FOURIER-TRANSFORM SPECTROSCOPY OF I$_2$ ($D$-$X$) EMISSION FOLLOWING OPTICAL-OPTICAL DOUBLE RESONANCE EXCITATION OF THE I$_2$ ($E$) STATE

Author Institution: Institute of Physics, St.Petersburg State University, Peterhof, 198504 Russia; Laboratoire de Spectrometrie Ionique et Moleculaire, Universite' de Lyon (Lyon 1), 69622 Villeurbanne, FranceThe $E$ 0$^+_g$ ion-pair state of I$_2$ molecule may be excited by the optical-optical double resonance (OODR) via the valence state $B$ 0$^+_u$, I$_2$($X$ 0$^+_g$) + $hv_1$ $\rightarrow$ I$_2$ ($B$ 0$^+_u$ ) + $hv_2$ $\rightarrow$ I$_2$ ( $E$ 0$^+_g$ ). Emission spectra of pure I$_2$ vapor selectively excited in this way show not only ransitions from the $E$ 0$^+_g$ state, but also $D$ 0$^+_u$ $\rightarrow$ $X$ 0$^+_g$ transition in the near ultraviolet. Typically the populated levels in the $D$ 0$^+_u$ state are located less than ~ 300 cm$^{-1}$ below the laser-excited level. In this contribution we examine rotationally resolved Fourier-transform spectra of the $D$ 0$^+_u$ $\rightarrow$ $X$ 0$^+_g$ emission following OODR excitation of various $E$ 0$^+_g$ ${v, J}$ levels with single mode cw dye lasers. We show that the $E$ 0$^+_g$ $\rightarrow$ $D$ 0$^+_u$ population transfer obeys the $J_D$=$J_E$ $^+_-$ 1, 3, 5 ... propensity rule. Transfer to the $J_D$=$J_E$ $^+_-$1 levels is the most probable although in some cases, $J_D$=$J_E$-1 and $J_D$=$J_E$+1 have significantly different populations