He I photoelectron spectroscopy of formic acid isotopomers HCOOH and DCOOD

He I photoelectron spectra (PES) of HCOOH and DCOOD have been measured with an electron kinetic energy resolution of 15 meV. Analysis of observed vibr onic structure in the two PES bands in the 11.3-13.5 eV energy region provi ded values for several vibrational modes of the (X) over tilde (2)A' and (A ) over tilde 2A " electronic states of HCOOH+ and DCOOD+. This analysis is based on the implications of structural changes expected to occur on ioniza tion, using literature values for calculated geometries of these states. Th e vibronic assignments of both PES bands were consistent with isotopic effe cts on vibrational mode frequencies and were helped by considerations on re lations between carbon-oxygen bond lengths and stretch mode frequencies. Th e analysis of the second PES band indicates that the calculated structure o f the (A) over tilde (2)A " state of the ion requires more refined theoreti cal treatment. The PES first ionization energy of HCOOH agrees well with th at obtained from Rydberg series in absorption spectra, and values more prec ise than hitherto were obtained for the first and second ionization energie s of DCOOD, 11.335 +/- 0.002 eV and 12.380 +/- 0.002 eV respectively, and t he second ionization energy of HCOOH, 12.378 +/- 0.002 eV. (C) 2001 Elsevie r Science BN. All rights reserved.