AN ANALYSIS OF THE F'OU-PAIR STATE OF I-2 BY A SEQUENTIAL 3-PHOTON EXCITATION( ION)

A three-photon absorption technique that utilizes a visible B3PI(O(u))-X1SIGMA(g)+ transition followed by two-photon absorption was applied to characterize the high-lying vibrational levels of the F'O(u)+ ion- pair state of I2, which correlates with I-(1S) + I+(1D) at the dissoci ation limit. The multiphoton spectrum was examined at high resolution using either a single tunable dye laser (3P1C), or two independently t unable pump and probe dye lasers (3P2C), operating in the blue-green r egion. The one-color spectrum excited by a single laser shows transiti ons that have not been assigned by previous workers; these rotational lines are interpreted as being due partly to 3P1C transitions to the F 'O(u)+ ion-pair state, for which molecular constants are derived to be (for upsilon' = 60-74, J' = 3- 100): Y00 = 58 150.76(1) Y10 = 92.98(1 1) Y20 = -1.305(37) X 10(-2) Y01 = 0.017 657 8(25) Y11 = -9.098(05) x 10(-5) Y02 = -2.06 X 10(-9) (all in cm-1 with one standard deviation i n the last quoted digit in parentheses).