Anal. Chem. 1989, 61,695-697
695
Diode Laser Initiated Resonance Ionization Mass Spectrometry of Lanthanum Robert W. Shaw,* J. P. Young, and D. H. Smith
Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
+ +
Three two-color 1 1 1 resonance ionization processes were demonstrated for lanthanum, with the Initial optical transltion excited by an inexpensive GaAiAs diode laser operating at 753.93 nm. A 6-kHz copper vapor laser pumped dye laser was utilized for the subsequent excitation and ionlzatbn steps; lanthanum139 ions were created when 566.4&, 584.86-, or 586.54-nm photons were present in conjunction with the diode laser Ilght. A practical resonance ionization mass spectrometric instrument employing only dlode lasers for excitatbn of bound-barnd atomic transitions Is envisioned.
INTRODUCTION Wide-spread use of lasers for analytical chemical applications has been hindered by the complexity of tunable lasers and the level of sophistication required for their operation. Semiconductor diode lasers (1)possess the simple operating characteristics to reverse that situation, while maintaining the desirable properties of tunability, narrow bandwidth, and spatial coherence. Their low cost is attributable to high volume production for the consumer products market, where they have found commercial application. Diode lasers have also found spectroscopic uses for atomic physics (2) and analytical chemical applications (3). We feel that they will have an important impact on the latter field in the near future, particularly as they progress toward visible wavelengths. We report here the use of a diode laser for the initial step of three resonance ionization mass spectrometry (RIMS) processes of lanthanum. This study demonstrates that a diode laser can be used to pump one optical transition between bound atomic levels for a resonance ionization process that is 1 1 1overall. As such, it represents the f i t step toward using several diode lasers to excite sequentially an atom to an upper Rydberg state for subsequent ionization by a nonresonant process with, for example, an electric field (4), an infrared laser (5), or collisions. In this particular demonstration the second and third photons (further excitation and ionization, respectively) were provided by a pulsed dye laser. Multiple-photon resonance ionization mass spectrometry of lanthanum using excimer-pumped dye lasers has previously been reported by Green et al. (6).
+ +
EXPERIMENTAL SECTION The diode laser used in this study was a 750-nm GaAlAs device with a nominal power of 3 mW (Model ML-4405, Mitsubishi Electronics America, Inc., Sunnyvale, CA). Ita output wavelength was tuned by controlling its case temperature and operating current. The diode laser was powered by a precision current power supply (Model LDX-3207, ILX Lightwave Corp., Bozeman, MT) specified as stable to 7 PA root mean square at the operating current of 74.5 mA. The diode laser was epoxied into an aluminum block and its case temperature adjusted with a thermoelectric heater/cooler in thermal contact with the block. The operating temperature (45.0 "C for 753.9 nm) was controlled to hO.01 "C by using a temperature controller (Model LDT-5910, ILX Lightwave Corp.) that sensed a thermistor glued into the aluminum block. The cost of the entire diode laser apparatus in0003-2700/89/0361-0695$01.50/0
cluding diode laser ($70),laser mount, current power supply, and temperature controller was approximately $5000. The diode laser line width was measured by using an optical spectrum analyzer with a 7.5GHz free spectral range and a fiiesse of 100. Single longitudinal mode operation was verified and the line width was determined to be 150 MHz (full width at half maximum) for 27 "C operation at 60 mA and was probably limited by the current noise of the power supply. Careful determination of the diode laser wavelength was made with a wavemeter (Model WA-20, Burleigh Instruments, Inc., Fishers, NY) accurate to hO.001 nm. The diode laser output was collected and focused in front of the sample filament with a 0.5 N.A. lens, creating a spot size of 0.8 mm. The copper vapor laser (Model 351HR, Plasma Kinetics Group of Metalaser Technologies, Inc., Fremont, CA) was operated at 6 kHz and pumped an wiIlator/amplifier rhodamine 6G dye laser (Model DL13P, Molectron Corp., Sunnyvale, CA). The peak power of the 20-ns dye laser pulses was 2-5 kW, depending on the operating wavelength, and the tuning range