Multichannel Quantum Defect Theory (MQDT) Analysis of the (2 + 1‘) Mass Analyzed Threshold Ionization (MATI) Spectroscopy of NH₃

Abstract

MATI spectra of NH 3 have been recorded using two-color (2 + 1‘) multiphoton excitation via the B̃ and C̃ ‘ states and have been simulated using multichannel quantum defect theory. New vibrational bands in the MATI spectra are reported for intermediate levels up to ν 2 = 6 in the B̃ state. The spectra obtained should in principle be identical to ZEKE spectra recorded under the same resolution. A good agreement between experiment and theory is obtained by using quantum defects determined previously from optical spectra and optimization of unknown parameters. The effects of p π− d δ mixing in the core region are included to account for “forbidden” transitions, and the optimization of quantum defect parameters is used to quantify the strength of this mixing. Molecular symmetry group arguments are used to define which channels must be included in the quantum defect matrix. The paper demonstrates that in favorable cases a complete theory of ZEKE/MATI line intensities is achievable including the effects of bound-state couplings.