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Experimentally and theoretically determined line strengths are presented for E, [F.sup.1][[sigma].sup.+.sub.g](v' = 0, J' = J") - [X.sup.1][[sigma].sup.+.sub.g] (v", J") (2 + 1) REMPI transitions in [H.sub.2], HD, and [D.sub.2]. The experimental technique employs a hot filament source of internally excited hydrogen that allows experimental determination of line strengths for the low rotational states of highly excited vibrational manifolds (v" [less than or equal to] 4). The line strengths are found to depend only weakly on J" for the states measured here, and theoretical results indicate that the line strengths depend strongly on v". These values are combined with previously measured and calculated line strengths for these transitions (K.-D. Rinnen, M.A. Buntine, D.A.V. Kliner, R.N. Zare, and W.M. Huo. J. Chem. Phys. 95, 214 (1991)), resulting in a more complete compilation of REMPI line strengths for molecular hydrogen. Key words: hydrogen, spectroscopy. REMPI, line strength, multiphoton.

An eight-dimensional time-dependent quantum dynamics calculation is reported to study the isotopic reaction, HD+C2H, on a new modified potential energy surface. Initial-state-selected reaction probability, integral cross section, and rate constants are presented in this isotopic reaction study. Initial-state-selected reaction probability is obtained by summing over all the possible product’s arrangements in this isotopic reaction study. This study shows that vibrational excitations of HD enhance the reactivity, whereas stretching excitations of C2H only have a small effect on the reactivity. Furthermore, the bending excitations of C2H, compared to the ground-state reaction probability, hinder the reactivity. The present results are consistent with those obtained for the H2+C2H reaction. The comparison of these two reactions also shows the isotopic effect in the initial-state-selected reaction probability, integral cross section, and rate constants. The rate constant comparison shows that the HD+C2H reaction has a smaller reactivity than the H2+C2H reaction. [ABSTRACT FROM AUTHOR]

An eight-dimensional time-dependent quantum dynamics wave packet approach is performed for the study of the H2+C2H→H+C2H2 reaction system on a new modified potential energy surface (PES) [L.-P. Ju et al., Chem. Phys. Lett. 409, 249 (2005)]. This new potential energy surface is obtained by modifying Wang and Bowman’s old PES [J. Chem. Phys. 101, 8646 (1994)] based on the new ab initio calculation. This new modified PES has a much lower transition state barrier height at 2.29 kcal/mol than Wang and Bowman’s old PES at 4.3 kcal/mol. This study shows that the reactivity for this diatom-triatom reaction system is enhanced by vibrational excitations of H2, whereas the vibrational excitations of C2H only have a small effect on the reactivity. Furthermore, the bending excitations of C2H, compared to the ground state reaction probability, hinder the reactivity. The comparison of the rate constant between this calculation and experimental results agrees with each other very well. This comparison indicates that the new modified PES corrects the large barrier height problem in Wang and Bowman’s old PES. [ABSTRACT FROM AUTHOR]

We report a detailed three-dimensional time-dependent quantum dynamics study of the state-to-state N+N[sub 2] exchange scattering in the 2.1–3.2 eV range using a recently developed ab initio potential energy surface (PES). The reactive flux arrives at the dividing surface in the asymptotic product region in a series of six packets, instead of a single packet. Further study shows that these features arise from the “Lake Eyring” region of the PES, a region with a shallow well between two transition states. Trappings due to Feshbach resonances are found to be the major cause of the time delay. A detailed analysis of the Feshbach resonance features is carried out using an L[sup 2] calculation of the metastable states in the “Lake Eyring” region. Strong resonance features are found in the state-to-state and initial state selected reaction probabilities. The metastable states with bending motions and/or bending coupled with stretching motions are found to be the predominant source of the resonance structure. Initial state selected reaction probabilities further indicate that the lifetimes of the metastable states with bending motions in the “Lake Eyring” region are longer than those of states with stretching motions and thus dominate the reactive resonances. Resonance structures are also visible in some of the integral cross sections and should provide a means for future experimental observation of the resonance behavior. A study of the final rotational distributions shows that, for the energy range studied here, the final products are distributed toward high-rotational states. Final vibrational distributions at the temperatures 2000 and 10 000 K are also reported. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The N + N[SUB2] exchange rate is calculated using a time-dependent quantum dynamics method on a newly determined ab initio potential energy surface (PES) for the ground [SUP4]A″ state. This ab initio PES shows a double barrier feature in the interaction region with the barrier height at 47.2 kcal/mol, and a shallow well between these two barriers, with the minimum at 43.7 kcal/mol. A quantum dynamics wave packet calculation has been carried out using the fitted PES to compute the cumulative reaction probability for the exchange reaction of N + N[SUB2] (J = 0). The J-K shift method is then employed to obtain the rate constant for this reaction. The calculated rate constant is compared with experimental data and a recent quasiclassical calculation using a London-Eyring-Polanyi-Sato PES. Significant differences are found between the present and quasiclassical results. The present rate calculation is the first accurate three-dimensional quantal dynamics study for the N + N[SUB2] reaction system and the ab initio PES reported here is the first such surface for N[SUB3]. [ABSTRACT FROM AUTHOR]

