Infrared-ultraviolet Double Resonance Research Articles

Dynamical symmetry breaking in the 4 νCH rovibrational manifold of acetylene

Time-resolved, fluorescence-detected infrared-ultraviolet double resonance spectroscopy of acetylene in the 12700 cm −1 ‘4 ν CH’ rovibrational manifold reveals unusual symmetry-breaking energy transfer, induced (at least in part) by collisions. This takes the form of odd-numbered changes of the rotational quantum number J, despite the fact that intramolecular transfer between the ortho and para nuclear-spin modifications of such a molecule is usually forbidden.

Infrared spectroscopy of OH stretching vibrations of hydrogen-bonded tropolone-(H2O)n (n=1–3) and tropolone-(CH3OH)n (n=1 and 2) clusters

Infrared spectra of jet-cooled tropolone-(H2O)n (n=1–3) and tropolone-(CH3OH)n (n=1 and 2) clusters were observed in the OH stretching region by using infrared-ultraviolet double resonance techniques. Size separated electronic spectra of these clusters were also observed with hole-burning spectroscopy in which the infrared laser was used as hole light. Both the infrared and hole-burning spectra of the tropolone-methanol clusters were found to be quite similar to those of the corresponding tropolone-water clusters, indicating that a similar structure is expected for both the clusters. Structure of the n=1 and 2 clusters of tropolone-water and -methanol is discussed. The infrared (IR) spectra suggest that the intramolecular hydrogen bond of tropolone OH is not destroyed in tropolone-(H2O)n (n≤2) and -CH3OH, while the intermolecular hydrogen bond dominates in tropolone-(H2O)3 and -(CH3OH)2. The transformation of the intramolecular to intermolecular hydrogen bond induced by the solvation is also discussed.

Rovibrational spectroscopy of the C 2H 2Ar van der Waals complex, using a fluorescence depletion infrared-ultraviolet double resonance technique

A rovibrational spectrum of the acetylene-argon van der Waals (vdW) complex, C 2H 2Ar, has been measured in the 1.52 5 μm ‘2 ν CH’ overtone region by a time-resolved infrared-ultraviolet double resonance method. Infrared absorption by the vdW complex is recorded as a depletion of laser-induced rovibronic fluorescence intensity, providing a novel way to measure rovibrational spectra of simple vdW complexes and to study their predissociation dynamics.

Rate coefficients for state-to-state rovibronic relaxation in collisions between NO(X 2Π, ν=2, Ω, J) and NO, He, and Ar at 295, 200, and 80 K

The state-to-state rates of collisional energy transfer within and between the rotational level manifolds associated with the Ω=1/2 and Ω=3/2 spin–orbit states of NO(X 2Π, ν=2) have been measured using an infrared–ultraviolet double resonance (IRUVDR) technique. NO molecules were initially prepared in a specific rovibronic level, for example, ν=2, Ω=1/2, J=6.5, by tuning the output from an optical parametric oscillator (OPO) to a suitable line in the (2,0) overtone band. Laser-induced fluorescence (LIF) spectra of the A 2Σ+–X 2Π (2,2) band were then recorded at delay times corresponding to a small fraction of the average time between collisions in the gas sample. From such spectra, the relative concentrations of molecules in levels populated by single collisions from the initially prepared state could be estimated, as could the values of the rate coefficients for the state-to-state processes of collisional energy transfer. Measurements have been made with NO, He, and Ar as the collision partner, and at three temperatures: 295, 200, and 80 K. For all collision partners, the state-to-state rate coefficients decrease with increasing ΔJ (i.e., change in the rotational quantum number and rotational angular momentum) and increasing ΔErot (i.e., change in the rotational energy). In NO–NO collisions, there is little propensity for retention of the spin–orbit state of the excited molecule. On the other hand, with He or Ar as the collision partner, transfers within the same spin–orbit state are quite strongly preferred. For transfers between spin–orbit states induced by all collision partners, a propensity to retain the same rotational state was observed, despite the large change in internal energy due to the spin–orbit splitting of 121 cm−1. The results are compared with previous experimental data on rotational energy transfer, for both NO and other molecules, and with the results of theoretical studies. Our results are also discussed in the light of the continuing debate about whether retention of angular momentum or of internal energy is the dominant influence in determining the rates of state-to-state rotational energy transfer.