was 5 6 0 0 nm. The nominal line width was 0.3 cm-*. The dye laser output was focused in front of the sample fiiament (spot size approximately 1 mm) by using a telescope constructed from two lenses of 40and 160-mm focal lengths. The wavelengths of dye-laser-excited RIMS lines were calibrated by splitting a small portion of the laser beam into a uranium hollow cathode lamp (Ne fill gas) and using the optogalvanic effect to record the 585.25-nm neon line during the same scan. The resulting line positions are accurate to fO.O1 nm. The magnetic sector mass spectrometer for RIMS experiments is similar to one described previously (7). A single filament arrangement was used here that consisted of a boat-shaped rhenium filament mounted directly in front of the mass spectrometer entrance. The f i i e n t was loaded with approximately 1mg of lanthanum metal that was overcoated with aquadag. The diode and dye laser beams crossed at right angles at their respective foci between the hot f i i e n t and the ion lens of the mass spectrometer. The lasers were arranged for parallel linear polarization. Single ion counting was accomplishedwith an electron multiplier tube (Model R515, Hamamatsu Photonics Corp., Bridgewater, NJ) at 3.13 kV, a 100-MHz leading-edge discriminator (Model 436, EG&G Ortec, Oak Ridge, TN), and a rate meter (Model 449, EG&G Ortec). The dye laser wavelength was controlled, and spectra were acquired, by using an IBM personal computer (Model PC, IBM Corp., Boca Raton, FL) operating an Asyst language (version 1.51, Asyst Software Technologies, Rochester, NY) program. Initially the ion lens and mass spectrometer were tuned for lanthanum-139 thermal ions from the single filament at high temperature (>1500 "C) with no lasers present. The filament temperature was then reduced to approximately 1100 "C, the CVL/dye laser beam admitted, and the ion extraction focus reoptimized for photoions from single-colorresonance ionization (e.g., at 585.04 nm). Because the mass spectrometer was focused to the laser focal volume (and not the filament), the background from thermally generated ions was reduced to less than 0.1% of the total ion signal. Finally, the diode laser beam was admitted for double resonance experiments.
RESULTS AND DISCUSSION Before double resonance (i.e., diode laser plus CVL/dye laser) spectra were measured, it was first necessary to record the resonance ionization spectrum for the CVL/dye laser alone. The single-color spectrum serves as the base line for 0 1989 American Chemical Soclety
696
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
2.4
TWO-COLOR LANTHANUM RIMS
I
2.7 f
I
I
4D5!2
1
30354cm-'
584.9 nm
0.3 ..
+
UI
! 577
579
581
583 585 587 589 WAVELENGTH (nm)
k
591
I
r l
4F:12
+--
13260 cm-l
I
593
Figure 1. Single-color resonance ionization spectrum of I3'La over a portion of the rhodamine 6 0 tuning range. 2D32
753.9 nm
o o cm-'
Figure 3. Partial energy level diagram of lanthanum.
1
584.2 584.3 584.4 584.5 584.6 584.7 584.8 584.9 585.0 WAVELENGTH (nm)
Figure 2. Resonance ionization spectra of I3'La: 753.905-nm diode laser plus CVL/dye laser (lower trace); 753.93-nm diode laser plus CVtldye laser (upper trace). two-color experiments. Three photons are required in the rhodamine 6G tuning range to elevate lanthanum atoms from low-lying initial states to above their ionization potential (44981 cm-I). The first two would be resonant and promote either sequential bound-bound transitions (both within the linewidth of the laser) or simultaneous two-photon transitions; the third photon would be absorbed nonresonantly and would effect ionization by promoting the atom into the continuum (8). Figure 1 presents the single-color resonance ionization spectrum for lanthanum-139 between 575 and 595 nm. The count rate for the strongest line is 7000 counts/s for a dye laser peak power of 5.6 kW. Additional lines are observed in the remainder of the dye tuning range between 560 and 575 nm with about the same spectral density. Assignment of this plethora of lines is currently under way with the assistance of a computer program, ETRANS (9), that computes allowed transitions and multiple-photon processes by operating on the National Bureau of Standards energy level tables of the lanthanides (10). The diode laser was tuned to 753.905 nm, and the beam was admitted to the source region of the mass spectrometer. A 1-nm dye laser scan was made with both laser beams present; the resulting spectrum is shown as the lower trace of Figure 2. This spectrum is very similar to the corresponding portion of Figure 1. The diode laser was then retuned to 753.93 nm and the 1-nm dye laser scan repeated (Figure 2, upper trace, offset vertically for clarity). A new line is obvious at 584.86 nm that is the base peak of the spectrum. The peak of the two-color line corresponds to 13OOO counts/s for diode and dye laser powers of 8 mW and 2.1 kW, re-
