Wherever I May Roam: A Time-Resolved Wavelength-Dependent Study of the Roaming Dynamics in Acetaldehyde.

Acetaldehyde represents a key carbonyl compound in star-forming regions of the Sagittarius B2 molecular cloud, where photolysis generates reactive intermediates central to astrochemical complex molecule formation. Yet, its dominant photodissociation mechanisms remain poorly understood. Here, we use extreme ultraviolet (21.5 eV) time-resolved photoelectron spectroscopy, supported by high level ab initio calculations, to investigate the wavelength-dependent photodissociation dynamics of acetaldehyde following excitation at 295 and 308 nm. At 295 nm, population of the S1 ( n O → π C = O * ) state decays on a 60 fs time scale via an energetically accessible S1/S0 conical intersection seam, leading to both direct dissociation via conventional transition state dynamics and the formation of longer-lived (4.0 ps) CH3 roaming intermediates on S0. These roaming intermediates subsequently yield radical and molecular products over a hundreds-of-picoseconds time scale. In contrast, excitation at 308 nm produces low intensity structure between 7.0-8.5 eV at intermediate and late delay times, which, together with EOM-IP-CCSD calculations of representative geometries, suggest that additional pathways, such as H-atom roaming, may be active, leading to acetyl radical formation on a tens-of-picoseconds time scale. These findings demonstrate that access to competing S1 pathways in acetaldehyde may be strongly wavelength- and mode-dependent, controlling the balance between conventional dissociation and distinct roaming mechanisms.