Main Index: Main Index WG5 Index: WG5 Index

Electronic spectra

The laboratory astrophysics group at the CfA has continued to provide quantitative, high spectral resolution measurements of molecules of atmospheric and astrophysical interest in the ultraviolet. Theoretical studies of the predissociation of the Schumann-Runge (S-R) bands of $O$ $$₂ have been carried out for both $rotational$ 10$and\; isotopic$ dependency.$ 11$Absorption\; bands\; of\; the\; S-R\; system\; have\; been\; observed\; at\; 670 K\; at\; high\; resolution,\; and\; precise\; wavelength\; measurements\; have\; led\; to\; the\; molecular\; constants\; of\; the\; ground\; state\; of$ O$ $$$$₂.$12$ Two highly excited states of $O$ $$₂ have been identified in collaboration with colleagues at the Photon Factory, Japan, and NRC, $Canada.$ 13$Absorption\; measurements\; at\; Doppler\; limited\; resolution\; have\; been\; carried\; out\; by\; Yoshino\; and\; colleagues\; with\; a\; VUV\; Fourier\; transform\; spectrometer\; (FTS)\; in\; combination\; with\; a\; White\; cell\; at\; Imperial\; College\; (IC),\; London,\; U.K.\; The$ O$ $$$₂ Herzberg I band system () has been measured at a resolution of $0.06\; cm$ -1$;$ 14$band\; oscillator\; strengths\; and\; rotational\; line\; strengths\; have\; been$ obtained.$ 15$These\; are\; the\; first\; absorption\; intensity\; measurements\; of\; the\; Herzberg\; bands\; free\; of\; problems\; arising\; from\; inadequate\; spectral\; resolution.\; In\; addition,\; the\; NO$$ and bands have been measured at a resolution $0.06\; cm$ -1$with\; the\; VUV\; FTS\; at$ IC.$ 16$To\; extend\; the\; high\; resolution\; measurements\; to\; shorter\; wavelengths,\; the\; CfA/IC\; group\; moved\; this\; apparatus\; to\; the\; Photon\; Factory,\; Japan,\; where\; VUV\; radiation\; from\; a\; synchrotron\; source\; with\; a\; narrow\; band\; predisperser\; is$ available.$ 17$Analysis\; of\; the\; measurements\; of\; the\; NO\; bands\; and\; the\; S-R\; bands\; of$ O$ $$$$$₂ obtained with this facility are in progress. The absorption cross sections for the VUV diffuse bands of $CO$ $$₂$18$ and $H$ $$₂O$19$ have been measured.

The VUV spectrum of CO continues to be intensively studied by several groups. The CfA group has obtained photoabsorption cross sections of CO in the wavelength region 92.5 nm to 97.4 nm at $$ ~20$K,thus\; greatly\; simplifying\; their\; application\; to\; astrophysical$ models.$ 20$The\; band\; oscillator\; strengths\; of\; the$ $ A(v\text{'})-X(0)$fourth\; positive\; bands\; of\; CO\; for$ $ v\text{'}=11\; -\; 14$have\; been\; obtained\; with\; high$ resolution$ 21$and\; have\; confirmed\; the\; values\; of\; Chan$ $$$$$$et al.$22$ The Meudon group has also studied the AX transition for $$ 12$C$ 18$Oand$ $ 13$C$ 18$O.$ 23,24$Using\; the\; A\; state\; term\; values\; a\; new\; set\; of\; term\; levels\; for\; the\; B,\; C\; and\; E\; states\; has\; been$ established.$ 25$$$$

A new triplet state of CO (k) has been analyzed through its interactions with the E and B states and by direct absorption $measurements.$ 26,27,28$The\; B$ $($ v$=3)level\; situated\; above\; the\; B$D' crossing has been observed and calculated with the semi-empirical potentials established $earlier.$ 29$A\; pulsed\; jet\; has\; been\; used\; in\; conjunction\; with\; the\; 10m\; vacuum\; spectrograph\; and\; the\; BRV\; pulsed\; continuum\; to\; obtain\; cooled\; spectra\; of\; the\; CO\; Rydberg$ states.$ 30$$$

