Analysis of the High-Resolution Rotational Structure of the Origin and First Torsional Members of the 267-nm Band System of Formic Acid

Abstract

The fluorescence excitation spectrum of formic acid monomer (HCOOH) was recorded in the 268–257 nm region under relatively high resolution. The cooling conditions of a rotating slit-jet nozzle simplified the rotational structure and allowed for a combination line-by-line least-squares/band-contour analysis and the determination of the rotational constants. The 000 and 910 bands were each simultaneously fitted to a combination of c-type and b-type bands. The coexistence of the two band types is a consequence of the very small separation between the 0+ and 0− levels of the inversion mode, ν8, brought about by the pyramidal conformation of the upper electronic state. In contrast, the 920 band shows a splitting of 0.288 cm−1 and is a consequence of the increased torsion–inversion coupling at higher energies. The 810 band could only be approximately modeled using band-contour techniques with an estimated 1+–1− splitting of 0.5 cm−1. The assignments, the observed, and calculated line positions used in the least-squares analyses are available in electronic form from the publisher as supplementary material.