Anal. Chem. 1904, 56,827-828
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AIDS FOR ANALYTICAL CHEMISTS Resonance Ionizatlon Mass Spectrometry of Uranium with Intracavity Laser Ionization S. W. Downey and N. S. Nogar*
Chemistry Division, Los Alamos National Laboratory, Los Alamos, N e w Mexico 87545 C. M. Miller
Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Resonance ionization mass spectrometry (RIMS) is a highly selective and sensitive tool for elemental and isotopic analysis (1-9).While most RIMS work to date has been carried out with high-power pulsed lasers, it has been suggested (IO)and verified (11),that continuous wave (CW) lasers may prove to be a viable alternative. Two major differences exist between pulsed and CW excitation: in the former case, the duty cycle is usually low, and ionization is normally near saturation while the laser is on ( I ) ; in the latter case, the duty cycle is unity, while the ionization process is far from saturation (10). Thus, one expects that increases in laser intensity should lead to increased ion yields for CW excitation. For CW, and in some cases flashlamp-pumped dye lasers, it is well-known that the circulating power inside the cavity is much greater than that coupled out of the cavity. This fact has been put to good use in experiments on intracavity photothermal (I2),photoacoustic (13), and fluorescence (14) spectroscopy and photochemistry (15).We have recently undertaken experiments designed to exploit intracavity spectroscopy for resonance ionization; to our knowledge, there have been no previous reports outside this laboratory of intracavity RIMS. In our experiment, the cavity of a flashlamp-pumped dye laser was extended to contain the source region of a timeof-flight mass spectrometer. Experiments were carried out with uranium, using the three-photon doubly resonant ionization (16)at 591.5 nm. This process is particularly attractive for uranium because photons of a single nominal frequency are resonant with both bound-bound transitions. Thus, a single laser can be used to ionize uranium through sequential resonances.
EXPERIMENTAL SECTION A schematic of the experimental apparatus is shown in Figure 1. The sample, consisting of an acidic solution of uranium, was evaporated onto a Re sample filament. Samples typically contained 10 pg of uranium and produced signals for -10 h when operated at -1500 O C . With fresh samples, it was observed that the predominant signal generated with Rhodamine 6G was due to UO'. Subsequent outgassing produced a signal which was ' U with a sharp resonant wavelength dependence. predominantly, Base pressure in the mass spectrometer was 1 2 x torr. The laser cavity encompassed a beam steering periscope constructed with dielectric mirrors, antireflection coated windows, and an antireflection coated positive lens (500 mm focal length) in addition to the normal elements of the dye laser. Total cavity length was -1.5 m. The intracavity lens was necessary to initiate lasing (17),apparently by reducing the divergence of the spontaneous fluorescence. The normal (85% R) output coupler of the laser was used for both normal and extended cavity operation. For normal operation, the output coupler was mounted in the manufacturer's supplied gimbal mount on the front of the laser enclosure. For the extended cavity, an external gimbal mount was used. The laser was typically operated at 20 Hz,with a pulse duration of 1 ps, yielding an effective duty cycle of Performance of the laser was monitored by an averaging energy meter, while wavelength was determined via an optogalvanic signal 0003-2700/84/0356-0827$01.50/0
in a uranium hollow cathode lamp (18). Ions were produced in a field of -100 V/cm, and were accelerated into the field-freeflight tube. Signal was generated with a channel-electron multiplier, passed through a preamplifier, and processed with standard analog electronics. The boxcar gate could be either set on the U+ signal, in order to maximize that signal, or swept to produce a time-of-flight spectrum.
RESULTS AND DISCUSSIONS Most experiments were carried out by fixing the flash lamp discharge voltage in the laser and then performing the ionization experiments both intra- and extracavity. In both cases, the laser output was maximized by adjusting the orientation of the output coupler. Ion production was initiated by tuning the birefringent filter until an optogalvanic signal was observed; fine tuning was performed while monitoring the ion signal. In general, the energy coupled out of the extended cavity was less than that coupled out of the normal cavity, Figure 2. This may be due to a variety of reasons. Reflective losses at the various optical surfaces will account for some of the difference, as will any imperfections in the dielectric mirror. Sample absorption losses are insignificant for our estimated sample densities, lo7 ~ m - ~A .major difference is apt to be mismatch of the mode size and active volume in the extended cavity (19,20); we did not make extensive efforts to maximize the performance of this geometry. Spot sizes (-2 mm diameter) at the source of the mass spectrometer for the extended and normal cavities were the same (120%). We estimate that 10% of the atoms leaving the sample filament were exposed to the laser pulse in the field of view of the ion extraction optics. As the lamp discharge energy and hence the output energy of the normal cavity was increased, the ratio of extended cavity to normal cavity output energies increases. This behavior may be explained qualitatively. The laser output power will be directly related (21)to the pump energy available in excess of that required to reach the lasing threshold. Because of the losses noted above, the lasing threshold for the extended cavity will be higher than that for the normal cavity. Thus, at low pump energies, the stored energy above threshold will be much greater for the normal cavity than for the extended cavity. As the pump power is raised, the fractional difference in available energies for the two cavities will decrease. Simple theory would predict that the laser energy circulating within the cavity approximately equals 1/(1- R) times the energy coupled out. For the 85% reflective output coupler supplied with this laser, 1/(1- R) = 6.7. In the absence of saturation (ionization of all atoms within the focal volume) one expects the ion signal to depend on the laser fluence. Since the absorption cross section for bound-bound transitions is usually several orders of magnitude larger than that for bound-free transitions (22),it is often the case that the resonant steps in a multiphoton ionization process may be
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0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58,NO. 4, APRIL 1984 Floshlamp- Pumped D y e LOW,
Figure 1. Schematic of the experimental apparatus. Laser wavelength was monitored by optogalvanic signals generated in the uranium lamp. Base pressure in the mass spectrometer was typically - 2 X lo-' torr.
