An analysis of the ion-pair state of I2 by a sequential three-photon excitation

Abstract

A three-photon absorption technique that utilizes a visible [Formula: see text] transition followed by two-photon absorption was applied to characterize the high-lying vibrational levels of the [Formula: see text] ion-pair state of I2, which correlates with I−(1S) + I+(1D) at the dissociation limit. The multiphoton spectrum was examined at high resolution using either a single tunable dye laser (3P1C), or two independently tunable pump and probe dye lasers (3P2C), operating in the blue–green region. The one-color spectrum excited by a single laser shows transitions that have not been assigned by previous workers; these rotational lines are interpreted as being due partly to 3P1C transitions to the [Formula: see text] ion-pair state, for which molecular constants are derived to be (for ν′ = 60–74, J′ = 3–100):[Formula: see text](all in cm−1 with one standard deviation in the last quoted digit in parentheses).