Infrared Emission from Photodissociation of Methyl Formate [HC(O)OCH₃] at 248 and 193 nm: Absence of Roaming Signature

Following photodissociation at 248 nm of gaseous methyl formate (HC(O)OCH₃, 0.73 Torr) and Ar (0.14 Torr), temporally resolved vibration-rotational emission spectra of highly internally excited CO (ν ≤ 11, J ≤ 27) in the 1850-2250 cm^-1 region were recorded with a step-scan Fourier-transform spectrometer. The vibration-rotational distribution of CO is almost Boltzmann, with a nascent average rotational energy (ER0) of 3 ± 1 kJ mol^-1 and a vibrational energy (EV0) of 76 ± 9 kJ mol^-1. With 3 Torr of Ar added to the system, the average vibrational energy was decreased to EV0 = 61 ± 7 kJ mol^-1. We observed no distinct evidence of a bimodal rotational distribution for ν = 1 and 2, as reported previously [Lombardi et al., J. Phys. Chem. A 2016, 129, 5155], as evidence of a roaming mechanism. The vibrational distribution with a temperature of -13000 ± 1000 K, however, agrees satisfactorily with trajectory calculations of these authors, who took into account conical intersections from the S₁ state. Highly internally excited CH₃OH that is expected to be produced from a roaming mechanism was unobserved. Following photodissociation at 193 nm of gaseous HC(O)OCH₃ (0.42 Torr) and Ar (0.09 Torr), vibration-rotational emission spectra of CO (ν ≤ 4, J ≤ 38) and CO₂ (with two components of varied internal distributions) were observed, indicating that new channels are open. Quantum-chemical calculations, computed at varied levels of theory, on the ground electronic potential-energy schemes provide a possible explanation for some of our observations. At 193 nm, the CO₂ was produced from secondary dissociation of the products HC(O)O and CH₃OCO, and CO was produced primarily from secondary dissociation of the product HCO produced on the S₁ surface or the decomposition to CH₃OH + CO on the S₀ surface.