Excited State Structure of NEON(2)

Several Rydberg series attributable to transitions originating from the a^{3}Sigma _sp{u}{+} metastable state of the neon dimer have been observed by laser excitation spectroscopy in the afterglow of a pulsed corona discharge. Most of the observed bands in the excitation spectrum in the region 25000-34400 cm^{-1} are attributed to transitions from the a^ {3}Sigma_sp{u} {+}(v = 0) state, the lowest excited state of Ne_2. The observed bands are the n = 4-17 members of the nppi ^3 Pi_{g}>= a^3Sigma_sp{u }{+} (0-0) inter-Rydberg transitions, the n = 6-23 members of the npsigma ^3Sigma_sp{g}{+ }>= a^3Sigma_sp {u}{+} (0-0) transitions, and the n = 4 to 6 members of the nflambda (^3Sigma_{g}, ^3Pi_{ g}) >= a ^3Sigma_sp{u}{+ } (0-0) transitions. An unusual loss of intensity in the Q branches for the higher n member of the np pi ^3Pi_{g } >= a^{3} Sigma_sp{u}{+} (0-0) transitions and a weak perturbation between the v = 1 vibrational level of the 5fsigma ^3Sigma_sp{g} {+} state and the 7ppi ^3Pi_{rm g}, v = 0 level are observed. Also, rotationally resolved spectra of the transitions nf(^3Sigma _sp{g}{+}, ^3Pi_{g} ) >= a^{3 }Sigma_sp{u}{+ } (n = 4-6) show that the rotational structures of the upper states are strongly l-uncoupled; that is, there is an indication of a transition in the angular momentum coupling from Hund's case (b^' ) to case(d) with increasing N. A detailed characterization of numerous Rydberg transitions has been made from the rotationally resolved spectra, and molecular constants have been obtained for the states a^{3 }Sigma_sp{u}{+ }, nfsigma ^3 Sigma_sp{g}{+} (n = 4-6), nfpi ^3Pi_{g} (n = 4-6), npsigma ^3 Sigma_sp{g}{+} (n = 10, 12-17, and 19-23), and nppi ^3Pi_{g} (n = 4-11, 13-17). From the highly resolved spectra of Ne_2, the ionization potential of the a^{3}Sigma _sp{u}{+} state has been determined (I.P. = 34 409 +/- 2.0 cm^{-1}). The excitation spectrum beyond the X^2 Sigma_sp{u}{+} Ne_sp{2}{+} (v = 0) ionization limit reveals several series of suppressions (windows) superimposed onto predissociative continua which are attributed to the interference effects of the autoionizing superexcited states. The suppressions trace out the (1-0) and (2-0) bands of the nppi ^3Pi_{g} - a^{3 }Sigma_sp{u}{+ } Rydberg transitions and are confirmed by the isotopic studies involving ^{20 }Ne_2 and ^ {22}Ne_2. A two -laser, suppression of laser-induced fluorescence experiment has been employed to obtain an equivalent absorption spectrum in these regions; the absorption spectrum shows absorption windows overlapping the LIF suppressions for the series terminating on v = 1 core Rydberg states and absorption peaks overlapping the LIF suppressions for the series terminating on v = 2 of the ion core state. There is a strong indication of Fano interference between the superexcited Rydberg states and several continuum states, which includes the predissociative continuum and the autoionizing continuum.