High-resolutionK-shell photoabsorption in formaldehyde

Inner-shell soft-x-ray absorption of formaldehyde, ${\mathrm{H}}_2$CO, 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 1${\mathit{s}}^{\mathrm{-}1}$ ${\mathrm{\pi }}^{\mathrm{*}}$ resonance and weaker transitions into Rydberg states, each exhibiting vibrational fine structure that is quantitatively analyzed in terms of the normal vibrational modes of ${\mathrm{H}}_2$CO. Isotopic effects on the vibrational modes were studied by taking analogous spectra of ${\mathrm{D}}_2$CO. A Franck-Condon analysis of the vibrationally split spectra yields equilibrium distances, molecular bond angles, and vibrational frequencies of C 1${\mathit{s}}^{\mathrm{-}1}$–excited formaldehyde, which show strong isotopic effects. In addition, the ground vibrational level of the lowest Rydberg state (C 1${\mathit{s}}^{\mathrm{-}1}$3s-${\mathit{a}}_1$) exhibits a pronounced shift to higher excitation energies upon replacement of H by D. This shift, as well as a considerable isotope-dependent line broadening, are presumably caused by perturbations of this state by valence orbitals. Analogous isotopic effects were observed for the O 1${\mathit{s}}^{\mathrm{-}1}$3s-${\mathit{a}}_1$ Rydberg state.