H-atom detachment photochemistry of (hetero)aromatic molecules: ab initio quantum dynamics studies

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When aromatic or heteroaromatic molecules with acidic groups (NH, NH2, OH, SH) are excited with ultraviolet (UV) light, the detachment of slow as well as fast hydrogen atoms is observed. These photodissociation processes are currently under extensive experimental scrutiny by means of modern photofragment detection techniques, which provide detailed quantum state-resolved information on this novel nonstatistical unimolecular decay process. In this research project, the electronic and quantum dynamical mechanisms of ultrafast H-atom detachment processes in selected (hetero)aromatic molecules will be explored by means of accurate ab initio electronic-structure and nuclear-dynamics calculations. The three representative molecules to be studied are pyrrole, phenol and thiophenol. Their hallmark is the existence of a conical intersection of the repulsive potential-energy function of a dark electronic state of pisigma* character with the attractive potential-energy function of the electronic ground state in the hydrogen-detachment channel. In phenol and thiophenol, an additional conical intersection between the lowest pipi* state and the pisigma* state is located in the Franck-Condon region. The goal of the project is (a) the construction of accurate ab initio based multi-dimensional diabatic potential-energy surfaces of the coupled electronic states involved in the photodissociation reaction and (b) the elucidation of the photodetachment mechanisms with time-dependent quantum dynamical calculations, in particular the effects of strong nonadiabatic interactions on the absorption spectrum and the final-state distribution of the photofragments. The intention is the rationalization of the rich amount of recent experimental data and the clarification of controversies concerning the assignment of the vibrational structures of the high-resolution H-atom kinetic-energy spectra.