Harmonic and anharmonic vibrational frequencies, as well as infrared and Raman intensities, are calculated for MgSO[sub 4]·nH[sub 2]O (n = 1-3). Electronic structure theory at the second order MollerPlesset perturbation theory level with a triple-ζ+polarization basis set is used to determine the geometry, energetics, charge distributions, and vibrational spectra of pure and hydrated MgSO[sub 4] salts. The direct vibrational self-consistent field method and its correlation corrected extension are used to determine anharmonic corrections to vibrational frequencies and infrared intensities for the pure MgSO[sub 4] and its complex with one water molecule. Very significant differences are found between vibrational spectra of water molecules in complexes with MgSO[sub 4] and pure water. Some of the O-H stretching frequencies are shifted to the red very significantly (by up to 2000 cm[sup -1]) upon complexation with magnesium sulfate. They should be observed between 1700 and 3000 cm[sup -1], in a region very different from the corresponding O-H stretch frequency region of pure water (3700-3800 cm[sup -1]). In addition, the SO[sub 2] stretching vibrations are found at lower frequency regions than the water vibrations. They can serve as unique identifiers for the presence of sulfate salts. The predicted infrared and Raman spectra should be of valuable help in the design of future missions and analysis of observed data from the ice surface of Jupiter's moon Europa that possibly contains hydrated MgSO[sub 4] salts. [ABSTRACT FROM AUTHOR]

Examines theoretical and experimental electron-impact ionization cross sections of CF[sub 4], C[sub 2]F[sub 6] and C[sub 3]F[sub 8]. Measurement of the cross sections as a function of incident electron energy; Molecular polarizabilities; Theoretical estimates for multiple ionization; Correlations included in wave functions.

Rotational energy transfer in collisions of nitrogen molecules has been studied theoretically, using the N2–N2 rigid-rotor potential of van der Avoird et al. [J. Chem. Phys. 84, 1629 (1986)]. For benchmarking purposes, converged close coupling (CC) calculations have been carried out to a total energy of about 200 cm-1. Coupled states (CS) approximation calculations have been carried out to a total energy of 680 cm-1, and infinite order sudden (IOS) approximation calculations have also been carried out. The CC and CS cross sections have been obtained both with and without identical molecule exchange symmetry, whereas exchange was neglected in the IOS calculations. The CS results track the CC cross sections rather well: between 113–219 cm-1 the average deviation is 14%, with accuracy improving at higher energy. Comparison between the CS and IOS cross sections at the high energy end of the CS calculations, 500–680 cm-1, shows that IOS is sensitive to the amount of inelasticity and the results for large ΔJ transitions are subject to larger errors. State-to-state cross sections with even and odd exchange symmetry agree to better than 2% and are well represented as a sum of direct and exchange cross sections for distinguishable molecules, an indication of the applicability of a classical treatment for this system. This result, however, does not apply to partial cross sections for given total J, but arises from a near cancellation of the interference terms between even and odd exchange symmetries on summing over partial waves. In order to compare with experimental data for rotational excitation rates of N2 in the n=1 excited vibrational level colliding with ground vibrational level (n=0) bath N2 molecules, it is assumed that exchange scattering between molecules in different vibrational levels is negligible and direct scattering is independent of n so that distinguishable molecule rigid rotor rates may be used. With these assumptions good... [ABSTRACT FROM AUTHOR]

