High-resolution (∼0.1 eV) core-level photoelectron spectra of gas-phase ${\mathrm{SiH}}_4$, ${\mathrm{PH}}_3$, ${\mathrm{GeH}}_4$, and ${\mathrm{AsH}}_3$ have been recorded using monochromatized synchrotron radiation. These spectra, multiconfiguration self-consistent-field calculations of the vibrational profile, and the core-equivalent model all show that core-level vibrational structure decreases dramatically, both down a group of congeneric molecules and across a periodic row of isoelectronic molecules. The theoretical calculations also show that the Franck-Condon factors do not change for relatively shallow core levels (e.g., Si 2p,2s or Ge 3d,3p) but increase significantly for deep core levels (e.g., Si 1s).

• Received 21 February 1992

DOI:https://doi.org/10.1103/PhysRevA.46.1688