Vibrational excitation in ESCA and soft X-ray emission spectra of small polyatomic molecules

The vibrational envelopes of the core ESCA bands of H₂O and NH₃and of the 3a₁X-ray emission band of NH₃have been investigated by means of Franck-Condon analysis. The data for the equilibrium geometry and the force constants of the neutral ground, N_1sand 3a₁ionized states of NH₃have been computed from ab initio wave functions, whereas the data for the neutral ground and the O_1sstate of H₂O have been taken from literature [1]. The N_1sstate of NH₃is foundto be planar with a significantly shortened N-H bond length in comparison with the neutral ground state. The ionizationof the O_1sstate of H₂O leads to an increase of the HOH angle by ∼ 16° with negligible change in the O-H bond distance. The vibrational intensities and energies computed from the ab initio results explain the band profiles of the spectra.