A quantum dynamical description of an atomic collision pair interacting with the electric field of a short infrared laser pulse is developed. Inelastic processes in the electronic ground state are due to stimulated emission resulting in photoassociation, or absorption leading to photoacceleration. A perturbative approach based on a state space representation is compared with a numerical treatment using a grid representation in coordinate space. Special emphasis is on the role of rotations and, in particular, of shape resonances. It is shown that these quasibound states which are supported by the centrifugal barrier (for J ≠ 0) can be used as initial states to effectively populate a selected bound state with specific vibrational and rotational quantum number (photoassociation), or a partial wave of a scattering state with defined energy and rotational quantum number (photoacceleration). Simulations are carried out for the prototype H + Cl collision pair. Also the effect of averaging over initial conditions (velocity, angular momenta) is investigated for a supersonic beam experiment. For a narrow velocity distribution, we predict the presence of a resonance structure of the association and acceleration probability as a function of the mean collision energy.
A study is made of the influence of photon absorption (emission) processes on the dynamics of collisions between two atoms. Under certain conditions the radiation field can transfer colliding atoms to a bound molecular state. The main contribution to these processes is made by atoms colliding with relative velocities and impact parameters which ensure that these atoms spend a long time near each other (quasifinite motion). The appearance of quasifinite states in the absorption spectra of Rb and Cs vapors is reported and a determination is described of the matrix elements of the transitions from these states to a bound molecular state.
The spectrum for absorption and emission of radiation by colliding atoms or a diatomic molecule is calculated for spectral regions dominated by an avoided level crossing. Example processes are absorption during an atom-atom collision with separation to either crossing atomic state, total absorption to both crossing states, and spontaneous emission during a collision with either state initially populated. Absorption and emission by bound diatomic molecules (including photodissociation) is described by the theory, and as an example it is applied to the
Cs2 A-X band. Spectra due to single-velocity and impact-parameter collisions, as well as thermal and impact-parameter averages, are given. The shapes of the resulting spectra are parametrized by the interaction strength (or excitation transfer probability) between the crossing levels and the slopes of the ground- and excited-state potentials (assumed constant) in the region of the level crossing. The theory is applied to two observations of such regions that have been reported. We conclude that measurements of spectra in the region of level crossing is a very powerful diagnostic of the potentials and transfer probability in the level-crossing region, which is responsible for most inelastic atom-atom energy-transfer processes.