Direct evidences of scattering resonances enhancement in the vibrational relaxation of I2(B3Î 0u+,Ï â€²=21) by collisions with He at very low temperatures

The vibrational deactivation of I2(B, ν′ = 21) by He in the low energy regime, was studied under single collision conditions, in a supersonic expansion. The time resolved fluorescence signal (TRFS) from the (B; ν′ = 21)→ (X; ν″ = 1) transition was collected at several distances from the nozzle to access different local temperatures in the range (0.17-6) K. From the analysis of the TRFS the value of the relaxation cross-section was extracted and it showed to decrease with energy through a series of peaks. This experimental observation, confirms the role of scattering (mainly orbiting) resonances as theoretically postulated previously.

The Figure shows the dependence of the vibrational relaxation cross-section of I2(B, ν′ = 21) by He on the collision energy in the range (0.5-6.0) cm−1. Several scattering resonances appear in the lowest energy regime.