State-resolved collisional energy transfer in highly excited NO2. I. Crosssections and propensities for J, K, and m(J) changing collisions

State-resolved experiments probing the dynamics in NO2#- NO2 collisions at high internal energies (17 500<E<18 000 cm(-1)) are reported. A sequential optical double resonance technique with sensitive laser-induced fluorescenc e detection has been employed for the assignment of states of NO2 in the en ergy range between 17 500 and 18 000 cm(-1), a spectral region where the op tically "bright'' B-2(2) state is strongly coupled to high lying ("dark'') states of the (2)A(1) ground state and other electronic states. Subsequentl y, the decay of population and polarization following rotationally inelasti c and elastic collisions has been probed directly using a time- and polariz ation-resolved optical double resonance technique. Total depopulation rates have been determined to be about 2-3 times above the Lennard-Jones estimat e. The thermally averaged state-to-state cross sections have been derived f rom a master equation analysis of the kinetic traces. The rate constants ha ve been scaled by angular momentum scaling expressions based upon the infin ite order sudden approximation which were modified to account for dynamical restrictions on angular momentum and polarization transfer. Pure rotationa l energy transfer within a vibrational state turned out to be fast and domi nating the collision dynamics, whereas rovibrational energy transfer was sl ower and proceeded with a lower efficiency. In addition, interesting propen sity pattern for angular momentum and polarization transfer have been found . The individual state-to-state rate constants clearly indicated that rotat ional energy transfer in highly excited mixed (chaotic) states is still gov erned by pronounced propensities in J, K, and m(J) changing collisions. Her e m(j) is the projection of J on a space fixed axis, which is defined by th e laser, and K is the projection of J on the body-fixed symmetry axis of th e molecule. In particular, we have found a propensity for small changes of m(J) in elastic and inelastic collisions, in accord with recently suggested theoretical models. Interestingly, we also found a considerably lower prob ability for Delta K changes in these collisions. The propensities found for Delta m(J) and Delta K are discussed within the framework of dynamic (kine matic) collision models. The observed cross sections, their overall scaling behavior, as well as estimations of the Massey parameter are consistent wi th collisions following mostly a direct mechanism for rotational energy tra nsfer rather than a complex forming mechanism. (C) 1999 American Institute of Physics. [S0021-9606(99)00302-5].