Acceleration of intramolecular vibrational redistribution by methyl internal rotation. II. A comparison of m fluorotoluene and pfluorotolueneAn analysis of the methyl rotation dynamics in the S 0 (X1 A 1) and T 1 (a3 A 2) states of thioacetone, (CH3)2CS and (CD3)2CS from pyrolysis jet spectra J. Chem. Phys. 95, 3137 (1991); 10.1063/1.460871Nonlinear dynamics of methyl rotation and intramolecular energy diffusion in pfluorotoluene Supersonic jet ScSo spectroscopy (resonance-enhanced multiphoton ionization, fluorescence excitation, and dispersed single vibronic level fluorescence) has been used to determine the SI and So internal rotation energy level structure of p-fluorotoluene with a CD 3 methyl rotor as well as to extend observations of the CH 3 rotor structure. The observed rotor energy levels 2';;;m.;;;8 for both species in both states are fit by a simple sixfold hindered rotor Hamiltonian for which the rotor inertial constants B and the internal rotation potential energy barriers V6 are evaluated. V6 may be obtained independently from B by observations of aE 3 , the observed splitting of the 3ai' and 3a!{ rotor levels. Numerical solution of the wave equation shows that the perturbation theory relationship V6 = -211E3 holds well for any reasonable B value. Correspondingly, the B constant may be obtained from other level energies without appreciable sensitivity to (reasonably) assumed barrier heights. Earlier microwave and SI-S0 fluorescence results are combined with the present work to produce a set of preferred values for these constants. The values in cm -I for the So state are B = 5.46 (2.82) and V6 = -4.77 ( -4.77) for CH 3 (CD 3 ) rotors. The SI values are B = 4.90 (2.54) and V6 = -33.0 ( -25.2). The 20% barrier height reduction occurring on transformation from a CH 3 to a CD 3 rotor is similar to that observed in other systems. Calculation implies that the staggered conformer is the minimum energy configuration for both electronic states. Many of the SI-SO rotor transitions are forbidden, and a discussion is given of induced intensity mechanisms that involve coupling of internal rotation to overall rotation or coupling of internal rotation to electronic motion. Substantial energy-level perturbations often occur for states with m;;.5. A survey of B values and hindered rotation constants for 30 species with methyl rotors attached to aromatic rings reveals some general correlations. 6362

Abstract. Room temperature (300 K) experiments have earlier established that the replacement of a fluorine atom in p-difluorobenzene (pDFB) with a methyl group to make p-fluorotoluene @FT) generates a qualitative difference in intramolecular vibrational redistribution (IVR) characteristics as seen in the S, states. Here we report S , S , fluorescence excitation and dispersed single vibronic level (SVL) fluorescence spectra that have been obtained for IVR comparisons in the cold (5 K) environment of supersonic expansions. As in the 300 K experiments, the cold beam S, vibrational energy threshold for IVR is substantially lower in pFT. The vibrational congestion in dispersed fluorescence that reveals extensive S, level interactions first appears after pumping an S, pFT level near 800 cm-'. In contrast, congestion in pDFB spectra is still absent from levels with twice this energy. Attention is directed to the relative S, ring level structures as a potential contributor to the distinctive IVR behaviors. Dispersed fluorescence from the S, zero point levels and fluorescence excitation spectra are combined with published information about S, fundamentals to show that the S, vibrational level structures of the two molecules are as closely related as those of an isotopomer pair. It is argued that the small differences in S, fundamentals cannot be a principal contribution to the qualitative IVR differences.

