Imaging the radical channel in acetaldehyde photodissociation: Competing mechanisms at energies close to the triplet exit barrier

Abstract

The photodissociation of acetaldehyde in the radical channel has been studied at wavelengths between 315 and 325 nm using the velocity-map imaging technique. Upon one-photon absorption at 315 nm, the molecule is excited to the first singlet excited state S(1), which, in turn, undergoes intersystem crossing to the first excited triplet state T(1). On the triplet surface, the molecule dissociates into CH(3) and HCO radicals with large kinetic energy release (KER), in accordance with the well characterized exit barrier on T(1). However, at longer wavelengths (>320 nm), which correspond to excitation energies just below the triplet barrier, a sudden change in KER is observed. At these photolysis wavelengths, there is not enough energy to surpass the exit barrier on the triplet state, which leaves the possibility of unimolecular dissociation on S(0) after internal conversion from S(1). We have characterized the fragments' KER at these wavelengths, as well as determined the energy partitioning for the radical fragments. A new accurate estimate of the barrier height on T(1) is presented.

A general method to describe intersystem crossing dynamics in trajectory surface hopping

“…[31] More recently, also the role of ISC in organic molecules has captured the attention of researchers. Examples of molecules where ISC is important range from aldehydes [37] and small aromatic compounds like benzene, [38] naphthalene, anthracene, and their carbonylic derivatives [39][40][41][42][43][44][45][46][47][48][49][50][51] to nitrocompounds. [43,[52][53][54][55][56][57][58][59][60][61] Furthermore, ISC has been reported for thio-substituted, [62][63][64][65][66][67][68] aza-substituted, [69] bromo-substituted, [70] and canonical nucleobases.…”

Competing pathways in the near-UV photochemistry of acetaldehyde

“…25,26 Threshold wavelengths for dissociation on T1 between 318.3-323.0 nm have been estimated using a variety of experimental techniques, including fluorescence lifetime collapse and the onset of product formation. [27][28][29][30][31][32] Laserinduced fluorescence detection of HCO products following photolysis above the T1 barrier has been used to identify modest rotational excitation and a tendency for vibrational excitation at shorter wavelengths. 27,28,33,34 The energy disposal has been justified based on ab initio calculations of the T1 transition state geometry and trajectory calculations that explore the dissociation dynamics.…”

Communication: Photodissociation of CH3CHO at 308 nm: Observation of H-roaming, CH3-roaming, and transition state pathways together along the ground state surface

“…21 The previous results show that the S 1 /S o IC process is likely to play a minor role. 18,22 Most molecular products via the S o pathway are expected to follow the ISC-based routes. Thus, the molecular channel (CO+CH 4 ) retains a small available energy partitioning in the fragments such that the CO population may lie in lower vibrational states.…”