ALBERT

Notes: The technique of time-resolved infrared–ultraviolet double resonance is used to characterize the rates and propensity rules for mode-to-mode vibrational (V–V) energy transfer in D2CO/D2CO and HDCO/HDCO collisions. Such processes are found to be exceptionally efficient when collision-induced transfer is between the ν6 and ν4 modes of D2CO or between the ν5 and ν6 modes of HDCO: in the case of D2CO prepared in a specific ν6 rovibrational state by the 10R32 line of a CO2 laser, the rate of V–V transfer to specific states of the ν4 rovibrational manifold is approximately three times greater than the hard-sphere gas-kinetic collisional rate. This efficiency is much higher than for typical V–V transfer processes and approaches that of pure rotational relaxation, with the result that rotationally specific V–V transfer channels can be identified. The essential mechanism depends on the strong Coriolis coupling between the modes of D2CO or HDCO involved, as demonstrated by a semiclassical theoretical treatment whichconsiders only the electric dipole/dipole portion of the intermolecular potential. The combined effect of Coriolis and asymmetric-rotor perturbations causes mixing of rovibrational basis states and induces nonvanishing matrix elements of the permanent electric dipole moment between the vibrational modes of interest. These effects are most pronounced at moderate values of the rotational quantum number Ka (∼4), because quantum-mechanical interferences tend to annihilate the transition moment induced by Coriolis coupling alone at higher values of Ka. The theory also assumes that particularly efficient V–V transfer channels arise from very small energy differences between initial and final states of the state-selected molecule, owing to the abundance of collision-partner molecules then available to yield a zero overall energy defect for the pair of colliding molecules. The predictions of the simple long-range theory adopted yield order-of-magnitude agreement with the experimental results; possible deficiencies of the theory are discussed. Also discussed are the wider implications of the results, with regard to collision-induced V–V transfer between discrete rovibrational levels of small polyatomic molecules in general, to intramolecular vibrational redistribution in congested rovibrational and rovibronic manifolds, and to mechanisms of infrared multiple-photon excitation.

Notes: Time-resolved infrared-ultraviolet double resonance (IRUVDR) spectroscopy is used to study the kinetics of collision-induced rovibrational energy transfer between the ν6 and ν4 modes of D2CO in the vapor phase. As in paper I [J. Chem. Phys. 93, 8634 (1990)] of the series, attention rests on the existence of V–V transfer channels which are rotationally specific with respect to both J and Ka. Infrared excitation by the 10R(32) CO2 -laser line prepares D2CO in two discrete rovibrational states, (J,Ka,Kc)=(11,4,7) and (7,2,6), of the v6=1 vibrational manifold. D2CO/D2CO collisions then disperse this selected population to various states of the (ν4,ν6) rovibrational manifold, through a combination of rotational energy transfer (RET) and ν6→ν4 transfer. This yields an extensive range of (J,Ka) -resolved IRUVDR kinetic curves, demonstrating the collision-induced evolution of rovibrational population and enabling that evolution to be modeled by means of a master-equation approach.The features of the model of best fit are as follows: the dominant Ka -resolved channel of ν6→ν4 transfer is that with Ka=4→6; accompanying J-resolved ν6→ν4 transfer channels favor ΔJ=0, with state–to–state rate constants scaling as J3.4; additional (J,Ka) -resolved ν6→ν4 channels allow a spread of J- and Ka -changing V–V transfer. These features are consistent with the accepted mechanism of ν6→ν4 transfer in D2CO, involving enhancement by a combination of Coriolis coupling and rotor asymmetry perturbations. In addition to ν6→ν4 transfer, RET provides the predominant channels of collision-induced relaxation: J-changing RET is described by a conventional fitting law based on the energy gap ||ΔE|| for the state-selected molecule; Ka -changing RET favors even values of ΔKa and, contrary to previous expectations, is J selective with a propensity for ΔJ=0. The physical implications of these results are discussed.

Notes: Time-resolved infrared-ultraviolet double-resonance (IRUVDR) spectroscopy is used to look for rotationally specific channels in collision-induced vibrational energy transfer between the ν6 and ν4 modes of D2CO. The efficiency of such V-V transfer has been shown in previous work to be enhanced by a combination of Coriolis coupling and rotor asymmetry. IRUVDR spectra, recorded in pure D2CO vapor with a range of delay intervals between IR pump and UV probe laser pulses, reveal (J,Ka) -dependent propensities in the resulting ν6→ν4 transfer arising from D2CO/D2CO collisions. At the same time, rotational relaxation within the rovibrational manifold (v6=1) initially prepared by the IR pump laser is found to be more pronounced than the growth of population in the neighboring v4=1 manifold, due to ν6→ν4 transfer. This trend is shown to be reversed in the case of D2CO/N2O collisions, where the effects of rotational relaxation appear to be less pronounced than those of ν6→ν4 transfer. This work, performed with spectroscopic resolution superior to that in previous investigations, has demonstrated a number of new effects, including the identification of weakly allowed t-type (ΔKa=3) features in the IRUVDR spectra. It also provides the spectroscopic background to paper II of this series, which explores the detailed kinetics of (J,Ka) -resolved ν6→ν4 transfer in D2CO.

