K-SHELL EXCITATION OF THE WATER, AMMONIA, AND METHANE MOLECULES USING HIGH-RESOLUTION PHOTOABSORPTION SPECTROSCOPY

The K-shell excitation spectra of the hydrides water, ammonia, and methane have been measured in photoabsorption experiments using synchrotron radiation in combination with a high-resolution monochromator. For the case of methane, in particular, a wealth of spectral detail is observed which was not accessible in previous studies. The measured excitation energies and relative intensities compare well with values calculated using a complete second-order approximation for the polarization propagator. In order to determine the extent of admixing of valence excitations (i.e., transitions into virtual sigma* orbitals) to the Rydberg manifolds, the X-H bond lengths have been varied in the calculations. In the case of H2O, the two lowest-energy bands are due to the O 1s-4a1/3s and O 1s-2b2/3p transitions and have strong valence character; their width indicates that both excitations are dissociative. The NH3 and ND3 spectra are also broad which is not only due to possible dissociation but also to unresolved vibrational fine structure (nu2 mode) and a Jahn-Teller instability. Valence character is concentrated in the lowest excited state in the Rydberg ns manifold, but is distributed more uniformly over the np (e) manifold. The weak dipole-forbidden C 1s-3s(a1) transition in CH4 and CD4 is accompanied by vibrational structure due to the nu4 mode, indicating that it derives its intensity from vibronic coupling with the C 1s-3p(t2) transition. The structure on the latter band is extremely complicated due to Jahn-Teller coupling and cannot be assigned at present, as is the case for the Rydberg transitions at higher energies. The higher np Rydberg excitations contain considerable valence character.