High resolution photofragment translational spectroscopy of the near UV photolysis of indole: Dissociation via the 1πσ* state

The fragmentation dynamics of indole molecules following excitation at 193.3 nm, and at a number of different wavelengths in the range 240 ≤ λphot ≤ 286 nm, have been investigated by H Rydberg atom photofragment translational spectroscopy. The longer wavelength measurements have been complemented by measurements of excitation spectra for forming parent and fragment ions by two (or more) photon ionisation processes. Analysis identifies at least three distinct contributions to the observed H atom yield, two of which are attributable to dissociation of indole following radiationless transfer from the 1ππ* excited states (traditionally labelled 1Lb and 1La) prepared by UV single photon absorption. The structured channel evident in total kinetic energy release (TKER) spectra recorded at λphot ≤ 263 nm is rationalised in terms of N–H bond fission following initial π* ← π excitation and subsequent coupling to the 1πσ* potential energy surface via a conical intersection between the respective surfaces—thereby validating recent theoretical predictions regarding the importance of this process (Sobolewski et al., Phys. Chem. Chem. Phys., 2002, 4, 1093). Analysis provides an upper limit for the N–H bond strength in indole: D0(H–indolyl) ≤ 31 900 cm−1. Unimolecular decay of highly vibrationally excited ground state molecules formed by internal conversion from the initially prepared 1ππ* states is a source of (slow) H atoms but their contribution to the TKER spectra measured in the present work is dwarfed by that from H atoms generated by one or more (unintended but unavoidable) multiphoton processes.