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Inner-shell soft-x-ray absorption of formaldehyde, H2CO, in the region of the C E and 0 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 1sexcitation spectrum is characterized by a dominant C 1s ' n. resonance and weaker transitions into Rydberg states, each exhibiting vibrational fine structure that is quantitatively analyzed in terms of the normal vibrational modes of H&CO. Isotopic effects on the vibrational modes were studied by taking analogous spectra of D&CO. A Franck-Condon analysis of the vibrationally split spectra yields equilibrium distances, molecular bond angles, and vibrational frequencies of C ls -excited formaldehyde, which show strong isotopic effects. In addition, the ground vibrational level of the lowest Rydberg state (C 1s 3s-a& ) 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 0 1s 3s-a, Rydberg state.PACS number(s): 33.20. Rm, 33.70.w, 33.80.Rv, 35.20.i

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