In the Laboratory
Advanced Chemistry Classroom and Laboratory
Two-Photon Ionization Spectrometry of Alkali Atoms in Flames
edited by
Joseph J. BelBruno Dartmouth College Hanover, NH 03755
W
Matthew G. Comstock, Jeffrey R. Kerr, and Jeffrey A. Gray* Department of Chemistry, Ohio Northern University, Ada, OH 45810;
[email protected] Nonlinear optical spectroscopy (1–3) and laser ionization mass spectrometry (4–6 ) are important, well-established techniques that generally rely on intense, short-pulse lasers. Fortunately, such lasers are now inexpensive, user friendly, and widely available. This summary describes a new experiment that introduces students to pulsed laser-based instrumentation with an optogalvanic technique we call two-photon ionization spectrometry (TPIS). Educational experiments based on multiphoton spectroscopy are rare (6–10). Most standard vibrational or electronic spectroscopy lab procedures involve linear techniques such as absorption, fluorescence, or Raman scattering, in which molecules are excited by single photons from a continuous light source. In contrast, multiphoton excitation requires simultaneous absorption of two or more photons, which is practical only with short-pulse laser sources. Such excitation is generally advantageous, however, because it provides spectroscopic access to quantum states having a wider range of energy and angular momentum. Since stand-alone instruments for multiphoton ionization spectrometry are not yet commercially available, students and instructors in advanced undergraduate or beginning graduate-level courses will still find it necessary to go beyond the typical black-box approach to using an instrument. Apparatus Alkali atom samples for TPIS are prepared by aspirating aqueous salt solutions into an acetylene–air flame, as shown in Figure 1. A commercial 5-cm slot burner provides good path length and the stability needed for quantitative measurements, although a natural gas burner with homemade aspirator also provides sufficient sample. A stainless-steel
+HV 602 nm
energy meter
< 4 mJ
dye laser 0.2 cm -1
burner 532 nm
+
C2H2 /air/Na (aq) Nd:YAG 5 ns 10 Hz
preamp
strip chart
boxcar
Figure 1. Diagram of TPIS apparatus.
500
scope
anode mounted 10 mm above and parallel to the burner slot must withstand high temperatures and oxidation while being supplied at up to 1000 V. The burner surface serves as the cathode near ground potential. The flame current is normally less than 500 µA under these conditions even when salt solutions are being aspirated. We use the 532 nm output of a Q-switched, 10-Hz Nd–YAG laser to pump a rhodamine dye laser that creates probe pulses 5 ns in duration with a line width of 0.2 cm᎑1 at energies up to 4 mJ/pulse. We direct the probe laser beam through a lens (focal length 1000 mm) that focuses it to a diameter of ca. 0.5 mm through the flame, and then onto an energy meter. The probe light is tunable near λair = 571.075 nm for the lithium 4s←2s transition or near λair = 602.231 nm for the sodium 5s←3s transition. When the atoms are so highly excited, collisions with hot molecules in the flame cause ionization (11). The electric field present in the flame thus accelerates the cations toward the burner surface. A charge-coupled preamplifier circuit (12) connected to the burner surface collects photocation pulses, yet is relatively insensitive to low-frequency current fluctuations in the flame. Typical ion signals are 5 µs in duration when viewed on an oscilloscope. We use a gated integrator/boxcar averager and a strip chart (analog or LabVIEW) to monitor the TPIS signal and record spectra. Hazards Because of the complexity and numerous hazards of this procedure, students should be directly supervised at all times by the instructor or an experienced assistant. In addition to following usual safety procedures for open flames, we ventilate the exhaust using a flexible duct to eliminate flammable gases and any toxic vapors generated. Use caution handling acetylene and avoid flashback in the burner. Exposed high voltage presents obvious danger. We reduce this hazard by keeping the power supply off except while the flame is on and an experimenter is present. Our power supply trips off if the current exceeds 1 mA. The supply current should always be limited by a resistor. Since chemical hazards associated with laser dyes are typically unknown, these compounds, especially in pure, powdered form, should be handled using nitrile gloves. Preparations equipment should be wiped clean afterwards with an acetonemoistened towel to eliminate residue. Solvents such as methanol should be handled with care. This experiment uses class IV visible laser beams that are a serious eye hazard and may damage exposed skin. Appropriate laser safety goggles must be worn during all alignment procedures. The invisible but very dangerous 1064-nm fundamental
Journal of Chemical Education • Vol. 79 No. 4 April 2002 • JChemEd.chem.wisc.edu
In the Laboratory
beam from the YAG laser should be separated and dumped within an enclosure before the 2nd harmonic beam is output to the dye laser. Laser beams should be contained on the bench below eye level. No reflective jewelry should be worn. Access to the lab should be restricted to the experimenters, and all doors to the lab should have warning lights or signs.
