Collision induced state-to-state energy transfer dynamics between the 2u (1D2) and 2g (1D2) ion-pair states of I2.

We report the first observation of collision induced state-to-state energy transfer from the 2_u (¹D₂) (v_2u = 3–7) ion-pair state of I₂ using a perturbation facilitated optical–optical double resonance technique through the c ¹Π_g∼ B ³Π(0+u) hyperfine mixed double-faced valence state as the intermediate state. The excitation of the 2_u (¹D₂) state yielded the weak UV fluorescence from the wide range of vibrational levels in the nearby 2_g (¹D₂) state. The vibrational distribution in the 2_g (¹D₂) state derived by the Franck–Condon simulation of the UV fluorescence showed that the population in the 2_u (¹D₂) state transfers mostly to the 2_g (¹D₂) vibronic levels which are located energetically above the laser-prepared level. The radiative lifetimes and the self-quenching rate constants were determined to be 21.3 ± 0.1 and 44.6 ± 0.8 ns, and (1.30 ± 0.01) × 10⁻⁹ and (2.26 ± 0.17) × 10⁻⁹ cm³ molecule⁻¹ s⁻¹ for the 2_u (¹D₂) (v_2u = 3) and 2_g (¹D₂) (v_2g = 5) states, respectively. The rate constant for the 2_u (¹D₂) – 2_g (¹D₂) collision induced state-to-state energy transfer was also evaluated to be (1.89 ± 0.01), (3.07 ± 0.07), and (3.77 ± 0.05) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ for the v_2u = 3, 5, and 7 levels, respectively. The very large self-quenching cross sections for the ion-pair states of I₂ could be explained by the harpoon mechanism. [ABSTRACT FROM AUTHOR]