- Email: [email protected]
Relaxation in excited vibrational levels of NH3 by infrared triple-resonance spectroscopy
05848539/91 $3.00 + O.Ot Pergamon Press plc
Spectrochimico Acta, Vol. 47A, No. 7, pp. 973-911, 1991 Printed in Great Britain
Relaxation in Excited Vibrational Levels of NH3 by Infrared Triple-Resonance Spectroscopy B. ABEL, J.J. KLAASSBN, SL. COY, AND J.I. STBINPBLD Depattmentof Chemistry and G.R. Herriscn Spectroscopy Laboratory Massachusetts Institute of Technology Cambridge, MA 02139. U.S.A. (Recieved 7 March 1991; accepted 15 March 1991)
- A two step optical pumping technique in combination with transient diode laser absorption is described which provides a powerful tool for preparing selected vibrationally excited states beyond single photon excitation and state-and time-resolved analysis of collisional energy transfer processes. Two TE Cq lasers
Abstract
operating on the lOP(32) and 9P(24) transitions are used to pump the aQ(S.3) and sR(4.3) of the ~2 t v2 bands of NH3.
Tbe lOP(32) line also pumps two-photon transitions directly.
0 and 2~2 t
This technique relies on the
coupling of the intermediate states by rapid rotational energy transfer (AJ=l collisions) in the v2=1 region.
Time
resolved transient 3V2 c 2~2 absorption measurements probing 2~2 levels coupled to the initial state by collision:, provide the possibility of state resolved collisional energy transfer studies in an energy region which is difficult to access with other excitation techniques.
INl’RODUCIlON
The mechanisms of infrared multiple-photon excitation (IRMPE) are still not entirely clear, especially as regards to the initial resonant excitation steps through discrete vibrational levels [l-2]. The possibility of rotational compensation of the excitation bottleneck due to anharmonically shifted bands has often been suggested, particularly in the case of the IRMPE of SF6 [3,4]. However, the likelihood of exact compensation of the anharmonic shift for a single laser excitation frequency is small; efficient IRMPE generally requires either partial rotational relaxation in intermediate vibrational levels, or the use of two or more excitation frequencies in a “two-color” excitation scheme. Direct, time-resolved measurement of the intermediate level populations has rarely been achieved [l I. The ammonia molecule undergoes IRMPE when excited by the C@ lOP(32) laser line at 10.72 pm [5,61. Collisionless IRMPE requires laser intensities in excess of 10GW/cm2 [5]; at lower intensities, IRMPE efficiency increases with ammonia or buffer-gas pressure 163. These observations indicate that either rotational relaxation or direct multiphoton excitation to high levels is required to overcome the anharmonic bottleneck. The 16-pm ammonia laser also depends for its operation on a 2-photon (often “two-color”) pumping process to the v2=2 excited bending-mode state [7-121. Rotational de-excitation and relaxation between a and s symmetry levels is an essential part of the up-pumping process [11,12]. However, little or no information on state-to-state relaxation in excited vibrational levels of ammonia is available. Reid and co-workers [13,14] have measured vibrational relaxation in the v2 = 1 and v2 = 2 levels of ammonia, using a diode-infrared laser doubleresonance technique, at total pressures (2-25 Torr) at which rotational equilibration is essentially complete. More recent experiments on rotational relaxation within excited ammonia vibrational levels, using either an IR pump/REMPI probe technique [IS] or diode-infrared laser doubleresonance [16], have given conflicting results. We have used a novel “triple-resonance” technique to probe transient population in the v2 = 2 level of ammonia. Two CO2 lasers, operating on the lOP(32) and 9P(24) lines, pump NH3 molecules to the v2 = 2 level, which is probed by a tunable lead-salt diode laser. This technique, which is similar in spirit to other recent experiments [l7] using two pump frequencies in conjunction with a tunable probe, makes it possible to study state-resolved collisional excitation techniques in energy regions which are difficult to access by other methods.