Using previously described close coupling (CC) and coupled states (CS) cross sections for N2–N2 collisions [J. Chem. Phys. 104, 7572 (1996)], we have calculated CARS linewidths at room temperature and below. Agreement with experimental values at room temperature is quite good but predictions become increasingly too large at lower temperatures, with errors reaching 10%–30% at 113 K. We believe these low temperature discrepancies reflect errors in the intermolecular potential used here. To obtain linewidths at higher temperatures we have used the energy corrected sudden (ECS) approximation, taking the fundamental cross sections, σ(0.0→J1,J2), from the CC–CS calculations extended to higher collision energies with additional CS and infinite order sudden (IOS) calculations: the ECS scaling distance, lc was chosen by fitting to the 300 K CC–CS results. In general, we find rather good agreement with experimental values to 1500 K, although it appears that smaller values of lc are more appropriate for higher temperatures and for higher rotational levels. This variability of lc is reasonable from physical arguments but somewhat diminishes the predictive utility of this approach. Agreement of these purely ab initio predictions with experimental data is nearly as good as that obtained from the best rate law model whose parameters were fitted to these data. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A theoretical study is reported for the dependence of the E, F 1Σ+g–X 1Σ+g (vE=0, vX) Q-branch two-photon transitions on the initial vibrational and rotational quantum numbers vX and J for H2, HD, and D2. The magnitude of the two-photon transition moment |Mfo| increases with vX between 0 and 2, then decreases rapidly at higher vX. This behavior is ascribed to (constructive and destructive) interference effects in the two-photon transition, which vary with the nodal behavior of the initial-state vibrational wave function. The J dependence of |Mfo| is most strongly affected by the changes in vibrational wave functions caused by centrifugal distortion of the vibrational potentials. At low to intermediate J, the Franck–Condon overlap improves, resulting in a slow, monotonic increase of |Mfo| with J. At higher J, resonant tunneling between the inner and outer wells of the E, F state introduces sharp variations in the |Mfo| structure. Near the top of the barrier, nonresonant tunneling in the E-state vibrational wave function causes a general decrease in |Mfo| with J. [ABSTRACT FROM AUTHOR]

The relationship between quantum-state populations and ion signals in (2+1) resonance-enhanced multiphoton ionization (REMPI) detection of H2, HD, and D2 via the E, F 1Σ+g (v’E=0, J’=J‘)–X 1Σ+g (v‘,J‘) transition is determined by calibration against a thermal effusive source. Correction factors are obtained for 102 rovibrational levels for v‘=0, 1, and 2 and J‘ ranging from 0 to 17. Within a given v‘, rotational correction factors are nearly unity except for the highest J‘ levels. The vibrational correction factors vary with v‘; (2+1) REMPI detection is 2–3 times more sensitive to v‘=1 and 2 than to v‘=0. Experimental correction factors are compared with those derived from a theoretical calculation of the two-photon transition moments by Huo et al. [J. Chem. Phys. 95, xxxx (1991)]. In general, the agreement is excellent, which suggests that theoretical correction factors may be used when experimental ones are unavailable. [ABSTRACT FROM AUTHOR]

In this paper, we report the results of ab initio calculations of photoelectron angular distributions and vibrational branching ratios for the (2+1) REMPI of H2 via the E,F 1Σ+g state, and compare these with the experimental data of Anderson et al. [Chem. Phys. Lett. 105, 22 (1984)]. These results show that the observed non-Franck–Condon behavior is predominantly due to the R dependence of the transition matrix elements, and to a lesser degree to the energy dependence. This work presents the first molecular REMPI study employing a correlated wave function to describe the Rydberg–valence mixing in the resonant intermediate state. [ABSTRACT FROM AUTHOR]