spectively. By comparison with the lower trace of Figure 2, it is clear that detuning the diode laser by as little as 0.025 nm eliminates this new line from the spectrum. The RIMS process for the double resonance line can be explained by reference to the partial state diagram (10) for lanthanum shown in Figure 3. The diode laser excites 'D3,z ground-state atoms to the 4F$, level at 13 260 cm-'. The 584.86-nm dye laser promotes the atom from that level to the 4D6,zlevel at 30354 cm-I. Absorption of an additional dye laser photon effects ionization nonresonantly. Both of the bound-bound transitions are known lanthanum emission lines (11),and that knowledge led us to choose this set of wavelengths for a demonstration of diode-laser-initiated double resonance RIMS. In the sense that two lasers are individually tuned to excite two sequential bound-bound atomic transitions for subsequent nonresonant ioniaation, this demonstration is analogous to earlier continuous wave (CW) dye laser RIMS experiments (5). The different feature of this example is that an inexpensive diode laser was utilized for one of the excitation steps. The diode laser power employed to record the upper trace of Figure 2 was 8 mW. That high power (the maximum rated diode laser power is 5 mW) was necessary to produce a stable operating wavelength of 753.93 nm. To move the operating point away from the vicinity of a mode hop, it was necessary to tune by using a slightly lower temperature and higher current than otherwise might have been used. This high current operation resulted in high output power and limited the use of individual diodes to only a few days operating time before multimode character set in. These diode lasers are specified to operate a t 750 f 10 nm with individual devices falling somewhere within that range for 27 "C operation. Our diode lasers exhibited a 27 "C operating wavelength of 748 nm and substantial temperature tuning (i. e., heating) was required to attain our operating wavelength. Additional diode lasers now in hand produce 753-nm light at 27 "C. A RIMS count rate vs diode laser power experiment showed that 8 mW (Le., 400 mW/cm2) was sufficient to saturate the 2D3/2-4F&2 optical transition; it was necessary to reduce the diode laser power to below 2 mW (100 mW/cm2) to attain a linear count rate response. Two additional double resonance lines were found within the dye tuning range for a diode laser wavelength of 753.93 nm. Figure 4 presents a 1.8-nm region near the blue end of the dye tuning curve. The lower trace represents the ion count rate for CVL/dye laser pumping only; the upper trace is a two-color experiment with the diode laser tuned to 753.93 nm. A new line is evident at 566.46 nm that exhibits a peak count
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
t
I\ 565.6 565.8 566.0 566.2 566.4 566.6 566.8 567.0 567.2 WAVELENGTH (nm)
Flgure 4. Resonance ionization spectra of '%a: CVL/dye laser only (lower trace); 753.93-nm diode laser plus CVL/dye laser (upper trace).
rate of 2800 counts/s for diode and dye laser powers of 8 mW and 6.1 kW, respectively. CVL/dye laser excitation of the 4F&2diode-laser-pumped state to a 4D5/2level at 30 909 cm-' is responsible for this second new line. This intermediate state is known (IO),but this particular transition is not observed in emission (11). The final double resonance line observed occurs for the same diode laser wavelength and a dye laser wavelength of 586.54 nm; this line corresponds to an interstate at 30 305.6 cm-*. mediate 2F5/2 Our lanthanum sample is of near natural abundance and as such contains approximately 0.1% '%La. We are able to detect mass 138 photoions for the two-color RIMS conditions, but the small signal-to-background ratio has prevented acquisition of a presentable '%La resonance ionization spectrum. In any event, the line width of our laser precludes an attempt to selectively excite and ionize either isotope individually. The reported 139La/138Laisotope shift for several transitions is approximately 