Photophysical studies of formic acid in the VUV. Absorption spectrum in the 6–22 eV region

Abstract

Absorption spectra of HCOOH were measured between 6 and 22 eV at a maximum resolution of 3 meV. Previous measurements had a spectral limit of 11.7 eV. Analysis and band assignment were aided by data from theoretical calculations on valence and Rydberg states and from photoelectron spectroscopy. Five valence transitions and the different types of Rydberg transitions converging to the ground and the first excited electronic state of HCOOH⁺ are discussed and assigned in the spectral region below 12.3 eV. Our assignments differ considerably in many aspects from those of previous studies. Observation, analysis and assignment of absorption features between 12 and 22 eV were carried out for the first time. Rydberg bands converging to five expected ionization limits were not observed as discrete features, except for the 3 ²A′ ion state, corresponding to promotion of an 8a′ electron. The Rydberg bands converging to the other ionization limits are broad and merge to form broad absorption features. Assignments are made for npa′ ← 8a′ and nda′ ← 8a′ Rydberg transitions, which exhibit discrete absorption bands. It is shown that Rydberg states in the 15.7–16.8 eV region undergo autoionization with a rate k_ai ≈ 7.5 × 10¹³ s⁻¹ and that this autoionization is probably dissociative.