Mechanism of vibrational relaxation and intersystem crossing within excited ion-pair states of I₂

Visible and ultraviolet fluorescence of I₂, following excitation by ArF/193nm excimer laser pulses, was recorded for different pressures of argon buffer gas in a flow system. Dispersed fluorescence spectra due to the transitions D'(2_g) → A'(2_y and D(0 _n ⁺ )→X0 _g ⁺ ) were analysed by inversion and spectral simulations. Thus vibrational distributions in the emitting states were obtained as a function of pressure to determine the mechanism of relaxation to populate the lowest quantum levels of the D' state, which are the emitting states in the iodine laser. Fast intersystem crossing is found to occur from initially populated vibrational levels of the D state to other ion-pair states correlating with the ground state ions, followed by rapid relaxation, involving both direct vibrational relaxation within individual states and intersystem crossing between states.