High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of imidazole

Abstract

The fragmentation dynamics of imidazole molecules following excitation at 193.3 nm and at many wavelengths in the range of 210< or =lambda(phot)< or =240 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Long wavelength excitation within this range results in population of the 1 (1)A(")((1)pisigma(*)) excited state, but the 2 (1)A(')<--X (1)A(')(pi(*)<--pi) transition becomes the dominant absorption once lambda(phot)< or =220 nm. The measured energy disposals show parallels with those found in recent studies of the UV photolysis of pyrrole [Cronin et al., Phys Chem. Chem. Phys. 6, 5031 (2004)]. The total kinetic energy release (TKER) spectra display a "fast" feature, centred at TKER approximately 9200 cm(-1). The analysis of the structure evident in the fast feature reveals the selective population of specific in-plane stretching vibrational levels of the imidazolyl cofragment; these fragments are deduced to carry only modest amounts of rotational excitation. Comparison with calculated normal mode vibrational frequencies allows the assignment of the populated levels and a precise determination of the N-H bond strength in imidazole: D(0)=33,240+/-40 cm(-1). The observed energy disposal can be rationalized using Franck-Condon arguments, assuming that the potential energy surface (PES) for the 1 (1)A(")((1)pisigma(*)) state has a topology similar to that of the corresponding (1)pisigma(*) state of pyrrole. As in pyrrole, photoexcitation populates skeletal motions in the S(1) state (in-plane motions in the present case) that are only weakly coupled to the N-H dissociation coordinate and thus map through into the corresponding product vibrations. A second, "slow" feature is increasingly evident in TKER spectra recorded at shorter lambda(phot). This component, which exhibits no recoil anisotropy, is attributed to H atoms formed by the "statistical" decay of highly vibrationally excited ground state molecules. The form of the TKER spectra observed at short lambda(phot) is rationalized by assuming two possible decay routes for imidazole molecules excited to the 2 (1)A(')((1)pipi(*)) state. One involves fast 2 (1)A(')((1)pipi(*)) right arrow-wavy 1 (1)A(")((1)pisigma(*)) radiationless transfer and subsequent fragmentation on the 1 (1)A(')((1)pisigma(*)) PES, yielding fast H atoms (and imidazolyl cofragments)-reminiscent of behavior seen at longer excitation wavelengths where the 1 (1)A(")((1)pisigma(*)) PES is accessed directly. The second is assumed to involve radiationless transfer to the ground state, most probably by successive 2 (1)A(') right arrow-wavy 1 (1)A(") right arrow-wavy X (1)A(') couplings, mediated by conical intersections between the relevant PESs and the subsequent unimolecular decay of the resulting highly vibrationally excited ground state molecules yielding slow H atoms.