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Laser-induced fluorescence detection is used in studying the electronic ground-state products formed in the reaction of electronically excited Sr((super3)P(sub 1)) with HF. The research uses a beam-gas setup in which a beam of Sr atoms is excited using a frequency-stabilized continuous wave titanium-sapphire laser. Results show the SrF internal energy distribution is dependent on the impact parameter while the light H atom is ejected with high translational energy.

A beam of rotating dipolar particles (molecules or clusters) will broaden when passed through an electric or magnetic field gradient region. This broadening, which is a common experimental observable, can be expressed in terms of the variance of the distribution of the resulting polarization orientation (the direction cosine). Here, the broadening for symmetric-top and linear rotors is discussed. These two types of rotors have qualitatively different low-field orientation distribution functions, but behave similarly in a strong field. While analytical expressions for the polarization variance can be derived from first-order perturbation theory, for experimental guidance it is important to identify the applicability and limitations of these expressions, and the general dependence of the broadening on the experimental parameters. For this purpose, the analytical results are compared with the full diagonalization of the rotational Stark-effect matrices. Conveniently for experimental estimations, it is found that for symmetric tops, the dependence of the broadening parameter on the rotational constant, the axial ratio, and the field strength remains similar to the analytical expression even outside of the perturbative regime. Also, it is observed that the shape envelope, the centroid, and the width of the orientation distribution function for a symmetric top are quite insensitive to the value of its rotational constant (except at low rotational temperatures). [ABSTRACT FROM AUTHOR]

The induced polarization of a beam of polar clusters or molecules passing through an electric or magnetic field region differs from the textbook Langevin–Debye susceptibility. This distinction, which is important for the interpretation of deflection and focusing experiments, arises because instead of acquiring thermal equilibrium in the field region, the beam ensemble typically enters the field adiabatically, i.e., with a previously fixed distribution of rotational states. We discuss the orientation of rigid symmetric top systems with a body-fixed electric or magnetic dipole moment. The analytical expression for their “adiabatic-entry” orientation is elucidated and compared with exact numerical results for a range of parameters. The differences between the polarization of thermodynamic and “adiabatic-entry” ensembles of prolate and oblate tops, and of symmetric top and linear rotators, are illustrated and identified. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence spectra reveal the internal energy distributions of SrF(X²Σ) formed in the reactions of electronically excited Sr(³P[sub 1]) with various unsaturated fluorohydrocarbons, i.e., CHF=CH[sub 2], CF[sub 2]=CH[sub 2], CHF=CHF, and C[sub 6]H[sub 5]F. The internal energy distribution of the ground state diatomic product typically shows less vibrational excitation, without inversion, and somewhat lower rotational excitation than the reactions of Sr(³P[sub 1]) with HF and saturated hydrocarbons. The different behavior of the two groups of reactants is rationalized by a simple MO picture, assuming that an electron from Sr is transferred to a σ[sup *] orbital in HF and the saturated fluorohydrocarbons and to a π[sup *] orbital in the unsaturated fluorohydrocarbons with a subsequent transfer to a σ[sup *] orbital of the C-F bond. The latter transfer constitutes an extension of the reaction path, leading to less vibrational excitation. This would explain why the energy disposal in the reaction with C[sub 6]H[sub 5]F behaves similar to that in the reactions with the fluoroethenes. Even if the shape of the vibrational distribution of the SrF product is the same for all unsaturated fluorohydrocarbons studied, the degree of vibrational excitation varies strongly. This even holds when comparing cis- and transCHF=CHF, where the distributions can be characterized by distinct surprisal parameters. [ABSTRACT FROM AUTHOR]

Examines high-resolution laser spectroscopy and transitions of K-=O stacks. Descriptions of the experimental setup and conditions; Outlines the theory of vibronic interactions, fine and hyperfine structure of rotational transitions; Discussion on the bolometric and fluorescence measurements.

