He I photoelectron spectroscopy of four isotopologues of formic acid: HCOOH, HCOOD, DCOOH and DCOOD

Abstract

Abstract He I photoelectron spectra of four isotopologues of formic acid, HCOOH, HCOOD, DCOOH and DCOOD have been measured, mainly with an electron kinetic energy resolution of 15 meV. Quantum chemical calculations of geometries, vibrational mode frequencies and the potential energy distributions characterising each normal mode were made for the neutral 1 1 A ′ ground state, the cation ground state 1 2 A ′ and first excited ion state 1 2 A ″ of the four isotopologues. The results were used to analyse observed vibronic structure in the two PES bands in the 11.3–13.5 eV energy region and this analysis provided values for several vibrational mode frequencies of the ion states. The calculated structure and internal dynamics of the 1 2 A ′ ground state of the ion provide satisfactory agreement with experiment but for the 1 2 A ″ excited state a more refined theoretical treatment, permitting greater structural flexibility, is required. The PES first ionization energy of HCOOH agrees well with that obtained from Rydberg series in absorption spectra, and values more precise than hitherto were obtained for the first and second ionization energies of the four isotopologues. Analysis is also made of PES features concerning the higher energy states of the ions between 12.2 and 21 eV. The adabatic ionization energies of the 2 2 A ′ , 2 2 A ″ , 3 2 A ′ and 4 2 A ′ states in this energy region were determined. Vibrational frequencies were obtained for the 3 2 A ′ ion state of the isotopologues and new assignments were made concerning the energy and structure of the 4 2 A ′ ion state, whose adiabatic energy was found to lie about 60 meV below the 3 2 A ′ state.