Photophysical Studies of Formic Acid in the Vacuum UV: Fragmentation, Fluorescence, and Ionization in the 6−23 eV Photon Energy Range

Abstract

Vacuum-UV (VUV) photodissociation of gaseous formic acid (HCOOH) has been studied in the 6−23 eV range using photofragment fluorescence spectroscopy and synchrotron radiation excitation. Previous studies were limited to the region below 11.4 eV. Emission of OH (A), CH (A, B), HCOO, and H Balmer radiation is observed at various excitation energies in the VUV. Bands in the corresponding fluorescence excitation spectra can be associated with specific absorption bands of HCOOH. The HCOO fluorescence between 330 and 480 nm is assigned to the 2B1 → 2A‘ ‘ transition. The quantum yield of photoionization, γ, measured from 10 to 23 eV, reaches unity at 18 ± 0.1 eV. The energy domain, ΔE, of competitive nonionic decay processes of superexcited molecular states above the ionization threshold is thus found to be ΔE(HCOOH) = 6.68 ± 0.1 eV. Between 15.7 and 17 eV, Rydberg absorption bands converging to the 32A‘ state of HCOOH+ are mirrored in the γ(Eexc) spectrum, from which an autoionization rate kai ≈ 7.5 × 1013 s-1 was determined for the Rydberg levels. Below the astrophysical HI limit (13.6 eV), the neutral processes HCOOH → OH + HCO and HCOOH → CO2 + 2H dominate the fragmentation of formic acid. The quantum yield of ionic dissociation processes HCOOH+ → COOH+ + H and HCOOH+ → HCO+ + OH is determined to be less than 25% in this energy region. Although the ionic processes become more important between 13.6 and 18 eV, neutral dissociation processes with large quantum yields still occur. The astrophysical relevance of the photophysical properties of formic acid is briefly discussed.