Near UV photodissociation of CD3SCD3: CD3 fragment (v, J) vector correlations

Photodissociation Dynamics and Stereodynamics of Methyl Mercaptan and Dimethyl Sulfide from the Second Absorption Band at 201 and 210 nm

The role of promoting and spectator modes vs energy randomization in nonadiabatic dynamics is interrogated in the photodissociation of methyl mercaptan, CH₃SH, and dimethyl sulfide, CH₃SCH₃ or DMS, in the second absorption band. The primary CH₃(ν) radicals produced in the dissociation of both systems at 210 nm have been resonantly detected in slice-imaging experiments, and the corresponding translational energy and angular distributions have been obtained. The stereodynamical information provided by Dixon's bipolar moments in conjunction with the energy partitioning among the different degrees of freedom of the primary CH₃(ν) products offers a panoramic picture of the photodissociation process of both systems. The remarkable similitude found between the two systems related to both vector correlations and internal energy content of the corresponding counterparts-SH for methyl mercaptan and SCH₃ for DMS-indicates that despite the diabaticity of the process, no efficient energy randomization of the available energy takes place. More specifically, only the parent vibrational modes whose participation in the initial absorption step is imposed by the conical intersection-i.e., the promoting modes-are adiabatically preserved during the process, while the rest of the vibrational modes play the spectator role. The results for both molecules at 210 nm are complemented with experiments carried out for DMS at 201 nm to explore the internal mechanism of the conical intersection in different zones of the absorption region.

Vibronic structures and dynamics of the predissociating dimethyl sulfide and its isotopomers (CH₃SCH₃, CD₃SCD₃, CH₃SCD₃) at the conical intersection

Conical intersection seam comprised of crossing surfaces of two lowest excited states of dimethyl sulfide (DMS) has been directly accessed by the one-photon excitation from the ground equilibrium state. Since the S-C bond rupture takes place promptly, the molecular structure on the excited state effectively belongs to C(S) symmetry. Namely, excited states of 1(1)B1 and 1(1)A2 in C(2)V become 1(1)A'' and 2(1)A'' states in C(S), respectively, and the optical transition from the ground equilibrium state to the dissociating molecule at the conical intersection seam is symmetry-allowed to facilitate the nonadiabatic transition on the 2(1)A'' state, leading eventually to the CH3S + CH3 products. The dynamic study of DMS, in this sense, gives the great opportunity to unravel the vibronic structure of the conical intersection seam by the conventional one-photon excitation method. In this work, utilizing the photofragment excitation (PHOFEX) spectroscopic method, the vibronic structures of DMS and its isotope analogs (CD3SCD3, CH3SCD3) at the conical intersection seam have been revealed, providing accurate lifetimes and detailed dynamics associated with individual vibronic transitions. The lifetime of the excited DMS is estimated to be ~100 fs, indicating that the dissociation is complete within one single oscillation in the conical intersection region. It is also found that the symmetric CSC stretching mode is strongly coupled to the reaction coordinate, as manifested by our experimental finding that the fragmentation yield of the S-CD3 bond is enhanced compared to that of the S-CH3 bond in the CH3SCD3 dissociation reaction only when the CSC symmetric stretching vibrational mode is excited at the conical intersection region. This work demonstrates that the better understanding of the excited state could make the bond-selective chemistry into reality.

Imaging photon-initiated reactions: A study of the Cl(P3∕22)+CH4→HCl+CH3 reaction

The hydrogen or deuterium atom abstraction reactions between Cl((2)P(3/2)) and methane, or its deuterated analogues CD(4) and CH(2)D(2), have been studied at mean collision energies around 0.34 eV. The experiments were performed in a coexpansion of molecular chlorine and methane in helium, with the atomic Cl reactants generated by polarized laser photodissociation of Cl(2) at 308 nm. The Cl-atom reactants and the methyl radical products were detected using (2+1) resonantly enhanced multiphoton ionization, coupled with velocity-map ion imaging. Analysis of the ion images reveals that in single-beam experiments of this type, careful consideration must be given to the spread of reagent velocities and collision energies. Using the reactions of Cl with CH(4), CD(4), and CH(2)D(2), as examples, it is shown that the data can be fitted well if the reagent motion is correctly described, and the angular scattering distributions can be obtained with confidence. New evidence is also provided that the CD(3) radicals from the Cl+CD(4) reaction possess significant rotational alignment under the conditions of the present study. The results are compared with previous experimental and theoretical works, where these are available.

Velocity-map imaging study of the photodissociation of acetaldehyde

Velocity-map imaging studies are reported for the photodissociation of acetaldehyde over a range of photolysis wavelengths (317.5-282.5 nm). Images are obtained for both the HCO and CH3 fragments. The mean rotational energy of both fragments increases with photodissociation energy, with a lesser degree of excitation in the CH3 fragment. The CH3 images demonstrate that the CH3 fragments are rotationally aligned with respect to the recoil direction and this is interpreted, and well modeled, on the basis of a propensity for forming CH3 fragments with M approximately K, where M is the projection of the rotational angular momentum along the recoil direction. The origin of the CH3 rotation is conserved motion from the torsional and methyl-rocking modes of the parent molecule. Nonstatistical vibrational distributions for the CH3 fragment are obtained at higher energies.