EXPERIMEWAL The triple resonance setup requires the use of two synchronized pump lasers and a tunable diode laser probe. Approximately collinear (within 1-2 deg) counter-propagating TEMoo 973
974
B&m.
et al.
multimode pulses from two grating tuned TE CG2 lasers (one a Laser Science Inc. PRF-150s: the second a home-built unit described in Ref. 18) were overlapped in a static Pyrex glass cell containing between 0.6 and 2.0 Torr NH3 (Ma&son). The sample was purified by several pump-thaw cycles prior to each measurement. NH3 pressure was measured with a MKS Baratron capacitance manometer. The output of the two lasers is typically 50 mJ/pulse (FWHM: 150ns) for the Laser Science PRF-ISOS, and lOmJ/pulse (FWHM: 1OOns)for the home-built laser, and the estimated bandwidths are about 4 GHz and 1 GI-Iz respectively. Both lasers were operated at low N2 partial pressure to avoid complications arising from long afterpulses. Unless otherwise noted, the PRF-150s laser is set to the CO2 lOP(32) line at 932.960 cm-l, and the second laser, delayed with respect to the first, is set to the 9P(24) line at 1042.1633 cm-l. The mismatches between the lOP(32) and 9P(24) lines, and the pumped aQ(5,3) and sR(4,3) [2v2 - v2] transitions of NH3, are 0.95 GHz and 0.36 GHz respectively, ensuring close resonance pumping conditions [ 19,201. The synchronization and delay of the two lasers was achieved by a digital delay generator (Berkeley Nucleonics Corp. BN7050) and a Hewlett-Packard 80-IIA Pulse Generator. The delay was varied and adjusted for maximum triple resonance signal amplitude. For transient absorption measurement of population prepared in the 2~2 level, we used a PbSnTe diode laser (Laser Photonics Analytics Division) operated at 35 to 50K at a current of 200 to 700 mA. The multimode output of the 8l.trn diode was passed through the sample cell, overlapping with the two CO2 laser pulses. ‘Ihe overlap of the three beams inside the sample cell is critical to the success of the triple resonance experiment. Good spatial overlap inside the cell was achieved by aligning the beams with an external aperture system. It was not possible to have the pump and probe beams completely collinear, however, in order to avoid scattered light on the diode and the monochromator. After exiting the sample cell, the diode probe beam passed into a 0.5 m monochromator (Bausch & Lomb) which isolated the mode of interest in the desired wavelength region between 1250 and 1450 cm-l. The diode laser beam was then focussed onto a HgCdTe detector (Santa Barbara Research, -501~ rise time). A confocal FabryPerot etalon (Tee-Optics, FSR 0.01005 cm-l) was inserted into the beam path after the monochromator to provide a frequency reference, calibrated against standard lines taken from Ref. 20. The wavenumber resolution of the diode laser system is on the order of 0.0004 cm-l. The detector output was amplified and monitored on a digital averaging oscilloscope (LeCroy 9400A). Typically, 10,000 traces are averaged to improve the signal-to-noise ratio to an acceptable level. The data were transferred to a microcomputer for further processing.
RESULTS AND DISCUSSION
The molecular level scheme pertinent to these experiments is shown in Fig.1. The CO2 laser 9P(24) line, designated L2, pumps ammonia molecules from the thermally populated v2 = 1, s(4,3) level to the v2 = 2, 2a(5,3) level, Collisions move population from there to the 2s(5,3) and 2s(4,3) levels, resulting in a transient absorption signal such as shown in Fig. 2(a). (The ammonia pressure and Co;! laser pulse energy were both increased to record this signal, which would otherwise be so small as to be indistinguishable from noise, as in Fig. 2(e)). Pumping with the CO2 laser lOP(32) line [Ll] above produces the strong transient absorption shown in Fig. 2(b). By comparing the sum frequency with (~2 = 2) - ground state combination differences, [20,22] we have identified this as arising from a direct 2-photon process populating the v2 = 2, 2a(8,0) state. Two-photon double-resonance was also detected in Hetzler’s work on silane [23], but in that case as a transient gain signal. The double-excitation sequence of lOP(32) followed by 9P(24) CL1 + L2] produces the transient absorption signal shown in Fig. 2(c). Subtracting from this the 2-photon signal due to Ll above yields the transient shown in Fig. 2(d), which represents the enhancement in v2 3: 2 population resulting from the two-color sequential pumping process. Reversing the Ll and L2 pump sequence yields only the 2-photon signal due to Ll , verifying the triple-resonance effect. In order for absorption to be detected at 2sR(J,3) transitions, following population of 2a(5,3) levels by the CO2 lasers (or 2a(8,0) level, in the case of 2-photon pumping), a collision-induced symmetry-changing a + s collision must occur. By modeling the rise and fall
Relaxation
in excited vibratiaml
levels of NH,
975
tr3aKQ) 3aW)
v2=3
2WW
2sR(5,3)
v2=2
v2=1 ?