TPIS Signal
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Results
1
laser energy 1.0 mJ/pulse anode potential 700 V 0.1
1
10
100
1000
+
Na (aq) Concentration / (µg/mL) Figure 2. Two-photon ionization spectrometry (TPIS) of dissolved sodium operates over a range of several decades. Plot symbols represent different scale settings for the gated integrator.
Laser Pulse Energy / mJ 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
TPIS Signal (arb. unit)
3.0
2.5
2.0
signal ∝ 1.5
I2 1+(I/ Isat)2
1.0
0.5
After assisting in the start-up procedure, students should begin by adjusting the alignment of the probe laser beam ca. 4 mm above the burner slot to optimize the signal. They will observe a large, nonresonant current pulse if the focused probe beam strikes either electrode. The TPIS signal pulse is noticeably broader and weaker for anode potentials below approximately 500 V because ions are less efficiently accelerated. If the gas flows are varied to find the best flame conditions, the beam alignment may need to be readjusted. TPIS can be used to measure dissolved alkali concentration over a wide linear dynamic range. Students should explore instrument settings such as integrator gate width and delay, and they should mind the baseline when changing scales. Figure 2 shows the typical variation of ion pulse signal with Na+(aq) concentration. The lower detection limit is due to nonresonant photoionization of unidentified substances in the flame, whereas the upper limit is due to excessive conductivity of the flame for high salt loading, which reduces the effective electric field in the probe region (13, 14 ). Figure 3 shows how the TPIS signal varies with optical pulse intensity (I ). When this is compared with the following nonlinear saturation relation in terms of a saturation parameter Isat
0.0
0
50
100
150
200
250
300
350
signal ∝
400
Laser Pulse Intensity / (MW cm᎑2) Figure 3. TPIS for sodium varies quadratically at low probe laser beam intensity and saturates at higher intensity. The line is a leastsquares fit of the data points to eq 1 (inset) with the parameter Isat = 52 MW/cm2. The sample is tap water.
2.6 mJ/pulse (÷5) 1.2 mJ/pulse (÷8) 0.15 mJ/pulse
In 1+ I I sat
n
(1)
the best fit is obtained for integer n = 2, which is consistent with a two-photon excitation process (7). TPIS spectra are recorded stepwise by averaging 10 pulses at each wavelength. Students should choose appropriate step size and scan rate to avoid distorting the line shape. Figure 4 shows scans of the sodium 5s←3s transition at three probe energy settings. At 0.15 mJ/pulse, the line width is 1.0 cm᎑1. Discussion
16598
16599
16600
16601
16602
Laser Wavenumber / cm
16603
16604
᎑1
Figure 4. High laser pulse intensity causes line broadening in TPIS of sodium. At low intensity, the line has a Gaussian shape due to the Doppler effect at 2νlaser (0.23 cm᎑1) and the laser bandwidth (0.2 cm᎑1). Saturation broadens the line and adds a Lorentzian component to the shape, which is most noticeable in the wings. The baseline also changes at higher intensity because of increased nonresonant photoionization.
It is important to understand the photoionization mechanism underlying TPIS. A constant fraction of ions in solution is first atomized by the flame. If the laser pulse energy is large, a significant fraction (5% to 30%) of the irradiated atoms absorb two photons to reach a highly excited state. Collisions with molecules in the flame de-excite (quench) some of the atoms. Efficient ionization from the excited state occurs either by collision or by absorption of one additional photon. These processes are widely known as Laser Enhanced Ionization (LEI) (8, 11, 14, 16–18) and 2+1 Resonance Enhanced Multi-Photon Ionization (2+1 REMPI), or more gen-
JChemEd.chem.wisc.edu • Vol. 79 No. 4 April 2002 • Journal of Chemical Education
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In the Laboratory 50
20
TPA
TPSE
(3s) 2S 1/2
Figure 5. Energy-level diagram for atomic sodium showing the twophoton absorption (TPA), two-photon stimulated emission (TPSE), and collisional ionization (CI) steps in the overall TPIS process.
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
13. 14. 15.
We thank the National Science Foundation for instrumentation grant (#DUE 96-50233) and R. W. Field of MIT for a generous gift of equipment that made the development of this experiment possible.
17. 18. 19.
502
(5s) 2S 1/2
30
0
16.
Material
Notes for instructors and a handout for students are available in this issue of JCE Online.
