Degree Of Rotational Excitation Research Articles

Near infrared second overtone cw-cavity ringdown spectroscopy of H2D+ ions

A study of H2D+ ions in their lowest rotational states is presented. The ions are generated in pulsed discharge in liquid N2 cooled He/Ar/H2/D2 gas mixture. Near infrared (NIR) second overtone transitions in the 6459–6467 and 6490–6492 cm−1 (1.540, 1.548 µm) region are used to identify the ions and determine their degree of rotational excitation. The data were obtained using NIR Cavity Ringdown Absorption Spectroscopy (NIR-CRDS). The sensitivity obtained was typically 5 × 10−9 cm−1. The measured second overtone transition frequencies are in good agreement with ab initio predictions with systematic shift −0:1 cm−1. The kinetics and rotational temperatures during discharge period was determined. The absolute number density of H2D+ as a function of H2/D2 mixing ratio is measured and results are compared to previous measurement of D2H+ ion.

Near infrared second overtone cw-cavity ringdown spectroscopy of D 2H + ions

A study of D 2H + ions in their lowest rotational states is presented. The ions are generated in pulsed discharge in liquid N 2 cooled He/Ar/H 2/D 2 gas mixture. Near infrared (NIR) second overtone transitions in the 6534–6536 cm −1 (1.529–1.530 μm) region are used to identify the ions and determine their degree of rotational excitation. The data were obtained using NIR cavity ringdown absorption spectroscopy (NIR-CRDS). The sensitivity obtained was typically 5 × 10 −9 cm −1. The measured second overtone transition frequencies are in very good agreement (better than 0.02 cm −1) with ab initio predictions. From the Doppler broadening the kinetic temperature of ions is estimated to be (220 ± 50) K. The absolute number density of D 2H + as a function of H 2/D 2 mixing ratio and time is measured.

Statistical mechanics of energy transfer in gas–surface scattering: A dynamical Lie algebraic approach

AbstractThe dynamical Lie algebraic (DLA) method is used to describe statistical mechanics of energy transfer in rotationally inelastic molecule–surface scattering. Statistical average values of an observable for the scattering system are calculated in terms of density operator formalism in statistical mechanics. Employing a cubic expansion procedure of molecule–surface interaction potential leads to generation of a dynamical Lie algebra. Thus these statistical average values as a function of the group parameters can be obtained analytically in this formulation. The group parameters can be found from solving a set of coupled nonlinear differential equations. The DLA method, which has no need for determination of transition probabilities in advance as made routinely in the calculation, offers an efficient alternative to the method for computing the statistical average values. This method is much less computationally intensive because most of calculations can be analytically carried out. The average final rotational energies and their dependence on the main dynamic variables and the average interaction potential are presented for the rotationally inelastic scattering of NO molecules from a flat, static Ag(111) surface. Direct comparison is made between the predictions of this model calculation and experiment. The model reproduces well the degree of rotational excitation and correlation between the average final translational and the average rotational energies. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Free-Jet Electronic Spectroscopy of the PO2 Radical

The laser fluorescence excitation spectrum of the jet-cooled PO2 radical has been recorded in the wavelength range 325−284 nm. This work extends previous flash photolysis and laser fluorescence studies of the UV electronic transition in PO2. Resolved emission spectra were recorded for excitation of a number of bands. The previous vibrational analyses of both the absorption and emission spectra were extended. The rotational structure of the bands was investigated and found to be very complicated, defying an analysis. The radiative decays of the excited vibronic levels were found to be nonexponential but could be fitted to a double exponential form. The lifetimes were found to be much shorter than those observed in a discharge-flow system by Hamilton [J. Chem. Phys. 1987, 86, 33]. The inferred dependence of the lifetime upon the degree of rotational excitation and the complicated rotational structure of the bands suggest that there is significant electronic state mixing, as in the isovalent NO2 molecule.

