Quenching and excitation transfer for the c 3Pi - u and a 3Sigma + g states of H2 in collisions with H2

Collisional destruction rate coefficients for c ³Π^-_u metastable states by thermal energy hydrogen molecules have been measured for various vibrational and rotational levels (N=1, v=0,1,2,3 and v=1, N=1,2,3). The rate coefficients were found to be independent of vibrational and rotational quantum number and had a mean value of (1.88±0.10)×10^-15 m³ s^-1 at 300 K. Destruction rate coefficients and radiative lifetimes of the a ³Σ⁺_g state have also been measured. Collisional quenching rate coefficients at 300 K of (11.5±1.0) and (6.3±0.8)×10^-16 m³ s^-1 were obtained for the v=0 and 1 levels, respectively. Radiative lifetimes of 11.1±0.3 ns for v=0 and 10.4±0.3 ns for a a ³Σ⁺_g (v=1) are in good agreement with recent experiment and theory. Collisional excitation transfer between the N=1 levels of the a ³Σ⁺_g and c ³Π^-_u states was observed for v=0 and 1. No collisional excitation transfer between other a ³Σ⁺_g and c ³Π^-_u levels or between two c ³Π^-_u levels was detected. Measured fractional absorption signals and detailed balancing arguments were used to determine the excitation transfer rate coefficient for the v=1, N=1 level of the a ³Σ⁺_g state to the v=1, N=1 level of the c ³Π^-_u state as (3±2)×10^-16 m³ s^-1 and that for the reverse process as (1.8±1)×10^-16 m³ s^-1 for a calculated gas temperature of 410 K. Excitation transfer to the c ³Π_u state and subsequent rapid quenching accounts for a large fraction, if not all, of the collisional quenching of the a ³Σ⁺_g (v=1, N=1) state.