First observation and analysis of the ion-pair state of Br2 by optical–optical double resonance

Abstract

The technique of optical–optical double resonance has been applied to a study of the [Formula: see text] ion-pair state of Br2. Two pulsed dye lasers excite the molecules from the ground state to the [Formula: see text] ion-pair state stepwise through the [Formula: see text] state in the pump-probe scheme: [Formula: see text]. The [Formula: see text] emission identified at around 215 nm is used to detect the double resonance signals when the probe laser frequency (ν2) is scanned by fixing the pump laser frequency (v1) to appropriate [Formula: see text] transitions. The emission spectra are resolved to establish the absolute vibrational numbering of the ion-pair state in terms of the Franck–Condon calculations. The Dunham coefficients of the [Formula: see text] state obtained in a global least-squares fit analysis of 377 transitions [Formula: see text] are Y00 = 57929.724(24), Y10 = 212.227(23), Y20 = −1.5683(56), Y30 = 1.606(40) × 10−2, Y01 = 0.0465652(89), Y11 = −4.500(36) × 10−4, Y21 = 1.133(41) × 10−5, and Y02 = −9.37(94) × 10−9 for 79Br2 isotope species (all in cm−1 with 3σ in parentheses).