Centrifugal sudden approximation

The original semiclassical version of the centrifugal sudden approximation (SCS) developed by Strekalov [198, 199] consistently takes into account adiabatic corrections to IOS. Since the orbital angular momentum transfer is supposed to be small, scattering occurs in the collision plane. The body-fixed correspondence principle method (BFCP) [200] was used to write the S-matrix for f — jf Massey parameter a>xc. At low quantum numbers, when 0)zc —> 0, it reduces to the usual non-adiabatic expression, which is valid for any Though more complicated, this method is the necessary extension of the previous one adapted to account for adiabatic corrections at higher excitation... 

At low temperature the classical approximation fails, but a quantum generalization of the long-range-force-law collision theories has been provided by Clary (1984,1985,1990). His capture-rate approximation (called adiabatic capture centrifugal sudden approximation or ACCSA) is closely related to the statistical adiabatic channel model of Quack and Troe (1975). Both theories calculate the capture rate from vibrationally and rotationally adiabatic potentials, but these are obtained by interpolation in the earlier work (Quack and Troe 1975) and by quantum mechanical sudden approximations in the later work (Clary 1984, 1985). 

Figure 6. Temperature dependence of the reaction efficiency per collision for the reactions of OD + CH3Cl (open circles), 0D D20 + CH3Cl (filled circles and 0D (D20)2 + CH3Cl (half-filled circles). The reaction efficiency per collision is the experimental rate constant divided by the calculated collision rate constant, calculated by Clary using the adiabatic capture centrifugal sudden approximation (ACCSA) (28). For experimental reasons (29), the measurements were made with completely deuterated...

In the centrifugal sudden approximation (CSA), which is also known as the coupled states approximation, the operator j j is replaced by a value such as [j +j - 2 J j ] such that there is no coupling between different" 2 states 07 4,25]. The S matrix elements are then of the form... 

Centrifugal-Sudden Approximation CSA Neglect Q coupling n X j for each... 

Key words Centrifugal sudden approximation - Infinite order sudden approximation - Sudden approximations -Inelastic collisions - Rotationally inelastic collisions... 

Neglect of these particular operators removes the coupling in basis functions with different Q. quantum numbers and drastically reduces the size of die basis set[7,25]. This is the centrifugal sudden approximation (CSA) and is often known as the coupled-states approximation, although that is a term we prefer not to use as it is easily confused with "close-coupling". The basis set expansion for a CSA calculation of vibrational-rotational excitation cross sections would thus be... 

CSA Centrifugal Sudden Approximation (Coupled States Approximation) V xjxk Fixed0... 

ABSTRACT. Calculation of the rate constant at several temperatures for the reaction +(2p) HCl X are presented. A quantum mechanical dynamical treatment of ion-dipole reactions which combines a rotationally adiabatic capture and centrifugal sudden approximation is used to obtain rotational state-selective cross sections and rate constants. Ah initio SCF (TZ2P) methods are employed to obtain the long- and short-range electronic potential energy surfaces. This study indicates the necessity to incorporate the multi-surface nature of open-shell systems. The spin-orbit interactions are treated within a semiquantitative model. Results fare better than previous calculations which used only classical electrostatic forces, and are in good agreement with CRESU and SIFT measurements at 27, 68, and 300 K. ... 

In the present study, we employ the ACCSA method which involves a combined rotationally adiabatic capture and centrifugal sudden approximation. The method enables the computation of cross-sections and rate constants which are state-selective in the initial rotational states of the reactant molecule. The ACCSA approach has been successful for ion-dipole reactions which involve proton transfer. Predicted rate constants for the reactions -f HCN H2CN + H2 and HCO + HCN - - CO are in... 

Employing the centrifugal sudden approximation (CSA ), we replace J j by the diagonal value 1) -h — 2ft ], which combined with the potential depending only... 

This is a very simple procedure that enables the reaction cross sections and rate coefficients to be determined simply by a small number of matrix dlagonalizations. Furthermore, by applying an approximation such as the centrifugal sudden approximation (CSA) in which the coupling in the projection quantum numbers of the rotational basis sets is neglected [11], the rotationally adiabatic potential curves for J > 0 are obtained from those calculated for J = 0 simply by adding the centrifugal term of equation (3). 

Zhang D, Lee S (1999) Fully converged integral cross sections of diatom-diatom reactions and the accuracy of the centrifugal sudden approximation in the H2 -F OH reaction. J Chem Phys 110 4435... 

Zhang D, Yang M, Lee S (2003) Accuracy of the centrifugal sudden approximation in the H + H2O reaction and accurate integral cross sections for the H + H2O —> H2 + OH abstraction reaction. J Chem Phys 117 10067... 

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