Predissociative and radiative lifetimes of the excited states of CO have been determined from line broadening measurements on singly resolved rotational states using a coherent and tunable VUV source by Ubachs and $co-workers.$ 31,32,33$For\; several\; states,\; the\; predissociation\; rates\; were\; found\; to\; depend\; on\; rotational\; state,\; parity\; and\; isotope.\; The\; E$ $v$=0 and 1 levels were carefully studied for $$ 12$COand$ $ 13$COand\; were\; found\; to\; be\; strongly\; predissociated\; by\; the\; k$$ bound state. The lifetime of the (4p)L $v=0$ state $($ f$parity)was\; determined\; at\; 1.0\; ns,\; typical\; of\; a\; radiative\; rather\; than\; a\; predissociative\; lifetime.$

Basic spectroscopic work on the most important astrophysical molecule, $H$ $$₂, continues by the Meudon group. The photographic atlas of the Lyman and Werner bands has been $published.$ 34$More\; than\; 12000\; lines\; are\; tabulated,\; 96\%\; of\; which\; have\; at\; least\; one\; assignment.\; New\; calculations\; concerning\; the$ B$ \text{'}$and\; D\; states\; have\; been$ published.$ 35$Together\; with\; previous\; work\; concerning\; the\; B\; and\; C,\; states\; these\; allow\; precise\; calculations\; of\; the\; Werner\; and\; Lyman\; bands.\; Comprehensive\; tables\; of\; line\; positions\; and\; intensities\; are\; now$ available.$ 36,37$$$$$

A new set of molecular constants for the ground state of $N$ $$₂ has been established using all the data available in the different spectral $ranges.$ 38$A\; comprehensive\; model\; of\; the\; first\; seven\; singlet\; Rydberg\; and\; valence\; states\; has\; been\; established.\; This\; will\; allow\; precise\; modeling\; of\; the\; VUV\; emission\; of$ N$ $$$₂ in the atmosphere of Titan and the Earth $around\; 100 nm.$ 39$The\; nf\; Rydberg\; levels\; of$ $ 14$N$ $$$₂ converging on the state of N have been $studied.$ 40$$

Studies of the N molecular ion have been made by the Berkeley group with a Pointolite arc which produces highly excited levels of the ion, and with a cooled hollow cathode lamp. The analysis was extended to much higher rotational energies as compared to the hollow cathode or furnace $spectra.$ 41$Bernath\; and\; co-workers\; used\; the\; Fourier\; Transform\; technique\; to\; study\; the\; electronic\; spectra\; of\; a\; number\; of\; astrophysically\; relevant\; molecules\; including$ CoH$ 45$,$ CH$ 46$and\; the\; Swan\; system\; of$ C$ $$$$$₂.$47$

Extensive electronic structure calculations continue to provide valuable information on potential energy surfaces, transition moments, spectroscopic constants and line intensities. A theoretical study of the NO , the $11000\; \AA ,and\; the$b bands has been made by the NASA-Ames $group,$ 42,43,44$who\; also\; computed\; the\; electronic\; states\; of$ MgC.$ 49$The\; ground\; and\; excited\; states\; of$ SiH$ +$$ 152$and$ CP$ 153$have\; been\; calculated\; by\; the\; Wuppertal\; group.\; Potential\; surfaces\; and\; line\; intensities\; of$ Si$ $$$$$$₂C,$48$ $N$ $$₃⁺,$50$ $NH$ $$₂⁺,$51$ and $CO$ $$₂$52$ have been investigated theoretically by the French group. The flat shape of the $N$ $$₃⁺ X ground state along the asymmetric displacement coordinate leads to strong anharmonic resonances. For $NH$ $$₂⁺, the quasilinear ground state and the bent quasi-linear $$ a1A$$₁ - b¹B₁ Renner-Teller system have been studied up to high energies. An exhaustive study of the excited electronic states of $CO$ $$₂ suggests that the observed peculiar splittings of the first dipole-allowed electronic transition X are due to complex avoided crossings and vibronic coupling effects between the Rydberg and valence states.

