ABINITIO STUDY OF MULTIPHOTON ABSORPTION PROPERTIES OF METHANE, ETHANE, PROPANE, AND BUTANE

The low-lying electronic excited states of methane, ethane, propane, and n-butane, accessed by multiphoton absorption, have been investigated at ab initio level according to quantum electrodynamical formalisms, by utilizing RPA vertical transition energies and amplitudes. For all four alkanes, two-photon spectroscopic properties which could be detected with various experimental arrangements have been reported. For methane, the transition probability coefficients and polarization ratios for two-, three-, and four-photon single-colour absorption from plane and circularly polarized light have been evaluated. All the low-lying excitations in methane and its higher homologues are found to bear Rydberg rather than valence nature.