On the formation of rotationally hot H2+. by dissociation of CH42+ dications

Dissociation of CH into CH ion pairs results in formation of H ions possessing a degree of rotational excitation which is high enough to enable formation of several rotational resonance states. These resonances give rise to unimolecular dissociation into H+ + H· fragments. Ion translational energy spectrometry has been applied to measure such dissociation spectra, and a number of rotational resonances are observed. Rotational resonances are not observed when H ions are generated from H2 precursors. Two models based upon ideas of impulsive energy release are investigated which attempt to correlate the centre-of-mass kinetic energy released upon dissociation of CH with the observation of the rotational resonances. There is good accord between the experimental results and the prediction of our models.

A rotationally resolved LIF study of the N+2 products of the thermal energy Penning ionization reaction: Ne*(3P2)+N2

Multiple temperature rotational distributions are observed in the N+2 products of the Penning ionization of N2 by Ne*(3P2). The rotational distributions for N=2–21 are well fit by a two-component Boltzmann distribution having a low temperature of 50 K±10 K and a high temperature ranging from 220 to 490 K±50 K. The relative contribution of each component varies smoothly with vibrational level from 62% Tlow:38% Thigh for v=0 to 43% Tlow:57% Thigh for v=4(±5%). We interpret the observed multiple rotational distributions as evidence for exit channel effects arising from details of the impulsive interaction in the electron transfer process. Since the overall degree of rotational excitation is small, observation of the multiple distributions requires suppression of the reactant initial rotation by using a supersonic molecular beam.

Dynamics of gas–surface energy transfer: Inelastic scattering of NO from Ag(111)

We have determined the velocity distributions of individual quantum states of NO scattering from Ag(111) using molecular-beam techniques to control the incidence energy Ei and angle θi, and detecting individual quantum states of NO at specific scattering angles θf and surface temperatures Ts by two-photon ionization. Here, Ei has been varied between 0.1 and 1.0 eV, θi from 15° to 65°, θf from 0° to 75°, and Ts from 155 to 760 K, with a total of over 1000 such velocity distributions. We report the observation of direct vibrational excitation and of an anticorrelation between translational energy loss to phonons and to rotation. We also show that this system is highly translationally inelastic with up to 55% of Ei being lost to phonons at Ei=1.0 eV and that the final translational energy is rather insensitive to θf. Angular distributions are found to depend on the degree of rotational excitation, but to a much lesser extent than would be expected for direct transfer of normal energy into rotation. Results are discussed in terms of an electronic mechanism for vibrational excitation, trajectory calculations, and simple models.

Energy Partitioning in Molecular Photodissociation: Methyl Nitrite Photolysis

The photodissociation of methyl nitrite CH3ONO in three different electronically excited states has been studied by determining the internal state distribution of one of the photofragments. The fluorescence of NO A Σ2+ produced for excitation of CH3ONO between 1200 and 1650 Å and of CH3O 2 A1 produced at 1930 Å has been investigated. The vibrational distribution in both species can be explained by statistical considerations. The rotational excitation of NO A Σ2+ is of Boltzmann type. For CH3ONO excited in the first nπ* excited state at 3550 Å the NO X fragment has been probed by a two photon laser excited fluorescence technique. The nascent NO X υ″ = 0, 1, 2, 3 exhibit a population inversion and a high degree of rotational excitation.

An optical model for exchange reactions

An optical model or complex potential has been used in a relatively simple fashion to provide an interpretation of several molecular dynamics experiments. Rather than attempting a quantitative curve fit to the available data using a phenomenological optical potential (which is certainly possible) we have correlated certain physically reasonable features of the complex potential with general trends in the reaction dynamics. As an explicit example, the relationship between the range characteristics of the optical potential and the dependence of the reaction probability upon the kinetic energy of the reactants is derived. Other correlations are also presented, such as the dependence of the reaction probability upon impact parameter and degree of rotational excitation. The power of such a treatment obviously lies in its general applicability to complex systems as well as in its ability to often provide a simple physical understanding of some rather anomalous features of the reaction dynamics.