Rotationally resolved spectroscopy of the ([Ctilde]-[Xtilde] band of 15NH3 by infrared-ultraviolet double resonance

Double resonance measurements on 15NH3 in a free expansion jet are reported. These were carried out using a continuous wave (CW) CO2 laser and a pulsed ultraviolet (UV) laser system. Infrared radiation was used to prepare molecules in individual rotational levels of the v2=1 vibrational state of the electronic ground state of [15N]ammonia. These molecules were the photoionized via transitions of the [Ctilde]-[Xtilde] band in a (2+1) process using the doubled output of a pulsed dye laser. The ion signal was detected using a time of flight mass spectrometer. Two known coincidences between CO2 laser lines and individual rovibrational transitions of 15NH3 were used to populate the (J, K)=(5, 4) and (3, 0) symmetric levels of v2=1. Double resonance signals corresponding to O, P, Q, R, and S branch two photon transitions in the [Ctilde]-[Xtilde] band were observed and measured accurately. These data were augmented by conventional (2+1) REMPI measurements on the 20/0, 20/1, and 20/2 sub-bands of 15NH3: A complet...

Collision-induced state-to-state energy transfer in the ν2+3ν3 rovibrational manifold of gas-phase acetylene

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.

Energy transfer in the 31,214151 Fermi-resonant states of acetylene. I. Rotational energy transfer

An infrared–ultraviolet double resonance technique is used to probe the state-to-state rotational energy transfer dynamics of self-relaxation in acetylene. The output of an optical parametric oscillator at ∼3 μm is used to excite C2H2 to a rotational level within one of its Fermi-resonant 31,214151 states. By fixing this wavelength and scanning the frequency-doubled output of a tunable dye laser, laser induced fluorescence signals arising from collisional population of rotational levels within both dyads are observed and state-to-state rate constants for rotational relaxation are obtained. Rotational relaxation to J levels within the pumped (upper energy) Fermi-dyad accounts for 74% of the total rate of loss of the population of the J=12 level, whereas relaxation to J levels in its partner accounts for only 16%. A further 7% of the absolute rotational relaxation rate is accounted for by vibrational relaxation out of the mixed levels, leaving only 3%–4% of the total relaxation to be accounted for.

Rotationally specific mode–to–mode vibrational energy transfer in D2CO/D2CO collisions. I. Spectroscopic aspects

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.

Rotationally specific mode–to–mode vibrational energy transfer in D2CO/D2CO collisions. II. Kinetics and modeling

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.

Rotational energy transfer in D2CO (v4=1): IR–UV double resonance studies of J-changing collisions

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 X̃ 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.

Coriolis-assisted vibrational energy transfer in D2CO/D2CO and HDCO/HDCO collisions: Experiment and theory

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 which considers 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.

Vibrational energy transfer study of C 6H 6 by IR-UV double resonance

Collision-induced vibrational energy transfer of benzene is studied by means of infrared-ultraviolet double resonance (IRUVDR). The time-dependent population and depopulation of the vibrational mode v 6 is measured after excitation of the vibrational mode v 18. The rate constant for v 18 → v 6 V-V energy transfer is obtained. The experiment.is also carried out using rare gases as collision partners. A simplified model for vibrational energy transfer is discussed.

Time-resolved infrared-ultraviolet double resonance studies of rotational relaxation in D 2CO

Rotational relaxation of the ν 4 = 1, J = 18, K a = 11 state of D 2CO is found to be faster than gas-kinetic and to involve multiple quantum changes in J within a single collision between D 2CO molecules. The results conform to scaling laws based on |Δ J| or the energy change | E| for the state-resolved molecule.