Notes: The technique of time-resolved infrared–ultraviolet double resonance (IRUVDR) spectroscopy is used to characterize the rate and mechanism of state-to-state rotational energy transfer (RET) in D2CO/D2CO collisions. The investigations employ CO2-laser irradiation to prepare a D2CO molecule in the v4=1, (J,Ka) =(18,11) rovibrational level of its X˜ 1A1 electronic ground state. Vapor-phase collisions with other D2CO (v=0) molecules then induce RET, with IRUVDR-monitored quantum-number changes ΔJ for the state-selected molecule ranging between +3 and −7. Kinetic modeling of the resulting experimental data shows that the inelastic cross sections for such J-changing rotational relaxation can be described adequately by simple scaling laws based on the rotational energy change ||ΔE|| for the state-selected molecule, with a power-gap fitting law proving marginally superior to an exponential-gap fitting law. The range of ||ΔJ|| monitored in these experiments is sufficiently extensive to discredit a simple propensity-rule fitting law, comprising consecutive collision-induced processes with individual changes ||ΔJ|| confined to values of 1 or 2. The microscopic rate constants derived reflect the dominance of ΔJ=±1 contributions for J-changing RET in D2CO/D2CO collisions, owing to long-range dipole/dipole interactions. These results elucidate RET in collisions between a pair of dipolar polyatomic (D2CO) molecules at a level of detail usually confined to studies of dipolar diatomic molecules, such as HF. Less detailed IRUVDR results, for RET in self-collisions of HDCO and for D2CO colliding with a variety of foreign-gas molecules, are also presented.

Notes: Infrared–ultraviolet double resonance is used to investigate fast (approximately gas–kinetic) vibrational energy transfer between the ν6 and ν4 modes of D2CO, arising from collisions with either D2CO or Ar. Relaxation channels specific to particular rotational states (J, Ka) are characterized and rationalized in terms of Coriolis coupling and quasielastic collisional interactions.

Notes: Time-resolved infrared-ultraviolet double resonance spectroscopy is used to measure collision-induced rovibrational energy transfer in the ν2+3ν3 region (∼11 600 cm−1) of gas-phase acetylene. Of particular interest is rotationally resolved V–V transfer between the Fermi-coupled 2133 and 11234351 levels, for which the rate is relatively high (approximately 13% of the Lennard-Jones collision rate) and the relevant rovibrational states markedly perturbed.

Notes: Injection seeding by a single-mode continuous-wave (cw) laser provides a convenient way to achieve narrowband tunable operation of a laser with a broad spectral gain profile, or of an optical parametric oscillator (OPO). Continuous single-mode tunability of the laser or OPO output usually requires the length of the optical cavity to be controlled as the injection-seeding wavelength is scanned. We report a novel variant on established methods of locking the optical cavity length to the seed wavelength. Our approach takes advantage of the resonance properties of an optical cavity. When the cavity is in resonance with the cw seed radiation, the total intensity of that radiation reflected off the cavity displays a pronounced dip; this intensity dip can be used as a locking signal to reset the cavity length piezoelectrically during each interval between the pump pulses that excite the laser or OPO. Our active cavity-locking scheme is realized in the case of a ring-cavity OPO, incorporating periodically poled lithium niobate (PPLN), pumped at 1.064 μm by a single-mode pulsed Nd:yttrium–aluminum–garnet laser and injection-seeded at its signal wavelength by a 1.55 μm single-mode tunable diode laser. The coherent infrared output of this injection-seeded PPLN OPO is shown to be continuously tunable, with an optical bandwidth of ∼130 MHz (0.0045 cm−1) and excellent spatial beam quality. © 1999 American Institute of Physics.

Notes: We report time-resolved optical double resonance spectroscopic experiments in which gas-phase acetylene molecules are selectively prepared and monitored in discrete rotational states of the v2=1 (C≡C stretch, 1974 cm−1) vibrational level. This is achieved by pulsed coherent Raman excitation and laser-induced fluorescence detection. State-selective spectra of single rovibrational states are presented under effectively collision-free conditions. Several new rovibronic bands in the A˜←X˜ absorption system of acetylene are identified in this way, owing to the enhanced sensitivity and spectral simplification of our Raman-optical double resonance technique. Investigations of C2H2(g) concentrate on rotationally resolved vibronic bands of the form 21030x (where x=1,2,3,...), exploring spectroscopic subtleties such as axis switching. The method has also been extended to the 21030x410 vibronic bands of C2H2(g), by Raman excitation in the (ν2+ν4−ν4) hot band, and to studies of the deuterated isotopomers, C2HD(g) and C2D2(g). Two distinct experimental strategies are demonstrated, in terms of their utility for spectroscopic assignment and energy transfer applications. One such approach comprises a rovibronic fluorescence excitation spectrum, recorded with fixed Raman excitation frequency. The alternative approach yields state-selected Raman spectra, with the Raman excitation frequency varied and the rovibronic excitation wavelength fixed.

Notes: Time-resolved infrared–ultraviolet (IR–UV) double resonance spectroscopy is employed for rotationally resolved kinetic studies of collision-induced energy transfer between the 4151 and 42 vibrational levels in the ground electronic state of acetylene-d2, C2D2. Second-order rate constants, for intra- and intermolecular V–V transfer and also for V–T,R transfer, are determined.