We show, by detailed electron–molecule scattering calculations, that the angular dependence of electron energy loss spectra in which an adsorbate is electronically excited can be used to identify the orientation of the molecule with respect to the surface and the nature of the final states. The calculations are exploratory and were carried out for an H2 molecule. The transition amplitude for electron–molecule scattering is calculated by using the Schwinger variational principle with two open channels. The effects of the surface were introduced through a semiquantitative model which treats the surface as a partly reflecting, flat mirror. [ABSTRACT FROM AUTHOR]

We report elastic differential and momentum transfer cross sections for the elastic scattering of electrons by H2O for collision energies from 2 to 20 eV. These fixed-nuclei static-exchange cross sections were obtained using the Schwinger variational approach. In these studies the exchange potential is directly evaluated and not approximated by local models. The calculated differential cross sections, obtained with a basis set expansion of the scattering wave function, agree well with available experimental data at intermediate and larger angles. As used here, the results cannot adequately describe the divergent cross sections at small angles. An interesting feature of the calculated cross sections, particularly at 15 and 20 eV, is their significant backward peaking. This peaking occurs in the experimentally inaccessible region beyond a scattering angle of 120°. The implication of this feature for the determination of momentum transfer cross sections is described. [ABSTRACT FROM AUTHOR]

The Le Roy–Hutson intermolecular potential for H2–Ar, which was derived from analysis of spectral data for the van der Waals complex and also shown to predict gas kinetic transport cross sections in good accord with experimental values, was subsequently found to provide rather poor predictions (small by nearly a factor of 2) for vibrational Q-branch Raman line shift cross sections. More recently, an ab initio intermolecular potential for this system has been calculated and scaled to agree with the earlier one in the region of the minimum where the latter is expected to be accurate [D. W. Schwenke et al., J. Chem. Phys. 98, 4738 (1993)]. We show here that this potential predicts line shifts in much better accord with experiment (about 10% large), but its predictions for linewidths are significantly less satisfactory. [ABSTRACT FROM AUTHOR]

An overview of the current status in electron collision cross section databases for plasma modeling is presented. Emphasis is on dissociation, ionization, and dissociative ionization. The effect of the operating conditions on the relative importance of different collision mechanisms is discussed. For high density plasmas with low gas pressure, a second collision with electrons becomes likely and metastable states may play a role in the dissociation and ionization processes. A number of outstanding theoretical and experimental issues in the cross-section data are discussed. For the total ionization cross section, a four-parameter fitting formula is found to represent the numerical data well and the parameters for some molecules used in plasma modeling are presented. Index Terms--Chemistry, collision processes, database reliability, electrons, impact ionization, plasma properties.

Abstract: Time-dependent quantum reaction dynamics calculations using eight degrees of freedom are reported in a study of the isotopic reaction, D2 +C2H, on a new modified potential energy surface. It shows that vibrational excitations of D2 enhance the reactivity, whereas the bending excitations of C2H hinder the reactivity. The comparison of the three isotopic reactions also shows the isotopic effect in the initial-state-selected reaction probability, integral cross section and rate constants. The rate constant comparison shows that the D2 +C2H reaction has the smallest reactivity among the three isotopic reactions D2/HD/H2 +C2H reaction, then HD+C2H, and H2 +C2H has the largest. [Copyright &y& Elsevier]

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Rich reactive resonances are found in a 3D quantum dynamics study of the N + N2 exchange reaction using our recently developed ab initio potential energy surface that has a shallow well between two transition states. An L2 analysis confirms that the quasibound states of ‘Lake Eyring’ are responsible for the reactive resonances with some quasibound states, mostly the bending motions, giving rise to strong reasonance peaks, whereas others contributing to the shoulders in the resonance structure. This is the first observation of reactive resonances from a ‘Lake Eyring’ feature in a potential energy surface. [Copyright &y& Elsevier]

The discrepancy between experimental and theoretical total electron-impact ionization cross-sections of the fluorides CFx and NFx (x=1–3) is attributed to inadequacies in previous theoretical models. Cross-sections calculated using a recently developed siBED model that takes into account the shielding of the long-range dipole potential in the collision are in agreement with experiment. The present study also re-analyzed the previously reported experimental data to account for incomplete collection of fragment ions and the presence of ion-pair formation channels. For NF3, our experimental and theoretical cross-sections compare well with the recent data of Haaland et al. in the region below dication formation. [Copyright &y& Elsevier]