GS
l-l
aQojlop:5,3~
Figure 1. Multiple resonance scheme for nv2 excitation in NH3. A variable delay is inserted between the CO2 laser lOP(32) [Ll] and 9P(24) [L2] pulses.
time of the triple-resonance signals, we have estimated a lower bound for this inelastic rate coefficient in the v2 = 2 level, viz., kas 2 2 ps~sec-~TOW l. This is much faster than the value found in the v2 = 1 level by earlier IR + REMPI experiments [15], which was 0.2 ttsec’1 Torr’! Since the a-s splitting in v2 = 2 is substantially larger than in v2 = 1, it is not likely that kas (v2 = 1) will be slower than kas (v2 = 2). Indeed, in extensive double resonance measurements on ammonia [161, we have found that kas (v2 = 1) is actually close to 20 psec-1 Torrl in accord with theoretical expectations. From the observation that the triple-resonance signal amplitudes are maximized at L2-Ll delay times on the order of 0.5-1.0 microsecond at p(NH3)cl Torr, we can estimate lower bounds on the rates for the (AI = 0 or -1, AK = 0) energy-transfer process. More precise values for these rates have been determined from the Wared double-resonance measurements on ammonia which are reported in Ref. 16. Note in Fig. 2 that the rise times of the 2-photon-pumped signals are appreciably slower than those of the triple-resonance signals. This reflects the slower rate of arrival of population from the 2a(8,0) level, (a AJ = -3, AK = +3 process), as compared with the 2a(5,3) level, into 2s(5,3) and 2s(4,3) levels, consistent with the assignment made previously for the 2-photon resonance. CONCLUSIONS
A ttiple-resonance technique has been described which combines two-step optical pumping from the ground state with transient diode laser absorption. This technique provides a powerful
diode laser transm.
!2mV/divJ diode laser transm.
[IOmV/div] [ZOmV/div]
diode laser transm. diode laser transm.
[2mV/div]
Figtue 2 Triple-mamance a@ala in ammonia. NH3 pruam ia 0.95 Tom for all traces except (a). which is 1.7 Tom The diode probes the 3v2 - 2~2. sR(4.3) transition at 1382376 cm-’ in a cues. (a) Pumping of themally pop&ted v2 = 1. ~(43) by 9P(24) line alone. (b) Two-photon dcuble-resonance resulting from lOP(32) pump. (c) Triple-monance [ lOP(32) + 9P(24)] signal_ (d) Differan of (c) - (b). iwhiq uipk-rcsoaan cc effect. (e) Revemd-time signal, i.e., 9P(24) precedes lOP(32): no triple-resonance is observed in this case.
diode laser transm. 120mVldivl
rr ?
P
P
.PJ
s m
Relaxation in excited vibrational levels of NH,
917
tool for preparing selected vibrationally excited states beyond single photon excitation and stateand time-resolved analysis of collisional energy transfer processes in those states. A variety of pumping schemes may be envisioned, such as 1 + 1, or 2 + 1, or “IRMA + 1” excitation processes. In the case of the ammonia v2 manifold, we found that collisional energy transfer between a and s symmetry levels is extremely rapid, as predicted by the dipolar - interaction model.
Acknowledgements - The work was supported by NASA office of Space Science and Applications Upper Atmosphere Research Program and Planetary Atmospheres Program, and by N.S.P. Grant CHES9-14953 to the G.R. Hartison Spectroscopy Laboratory. Dr. Abel has been supported by a fellowship from the Deutsche Fonchungsgemeinschaft.
