An adiabatic model for the photodissociation of CH3SH in the first ultraviolet absorption band

Vibration mediated photodissociation dynamics of CH3SH: manipulation of the dynamic energy disposal into products

The S-H bond dissociation dynamics of CH3SH have been investigated for the S1-S0 transition mediated by either the S-H stretching (2608 cm-1) or CH3 symmetric stretching (2951 cm-1) mode excitation in the S0 state. The S-H and C-S bond extensions are strongly coupled in the S1 state through the S1/S2 same-symmetry conical intersection, giving the C-S stretching mode excitation of the CH3S˙ fragment during the prompt S-H bond rupture on S1. In the IR + UV transition mediated by the S-H stretching mode, the vertical transition seems to access the Franck-Condon region where the S-H bond is shortened while the coupling to the C-S bond stretching becomes stronger compared to the case of one-photon UV transition, indicating that the intramolecular vibrational redistribution (IVR) is little activated in S0. When the IR + UV excitation is mediated by the CH3 symmetric stretching mode, on the other hand, the Franck-Condon region in S1 encompasses the enlarged molecular structures with respect to both S-H and C-S bond extensions, presumably due to the rapid IVR in S0 prior to the vertical transition. This leads to the inverted vibrational state population of the C-S bond stretching mode of the CH3S˙ fragment. This work demonstrates that the reaction dynamics upon the IR + UV excitation of CH3SH is highly mode dependent and the energy disposal dynamics could be controlled by the manipulation of the Franck-Condon region through the particular vibrational-state mediation in the ground state, shedding new light on the structure-dynamics relationship.

Photodissociation dynamics of ethanethiol in clusters: complementary information from velocity map imaging, mass spectrometry and calculations

We investigate the solvent effects on photodissociation dynamics of the S–H bond in ethanethiol CH₃CH₂SH (EtSH).

Imaging CH3SH photodissociation at 204 nm: the SH + CH3 channel

The SH + CH(3) product channel for the photodissociation of CH(3)SH at 204 nm was investigated using the sliced velocity map ion imaging technique with the detection of CH(3) products using state selective (2+1) resonance enhanced multiphoton ionization (REMPI). Images were measured for CH(3) formed in the ground and excited vibrational states (v(2) = 0, 1, and 2) of the umbrella mode from which the correlated SH vibrational state distributions were determined. The vibrational distribution of the SH fragment in the SH + CH(3) channel at 204 nm is clearly inverted and peaks at v = 1. The highly negative anisotropy parameter of the CH(3) (v(2) = 0, 1, and 2) products is indicative of a fast dissociation process for C-S bond cleavage. Two kinds of slower CH(3) products were also observed (one of which was partly vibrationally resolved) that are assigned to a two-step photodissociation processes, in which the first step is the production of the CH(3)S (X(2)E) radical via cleavage of the S-H bond in CH(3)SH, followed by probe laser photodissociation of nascent CH(3)S radicals yielding CH(3)(X(2)A(1), v(2) = 0-2) + S((3)P(j)/(1)D) products.

Imaging the photodissociation of CH3SH in the first and second absorption bands: The CH3(X̃A12)+SH(XΠ2) channel

The CH3(X2A1)+SH(X2Pi) channel of the photodissociation of CH3SH has been investigated at several wavelengths in the first 1 1A"<--X 1A' and second 2 1A"<--X1A' absorption bands by means of velocity map imaging of the CH3 fragment. A fast highly anisotropic (beta=-1+/-0.1) CH3(X2A1) signal has been observed in the images at all the photolysis wavelengths studied, which is consistent with a direct dissociation process from an electronically excited state by cleavage of the C-S bond in the parent molecule. From the analysis of the CH3 images, vibrational populations of the SH(X2Pi) counterfragment have been extracted. In the second absorption band, the SH fragment is formed with an inverted vibrational distribution as a consequence of the forces acting in the crossing from the bound 2 1A" second excited state to the unbound 1 1A" first excited state. The internal energy of the SH radical increases as the photolysis wavelength decreases. In the case of photodissociation via the first excited state, the direct production of CH3 leaves the SH counterfragment with little internal excitation. Moreover, at the longer photolysis wavelengths corresponding to excitation to the 1 1A" state, a slower anisotropic CH3 channel has been observed (beta=-0.8+/-0.1) consistent with a two step photodissociation process, where the first step corresponds to the production of CH3S(X2E) radicals via cleavage of the S-H bond in CH3SH, followed by photodissociation of the nascent CH3S radicals yielding CH3(X2A1)+S(X3P0,1,2).

Chemical reaction dynamics beyond the Born-Oppenheimer approximation

To predict the branching between energetically allowed product channels, chemists often rely on statistical transition state theories or exact quantum scattering calculations on a single adiabatic potential energy surface. The potential energy surface gives the energetic barriers to each chemical reaction and allows prediction of the reaction rates. Yet, chemical reactions evolve on a single potential energy surface only if, in simple terms, the electronic wavefunction can evolve from the reactant electronic configuration to the product electronic configuration on a time scale that is fast compared to the nuclear dynamics through the transition state. The experiments reviewed here investigate how the breakdown of the Born-Oppenheimer approximation at a barrier along an adiabatic reaction coordinate can alter the dynamics of and the expected branching between molecular dissociation pathways. The work reviewed focuses on three questions that have come to the forefront with recent theory and experiments: Which classes of chemical reactions evidence dramatic nonadiabatic behavior that influences the branching between energetically allowed reaction pathways? How do the intramolecular distance and orientation between the electronic orbitals involved influence the nonadiabaticity in the reaction? How can the detailed nuclear dynamics mediate the effective nonadiabatic coupling encountered in a chemical reaction?