Propyne and allene photolysis at 193.3 nm and at 121.6 nm

The fragmentation dynamics of allene and propyne molecules following photoexcitation at 193.3 nm and at 121.6 nm have been investigated by H(D) Rydberg atom photofragment translational spectroscopy. The total kinetic energy release (TKER) spectra of the H (and D) atoms resulting from H2CCCH2, H3CCCH, and D3CCCH photolysis at 193.3 nm are found to be essentially identical. The results contradict conclusions reached in several previous studies of propyne photochemistry at this wavelength. The observed energy disposal, and the isomer independence, are most readily rationalized by assuming that the fragmentation of both molecules following excitation at 193.3 nm is preceded by internal conversion to the ground (S0) state potential energy surface, and that the isomerization rate of the resulting highly vibrationally excited S0 molecules is faster than their unimolecular decay rate. The time-of-flight (TOF) and TKER spectra of the H and D atoms resulting from 121.6 nm photolysis of allene, propyne and propyne-d3 show significant differences, however. The differences can be reconciled by assuming two competing pathways for forming H(D) atoms following photoexcitation of propyne. One, involving selective cleavage of the acetylenic H3CCC-H bond, is assumed to occur from the excited electronic state prepared by photon absorption or from a recognizably 'propynelike' state to which it couples efficiently. The other, which yields a slower distribution of H(D) atoms, is considered to arise via radiationless transfer to a lower electronic state, isomerization, and subsequent unimolecular decay. The TOF and TKER spectra of the H atoms resulting from 121.6 nm photolysis of allene are indistinguishable from those associated with this second, 'statistical' fragmentation channel in propyne