Time-resolved infrared-ultraviolet double resonance in D 2CO and HDCO: Spectroscopic applications

Spectroscopic results, from an extensive study of rotationally resolved infrared-ultraviolet double resonance (IRUVDR) effects in the molecules D 2CO and HDCO, are reported. The experiments involve rovibrational pumping (in the ν 4 or ν 6 bands of D 2CO and the ν 5 or ν 6 bands of HDCO) and rovibronic probing of the A 1A 2 ← X 1A 1 electronic system (in the 4 0 1 or 6 0 1 bands of D 2CO and the 5 0 1 or 6 0 1 bands of HDCO) with visible-fluorescence detection, using sequential pulsed excitation by CO 2 and dye lasers. A remarkable result is the observation of IRUVDR rovibronic features in the very weak and heavily overlapped 6 0 1 band of D 2CO, which has not previously been detected by conventional means. Further experiments which probe the 4 1 2 band of D 2CO are interpreted in terms of single-photon rovibrational pumping in the (2 ν 4 - ν 4) hot band, rather than two-photon pumping in the 2 ν 4 band. The results reported are obtained under effectively collision-free conditions, achieved by maintaining the product of pump-probe delay and sample pressure below ∼10 nsec Torr; collision-induced rotational and rovibrational relaxation is apparent when this product is increased. Other aspects of the IRUVDR technique which are considered include: the relation of IRUVDR results for D 2CO and HDCO to coincidences between CO 2-laser and molecular rovibrational frequencies, previously demonstrated by sub-millimeter optically pumped laser emission, infrared-radiofrequency double resonance, and Lambdip absorption studies; the role of saturation broadening in promoting multiple coincidences of molecular rovibrational transitions with a given CO 2-laser line; relevance to mechanisms of infrared multiple-photon excitation of D 2CO; inconsistencies between observed IRUVDR signal intensities and those anticipated from rovibronic transition probabilities.

Vibrational energy transfer in ozone by infrared-ultraviolet double resonance

Absorption transients at 254 nm have been observed in O 3-O 2 mixtures following laser irradiation at 9.64 μm. From analysis of these transients, we are able to determine vibrational relaxation rate constants (O 3-O 2 λ 1 −1/[O 2] = (2560±370) Torr −1 S −1, λ 2 −1/[O 2] = (640±50) Torr −1 S −1, and also a v 1- v 3 equilibration rate constant (O 3-O 3) of (1.5±1.0) × 10 6 Torr −1 S −1.

Rotationally resolved infrared-ultraviolet double resonance spectroscopy in molecular D 2CO and HDCO

Rotationally resolved infrared-ultraviolet double resonance (IRUVDR), consisting of sequentially excited rovibrational and rovibronic transitions sharing a common intermediate molecular level, is demonstrated. The technique employs pulsed CO 2 and tunable dye lasers and is applied to the molecules D 2CO and HDCO. For D 2CO, infrared pumping produces 100fold population enhancement in specific rotational sublevels of the v 4 = 1 level of the X ̃ 1A 1 electronic ground state, followed by ultraviolet excitation in the 365-nm 4 1 0 band of the A ̃ 1A 2 ← X ̃ 1A 1 electronic system. This ultraviolet excitation occurs at a specific set of dye laser frequencies, determined by the preceding rovibrational transition, and is detected by molecular fluorescence in the visible region. Similar effects observed in HDCO involve rovibrational pumping to either the v 6 = 1 or v 5 = 1 levels and give rise to enhanced rovibronic transitions in the 6 1 0 and 5 1 0 bands of the A ̃ ← X ̃ system, respectively. The resulting IRUVDR spectra enable detailed spectroscopic assignments to be made and are consistent with previous results from infrared and ultraviolet absorption, laser Stark, and infrared-radiofrequency double resonance spectroscopy. Collision-induced satellite structure, arising from rotational relaxation of the intermediate rovibrational level in the IRUVDR sequence, is also reported.

Infrared/ultraviolet double resonance in biacetyl vapour

A novel infrared/ultraviolet double resonance (IRUVDR) technique, involving stepwise molecular absorption of pulsed 10.6μm CO 2 and cw 441.6 nm HeCd laser radiation and detection of the resulting luminescence, is used to monitor energy transfer processes in biacetyl vapour. The infrared-induced variation in the intensity of biacetyl fluorescence at 460 nm is attributed to a specific hot band and subsequent vibrational equilibrium within the ground electronic state. Infrared fluorescence (6μm) and IRUVDR-induced phosphorescence (510 nm) have also been observed.