300 MHz, with the spectra of both isotopes exhibiting hyperfine splitting larger than the isotope shift (12). This demonstration used a diode laser for only one step of the ionization scheme. Multiple diode laser schemes are feasible, and a practical instrument for isotope ratio measurements can be envisioned. The most likely elemental candidates amenable to such an instrument are the lanthanides and actinides (f transition elements) where the density of electronic states is high; other elements are, however, not precluded. The ionization potentials of the lanthanides and actinides range from 5.4 to 6.6 eV, so several diode lasers will be required. Depending on the wavelengths utilized, which of course depend on the specific electronic states excited, as many as four to six diode lasers may be required to reach a high-lying level. This situation would be prohibitive for CW ring dye lasers but is reasonable for diode lasers at $5000 each. The wavelengths that can be attained by using currently available diode lasers center around 670, 750, 780,815, 850, 1200, and 1500 nm; individual device selection and temperature tuning help to fill the gaps between the nominal
897
wavelengths. We feel that the commercial progression toward visible wavelengths will continue at a rapid pace, especially if nonlinear optical methods are considered. For efficient production of photoions, the final step, i.e., ionization, will probably require electric field or carbon dioxide laser ionization from a Rydberg level; currently available, single stripe diode lasers lack sufficient power to saturate bound-continuum atomic transitions. Careful tuning of the diode laser along with counterpropagating beam directions (for Doppler-free line widths) should permit isotopic selectivity that will reduce the instrumental dynamic range required for elements with large isotopic abundance differences. The practical isotope ratio instrument envisioned would most likely incorporate a simpler mass spectrometer than used here, e.g., a quadrupole mass filter. If complete isotopic selectivity could be attained in the optical process, no mass spectrometer would be required at all. CONCLUSIONS The utility of an inexpensive diode laser has been demonstrated for two-color resonance ionization mass spectrometry of lanthanum. Experiments are under way to demonstrate a resonance ionization example for which all of the lasers utilized are diode lasers. If that can be accomplished, then a practical isotope ratio RIMS instrument can be realized. ACKNOWLEDGMENT We thank A. S. Bonnano for programming assistance. Registry No. La, 7439-91-0. LITERATURE CITED Physics of New Laser Sources; Abraham, N. B., Arecchi, F. T., Mwradian, A., Sona, A.; Eds.; NATO Advanced Science Institutes Series; Plenum Press: New York, 1985; Voi. 132. Nakanishi, S.; Arti, H.; Itoh, H.; Kondo, K. Opt. Lett. 1987, 12, 864. Lawrenz, J.; Obrebski, A.; Niemax, K. Anal. Chem. 1987, 5 9 , 1232. Bekov, G. I.; Letokhov, V. S. Appl. Phys. 8 1983. 3 0 , 161. Cannon, B. D.; Bushaw, B. A.; Whitaker, T. J. J . Opt. Soc. Am. 8 1985, 2 , 1542. Green, L. W.; Macdonald, R. G.; Sopchyshyn, F. C.; Bonneli, L. J. Resonance Ionization Spectroscopy- 1986; Conference Series No. 8 4 Hurst. G. S., Morgan, C. G., Eds.; The Institute of Physics: Brlstoi, 1987; p 133. Donohue, D. L.; Young, J. P.; Smith, D. H. Int. J . Mess Spectrom. Ion Phys. 1982, 4 3 , 293. Donohue, D. L.; Smith, D. H.; Young, J. P.; Ramsey, J. M. Resonance Ionization Spectroscopy- 1984; Conference Series No. 71, Hurst, G. S., Payne, M. G., Eds.; The Institute of Physics: Bristol, 1984; p 83. Smith, D. H.; McKown, H. S.; Young, J. P.; Shaw, R. W.; Donohue, D. L ~ p p i spectrosc. . ~ 8 8 4,2 , 1057. Martin, W. C.; Zalubas, R.; Hagan, L. Atomic Energy Levels-The Rare Earth Elements ; National Standard Reference Data System, NBS-60; National Bureau of Standards: Washington, DC, 1978. Meggers, W. F.; Corliss, C. H.; Scribner, B. F. Tables of Spectral-Line Intenslties; NBS Monograph 145, Part 1; National Bureau of Standards: Washington, DC. 1975. Fischer, W.; Huhnermann, H.; Mandrek, K. Z . Phys. 1974, 269, 245.
RECEIVED for review October 21, 1988. Accepted December 27, 1988. Research sponsored by the Office of Energy Research, U.S.Department of Energy, under Contract DEAC05-840R21400, with Martin Marietta Energy Systems, Inc. These results were presented at the International Laser Science Conference IV, Atlanta, GA, October 3, 1988.
