Vibrational and Electronic Spectroscopy of Sulfuric Acid Vapor

We have measured and analyzed the infrared (IR), near-infrared (NIR) and vacuum ultraviolet (VUV) absorption spectra of vapor-phase sulfuric acid (H 2 SO 4 ). Transitions associated with the fundamental vibrations and the first and second OH stretching overtones of this molecule have been identified. Our measured vibrational spectrum extends and complements those in the literature and agrees well with ab initio calculated spectra. We have calculated the fundamental and overtone OH stretching intensities with the use of a simple anharmonic oscillator local-mode model with ab initio calculated dipole-moment functions. Theory and experiment have been used to investigate the OH stretching vibrational overtones of H 2 SO 4 and indicate that the OH stretching mode in H 2 SO 4 is an important aspect of the spectroscopy of this atmospheric chromophore. We have attempted to measure the VUV spectrum of vapor-phase sulfuric acid and, in the absence of observed bands, give upper bounds to the photoabsorption cross section. We conclude that H 2 SO 4 absorbs in the IR and NIR regions, with the OH stretching vibration playing the dominant role, and that the electronic excitation of H 2 SO 4 will only be significant at very high energies, well above those available from the sun in the earth's atmosphere.