Cl
10
Acknowledgments
WSupplemental
+ − Na + e
40
Energy / (103 cm᎑ 1)
erally as Resonance Ionization Spectroscopy (RIS). Since we observe signals that vary as I 2 rather than as I 3, LEI appears to dominate for our conditions. 2+1 REMPI would be more important if the sample were at lower pressures, if a more intense laser beam were used, or if the excited state energy were not within approximately 2 eV of the ionization limit (11). In any case, the integrated TPIS signal per pulse should be directly proportional to the analyte concentration (7, 8). This experiment should also work well for other elements having low ionization potentials provided that suitable wavelengths can be generated. For instance, the 6s←4s transition in potassium occurs at λair = 728.380 nm. Other transitions may be identified using tables of atomic energy levels (19), and it is a good exercise to compute the desired laser wavelength in air (20) from the vacuum wavenumber of the excited state. TPIS extends students’ understanding of selection rules for optical transitions. Conservation of angular momentum in one-photon transitions requires that ∆L = ∆J = ±1, as in the familiar 2PJ ↔ 2S1/2 lines of alkali atoms. Alternatively, ∆L = ∆J = 0, ±2 for two-photon transitions, so that 2S1/2← 2S1/2 and 2 DJ ← 2S1/2 but not 2PJ ← 2S1/2 excitations are allowed. Larger spin–orbit coupling in heavy atoms relaxes the ∆S = 0 rule. Optical saturation is not normally explored in traditional spectroscopy exercises. Even though two-photon transitions are generally much weaker than one-photon transitions, our results show that alkali atoms saturate easily. Figure 5 shows a simplified energy level diagram for TPIS of sodium. Stimulated emission adds to the loss rate for the excited state, thus decreasing its lifetime and increasing the observed line width. TPIS offers higher spectral, temporal, and spatial resolution than traditional atomic absorption (AA) or atomic emission (AE) techniques. For example, TPIS could allow students to analyze line shapes (15) for gas samples, to map concentration profiles with submillimeter precision, or to study fast kinetics. TPIS may have a wider dynamic range than either AA or AE for alkali elements, though it may be more difficult to ensure linearity. LEI methods related to TPIS have achieved the ultimate detection limits for certain elements (8, 18). Students do this procedure in one afternoon session of our undergraduate instrumental analysis course. They have some prior experience using a pulsed N2 laser, oscilloscopes, and commercial flame AA instrument. This experiment generates much enthusiasm, and its complexity demands significant patience. It also has encouraged students to connect to current research literature.
20.
Bloembergen, N. J. Chem. Educ. 1998, 75, 555–558. Hochstrasser, R. M. J. Chem. Educ. 1998, 75, 559–564. Mourou, G. J. Chem. Educ. 1998, 75, 565–570. Muddiman, D. C.; Bakhtiar, R.; Hofstadler, S. A.; Smith, R. D. J. Chem. Educ. 1997, 74, 1288–1292. Ledingham, K. W. D.; Singhal, R. P. Int. J. Mass Spectrom. Ion Processes 1997, 163, 149–168. Smalley, R. E. J. Chem. Educ. 1982, 59, 934–939. Friedrich, D. M. J. Chem. Educ. 1982, 59, 472–481. Travis, J. C. J. Chem. Educ. 1982, 59, 909–914. Quick, C. R.; Wittig, C. J. Chem. Educ. 1977, 54, 705–706. Crosley, D. R. J. Chem. Educ. 1982, 59, 446–455. Curran, F. M.; Lin, K. C.; Leroi, G. E.; Hunt, P. M.; Crouch, S. R. Anal. Chem. 1983, 55, 2382–2387. Malmstadt, H. V.; Enke, C. G.; Crouch, S. R. Microcomputers and Electronic Instrumentation: Making the Right Connections; American Chemical Society: Washington, DC, 1994; p 215. Coche, M.; Berthoud, T.; Mauchien, P.; Camus, P. Appl. Spectrosc. 1989, 43, 646–650. Havrilla, G. J.; Schenck, P. K.; Travis, J. C.; Turk, G. C. Anal. Chem. 1984, 56, 186–193. Steinfeld, J. I. Molecules and Radiation; The MIT Press: Cambridge, MA, 1978; Chapter 1. Travis, J. C.; Schenck, P. K.; Turk, G. C.; Mallard, W. G. Anal. Chem. 1979, 51, 1516–1520. Omenetto, N.; Berthoud, T.; Cavalli, P.; Rossi, G. Anal. Chem. 1985, 57, 1256–1261. Szabo, N. J.; Latz, H. W.; Petrucci, G. A.; Winefordner, J. D. Anal. Chem. 1991, 63, 704–707. Moore, C. E. Atomic Energy Levels as Derived from the Analyses of Optical Spectra, Vol. 1; NSRDS-NBS; U.S. National Bureau of Standards: Washington, DC, 1971. CRC Handbook of Chemistry and Physics, 69th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1988; p E-383.
Journal of Chemical Education • Vol. 79 No. 4 April 2002 • JChemEd.chem.wisc.edu
