Bibliothek

Notizen: We present a simple model to evaluate the degree of l and ml mixing in high Rydberg states that results from perturbations caused by weak, homogeneous dc electric fields and static ions. This model predicts the lifetime of these states qualitatively and explains several seemingly contradictory observations obtained using zero-kinetic-energy (ZEKE) photoelectron spectroscopy. The presence of a small homogeneous dc electric field and a few ions in the sample volume causes ml mixing in general as well as l mixing, both of which contribute to the lengthening of the lifetimes. Consequently, the lifetime lengthening appears to be insensitive to the sample pressure. The effect of the dc electric field on the lifetime is complex. Although the electric field results in l mixing, with increasing field strength it inhibits ml mixing, and, at still higher field strength, induces ionization. The variation of the lifetimes with ion concentration is also complicated. At low ion concentration, the ml mixing varies across the Stark manifold of Rydberg states that belong to the same principal quantum number, so that different states have different lifetimes. At higher ion concentration, l and ml mixing are more uniform, which lengthens the lifetimes and makes them more similar across the Stark manifold. At still higher concentrations, collisional ionization dominates, which shortens the lifetimes.

Notizen: A comprehensive first principles theoretical investigation of the gas phase reaction Ca+HF→CaF+H is reported. The overall study involves three distinct elements: (a) generation of an accurate ab initio potential energy surface for the ground electronic state of the Ca–F–H system, (b) careful fitting of the computed surface to an analytical form suitable for three-dimensional reactive scattering calculations, and (c) execution of classical trajectory calculations for Ca+HF collisions using the fitted potential surface. Ab initio potential energy calculations were performed for 175 Ca–F–H geometries using an MCSCF-CI method with a large Gaussian orbital basis set. The error in the computed endothermicity for the reaction of Ca and HF is less than 1 kcal/mol and the errors in the computed saddle point energies are believed to be less than 3 kcal/mol. The potential energy surface is dominated by a deep well corresponding to a stable linear H–Ca–F intermediate with an extremely small bending force constant. The calculations clearly demonstrate that the preferred geometry for Ca attack on HF is markedly noncollinear. The saddle point for both fluorine exchange reaction and insertion into the H–Ca–F well occurs for a Ca–F–H angle of 75° and has an energy of 16.1 kcal/mol relative to Ca+HF. The energy barrier for collinear reaction, 30.0 kcal/mol, is nearly twice as high. The analytical representation of the ab initio potential energy surface isbased on a polynomial expansion in the three diatomic bond lengths that reproduces the values of the computed energies to within a root mean square deviation of 1.2 kcal/mol and reduces to the appropriate diatomic potentials in the asymptotic limits. Classical trajectory calculations for Ca+HF(v=1) utilizing the fitted surface establish the fact that the H–Ca–F potential well dominates the collision dynamics thus qualifying Ca+HF as a bona fide example of a chemical insertion reaction. Because of the extensive sampling of the H–Ca–F well, many trajectories formed rather long-lived intermediate complexes before reaching diatomic end products. A significant number of these trajectories were not converged with respect to changes in the integration time step. Despite uncertainties associated with the ultimate fates of the nonconverged trajectories, the results obtained support a number of generalizations relating to microscopic features of Ca+HF collisions. Among these are: (1) at fixed total collision energy, excitation of HF to v=1 is much more effective in promoting reaction than is placing the corresponding amount of energy in Ca,HF translation, (2) at fixed initial translational energy, reaction cross sections increase with increasing HF rotational quantum number J, (3) for trajectories which enter the H–Ca–F well, escape to form products is favored by increasing initial HF rotation and escape back to reactants is favored by increasing the initial relative translational energy, and (4) the CaF fractional product energy disposals are remarkably independent of initial collision conditions. These conclusions are compatible with the observation that significant intermode vibrational energy transfer does not occur in the H–Ca–F intermediate on the collision time scale (1–2 ps).

Notizen: The rotational and vibrational distributions of H2 and D2 recombinatively desorbing from clean Cu(110) and Cu(111) surfaces following atomic permeation are studied using multiphoton ionization combined with time-of-flight mass spectrometry. Rotational distributions are found to be non-Boltzmann and to possess mean rotational energies which are 80%–90% of the surface temperature, Ts. These distributions are identical to within the experimental accuracy for H2 and D2 and also for desorption from the (110) and (111) faces. Moreover, the ortho and para nuclear spin modifications of both isotopes are statistically populated. In contrast, the vibrational population ratio, Pv‘=1/Pv‘=0, is found to be as much as 100 times greater than the ratio corresponding to a Boltzmann vibrational population at Ts. Specifically, the Pv‘=1/Pv‘=0 ratio for H2 (D2) is 0.052±0.014 (0.24±0.20) desorbing from Cu(110), and 0.084±0.030 (0.35±0.20) desorbing from Cu(111). For comparison the Boltzmann-at-Ts ratios would be 0.0009 for H2 and 0.0063 for D2 at T=850 K. Simple models are discussed which attempt to account for the qualitative trends of these results. Detailed balance arguments applied to the vibrational distributions measured in recombinative desorption are unable to predict correctly the dissociative adsorption probability as a function of vibration, indicating that these two processes are dynamically different for this system.

