Oscillator strengths and cross sections of high Rydberg states in ammonia studied by fast electron scattering

Abstract

Abstract We report the experimental generalized oscillator strengths (GOSs) for transitions from the ground state X ~ A 1 ′ 1 to the Rydberg states B ~ E ′ ′ 1 , C ~ ′ A 1 ′ 1 , D ~ E ′ 1 + D ~ ″ A 2 ′ ′ 1 , E ~ E ′ 1 ( v 2 ′ = 0 – 7 ) + ( F ~ E ′ 1 + G ~ A 2 ′ ′ 1 ) ( v 2 ′ = 0 – 4 ) and an ‘undefined’ state U ~ ( v 2 ′ = 0 – 4 ) in ammonia. The experiment was performed using an angle-resolved electron energy loss spectrometer operated at an incident energy of 1500 eV and an energy resolution of about 80 meV. The experimental observation confirms the recent theoretically predicted appreciable vibrational effects on the GOS for the transition B ~ E ′ ′ 1 ← X ~ A 1 ′ 1 (2021 J. Phys. B: At. Mol. Opt. Phys. 54 135202). The measured GOS data are fitted with the Lassettre formula to determine the corresponding optical oscillator strengths for dipole-allowed transitions. Furthermore, the fitted curves enable the integrations of GOSs over the momentum transfer squares to obtain the Born excitation cross sections. The integral cross sections are scaled to an accurate scale by using the BE-scaling method. The data in this work can supplement the molecular database and deepen our understanding of the scattering mechanism.