Multichannel quantum defect theory (MQDT) analysis of the (2+1 ') mass analyzed threshold ionization (MATI) spectroscopy of NH3

MATI spectra of NH3 have been recorded using two-color (2 + 1') multiphoton excitation via the (B) over tilde and (C) over tilde states and have been simulated using multichannel quantum defect theory. New vibrational bands i n the MATI spectra are reported for intermediate levels up to nu (2) = 6 in the (B) over tilde state. The spectra obtained-should in principle be iden tical to ZEKE: spectra recorded under the same resolution. A good agreement :,between experiment and theory-is obtained by using quantum defects determ ined previously from optical spectra and optimization of unknown parameters ; The effects of p pi -d delta mixing in the core region are included to ac count for "forbidden" transitions, and the optimization of quantum defect p arameters is used to quantify the strength of this mixing. Molecular symmet ry group arguments are used to define which channels must be included in th e quantum-defect matrix. The paper demonstrates that in favorable cases a c omplete theory of ZEKE/MATI line intensities is achievable including the ef fects of bound-state couplings.