Laser induced amplified spontaneous emission from the |$f\;0_g^ +$|$fg+$ (³P) ion-pair state of I₂

Laser induced amplified spontaneous emission (ASE) from the |$f\;0_g^ +$|$f0g+$ (³P₀) (v_f = 1-7) ion-pair state of I₂ was directly observed using an optical-optical double resonance technique with the B |$0_u^ +$|$0u+$ (v_B = 21) valence state as the intermediate state. The emission detected at ∼1660 nm was assigned to transitions from the |$f\;0_g^ +$|$f0g+$ state to the D |$0_u^ +$|$0u+$ (³P₂) ion-pair state. The transitions observed in the dispersed IR emission spectra were found to be between vibrational levels having the same vibrational quantum numbers in both electronic states, v_f = v_D. This is due to the almost parallel nature of the potential energy functions of the |$f\;0_g^ +$|$f0g+$ and D |$0_u^ +$|$0u+$ states, leading to almost unit values for the Franck-Condon factors for v_f = v_D. That the observed infrared emission is due to ASE is shown by the facts that it propagated in a limited range of solid angles, exhibited a clear threshold against the input-laser power, and had different polarization to that of laser induced fluorescence.