The 212.8-nm photodissociation of formic acid: Degenerate four-wave mixing spectroscopy of the nascent OH(X 2Πi) radicals

Abstract

The 212.8-nm photodissociation dynamics of formic acid was investigated utilizing degenerate four-wave mixing spectroscopy. The background-free rotational spectrum of the nascent OH radicals was obtained, and a cold rotational energy distribution peaking at N″=3 was extracted from the DFWM spectrum. The distribution was well approximated by a Boltzmann distribution with a rotational temperature of Trot∼716 K, which corresponds to an average rotational energy of ∼498 cm−1. The observation of a nonstatistical spin–orbit state distribution, with a preference for the low-energy F1 manifold, implies the absence of any interactions with nearby triplet states during dissociation. Preferential population of the Λ-doublet was observed, indicating that the ν7 H–O–C bending vibration in HCOOH(Ã) and the recoil impulse are the principal sources of the OH rotation.