Laser-induced molecular predissociation by stimulated single-photon or multiphoton absorption or emission of infrared photons

Diatomic or polyatomic molecules in discrete molecular states imbedded in vibrational continuum(s) can be induced by available infrared laser beams to make significant transitions to the continuum(s), leading to predissociation of the molecules. These laser-induced predissociative transitions can be one of three kinds of photoprocesses studied: (i) stimulated single-photon absorption or emission of the laser photons, (ii) stimulated multiphoton absorption or emission of the laser photons, and (iii) stimulated transition without actual absorption or emission of the laser photons. The probability rates and the selection rules for these three processes are given and compared. Type I, II, and III of the above laser-induced molecular predissociation (LIMP) are defined and treated. Spectroscopic applications of these processes, such as deducing unstable electronic potential surfaces, are suggested. One such application is the locations of the potential curves of the unstable $1g\left({}_{}{}^3{}_{}{}^{}\pi \right)$ and $0^{+}g\left({}_{}{}^3{}_{}{}^{}\pi \right)$ states of diatomic halogens (${\mathrm{I}}_2$ or ${\mathrm{Br}}_2$), since LIMP of the bound states in the metastable $B{0}^{+}u\left({}_{}{}^3{}_{}{}^{}\pi \right)$ into these unstable states by process (i) is dominant. As a new mechanism of unimolecular decompositions or reactions, LIMP can open up new channels or selectively enhance particular channels of molecular decomposition with laser-field-dependent rate "constants," leading to new dissociated products or higher product yields. It provides a means of decomposing homonuclear diatoms or polyatoms in infrared-inactive modes, that are already excited in the initial discrete state.