Reports on the laser-induced fluorescence studies of excited strontium reactions. Reactive scattering experiments; Frequency-stabilized Titanium-Sapphire laser system; Vibrational and rotational population of the strontium products.

The energies and eigenfunctions of hyperfine levels belonging to the J=|K|=1 rotational manifold of the methylhalides have been calculated as a function of the strength of an applied electric (dc) field. From the eigenfunctions, the molecular orientational distribution function P(cos [variant_greek_theta]), where [variant_greek_theta] is the angle of the principal axis relative to the electric field, has been obtained. In the case of interest (J=1), the Legendre expansion of P(ρ), with ρ≡cos [variant_greek_theta], can be characterized by two moments only: the orientation and the alignment. The calculated moments have been compared with data available from different molecular beam experiments, without recourse to a specific collision model. For the passage of the molecules through different electric fields, the adiabatic approximation has been assumed to apply, except for curve crossings with extremely small gaps. For methylfluoride and methylchloride, the calculated orientations are in agreement with the measured steric effects of the reactive collisions with Ca atoms, down to a field strength of a few volts per centimeter.At lower field strengths the orientation falls off more rapidly than predicted by the theory. This behavior is tentatively attributed to the variation in the direction of the electric field, and hence in the axis of quantization, across the reaction volume, at low field strengths. This may, in addition, cause a breakdown of the adiabatic approximation. In the case of methyliodide, quantitative agreement between the theoretical and the experimental asymmetry parameters of photodissociation is obtained if the latter ones are provisionally corrected for systematic errors. Contrary to earlier preliminary conclusions, the results show that within experimental error the adiabatic approximation is applicable. Up until now, no experimental data has been available for methylbromide. The calculations... [ABSTRACT FROM AUTHOR]

Deuterium spin-lattice relaxation rates in toluene-d8 and p-xylene-d10, dissolved in nematic Phase V, have been measured at five different field strengths. The frequency-dependent relaxation behavior can be satisfactorily described by a model in which the molecules undergo small step rotational diffusion in an axially symmetric orienting potential, with contributions from slow director fluctuations. The parameters defining the model are fitted to the relaxation rates of para and methyl deuterons, using a least-squares parameter estimation program. Parameter values obtained are consistent with a physically acceptable set of values of the associated hydrodynamic properties of the solutions, and are in agreement with results obtained previously on 14N relaxation in methylisocyanide. Relaxation rates of ortho and meta deuterons in toluene, and of the ring deuterons in p-xylene lead to a J1/J2 ratio smaller than unity. Moreover, both J1 and J2 are weakly frequency dependent. The former effect may turn up as a consequence of the assumption of axially symmetric diffusion; the frequency dependence of J2 is probably due to second order effects of the director fluctuations. [ABSTRACT FROM AUTHOR]

The dynamical behavior of methylisocyanide, CH3NC, dissolved in two different nematic liquid crystals, has been studied for the first time by direct measurement of the spin-lattice relaxation of its 14N nucleus, at three different field strengths. The results show a frequency dependent relaxation behavior, that can be described by director fluctuations in the nematic solvents. The parameters of the relaxation model: the reorientational correlation time τ⊥, and the amplitude factor A and cut-off frequency ωc of the director fluctuation contributions, were estimated in dependence of the renormalization of the probe order parameter Sz’z. It is shown that the influence of the mean square of fluctuations 〈θ2〉 on the relaxation model can be far from negligible. Choosing literature values for the average elastic constant K and effective viscosity η in one solution (phase V), a reasonable value for the translational diffusion constant Dtr is found. The corresponding cut-off wavelength is L∼50 Å. For the other solution (ZLI 1167), using the same L and an estimated viscosity η, reasonable values for K and Dtr are obtained. Physical arguments are used to account for the differences between the two solutions and a comparison is made with previous studies of analogous systems. It is emphasized that the value of the director fluctuation model could be enhanced by carrying out more experiments to improve the accuracy of ωc, and by refining the theoretical description of the cut-off behavior. [ABSTRACT FROM AUTHOR]