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 ${\mathrm{\sigma }}^{\mathrm{*}}$ orbitals) to the Rydberg manifolds, the X-H bond lengths have been varied in the calculations. In the case of ${\mathrm{H}}_2$O, the two lowest-energy bands are due to the O 1s-4${\mathit{a}}_1$/3s and O 1s-2${\mathit{b}}_2$/3p transitions and have strong valence character; their width indicates that both excitations are dissociative. The ${\mathrm{NH}}_3$ and ${\mathrm{ND}}_3$ spectra are also broad which is not only due to possible dissociation but also to unresolved vibrational fine structure (${\nu }_2$ 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(${\mathit{a}}_1$) transition in ${\mathrm{CH}}_4$ and ${\mathrm{CD}}_4$ is accompanied by vibrational structure due to the ${\nu }_4$ mode, indicating that it derives its intensity from vibronic coupling with the C 1s-3p(${\mathit{t}}_2$) 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.