of the final state (whichever is larger) Bohr radius, Planck's constant, speed of light under vacuum ground, enhancing, and resonant states laser wavelengths for associated transitions laser angular frequencies for associated transitions detuning of the laser frequency from the enhancing state oscillator strength for associated transitions Clebsch-Gordon coefficients for excitation of the appropriate transitions with linearly polarized light
ionization signal from the J = 1/20 state (vide supra) must be accounted for in the enhancer-to-resonant states' oscillator strengths and the factor of 2 decrease in laser power at this wavelength. On the basis of these considerations, we estimate the 6p2 (3P0)7p J = 1/20 6p27s4P1,aoscillator strength to be 0.005, within a factor of 2. Clearly, as seen from the power-dependence studies (Figure 4), saturation is nearly reached for P b with only modest laser pulse energies. These results suggest that for P b a high efficiency of ionization is easily achieved, which leads not only to excellent analytical sensitivity but also to insensitivity of ion signal to laser power fluctuations. The same is not true for Bi. Since Bi shows a lower level of saturation, it will show an increased sensitivity to laser noise, but with -50% ionization (number of ions generated per number of atoms in interaction volume per pulse) good sensitivity is still obtainable. With the laser operating with the indicated experimental line width and geometry (vide supra), a pulse energy of 55 mJ (attainable with commercial lasers) will give -90% saturation, thus removing the laser noise problem. Further, it is important to point out that, since the two-photon cross section is predicted to increase with the inverse of the laser line width, nearly 100% saturation for both species can be achieved easily with only very modest powers if one can realize a 10-fold reduction in laser line width (attainable by using commercial etalons). In fact, parallel experiments with the narrower bandwidth laser (0.3 cm-') exhibited near-saturation with comparable pulse energies. As the laser line width is decreased, it also becomes feasible to perform isotopically selective resonance ionization. This may be highly desirable in cases of very large isotope ratios, e.g., measurements of zlOPb. For Pb, the 6p7p 3P0 6p2 3P, transition will have no hyperfine structure ( J = 0 J = 0) and only isotope shifts will determine line positions. These shifts amount to a -200-mK energy spread for 204Pb-210Pb (- 100-mK laser tuning) for the present two-photon process (22-24). For Bi, both hyperfine structure and isotope shifts contribute to the optical spectra. The hyperfine pattern will be -250 m K wide for all cases (25-30); the isotope shift, although unmeasured at present, can be expected to be similar to that of P b (29). Of course, in both instances, the atom source conditions must be such that Doppler broadening is
-
--
not extreme. Unfortunately, as is the case for most pulsed RIMS experiments, the duty cycle (fraction of the atoms exposed to the laser) is only -10-5(temporal overlap) X 10-2(spatial This effectively limits the useful dynamic overlap) = range in isotope ratios to greater than 0.01% for microgramsize samples. Clearly, the present experiments fall short of most analytical requirements, especially those for Bi, and suggest the desirability of performing the experiments with high repetition rate pulsed lasers or continuous wave (CW) lasers to increase the duty cycle and, in turn, the analytical sensitivity (9). High repetition rates are available with copper vapor laser pumping, but the desired wavelengths are difficult to achieve. Thus, for good sensitivity with pulsed systems, further development in laser hardware and/or pulsed evaporation sources is needed. Resonance ionization via CW lasers also poses some difficulties: recent experiments in tantalum (2 1) CW RIMS have not been successful, owing to the small two-photon cross section (31). A potential alternative is a (1 + I) scheme with the resonant laser operating in the ultraviolet. This requires frequency-doubling of the CW dye laser, which for ring lasers produces 1 mW a t the desired wavelengths. Although this power is sufficient to saturate the resonant one-photon transition, the ionization cross section is many orders of magnitude smaller than the one-photon transition cross section, and thus, only negligible ionization can be expected. Fortunately, this problem is not a limiting factor, since secondary laser ionization can easily be achieved (Le.,by using another laser at high power for ionization). This method has been shown previously to operate effectively (32). The exact conditions required for such a method are currently under consideration.
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+
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ACKNOWLEDGMENT We would like to thank M. Murre11 for his advice on use of the plating apparatus. R e g i s t r y No. Pb, 7439-92-1;Bi, 7440-69-9. LITERATURE CITED Miller, C. M.; Nogar, N. S.;Apel, E. C.; Downey, S. W. I n Resonance Ionization Spectroscopy, 7986; Hurst, G. S., and Morgan, C. G., Eds.; Institute of Physics: Bristol, England, 1987; p. 109. Hurst, G. S. I n Resonance Ionization Spectroscopy 7986; Hurst, G. S . , Morgan, C. G., Eds.; Institute of Physics: Bristol, England, 1987; p 1. Moore, L. J.; Fassett. J. D.; Travis, J. C. Anal. Cbem. 1984, 56, 2770. Keller, R. A., Los Alamos National Laboratory, personal communication, 1986. Apel, E. C.; Anderson, J. E.; Estler, R. C.; Nogar, N. S.; Miller, C. M, Appl. Opt. 1987, 26, 1045. Avizonis, P. V.; Doss. T. T.; Heimlich, R. Rev. Sci. Instrum. 1987, 3 8 , 331. Rokop, D. J.; Perrin, R. E.; Knobeloch, G. W.; Armijo, V. M.; Shields, W. R. Anal. Chem. 1982, 5 4 , 957. Moore, C. E. Natl. Bur. Stand. ( U S . ) Circ. 1958, No. 467. Miller, C. M.; Nogar, N. S. Anal. Cbem. 1983, 55, 481. Grynberg, G.; Cagnac, B. Rep. f r o g . Phys. 1977, 4 0 , 791. Bell, G. D.; King, R. 8. Astropbys. J . 1981, 733,718. Corliss, C. H.; Bozman, W. R. N6S Monogr. ( U . S . ) 1982, No. 53. Penkin. N. P.; Slavenas, I.Opt. Specfrosc. (Engi. Transl.) 1963, 15, 83.
Saloman, E. B.; Happer, W. Phys. Rev. 1988, 144, 7. de Zafra, R. L.; Marshall, A. Phys. Rev. 1968, 770, 28. Gibbs, H. M.; Churchill, G. G.; Salamo, G. J. Pbys. Rev. A 1973, 7, 1766. Guern. Y.; Lotrian, J.; Bideau-Mehu, A.; Johannin-Gilies. A. J. Pbys. 6 1978. 1 1 . 3821. Koz