3
O0
i -1
PULSE ENERGY ( m J )
(normal cavity) Figure 2. Ratios of extended cavity to normal cavity output pulse energies (lower half) and extended (intra)cavity to normal cavity ionization signals (upper half) vs. normal cavity pulse energy. Pulse energies and ion signals were both integrated for 100 shots.
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saturated while the ionization step is still dependent on the fluence. Thus, even for a multiphoton process such as the uranium ionization, the ion yield may vary nearly linearly with fluence over a limited range of fluences. Obviously, for sufficiently high fluences, the ionization will be saturated and the signal will be fluence independent, while a t very low fluences, the ion yield will exhibit a higher order dependence on the fluence. For the range where the signal is linear with fluence, the ratio of intra- to extracavity ion yields should be proportional to the corresponding ratio of pulse energies. This behavior can be seen in the upper half of Figure 2. Further, the ratio of intra- to extracavity ion yields should be given by S(intra)/S(extra) = 6.7[E(intra)/E(extra)], where E(intra) is the measured pulse energy coupled out of the extended cavity and E(extra) is the same quantity for the normal cavity. This relationship can also be verified qualitatively by a comparison of the upper and lower halves of Figure 2.
It thus appears that it is possible to significantly improve the rate of ionization for RIMS by placing the mass spectrometer inside the laser cavity. This enhancement may be particularly significant for ionization with CW lasers. In this case, even the resonant transitions may not be saturated, and the ion yield will exhibit (IO) a greater than first-order dependence on fluence. In addition, the output coupler reflectivity for CW lasers is generally >85%, so the intracavity power enhancement can be even greater. It is not unreasonable to expect an order of magnitude improvement in ionization rate for CW intracavity ionization, although technical difficulties associated with the operation of extended cavity CW lasers may render it less experimentally tractable. Two caveats must be considered in the application of this technique. First, it is obviously suited only to systems in which ionization can be affected by photons of a single wavelength. This will include 1+ 1 , 2 + 1,and 2 + 2 (photons to resonance and photons to ionization) ionization schemes, as well as some special circumstances such as the uranium ionization. Second, selectivity of the ionization process may be reduced at high intensities for pulsed lasers. In a separate set of experiments involving ionization of 233U/242Pu mixtures, it was found that at moderate intensities, C100 kW/cm2, the ionization process was highly selective, while at higher intensities, 400 kW/cm2, both U and PUions were observed. With CW lasers, however, such a limitation is not anticipated.
ACKNOWLEDGMENT Helpful comments from R. A. Keller are gratefully acknowledged. Registry No. Uranium, 7440-61-1. LITERATURE CITED ( I ) Hurst, G. S.;Payne, M. G.; Kramer, S. D.; Young, J . P. Rev. Mod. PhyS. 1979, 51, 767-819. (2) Beekman, D. J. W.; Callcott, T. A,; Kramer, S. D.; Arakawa, E. T.; Hurst, G. S. Int. J . Mass Spectrom. Ion Phys. 1980, 89-97. (3) Miller, C.M.; Nogar, N. S.;Gancarz, A. J.; Shlelds, W. R. Anal. Chem. 1982, 5 4 , 2377-2378. (4) Donahue, D. L.; Young, J. P.; Smith, D. H. Int. J . Mass Spectrom. Ion Phys. 1982, 43, 293-307. (5) Young, J. P.; Donahue, D. L. Anal. Chem. 1983, 55, 88-91. (6) Donahue, D. L.; Young, J . P. Anal. Chem. 1983, 55, 378-379. (7) Kimock, F. M.; Baxter, J. P.; Wlnograd, N. Surf. Scl. 1983, 124, L42L48. (8) Winograd, N.; Baxter, J. P.; Kimock, F . M. Chem. Phys. Len. 1982, 88, 581-584. (9) Fassett, J. D.; Travis, J. C.;Moore, L. J.; Lytle, F . E. Anal. Chem. 1983, 55, 765-770. (IO) Miller, C. M.; Nogar, N. S.Anal. Chem. 1983, 55, 481-488. (11) Miller, C. M.; Nogar, N. S . Anal. Chem. 1983, 5 5 , 1606-1608. (12) Reddy, K. V. Rev. Sci. Instrum. 1983, 54, 422-424. (13) Bray, R. G.;Berry, M. J. J . Chem. Phys. 1979, 77, 4909-4922. (14) Anderson, W. R.; Vanderhoff, J. A,; Hotlar, A. J.; Dewilde, M. A.; Beyer, R. A. J . Chem. Phys. 1982, 77, 1677-1685. (15) Reddy, K. V.; Berry, M. J. Chem. Phys. Lett. 1979, 66, 223-229. (18) Chen, H. L.; Borzileri, J. Chem. Phys. 1981, 7 4 , 6063-6069. (17) Dawney, S. W.; Nogar, N. S . , submitted to Appl. Spectrosc. (18) Keller, R. A,; Engleman, R., Jr.; Zalewski, E. F. J . Opt. SOC. A m . 1979, 69,738-742. (19) Collins, S. A., Jr. Appl. Opt. 1964, 3 , 1263-1275. (20) Kogelnik, H.;Li, T. Appl. Opt. 1966, 5 , 1550-1567. (21) Yariv, A. "Quantum Electronics"; Wiley: New York, 1975. (22) Bekov, G. I.; Letokhov, V. S . Appl. Phys. B 1983, 30, 161-176.
RECEIVED for review October 24, 1983. Accepted December 8, 1983. Financial support by the DOE under the auspices of Los Alamos National Laboratory is gratefully acknowledged.
