Infrared Emission from Photodissociation of Methyl Formate [HC(O)OCH 3 ] 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 ( v ≤ 11, J ≤ 27) in region 1850‒2250 cm‒1 were recorded with a step-scan Fourier-transform spectrometer. The vibration-rotational distribution of CO is near Boltzmann with nascent average rotational energy ER0 = 3±1 kJ mol‒1 and vibrational energy EV0 = 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 v = 1 and 2 as reported previously by Lombardi et al. [J. Phys. Chem. A 129, 5155 (2016)], as evidence of a roaming mechanism. The vibrational distribution with a temperature ~13000±1000 K, however, agrees satisfactorily with trajectory calculations of these authors who took into account conical intersections from the S1 state. Some extremely weak emission likely from H2 CO was observed in region 2600‒3000 cm-1 ; highly internally excited CH3 OH 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 ( v ≤ 4, J ≤ 38) and CO2 (with two components of varied internal distributions) were observed, indicating new channels are open. Quantum-chemical calculations, computed at varied levels of theory on the ground electronic potential-energy schemes provide 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 CH3 OCO, and CO was produced from secondary dissociation of the product HCO.