HIGH-RESOLUTION K-SHELL PHOTOABSORPTION IN FORMALDEHYDE

Inner-shell soft-x-ray absorption of formaldehyde, H2CO, in the region of the C K and O K absorption thresholds was studied with high energy resolution using synchrotron radiation from the SX700/II monochromator at BESSY. The absorption spectra were recorded via the total photocurrent yield. The C 1s-excitation spectrum is characterized by a dominant C 1s-1 pi* resonance and weaker transitions into Rydberg states, each exhibiting vibrational fine structure that is quantitatively analyzed in terms of the normal vibrational modes of H2CO. Isotopic effects on the vibrational modes were studied by taking analogous spectra of D2CO. A Franck-Condon analysis of the vibrationally split spectra yields equilibrium distances, molecular bond angles, and vibrational frequencies of C 1s-1-excited formaldehyde, which show strong isotopic effects. In addition, the ground vibrational level of the lowest Rydberg state (C 1s(-1)3s-a1) exhibits a pronounced shift to higher excitation energies upon replacement of H by D. This shift, as well as a considerable isotope-dependent line broadening, are presumably caused by perturbations of this state by valence orbitals. Analogous isotopic effects were observed for the O 1s(-1)3s-a1 Rydberg state.