The photodissociation of small Aldehydes: a refined understanding of prototypical dynamical systems

Abstract

H2CO is photolysed between 30000 cm-1 to 37500 cm-1 ascertaining the individual quantum yield of all five radical, molecular and triple fragmentation channels. Branching fractions are extracted from H atom velocity mapped ion (VMI) images and speed correlated REMPI spectra of CO(v = 0, 1). The traditional transition state (TS) mechanism is dominant at the extremes of this energy range, with the radical channels on the T1 and So surfaces dominant elsewhere. The roaming channel branching fraction is constant, accounting for -20 % of CO produced. Roaming occurs on a timescale shorter than the vibrational predissociation of HCO. The quantum yield of the triple fragmentation channel grows exponentially above threshold and is well modelled by a modified double phase space theory (PST) model. The (2+1) REMPI spectrum of H2 via the E,F state was collected and assigned. Nuclear spin is conserved in both the 'tight'-TS and roaming pathways. VMI images of H2 (v = 3 - 9, J = 0 -12) are collected following the photodissociation of H2CO via the rR0(0), rR0(1), and rR1(1) transitions to the 2143 state. CO(v, J) distributions correlated with individual H2 (v, J) states reveal that the CO(v) rotational distributions from the 'tight'- transition state (TS) and roaming pathways are multimodal. The multimodal nature of these distributions is caused by structure in the nuclear wave function mapping onto the product rotational distribution (i.e. rotation-reflection principle). Two roaming pathways are identified as the major pathways for the production of CO and CH4 from the photodissociation of CH3CHO. The two components of the bimodal CO product state distribution are attributed to the H- and CH3-roaming channels. No clear signature of the 'tight'-TS channel is observed. H2 is detected for the first time as a product of the unimolecular decay of CH3CHO in a jet. These products are assigned to the H-roaming mechanism, similar to that producing CO + CH4. This is the first example of multiple roaming channels on the same electronic state and of two roaming channels associated with a single barrierless dissociation channel. The results demonstrate that roaming can be significant in chemical reactions and outweigh traditional pathways.