The Leiden group has performed comprehensive theoretical calculations of the absorption spectra of the lowest 10 electronic states of $CH$ $$₂ and its $isotopes.$ 53-57$Similar\; work\; on$ NH$ $$$₂ is in $progress.$ 58$The$ HCO$ +$ion\; appears\; to\; be\; remarkably\; transparent\; to\; ultraviolet\; radiation,\; since\; all\; its\; dipole-allowed\; transitions\; lie\; at\; very\; high$ energies.$ 59,60$$$$

The transition rates for ZrO have been measured from a laboratory absorption spectrum by the Berkeley group, for the purpose of resolving differences between an earlier analysis of an emission spectrum, and experimental results and theoretical $treatments.$ 61$The\; spectrum\; of\; LaS\; from\; a\; carbon\; tube\; furnace\; has\; been\; produced\; and$ analyzed.$ 62$A\; major\; theoretical\; effort\; by\; Langhoff\; on\; the$ g\; f$values\; for\; all\; absorption\; lines\; of\; low-lying\; electronic\; states\; of\; TiO\; that\; would\; contribute\; to\; the\; opacity\; of\; cool\; evolved\; stars\; has\; just\; been$ finished.$ 63,64$$$$

The SCAN data base by $Jørgenson$ 9$contains\; information\; about\; the\; identification,\; frequency,\; excitation\; energy\; and\; strength\; of$ $ $~70\; millionlines\; from\; the\; molecules\; CN,$ CH$ 65$,TiO,$ H$ $$$$$₂O, HCN, $C$ $$₂H$$₂ and $C$ $$₃. The Harvard-Smithsonian data base on TiO, $C$ $$₂, SiO, CN, CO, $H$ $$₂, CH, NH, OH, MgH and SiH was recently reviewed by $Kurucz.$ 66$$

There has been major progress in experimental and theoretical studies of larger molecules. The photofragmentation of model PAH monocations such as deuterated $naphtalene$ 67$and$ benzene$ 68$has\; been\; studied\; using\; synchrotron\; radiation\; in\; the\; range\; 15-35\; eV.\; The\; effect\; of\; molecular\; size\; has\; been$ evaluated.$ 69$The\; single\; and\; double\; ionization\; potentials\; of\; PAH\text{'}s\; have\; been\; modeled\; by\; an\; empirical$ expression.$ 70$The\; formation,\; relaxation\; and\; destruction\; of\; negative\; PAH\; ions\; has\; been\; studied\; by\; an\; electron\; diffusion$ technique.$ 71$The\; formation\; of\; PAH\text{'}s\; in\; stellar\; envelopes\; has\; been\; modeled\; on\; the\; basis\; of\; the\; laboratory\; data.\; Small\; PAH\text{'}s\; appear\; to\; be\; formed\; near\; the\; star\; and\; destroyed\; further\; out.\; Larger\; ones\; are\; formed\; at\; larger\; distances\; by\; condensation\; of\; the\; small\; PAH\text{'}s\; produced$ initially.$ 72,73$Ab\; initio\; studies\; of\; the\; electronic\; absorption\; spectra\; of\; some\; PAH\; cations\; have\; been\; made\; in\; the\; Bonn$ group.$ 57$$$$$$$$

The possibility that linear carbon chains are responsible for some of the diffuse interstellar bands has received renewed $attention.$ 74,75$Detailed\; absorption\; spectra\; of\; these\; chains\; and\; their\; cations\; in\; neon\; matrices\; have\; been\; obtained\; by\; Maier\; and$ co-workers.$ 76-82$A\; possible\; assignment\; of\; two\; diffuse\; interstellar\; bands\; observed\; at\; near-infrared\; wavelengths\; with$ C$ $$$$₆₀⁺ was made by et al.$84$ The electronic spectroscopy of $C$ $$₆₀ has been studied in the Meudon laboratory between 200 and 750 nm by electron energy loss $spectroscopy.$ 85,70$Rotational\; band\; contours\; of$ C$ $$$₆₀ and linear carbon chains have been calculated and compared to the diffuse interstellar $bands.$ 86$It\; is\; concluded\; that\; most\; DIB\text{'}s\; would\; be\; due\; to\; molecules\; containing\; more\; than\; 10\; atoms\; provided\; the\; excited\; states\; are\; coupled\; with\; isoenergetic\; vibronic\; levels\; of\; lower\; electronic\; states.$

Main Index: Main Index WG5 Index: WG5 Index