MATI spectra of NH 3 have been recorded using two-color (2 + 1′) multiphoton excitation via the B ˜and C ˜′ 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 B ˜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 pπ-dδ 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. † Part of the special issue "Edward M. Schlag Festschrift".

NH +3 ions have been prepared in unique vibration-rotation states v + 2 , N + , K + , by pulsed-field ionization of high Rydberg states of NH 3 , populated by two-colour multiphoton excitation. The selected states, ranging from v + 2 = 0 to 7 and N + , K + 4 are accessed via the B and C states as intermediates. The application of multichannel quantum defect theory to the understanding of the rotational propensities in the mass-analysed threshold ionization spectra is discussed. It is demonstrated that the interactions of the Rydberg pseudo-continuum both with bound Rydberg states and with other continua can be modelled.

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