REFERENCES Adv. Multi-Photon Processes and Spectroscopy, Vol. 2 (S.H.Lin ed.), World Scientific Publishing Co Pte Ltd. Singapore (1986). pp. 79-173. PI P.M. Keehn. M.J. Pilling. and J. Pola, eds., Proc. Luscr Induced Chemistry Con& (BcchynC 1989): Spcctrochim. Acta, 46A, 44 1669. [31 R.V. Amba&umian and V.S. Letokhov, in Chemical and Biochemical Application of Lasers. VoI. 3, (C.B. Moore, cd.) Academic Press, New York, (1977). p. 228. C.C. Jensen, W. B. Person, B.J. Kmhn, and J. Gverend. Opt. Commtm. 20,275 (1977). ;; J.D. Campbell, G. Hancock, J.B. Halpem. and K.H. Welge. C&m. Phys. Lefts. 44, 404 (1976). [al ph. Avouris, M.M.T. Lay, and I.Y. Ghan, Chem. Phys. L.&s. 63.624 (1979). J.Eggleston, J. Dallarossa. W. K. Bischel. J. Bokor. and C.K. Rhodes, J. Appl. Phys. 50, 3867 (1979). H. Pummer, W.K. Bischel, and C.K. Rhodes, J. Appl. Phys. 49, 976 (1978). 9 R.R. Jacob, D. Prosnitx, W.K. Bischel, and C.K. Rhodes, Appl. Phys. Le:fs. 29, 710 (1976). WI H.D. Morrison, J. Reid, and B.K. Garside, Appl. Phys. L&s. 45, 321 (1984). HII A.N. Bobrovskii. A.A. Vedenov. A.V. Kozhevnikov. and D.N. Soblenko, Sov. Phys. JETP Letts. 29,536
[II J.S. Pmncisw and J.I. Steinfeld. in
(1979).
WI P.Pinson. A. Delage, G. Guard. and M. Michon, J. Appl. Phys. 52, 2634 (1981). [131 D.J. Danagher and J. Reid. J. Chem. Phys. 86. 5449 (1987). r141 P. Dub6 and J. Reid, J. C&m. Phys. 90,-2892 (1989): MJ. Shultx and J. Wei. J. Chem. Phvs. 92. 5951 (1990). t::; S.L. Coy, B. Abel, J.J.‘Klaassen. and J.I. Steinfeld (to be published). 1171 X. Luo and T.R. Rizro, J. Chem. Phys. 94, 889 (1991). WI B. Foy, J. Hetxler. G. Millot. and J.I. Steinfeld. J. Chem. Phys. 88, 6838 (1988). 1191 F. Shimixu. J. Chem. Phys. 52, 3272 (1970); Y. Ueda and J. Iwahori, J. Mol. Spectroscopy 116, 191 (1986). G. Guelachivili and K.N. Rae. Handbook of Infrared Standor&, Academic Pmss, Orlando (1986). t;:; K.K. Plyer and E.D. Tidwell, J. Chem. Phys. 29, 829 (1958). I=1 S. Urban, V. Spirko, and D. Papousek, J. Mof. Specfroscopy 101, 1 (1983). f231 J.R Hetxler and J.I. Steinfeld, J. Chem. Phys. 92, 7135 (1990). T. Oka. J. Chem. Phys. 48, 4919 (1968). El S. Green, J. Chem. Phys. 73. 2740 (1980).
Spectrochimico Acta, Vol. 47A, No. 7, pp. 973-911, 1991 Printed in Great Britain
Relaxation in Excited Vibrational Levels of NH3 by Infrared Triple-Resonance Spectroscopy B. ABEL, J.J. KLAASSBN, SL. COY, AND J.I. STBINPBLD Depattmentof Chemistry and G.R. Herriscn Spectroscopy Laboratory Massachusetts Institute of Technology Cambridge, MA 02139. U.S.A. (Recieved 7 March 1991; accepted 15 March 1991)
- A two step optical pumping technique in combination with transient diode laser absorption is described which provides a powerful tool for preparing selected vibrationally excited states beyond single photon excitation and state-and time-resolved analysis of collisional energy transfer processes. Two TE Cq lasers
Abstract
operating on the lOP(32) and 9P(24) transitions are used to pump the aQ(S.3) and sR(4.3) of the ~2 t v2 bands of NH3.
Tbe lOP(32) line also pumps two-photon transitions directly.
0 and 2~2 t
This technique relies on the
coupling of the intermediate states by rapid rotational energy transfer (AJ=l collisions) in the v2=1 region.
