photoassociation

Photoassociation is a process in which two colliding atoms absorb a photon to form an excited diatomic molecule. In ultracold atomic gases, the kinetic energy is very small, and a laser tuned near a molecular resonance couples the scattering state of two atoms to a bound electronic excited state. The resulting excited molecule may decay radiatively to bound rovibrational levels of the ground electronic state, yielding ultracold molecules, or may dissociate. The rate and final outcomes depend on the Franck-Condon factors, long-range potentials, and transition dipole moments.

Two main variants exist: one-photon photoassociation, where a single photon couples to an excited state; and two-photon photoassociation or stimulated Raman adiabatic passage (STIRAP), which transfers population from the colliding pair to a selected ground-state bound level via an intermediate excited state with coherent control.

Photoassociation spectroscopy uses laser detuning to map excited-state potentials and scattering properties; the linewidths and resonance positions provide information on interatomic interactions and molecular structure. Historically, PA experiments began in the 1990s with alkali atoms and have since enabled production of ultracold molecules in a variety of species.

Applications include formation of ultracold heteronuclear and homonuclear molecules, studies of chemical reactions at ultralow temperature, precision measurement, quantum simulation, and investigations of molecular potentials. Experimental signatures include trap loss, emission, or ionization signals; optical lattices provide control of PA rates and state selectivity.