Vibrational-rotational energy transfer in H-2-H-2 collisions. I. Semiclassical decoupling approximation

A new semiclassical decoupling procedure for rotational projection states i n rovibrationally inelastic atom-diatom and diatom-diatom collisions is dev eloped. Computed vibrational self-relaxation rate constants for para-H-2 an d ortho-H-2 are in good quantitative agreement (within a factor of 1.5, exc ept for the lowest temperatures) with experimental data over the investigat ed temperature range 50-2000 K. This allows us to hope that also more detai led (nonmeasured) rate constants for rovibrational state-to-state transitio ns in molecular hydrogen, calculated by our new model, are sufficiently acc urate for astrophysical applications. (C) 1999 American Institute of Physic s. [S0021-9606(99)02330-2].