Time
resolved transient 3V2 c 2~2 absorption measurements probing 2~2 levels coupled to the initial state by collision:, provide the possibility of state resolved collisional energy transfer studies in an energy region which is difficult to access with other excitation techniques.
INl’RODUCIlON
The mechanisms of infrared multiple-photon excitation (IRMPE) are still not entirely clear, especially as regards to the initial resonant excitation steps through discrete vibrational levels [l-2]. The possibility of rotational compensation of the excitation bottleneck due to anharmonically shifted bands has often been suggested, particularly in the case of the IRMPE of SF6 [3,4]. However, the likelihood of exact compensation of the anharmonic shift for a single laser excitation frequency is small; efficient IRMPE generally requires either partial rotational relaxation in intermediate vibrational levels, or the use of two or more excitation frequencies in a “two-color” excitation scheme. Direct, time-resolved measurement of the intermediate level populations has rarely been achieved [l I. The ammonia molecule undergoes IRMPE when excited by the C@ lOP(32) laser line at 10.72 pm [5,61. Collisionless IRMPE requires laser intensities in excess of 10GW/cm2 [5]; at lower intensities, IRMPE efficiency increases with ammonia or buffer-gas pressure 163. These observations indicate that either rotational relaxation or direct multiphoton excitation to high levels is required to overcome the anharmonic bottleneck. The 16-pm ammonia laser also depends for its operation on a 2-photon (often “two-color”) pumping process to the v2=2 excited bending-mode state [7-121. Rotational de-excitation and relaxation between a and s symmetry levels is an essential part of the up-pumping process [11,12]. However, little or no information on state-to-state relaxation in excited vibrational levels of ammonia is available. Reid and co-workers [13,14] have measured vibrational relaxation in the v2 = 1 and v2 = 2 levels of ammonia, using a diode-infrared laser doubleresonance technique, at total pressures (2-25 Torr) at which rotational equilibration is essentially complete. More recent experiments on rotational relaxation within excited ammonia vibrational levels, using either an IR pump/REMPI probe technique [IS] or diode-infrared laser doubleresonance [16], have given conflicting results. We have used a novel “triple-resonance” technique to probe transient population in the v2 = 2 level of ammonia. Two CO2 lasers, operating on the lOP(32) and 9P(24) lines, pump NH3 molecules to the v2 = 2 level, which is probed by a tunable lead-salt diode laser. This technique, which is similar in spirit to other recent experiments [l7] using two pump frequencies in conjunction with a tunable probe, makes it possible to study state-resolved collisional excitation techniques in energy regions which are difficult to access by other methods.
EXPERIMEWAL The triple resonance setup requires the use of two synchronized pump lasers and a tunable diode laser probe. Approximately collinear (within 1-2 deg) counter-propagating TEMoo 973
974
B&m.
et al.
multimode pulses from two grating tuned TE CG2 lasers (one a Laser Science Inc. PRF-150s: the second a home-built unit described in Ref. 18) were overlapped in a static Pyrex glass cell containing between 0.6 and 2.0 Torr NH3 (Ma&son). The sample was purified by several pump-thaw cycles prior to each measurement. NH3 pressure was measured with a MKS Baratron capacitance manometer. The output of the two lasers is typically 50 mJ/pulse (FWHM: 150ns) for the Laser Science PRF-ISOS, and lOmJ/pulse (FWHM: 1OOns)for the home-built laser, and the estimated bandwidths are about 4 GHz and 1 GI-Iz respectively. Both lasers were operated at low N2 partial pressure to avoid complications arising from long afterpulses. Unless otherwise noted, the PRF-150s laser is set to the CO2 lOP(32) line at 932.960 cm-l, and the second laser, delayed with respect to the first, is set to the 9P(24) line at 1042.1633 cm-l. The mismatches between the lOP(32) and 9P(24) lines, and the pumped aQ(5,3) and sR(4,3) [2v2 - v2] transitions of NH3, are 0.95 GHz and 0.36 GHz respectively, ensuring close resonance pumping conditions [ 19,201. The synchronization and delay of the two lasers was achieved by a digital delay generator (Berkeley Nucleonics Corp. BN7050) and a Hewlett-Packard 80-IIA Pulse Generator. The delay was varied and adjusted for maximum triple resonance signal amplitude. For transient absorption measurement of population prepared in the 2~2 level, we used a PbSnTe diode laser (Laser Photonics Analytics Division) operated at 35 to 50K at a current of 200 to 700 mA. The multimode output of the 8l.trn diode was passed through the sample cell, overlapping with the two CO2 laser pulses. ‘Ihe