a Quantum-Mechanical Study of Atom-Diatom Collisions in a Laser Field.

A quantum-mechanical formalism, in both space -fixed (SF) and body-fixed (BF) coordinate systems, is developed for describing an S-state structureless atom (A) colliding with a Sigma-state vibrating rotor diatomic molecule (BC) in the presence of a laser field. The additional Hamiltonians H _{rad} and H_{int }, which describe the laser field and its interaction with the atom-diatom collision system, have been added to the field-free Hamiltonian H_0. And the collision problem can be solved by this extended Hamiltonian. The laser field Hamiltonian is represented by the number state representation. The interaction Hamiltonian is expressed by -vecmu_{BC} cdot vec{cal E}, where vecmu_ {BC} is the dipole moment of the diatomic molecule BC, and vec{cal E} is the electric field strength of the laser field. Since the field-free total angular momentum J is coupling with the laser field, J and its z-axis projection M are no longer conserved. To facilitate the collision problem, the laser field is restricted to a single mode, and its interaction with the collision only involves dipole allowed transitions in which a single photon is absorbed or emitted. For convenience, the coupled -channel equations are solved by the real boundary conditions instead of the complex boundary conditions. On applying the real boundary conditions, we obtain the K-matrix, which is related to the S-matrix by S = (I + iK)(I - iK)^{ -1}. A model calculation is discussed for the Ar + CO collision system in a laser intensity of 10 ^9 W/cm^2.