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Time- and frequency-resolved infrared (IR) laser absorption methods are used to probe hot atom energy transfer in open shell interactions of Cl(2P3/2)+HCl(J) in the single collision regime. The Cl(2P3/2) atoms are prepared by 308 nm laser photolysis of Cl2, and suffer collisions at Erel̄∼3500 cm−1 with a room temperature HCl distribution in a fast flow cell. Selective collisional excitation of final HCl(Jf) states is monitored by transient IR absorption on R(J≥4) branch lines in the HCl(v=1←0) band, while depletion of the initial HCl(Ji) states is monitored by transient bleaching of the room temperature Doppler profiles. Analysis of the J dependent Doppler profiles permits extraction of rotational loss [σloss(Ji)=∑fP(Ji)⋅σf←i] and gain [σgain(Jf)=∑iP(Ji)⋅σf←i] cross sections, as a function of initial and final J states, respectively. Absolute transient concentrations of the HCl(Ji) and HCl(Jf) are measured directly from absorbances via Beer’s Law, and used to extract absolute collisional cross sections. The results are compared with quasiclassical trajectory (QCT) calculations on a hybrid ab initio/LEPS surface of Schatz and Gordon, which prove remarkably successful in reproducing both the J dependent trends and absolute values of the state-resolved gain and loss collision cross sections.

Time- and frequency-resolved IR-laser absorption methods are used to probe state-resolved collisional energy transfer in open-shell collisions of Cl(2P3/2) radicals with HCl(J) in the near single-collision regime. Translationally ‘‘hot’’ Cl(2P3/2) radicals are formed by excimer laser photolysis of Cl2, which then collide with a room-temperature distribution of HCl peaked at J≂3. Final-state distributions of the HCl are monitored via transient absorption detection of a cw IR probe laser by the collisionally populated states (J=4, 5, 6,...,12). In previous work [J. Chem. Phys. 102, 7046 (1995)], these transient signals are used to extract absolute integral collisional cross sections for state-resolved rotational energy transfer into final J states. In the present study, high-resolution IR Dopplerimetry with the single-mode probe laser is used to measure translational distributions of the collisionally populated HCl(J) as a function of final J state. Analysis of these translational distributions leads to state-resolved differential scattering cross sections for rotational energy transfer, which exhibit a strong propensity for forward scattering into all levels observed (J≤12). These results are compared with quasiclassical trajectory calculations (QCT) on a recently modified potential energy surface of Schatz and Gordon. The theoretical analysis is in good agreement with experiment, with the angular distribution dominated by forward scattering for most of the final HCl rotational states. However, for the very highest J states collisionally populated, the QCT calculations predict a shift from predominantly forward to more isotropic scattering that is not evident in the experimental results.

1992 luminescence, fluorescence luminescence, fluorescence (organic substances) K 2540 37 -039 p-Fluorotoluene. Part 1. Methyl (CH3 and CD3) Internal Rotation in the S1 and S0 States -(fluorescence excitation and resonance-enhanced multiphoton ionization) . -(ZHAO, Z.-Q.; PARMENTER, C. S.; MOSS, D. B.; BRADLEY, A. J.; KNIGHT, A. E. W.; OWENS, K. G.; J. Chem. Phys. 96 (1992) 9, 6362-6377;

S1‐S0 fluorescence excitation and dispersed fluorescence spectra from p‐fluorotoluene‐Ar (pFT‐Ar) formed in a supersonic expansion are used to characterize the vibrational predissociation (VP) from S1 complex levels with 800, 1200 and 2000 cm−1 of vibrational energy. VP from the lower level occurs in at least 12 vibrational channels that involve only the three lowest energy pFT ring modes. VP from the higher levels is different, but those two levels are a matched pair with respect to their VP characteristics. They have almost identical channels and branching ratios, including a dominant channel producing free pFT in its zero point level. That channel requires conversion of as much as 1800 cm−1 of ring mode vibrational energy into translational and rotational energy. These behaviors appear to be unique among the aromatic van der Waals complexes so far studied, particularly with respect to the non‐selective nature of the VP channels. Evidence pointing to the active participation of the methyl internal rotation in the VP dynamics is presented. Conjecture is offered about how this internal rotation contributes to the highly unusual VP characteristics.

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