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Title:Time-resolved absorption spectroscopy of the rare gas dimer Rydberg states
Author(s):Shannon, David Charles
Doctoral Committee Chair(s):Eden, James G.
Department / Program:Electrical and Computer Engineering
Discipline:Electrical and Computer Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Physical
Abstract:Molecular inter-Rydberg transitions of the rare gas dimers (He$\sb2$ through Kr$\sb2$) have been observed both through intra-cavity and multi-pass laser absorption spectroscopy.
All five Hund's case (c) members of the A5p$\pi\sb{\rm u}$ manifold ($\sim$490-530 nm), predicted by the guidelines established by Mulliken (J. Chem. Phys., 52, 5170 (1970)) for Xe$\sb2$ and later adapted by Chang and Setser (J. Chem. Phys., 69, 3885 (1978)) for Ar$\sb2$, have been tentatively assigned. The molecular transitions originate from A4s$\sigma\sb{\rm g}$ (1$\sb{\rm u},0\sbsp{\rm u}{-}$) and terminate on the A$\sp2\Sigma\sbsp{\rm 1/2u}{+}$ ion core, $\sp3\Pi\sb{\rm g}$-derived (5p Rydberg electron) states (ranked in ascending energy): $0\sbsp{\rm g}{-},\ 0\sbsp{\rm g}{+},\ 2\sb{\rm g},\ 1\sb{\rm g},$ and $1\sb{\rm g}$. Transitions involving three of these are strongly red degraded (revealing that $\rm R\sb{e}4s(1\sb{u},\ 0\sbsp{u}{-}) < R\sb{e}5p(0\sbsp{g}{-})\ \leq\ R\sb{e}(0\sbsp{g}{+})\ <\ R\sb{e}(1\sb{g}-upper))$ whereas the bands associated with the 2$\sb{\rm g}$ and lowest 1$\sb{\rm g}$ levels are not. Analysis of newly-observed hot bands ($\Delta$v $\equiv$ v$\sp\prime$ - v$\sp{\prime\prime}$ $<$0) yields the following vibrational constants for A4s$\sigma\sb{\rm g}$(1$\sb{\rm u},\ 0\sbsp{\rm u}{-}):\ \omega\sb{\rm e}\sp{\prime\prime}$ = 297.2 $\pm$ 1.0 cm$\sp{-1}$, $\omega\sb{\rm e}\sp{\prime\prime}\rm x\sb{\rm e}\sp{\prime\prime}$ = 3.3 $\pm$ 0.4 cm$\sp{-1}$, $\omega\sb{\rm e}\sp{\prime\prime}\rm y\sb{\rm e}\sp{\prime\prime}$ = 0.08 $\pm$ 0.05 cm$\sp{-1}$. Several clear vibrational sequences are observed and tentative assignments for the A5p$\pi\sb{\rm u}$ separated atom limits are reported. The $0\sbsp{\rm g}{-},\ 0\sbsp{\rm g}{+},\ 2\sb{\rm g},\ 1\sb{\rm g}$, and $1\sb{\rm g}$ (upper) states apparently correlates (in the separated atom limit) with Ar($\sp1$S$\sb0$) + Ar5p (1/2) $\sb1$, (5/2) $\sb2$, (5/2) $\sb3$, and (3/2) $\sb1$, respectively, which is consistent with the known 4p, 5p $\to$ 4s oscillator strengths, the Ar 3p$\sp5$ 5p fine structure splittings, and the measured energy defects between the A5p$\pi\sb{\rm u}$ substates. Rather than being degenerate at small R, the 0$\sbsp{\rm g}{-}$ and 0$\sbsp{\rm g}{+}$ states of the A5$\pi\sb{\rm u}$ manifold are split by 247 cm$\sp{-1}$ near R$\sb{\rm e}$. Additionally, the A5p$\sigma\sb{\rm u}$ $\gets$ A4s$\sigma\sb{\rm g}$(1$\sb{\rm u}$, 0$\sbsp{\rm u}{-}$) (v = 0,1) transitions are also observed. Presumably correlated with Ar 5p (5/2) $\sb2$ + $\sp1$S$\sb0$ in the separated atom limit, calculations show that the A5p$\sigma\sb{\rm u}$ (0$\sbsp{\rm g}{-}$) state dissociation energy (D$\sb0$ $\cong$ 1.0 eV) is noticeably smaller than that for the A5p$\pi\sb{\rm u}$ levels (D$\sb0$ $\cong$ 1.26 $\pm$ 0.01 eV).
The lowest energy Anp$\pi\sb{\rm u}$ $\gets$ Ans$\sigma\sb{\rm u}$(1$\sb{\rm u}$, 0$\sbsp{\rm u}{-}$) (n = 3$-$5) transitions in the heavier noble gases (Ne through Kr) have been observed with a multi-pass discharge absorption apparatus. Comparison of these transitions with regard to atomic number indicates that dissociative states arising from the ns$\sp\prime$ (1/2) $\sb0$ and ns$\sp\prime$ (1/2) $\sb1$ + $\sp1$S$\sb0$ atomic asymptotes are most likely responsible for the predissociation observed in the associated Anp$\pi\sb{\rm u}$ manifolds. The A4p$\pi\sb{\rm u}$ manifold of argon is perturbed to a far greater extent by dissociative curve crossings than the analogous manifolds in either neon or krypton. Attempts to investigate the ungerade molecular Rydberg states of Ar$\sb2$ using a resonant, two color sequential absorption process with the A4p$\pi\sb{\rm u}$(1$\sb{\rm g}$, 2$\sb{\rm g}$) states as intermediates were unsuccessful.
Finally, an as yet unidentified band of absorptions in discharge pumped helium, requiring the presence of two strong optical fields of differing wavelengths, is reported and discussed.
Issue Date:1989
Rights Information:Copyright 1989 Shannon, David Charles
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9011013
OCLC Identifier:(UMI)AAI9011013

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