Notizen: We report an experimental investigation of the reaction O(3P)+HCl(v=2,J)→OH(v′=0 and 1,N′)+Cl(2P), where HCl is prepared in a single rovibronic level using a tunable optical parametric oscillator and the rovibronic levels of the OH reaction product are monitored using laser-induced fluorescence. The nascent rotational state distributions in OH(v′=0 and 1) have been measured as a function of the initial rotational state J of HCl(v=2). These distributions are substantially rotationally excited and exhibit a shift towards higher N′ as J is increased. Comparable branching into OH(v′=0) and OH(v′=1) is observed. The reaction cross section is found to increase markedly with reagent rotation: the increase is approximately a factor of 3 over the range J=1–10 of HCl(v=2), but is not necessarily monotonic for low J.

Notizen: Optical-microwave double resonance measurements were carried out to find the hyperfine structure constants of the v=0 level of the BaI X 2Σ+ state. These were combined with sub-Doppler optical measurements of the BaI C 2Π–X 2Σ+(0,0) band in order to derive the hyperfine structure constants of the excited state. We have determined the following molecular constants (in MHz) where the numbers in parentheses represent one standard deviation in a least squares fit: for the BaI X 2Σ+ state, γ‘=75.8501(33), b‘=93.117(19), c‘=52.170(54), and eQq‘=−33.62(12), and for the BaI C 2Π state, a'=263(53), b'+c'=−430(212), d'=−66.7(1.4), and eQq'=−214(11). The Fermi contact interaction and the electric quadrupole coupling constants for both the BaI X and C states appear to arise from the distortion of closed-shell I− orbitals by the field of the Ba+ ion. In the BaI X state, the charge distribution on the Ba+ center is directed away from I− while in the C state toward I−.

Notizen: Under single-collision beam–gas scattering conditions the Ba+CF3I→Bal+CF3 reaction has been studied using laser induced fluorescence to detect the BaI X 2∑+ product. The resulting BaI C 2Π–X 2∑+ excitation spectrum has a complex appearance owing to the similarity of the upper and lower state rotational constants, causing a reversal in band shading, and owing to a predissociation in the upper state, causing a break off in the fluorescence of high vibrational levels in each spin-orbit subband. From the predissociation onset an upper bound of 78.5±0.5 kcal/mol is placed on the BaI bond energy. The vibrational population distribution is shown to be bell shaped, peaking near v‘=50, and accounts for most of the available energy. The average energy appearing in BaI rotation decreases with increasing BaI vibrational excitation. The BaI rotational distribution has a width comparable to that of the vibrational distribution.

Notizen: A bivariate polynomial representation of rovibrational population distributions is developed. This representation permits direct reduction of diatomic fluorescence spectra from chemical dynamics experiments to estimates of rotational and vibrational parameters by means of a linear least squares procedure.

Notizen: The near IR and visible vibrational absorption spectra of CHF3 were recorded up to wave numbers of 17 500 cm−1 providing complete frequency coverage, together with paper I, from the low frequency fundamentals to the N=6 CH stretching–bending overtone multiplet. All strong bands in the high overtone spectra could be predicted and assigned by means of the tridiagonal Fermi resonance Hamiltonian, including a few combinations with intense CF3 stretching vibrations already observed for the low overtones. Improved vibrational Fermi resonance constants are presented on the basis of a fit to 35 assigned bands. An analysis of the rotational fine structure of the 2ν4 (E) overtone component and several Fermi resonance component bands result in values for αb and αs, which allow us to determine Be. In the high overtone bands no rotational fine structure is observed. The bands can be understood by introducing additional homogeneous rovibrational structures of phenomenological widths Γ≈1 to 10 cm−1. The results are discussed in relation to the separation of time scales for mode selective vibrational redistribution and further evolution. The overtone band strengths are reported and analyzed approximately with the empirical local Mecke dipole function.

Notizen: A theory is presented to reduce 1+1 resonance enhanced multiphoton ionization (REMPI) spectra to accurate rovibrational state population distributions. Classical and quantum mechanical treatments are developed to model the polarization dependence of the REMPI signal from an initially aligned ground state having cylindrical symmetry. The theory includes the effects of saturation and intermediate state alignment. It is demonstrated that, for favorable cases, 1+1 REMPI allows the determination of the relative population as well as the quadrupole and hexadecapole moments of the alignment for rovibrational levels of a linear molecule. The classical treatment differs from that of the quantum treatment by less than 5% for rotational quantum numbers greater than J=4, suggesting that the classical treatment suffices for 1+1 REMPI in most molecular systems.

Notizen: A two-step methodology is presented for extracting ground state population distributions and alignment factors from 1+1 resonance enhanced multiphoton ionization (REMPI) spectra. In the first step the ion signal is corrected for variation with laser intensity as it is collected, generating an isopower spectrum. In the second step populations and alignments are derived from the isopower spectrum by correcting for the interdependent effects of saturation and intermediate state alignment. This procedure is applied to a room temperature thermal distribution of nitric oxide using the 1+1 REMPI process in which lines of the NO A 2Σ+–X 2Π (0,0) band constitute the resonant transition. The present treatment is able to recover the known rovibrational population distribution, independent of branch choice, over a wide range of practical operating conditions.