overlap of the three beams inside the sample cell is critical to the success of the triple resonance experiment. Good spatial overlap inside the cell was achieved by aligning the beams with an external aperture system. It was not possible to have the pump and probe beams completely collinear, however, in order to avoid scattered light on the diode and the monochromator. After exiting the sample cell, the diode probe beam passed into a 0.5 m monochromator (Bausch & Lomb) which isolated the mode of interest in the desired wavelength region between 1250 and 1450 cm-l. The diode laser beam was then focussed onto a HgCdTe detector (Santa Barbara Research, -501~ rise time). A confocal FabryPerot etalon (Tee-Optics, FSR 0.01005 cm-l) was inserted into the beam path after the monochromator to provide a frequency reference, calibrated against standard lines taken from Ref. 20. The wavenumber resolution of the diode laser system is on the order of 0.0004 cm-l. The detector output was amplified and monitored on a digital averaging oscilloscope (LeCroy 9400A). Typically, 10,000 traces are averaged to improve the signal-to-noise ratio to an acceptable level. The data were transferred to a microcomputer for further processing.
RESULTS AND DISCUSSION
The molecular level scheme pertinent to these experiments is shown in Fig.1. The CO2 laser 9P(24) line, designated L2, pumps ammonia molecules from the thermally populated v2 = 1, s(4,3) level to the v2 = 2, 2a(5,3) level, Collisions move population from there to the 2s(5,3) and 2s(4,3) levels, resulting in a transient absorption signal such as shown in Fig. 2(a). (The ammonia pressure and Co;! laser pulse energy were both increased to record this signal, which would otherwise be so small as to be indistinguishable from noise, as in Fig. 2(e)). Pumping with the CO2 laser lOP(32) line [Ll] above produces the strong transient absorption shown in Fig. 2(b). By comparing the sum frequency with (~2 = 2) - ground state combination differences, [20,22] we have identified this as arising from a direct 2-photon process populating the v2 = 2, 2a(8,0) state. Two-photon double-resonance was also detected in Hetzler’s work on silane [23], but in that case as a transient gain signal. The double-excitation sequence of lOP(32) followed by 9P(24) CL1 + L2] produces the transient absorption signal shown in Fig. 2(c). Subtracting from this the 2-photon signal due to Ll above yields the transient shown in Fig. 2(d), which represents the enhancement in v2 3: 2 population resulting from the two-color sequential pumping process. Reversing the Ll and L2 pump sequence yields only the 2-photon signal due to Ll , verifying the triple-resonance effect. In order for absorption to be detected at 2sR(J,3) transitions, following population of 2a(5,3) levels by the CO2 lasers (or 2a(8,0) level, in the case of 2-photon pumping), a collision-induced symmetry-changing a + s collision must occur. By modeling the rise and fall
Relaxation
in excited vibratiaml
levels of NH,
975
tr3aKQ) 3aW)
v2=3
2WW
2sR(5,3)
v2=2
v2=1 ?
GS
l-l
aQojlop:5,3~
Figure 1. Multiple resonance scheme for nv2 excitation in NH3. A variable delay is inserted between the CO2 laser lOP(32) [Ll] and 9P(24) [L2] pulses.
time of the triple-resonance signals, we have estimated a lower bound for this inelastic rate coefficient in the v2 = 2 level, viz., kas 2 2 ps~sec-~TOW l. This is much faster than the value found in the v2 = 1 level by earlier IR + REMPI experiments [15], which was 0.2 ttsec’1 Torr’! Since the a-s splitting in v2 = 2 is substantially larger than in v2 = 1, it is not likely that kas (v2 = 1) will be slower than kas (v2 = 2). Indeed, in extensive double resonance measurements on ammonia [161, we have found that kas (v2 = 1) is actually close to 20 psec-1 Torrl in accord with theoretical expectations. From the observation that the triple-resonance signal amplitudes are maximized at L2-Ll delay times on the order of 0.5-1.0 microsecond at p(NH3)cl Torr, we can estimate lower bounds on the rates for the (AI = 0 or -1, AK = 0) energy-transfer process. More precise values for these rates have been determined from the Wared double-resonance measurements on ammonia which are reported in Ref. 16. Note in Fig. 2 that the rise times of the 2-photon-pumped signals are appreciably slower than those of the triple-resonance signals. This reflects the slower rate of arrival of population from the 2a(8,0) level, (a AJ = -3, AK = +3 process), as compared with the 2a(5,3) level, into 2s(5,3) and 2s(4,3) levels, consistent with the assignment made previously for the 2-photon resonance. CONCLUSIONS
A ttiple-resonance technique has been described which combines two-step optical pumping from the ground state with transient diode laser absorption. This technique provides a powerful
diode laser transm.
