Electronic Structure of Liquid Methanol and Ethanol from Polarization-Dependent Two-Photon Absorption Spectroscopy.

Two-photon absorption (2PA) spectra of liquid methanol and ethanol are reported for the energy range 7-10 eV from the first electronic excitation to close to the liquid-phase ionization potential. The spectra give detailed information on the electronic structures of these alcohols in the bulk liquid. The focus of this Article is to examine the electronic structure change compared with water on substitution of a hydrogen by an alkyl group. Continuous 2PA spectra are recorded in the broadband pump-probe fashion, with a fixed pump pulse in the UV region and a white-light continuum as a probe. Pump pulses of two different energies, 4.6 and 6.2 eV, are used to cover the spectral range up to 10 eV. In addition, theoretical 2PA cross sections for both molecules isolated in the gas phase are computed by the equation-of-motion coupled-cluster method with single and double substitutions (EOM-CCSD). These computational results are used to assign both the experimental 2PA and literature one-photon linear absorption spectra. The most intense spectral features are due to transitions to the Rydberg states, and the 2PA spectra are dominated by the totally symmetric 3p