Electronically excited states of formic acid investigated by theoretical and experimental methods

Absolute cross-section values are reported from high-resolution vacuum ultraviolet (VUV) photoabsorption measurements of gas-phase formic acid (HCOOH) in the photon energy range 4.7-10.8 eV (265-115 nm), together with quantum chemical calculations to provide vertical energies and oscillator strengths. The combination of experimental and theoretical methods has allowed a comprehensive assignment of the electronic transitions. The VUV spectrum reveals various vibronic features not previously reported in the literature, notably associated with (3pa ' <- 10a '), (3p ' a ' <- 10a '), (3sa ' <- 2a '') and (3pa ' <- 2a '') Rydberg transitions. The assignment of vibrational features in the absorption bands reveal that the C=O stretching, v ' 3(a '), the H '-O-C ' deformation, v5(a ' ), the C-O stretching, v ' 6(a ' ), and the O=C-O ' deformation, v ' 7(a ') modes are mainly active. The measured absolute photoabsorption cross sections have also been used to estimate the photolysis lifetime of HCOOH in the upper stratosphere (30-50 km), showing that solar photolysis is an important sink at altitudes above 30 km but not in the troposphere. Potential energy curves for the lowest-lying electronic excited states, as a function of the