!2mV/divJ diode laser transm.
[IOmV/div] [ZOmV/div]
diode laser transm. diode laser transm.
[2mV/div]
Figtue 2 Triple-mamance a@ala in ammonia. NH3 pruam ia 0.95 Tom for all traces except (a). which is 1.7 Tom The diode probes the 3v2 - 2~2. sR(4.3) transition at 1382376 cm-’ in a cues. (a) Pumping of themally pop&ted v2 = 1. ~(43) by 9P(24) line alone. (b) Two-photon dcuble-resonance resulting from lOP(32) pump. (c) Triple-monance [ lOP(32) + 9P(24)] signal_ (d) Differan of (c) - (b). iwhiq uipk-rcsoaan cc effect. (e) Revemd-time signal, i.e., 9P(24) precedes lOP(32): no triple-resonance is observed in this case.
diode laser transm. 120mVldivl
rr ?
P
P
.PJ
s m
Relaxation in excited vibrational levels of NH,
917
tool for preparing selected vibrationally excited states beyond single photon excitation and stateand time-resolved analysis of collisional energy transfer processes in those states. A variety of pumping schemes may be envisioned, such as 1 + 1, or 2 + 1, or “IRMA + 1” excitation processes. In the case of the ammonia v2 manifold, we found that collisional energy transfer between a and s symmetry levels is extremely rapid, as predicted by the dipolar - interaction model.
Acknowledgements - The work was supported by NASA office of Space Science and Applications Upper Atmosphere Research Program and Planetary Atmospheres Program, and by N.S.P. Grant CHES9-14953 to the G.R. Hartison Spectroscopy Laboratory. Dr. Abel has been supported by a fellowship from the Deutsche Fonchungsgemeinschaft.
REFERENCES Adv. Multi-Photon Processes and Spectroscopy, Vol. 2 (S.H.Lin ed.), World Scientific Publishing Co Pte Ltd. Singapore (1986). pp. 79-173. PI P.M. Keehn. M.J. Pilling. and J. Pola, eds., Proc. Luscr Induced Chemistry Con& (BcchynC 1989): Spcctrochim. Acta, 46A, 44 1669. [31 R.V. Amba&umian and V.S. Letokhov, in Chemical and Biochemical Application of Lasers. VoI. 3, (C.B. Moore, cd.) Academic Press, New York, (1977). p. 228. C.C. Jensen, W. B. Person, B.J. Kmhn, and J. Gverend. Opt. Commtm. 20,275 (1977). ;; J.D. Campbell, G. Hancock, J.B. Halpem. and K.H. Welge. C&m. Phys. Lefts. 44, 404 (1976). [al ph. Avouris, M.M.T. Lay, and I.Y. Ghan, Chem. Phys. L.&s. 63.624 (1979). J.Eggleston, J. Dallarossa. W. K. Bischel. J. Bokor. and C.K. Rhodes, J. Appl. Phys. 50, 3867 (1979). H. Pummer, W.K. Bischel, and C.K. Rhodes, J. Appl. Phys. 49, 976 (1978). 9 R.R. Jacob, D. Prosnitx, W.K. Bischel, and C.K. Rhodes, Appl. Phys. Le:fs. 29, 710 (1976). WI H.D. Morrison, J. Reid, and B.K. Garside, Appl. Phys. L&s. 45, 321 (1984). HII A.N. Bobrovskii. A.A. Vedenov. A.V. Kozhevnikov. and D.N. Soblenko, Sov. Phys. JETP Letts. 29,536
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