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We present a practical semiclassical method for computing the electron spin dynamics of a radical in which the electron spin is hyperfine coupled to a large number of nuclear spins. This can be used to calculate the singlet and triplet survival probabilities and quantum yields of radical recombination reactions in the presence of magnetic fields. Our method differs from the early semiclassical theory of Schulten and Wolynes [J. Chem. Phys. 68, 3292 (1978)] in allowing each individual nuclear spin to precess around the electron spin, rather than assuming that the hyperfine coupling-weighted sum of nuclear spin vectors is fixed in space. The downside of removing this assumption is that one can no longer obtain a simple closed-form expression for the electron spin correlation tensor: our method requires a numerical calculation. However, the computational effort increases only linearly with the number of nuclear spins, rather than exponentially as in an exact quantum mechanical calculation. The method is therefore applicable to arbitrarily large radicals. Moreover, it approaches quantitative agreement with quantum mechanics as the number of nuclear spins increases and the environment of the electron spin becomes more complex, owing to the rapid quantum decoherence in complex systems. Unlike the Schulten-Wolynes theory, the present semiclassical theory predicts the correct long-time behaviour of the electron spin correlation tensor, and it therefore correctly captures the low magnetic field effect in the singlet yield of a radical recombination reaction with a slow recombination rate. [ABSTRACT FROM AUTHOR]

A new method for solving the close coupled equations of inelastic scattering is presented. The method is based on Johnson’s log derivative algorithm, and uses the same quadrature for the solution of the corresponding integral equations. However it differs from the original method in the use of a piecewise constant diagonal reference potential. This results in a reduction in matrix operations at subsequent energies, and an improved convergence of the solution with respect to the number of grid points. These advantages are clearly demonstrated when our method is applied to an atom–diatom rotational excitation problem. [ABSTRACT FROM AUTHOR]

The water molecule, rotationally state selected in the third and fourth OH stretching overtone (|04>-, |05>-) and stretch–bend combination (|04-2>) levels, has been photodissociated via the A state at λ[bar_over_tilde:_approx._equal_to]282 nm. The photofragment rotational state distributions, determined by OH(A–X) laser induced fluorescence (LIF), differ from those reported previously by Andresen and co-workers, in which water was initially prepared in the |01>- level and photodissociated at 193 nm, and from those by Crim and co-workers, in which H2O was photodissociated via the |04>- level at wavelengths shorter than 282 nm. These differences become more pronounced with increasing angular momentum in the parent water molecule and with an increasing number of quanta in the intermediate OH stretching overtone state. The Franck–Condon theory of Balint-Kurti, previously employed successfully to account for the product state distributions arising from the 193 nm photodissociation of H2O|01>-, qualitatively reproduces the trends observed in the present study if it is assumed that dissociation occurs preferentially from extended RH-OH configurations of the |04>- and |05>- overtones. The product OH state distributions are thus shown to be sensitive indicators of the bending and rotational motions of H2O(X) in the wide amplitude stretching region of the ground state surface. [ABSTRACT FROM AUTHOR]

Close-coupling scattering calculations have been carried out to predict the magnitude of electric fields required to dissociate specific vibration-rotation levels in H2+. Results for electric fields up to 40 kV cm-1, which may dissociate levels up to 25 cm-1 below the dissociation limit, are presented. The results are used to simulate spectra which are compared with experimental spectra. A method for extracting the eigenphase sum from a coupled channel scattering calculation is described in the appendix and used in the calculations. [ABSTRACT FROM AUTHOR]

A statistical model based on the quasiclassical trajectory method is presented in this work for atom-diatom insertion reactions. The basic difference between this and the corresponding statistical quantum model (SQM) lies in the fact that trajectories instead of wave functions are propagated in the entrance and exit channels. Other than this the two formulations are entirely similar. In particular, it is shown that conservation of parity can be taken into account in a natural and precise way in the statistical quasiclassical trajectory (SQCT) model. Additionally, the SQCT model complies with the principle of detailed balance and overcomes the problem of the zero point energy in the products. As a test, the model is applied to the H3+ and H+D2 exchange reactions. The excellent agreement between the SQCT and SQM results, especially in the case of the differential cross sections, indicates that the effect of tunneling through the centrifugal barrier is negligible. The effect of ignoring quantum mechanical parity conservation is also investigated. [ABSTRACT FROM AUTHOR]