The `approach-inducedʹ I2(E0+g ↔ I2(X) D0+u) transition Original Research Article

The dependences of the I2(E, vE ↔I2(X) D, vD) collision-induced transition rate constant on the vibrational vE, vD, rotational JD numbers, energy gaps, and Franck–Condon factors (FCF) of the combined levels have been studied. Selected rovibrational levels of the E state have been produced by optical–optical double resonance (OODR) excitation via the B0u+ state. Rovibronic levels of the D state populated in collision-induced intramolecular transitions (CIIT) in the nearly single-collision conditions have been identified. Rate constants of CIIT derived from the E→B and D→X emission spectra simulation have been determined. It has been shown that the u↔g, ΔΩ=0 and ΔJ≤3 propensity rules are valid in the I2(E)+I2(X) → I2(ion-pair states)+I2(X) process. Its cross-sections are giant ((1.1–3.3)×103 Å2), and there is distinct correlation between them and energy gaps ΔE=f(Δv, ΔJ) of the combined rovibronic levels. The transitions between isoenergetic rovibronic levels are the most probable. There is no correlation between the cross-sections of transitions and FCF of the levels. An applicability of some models of CIIT to the processes under study has been discussed. It has been shown that they cannot explain the observed experimental data. It would be better to call the processes similar to the observed one as `approach-inducedʹ to emphasize their extremely long-range character.