Notes: 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 collision-induced symmetry change (a↔s) in v2=1 are on the order of kas=4 μs−1 torr−1, smaller than kas in the ground state, but over an order of magnitude larger than that recently reported in the literature. Depopulation rates for other collision partners (Ar, H2, N2, and He) can be understood in terms of the intermolecular potentials. Comparisons are made between the relaxation rates measured in this work and infrared pressure-broadening coefficients reported in the literature.

Notes: 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.

Notes: We propose a stability criterion applicable to eigenstates of approximate Hamiltonians. We define a "robustness'' of the physical properties of the underlying system under a variety of internal and external conditions that can be deduced from stability properties of the eigenstates of the approximate Hamiltonian. Stability properties are assigned to an ensemble of eigenstates according to the statistics of its response to an ensemble of random perturbations of given magnitude. The stability criterion is explicitly formulated for the eigenstates of polyatomic molecules exhibiting a normal to local mode transition. As an example, a stability analysis for the water molecule is carried out and experiments in the low density gas phase (rotational energy transfer), high density gas phase (collision-induced spectra), and condensed phase are suggested. © 1995 American Institute of Physics.

Notes: We argue that the physical significance of the normal to local mode transition in triatomic molecules AB2 lies in an enhanced susceptibility of eigenstates to symmetry breaking perturbations. States of local character have higher susceptibilities than states of normal character. The quantum mechanical condition for the normal to local mode transition is formulated. The issue of stability is addressed from a classical, semiclassical, and quantum mechanical point of view. The question of instability of the unperturbed quantum system is decided individually for each eigenstate. The relevance for experiments is outlined.

Notes: A time-resolved infrared double-resonance technique has been used to measure vibrationally and rotationally inelastic collision rates in ground and vibrational overtone levels of methane. A Raman-shifted Ti:sapphire laser is used to pump J=0 through 7 states in the 2ν3 and ν3+ν4 levels of 12CH4, and a tunable diode laser is used to probe the time-dependent level populations. Vibrational equilibration is observed among the octad, pentad, and dyad levels, with subsequent relaxation to the ground state. State-to-state rotational energy transfer rates are obtained in the ground and ν3+ν4 excited vibrational levels, and compared with theoretical predictions and with pressure-broadening measurements on the corresponding transitions. The probability of molecular reorientation in an inelastic collision is also inferred from the polarization dependence of the relaxation times. Parity-conserving and vibrational angular momentum propensity rules are inferred for the lower rotational levels of methane. © 1994 American Institute of Physics.

Notes: 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) A(ring). In addition, the harmonic force field for the A˜ state has been experimentally determined. fhh, fαα−fαα', and frr were found to be 1.06(4) aJ/A(ring)2, 0.25(2) aJ, and 4.9 aJ/A(ring)2, 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.

Notes: Polyad quantum numbers have been assigned to 134 vibrational levels of the X˜1Σg+ state of acetylene with internal energies from 3,000 to 15,000 cm−1. These polyad assignments have been made possible by two advances: (1) the recording of new, rigorously calibrated acetylene A˜1Au→X˜1Σg+ dispersed fluorescence spectra, and (2) the development of a numerical pattern recognition technique which identifies groups of transitions in the spectra that terminate on eigenstates with the same polyad quantum numbers. This pattern recognition technique is based on the Extended Cross-Correlation, which has been reported previously in this Journal [J. Chem. Phys. 107, 8349, 8357 (1997)], and requires neither a priori knowledge of the number of polyads in the spectra nor the pattern of spectral lines that is associated with each polyad. No evidence for the breakdown of the polyad quantum numbers is found, at the 7 cm−1 resolution of our spectra, at internal energies up to at least 15,000 cm−1. The ability to assign polyad quantum numbers to spectral features with up to 15,000 cm−1 of internal energy provides a panoramic perspective on the trends in the short-time (∼1 ps) dynamics of acetylene at high internal energy. © 1998 American Institute of Physics.

Notes: The analysis of current problems in physical chemistry often requires the identification of patterns that encode the composition, structure, and dynamics of a system. Overlapped patterns, unexpected patterns, and patterns whose forms are initially unknown are especially difficult to identify and to extract. We have developed two new techniques for pattern recognition and extraction designed for these situations. These techniques, extended cross correlation (XCC) and extended auto correlation (XAC), identify and extract multiple patterns from experimental data even when the number of derived patterns exceeds the number of experiments. The XCC, which is the focus of this paper, allows the rapid identification and extraction of patterns that are repeated in multiple experimental records. The related XAC technique permits the identification of complex patterns that are parameterized in a multidimensional way, even when the patterns are obscured by the presence of interfering data. The XCC and XAC provide straightforward methods for extracting the features which comprise a pattern, and can be applied in a model-free way. This paper provides a formal description of multidimensional forms of the XCC technique, and illustrates use of the XCC on large data sets with multiple patterns. © 1997 American Institute of Physics.