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Results of ab initio R-matrix calculations [S. N. Altunata et al., J. Chem. Phys. 123, 084319 (2005)] indicate the presence of a broad shape resonance in electron-CaF+ scattering for the 2Σ+ electronic symmetry near the ionization threshold. The properties of this shape resonance are analyzed using the adiabatic partial-wave expansion of the scattered electron wave function introduced by Le Dourneuf et al. [J. Phys. B 15, L685 (1982)]. The qualitative aspects of the shape resonance are explained by an adiabatic approximation on the electronic motion. Mulliken’s rule for the structure of the Rydberg state wave functions [R. S. Mulliken, J. Am. Chem. Soc. 86, 3183 (1964)] specifies that, except for an (n*)-3/2 amplitude scale factor, every excited state wave function within one Rydberg series is built on an innermost lobe that remains invariant in shape and nodal position as a function of the excitation energy. Mulliken’s rule implies a weak energy dependence of the quantum defects for an unperturbed molecular Rydberg series, which is given by the Rydberg-Ritz formula. This zero-order picture is violated by a single 2Σ+ CaF Rydberg series at all Rydberg state energies (n*=5→∞, more so with increasing n*) below the ionization threshold, under the broad width of the shape resonance. Such a violation is diagnostic of a global “scarring” of the Rydberg spectrum, which is distinct from the more familiar local level perturbations. [ABSTRACT FROM AUTHOR]

An infrared double-resonance laser spectroscopic technique is used to study state-resolved rotational (R–R, R–T) energy transfer in ammonia (14NH3) (self-collisions and between ammonia and foreign gases). NH3 molecules are prepared in selected rovibrational states of the v2=1 level using coincidences between CO2 -laser lines and ν2 fundamental transitions. Measurements of both the total rate of depopulation by collisions, and the rates of transfer into specific final rovibrational states (v,J,K) have been carried out using time-resolved tunable diode laser absorption spectroscopy. For NH3–NH3 collisions, measurements of total depopulation rates of selected JK states in v2=1 and ground-state recovery rates are found to be three and eight times larger, respectively, than the Lennard-Jones collision rate, in accord with theoretical expectations for polar molecules.A kinetic master-equation analysis of time-resolved level populations yields state-to-state rate constants and propensity rules for NH3–NH3 and NH3–Ar collisions. Individual rotational energy-transfer rates in v2=1 are slower than in the vibrational ground state, but still comparable to the Lennard-Jones collision frequency. Our experiments show that rotational energy transfer in v2=1 is not governed by simple ‘‘dipolelike’’ selection rules. They show fast rotational energy transfer, which can be related to long-range interaction potentials, but at the same time considerable amounts of ΔJ=2 and 3, ΔK=0, and ΔJ=1–4, ΔK=3, transitions, which may be attributed to higher-order terms in the multipole expansion of the intermolecular potential. No pronounced symmetry-state correlation and no preferred pathways were found except the preference for relaxation within a K stack and the expected separate relaxation of different nuclear-spin species, which can be labeled by their K-quantum number. Rates of... [ABSTRACT FROM AUTHOR]

Microwave detected, microwave-optical double resonance was used to record the A state electronic spectrum of NH3, NH2D, and NHD2 with both vibrational and rotational resolution. To investigate ND3 with the same resolution as we had with our hydrogen containing isotopomers, a strip-line cell was constructed allowing the simultaneous passage of radio-frequency and ultraviolet radiation. Rotational constants were obtained as a function of ν2 excitation and an A state equilibrium bond length was estimated at 1.055(8) Å. In addition, the harmonic force field for the à state has been experimentally determined. fhh, fαα-fαα’, and frr were found to be 1.06(4) aJ/Å2, 0.25(2) aJ, and 4.9 aJ/Å2, respectively. This calculated harmonic force field predicts that the asymmetry observed in the NH3 24 band is due to a strong anharmonic interaction with the 43 level and the broad feature observed in the dispersed fluorescence spectrum previously assigned to the 11 band is more likely attributable to the 42 level. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using microwave detected, microwave-optical double resonance, we have measured the homogeneous linewidths of individual rovibrational transitions in the A state of NH3, NH2D, NHD2, and ND3. We have used this excited state spectroscopic data to characterize the height of the dissociation barrier and the mechanisms by which the molecule uses its excess vibrational and rotational energies to help overcome this barrier. To interpret the observed vibronic widths, a one dimensional, local mode potential has been developed along a N–H(D) bond. These calculations suggest the barrier height is roughly 2100 cm-1, approximately 1000 cm-1 below the ab initio prediction. The observed vibronic dependence of levels containing two or more quanta in ν2 is explained by a Fermi resonance between 2ν2 and the N–H(D) stretch. This interaction also explains the observed trends due to isotopic substitution. The rotational enhancement of the predissociation rates in the NH3 21 level is dominated by Coriolis coupling while for the same level in ND3, centrifugal effects dominate. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The interaction between electronic and rotational motions in the Rydberg states of calcium monofluoride is investigated using multichannel quantum defect theory (MQDT). By examining the partial-ℓ and N+ composition of the molecular wavefunctions of successive members of each of the Rydberg series, we show that the nature of the interaction between the motion of the Rydberg electron and the rotational motion of the ion core undergoes complex and non-monotonic changes with increasing principal quantum number n* and total angular momentum N. Resonances between the Kepler motion of the Rydberg electron and the rotational motion of the ion core (the 'stroboscopic effect') are observed when the Kepler period is an integer multiple of the rotational period. These resonances result in strong mixing of both the electron orbital angular momentum ℓ and the ion core rotational angular momentum N+. At n* or N values between these resonances, the angular momenta of the electron and ion core subsystems are more nearly conserved. We also find evidence for a second type of resonance, which takes place between the precessional motion of the Rydberg electron and the rotation of the core. The qualitative features of these various dynamical regimes can be explained in terms of the classical frequencies of motion of the electron and ion and the nature of the electrostatic interactions between them. The results presented here provide general insights into the interaction between electronic and rotational motions in both polar and non-polar diatomic molecules. [ABSTRACT FROM AUTHOR]