Field-induced avoided crossing and new transition channels in atomic and molecular gases irradiated by lasers

We show that the presence of an oscillating electromagnetic field can induce a new avoided crossing between true crossing electronic energy surfaces of molecules without actual absorption or emission of photons. This avoided-crossing energy gap and the field-dressed energies and states are functions of the field intensity, frequency, and polarization. Thus the elastic and inelastic probabilities across these energy regions can be controlled by tuning these field parameters. This mechanism can be used to enhance or suppress field-free transitions by orders of magnitude. More importantly, the field can open up new inelastic channels so that the field-induced transitions, free of any competing field-free transitions, can be readily detected. We apply these new effects to molecular dissociation and predissociation and to atomic (ionic) and molecular collisions. The field-induced probabilities of these processes are studied as functions of the (quasi) molecular electronic energies, relative velocity or vibrational energies, laser field frequency and intensity. It is shown that far-away nonresonant states can contribute significantly to the field-induced avoided crossing and transitions. We give approximate but accurate formulas which enable rapid calculation of all these effects. Our method of solution of the charge-field interaction includes the stationary perturbation theory in the weak-field limit. Our modified Landau-Zener formulas of transition probability include the time-dependent perturbation results, the adiabatic limit, and the field-free limit. Improvements over earlier methods of solving the adiabatic eigenvalue problem are given, especially for charge systems with definite "parities." It is shown that the radiative-induced predissociation of I₂ in the B 0⁺u(³Π) state into the repulsive 1u(¹Π) state can broaden its vibrational spectrum and quench its fluorescence. The same mechanism can be used to increase the enrichment of laser isotope separation of Br₂ involving the same states. Formulas f\'or field-induced predissociation rate per second without or with tunneling are given. In these cases, the small laser-induced coupling controls the large internal energy flow as fluorescence or as kinetic energy. In other situations such as dissociation and collisions, the laser field is also a means of switching on the exchange of electronic or vibronic energies internally and/or externally as kinetic energy. All these are achieved at no expense of photon energy since no actual photo-absorption or -emission is involved in these nonresonant processes.