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Time-resolved studies of the photodissociation of adenine

By Kym Lewis Wells


A novel time-resolved velocity map ion imaging (TR-VMI) experiment has been\ud constructed and successfully applied to the study of non-statistical dissociation\ud processes.\ud The photodissociation of NH3 following the population of the ν2\ud ’ = 4 umbrella\ud vibrational mode of the first electronically excited, Ã1A2”, singlet state, was\ud initially studied. It was clearly observed that the N-H dissociation timescale was\ud inversely proportional to kinetic energy released to the H fragment. Assignment\ud of different kinetic energy regions of the TR-VMI transients to corresponding\ud bending vibrational modes (ν2) of the X2B1 state NH2 photoproduct clearly\ud suggests that dissociation into the vibrationless NH2 occurs in < 50 fs. Low\ud kinetic energy channels, show extended dissociation timescales, strongly\ud indicative of adiabatic dissociation to the first electronically excited state of NH2\ud (Ã2A1).\ud With an aim of modelling the photodissociation dynamics of adenine, the\ud photodissociation of pyrrole, imidazole, 2-methylimidazole, 4-methylimidazole\ud and 2,4-dimethylimidazole following excitation at 200 nm were studied using\ud time-resolved mass spectrometry (TR-MS) and VMI. In all cases ultrafast H\ud elimination was observed in < 130 fs, consistent with direct dissociation via the\ud repulsive 1πσ* potential energy surfaces. The photodissociation of 1-\ud methyimidazole at this wavelength was also studied. Once again ultrafast H\ud elimination was observed, but with greatly reduced yields, strongly suggesting H elimination from the non-heteroatom co-ordinates (C-H) also partaking in the\ud photodissociation dynamics at this wavelength.\ud TR-MS and VMI have also been applied to the study of the photodissociation of\ud adenine, 9-methyladenine and 6-dimethylaminopurine. In all measured kinetic\ud energy spectra a high kinetic energy channel has been observed, strongly\ud suggesting the participation of 1πσ* potential energy surfaces of both the azole\ud and amino co-ordinates in H elimination following excitation at 200 nm. Power\ud dependence studies at 266 nm suggest H elimination, but subsequent TR-MS\ud measurements seem to suggest that this is not due to the participation of the\ud 1πσ* potential energy surfaces at this excitation wavelength

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