Role of ion-pair states in the predissociation dynamics of Rydberg states of molecular iodine

Using femtosecond pump–probe ion imaging spectroscopy, we establish the key role of I⁺ + I⁻ ion-pair (IP) states in the predissociation dynamics of molecular iodine I₂ excited to Rydberg states. Two-photon excitation of Rydberg states lying above the lowest IP state dissociation threshold (1st tier) is found to be followed by direct parallel transitions into IP states of the 1st tier asymptotically correlating to a pair of I ions in their lowest states I⁺(³P₂) + I⁻(¹S₀), of the 2nd tier correlating to I⁺(³P₀) + I⁻(¹S₀), and of the 3rd tier correlating to I⁺(¹D₂) + I⁻(¹S₀). Predissociation via the 1st tier proceeds presumably with a delay of 1.6–1.7 ps which is close to the vibrational period in the 3rd tier state (3rd tier-mediated process). The 2nd tier IP state is concluded to be the main precursor for predissociation via lower lying Rydberg states proceeding with a characteristic time of 7–8 ps and giving rise to Rydberg atoms I(5s²5p⁴6s¹). The channel generating I(²P_3/2) + I(²P_1/2) atoms with total kinetic energy corresponding to one-photon excitation is found to proceed via a pump – dump mechanism with dramatic change of angular anisotropy of this channel as compared with earlier nanosecond experiments.