Photodissociation dynamics of the S1(nπ*) state of formic acid

Abstract

Formic acid monomer was excited to the S1(nπ*) region from 220 to 250 nm. The generated OH fragment was studied by the fluorescence excitation spectrum of the A 2∑+–X 2∏ transition under a high resolution and with different laser polarization geometries. The internal state distribution, translational energy and angular distribution of the OH fragment were determined by the observed results. No vibrational excitation was observed ( fv<0.009) for the OH fragment. Though the rotational excitation is also low ( fr<0.06), the fraction increases with the dissociation energy. The spin–orbit components are roughly in equilibrium. The degree of alignment of the OH fragment is very small and its π-lobe lies preferentially in the plane of rotation. The translational energy of the OH fragment is relatively high ( ft∼0.34) but the Doppler line shape does not show any clear sin2 θ or cos2 θ dependence. These results indicate that the S1 state dissociates directly through a pyramidal structure or that the dissociation is not direct dissociation but is predissociation. Compared to the OH fragment, the HCO fragment is thought to be internally very excited ( fint∼0.3) as a result of the character of S1(nπ*) state.