l-dominant study of rotationally inelastic Li+–H2 collisions

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Abstract
For rotationally inelastic collisions at large total angular momentum J, the orbital angular momentum l exerts considerable dynamical importance. The inelastic T matrix is dominated by transitions between states characterized by small centrifugal barriers. Recognizing this fact, we have recently developed an ’’l‐dominant’’ (LD) approximation to the space‐fixed quantum‐mechanical close‐coupled (CC) equations, which consists in neglecting all channels with l≳J. In the present work we have applied the LD method to the determination of partial and total cross sections for the j=0→2,4,6 processes in Li+–H2 collisions at an energy of 0.5 eV. For the 0→2 and 0→4 transitions the LD results agree well with exact CC values, particularly at high J where the l dominance is most pronounced. By contrast, agreement is less satisfactory for the weak 0→6 transition, where high J values do not contribute significantly to the total cross section. Compared with the results of coupled‐states (CS) calculations, the LD partial cross sections are considerably more accurate. In terms of computational expense, the LD method lies midway between CS and CC calculations.