The Stark effect in autoionizing Rydberg states of NO, n=10–20,v+=1, is studied by a combination of experimental and theoretical methods. Double resonant excitation via selected intermediate rotation levels of the A 2Σ+, v′=1 state is carried out in the presence of fields 0–1000 V/cm. The spectra are simulated using both matrix diagonalization and full multichannel quantum defeat theory (MQDT) approaches, providing a test of quantum defect parameters and transition dipoles for Rydberg series from l=0 to l=4. The use of the same input parameters in these two types of calculation allows comparison of the validity and utility of these methods.

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.

A novel experiment for near-threshold photodissociation studies is presented. State-selective excitation of the molecular photofragments to high-n Rydberg states is used in a variation of the ion imaging technique, allowing for undistorted detection of slow fragments produced close to the channel dissociation threshold. As a first demonstration of this method, the angular anisotropy parameter β for production of NO (J=17/2) and O 3P2 in the photodissociation of NO2 has been obtained as a function of excess energy. A classical model for β as a function of excess energy is presented, accounting for the decrease of anisotropy in the angular photofragment distribution upon approaching the channel threshold. The experimental values of β fluctuate substantially around the values predicted by the model, indicating strong underlying fluctuations in the state-to-state rate constant. This experiment offers in principle a unique route to measuring state-to-state reaction rate constants in situations where existing time- or frequency-resolved methods are inappropriate.

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