The optical spectrum of a large isolated polycyclic aromatic hydrocarbon: hexa-peri-hexabenzocoronene, C42H18

The first optical spectrum of an isolated polycyclic aromatic hydrocarbon large enough to survive the photophysical conditions of the interstellar medium is reported. Vibronic bands of the first electronic transition of the all benzenoid polycyclic aromatic hydrocarbon hexa-peri-hexabenzocoronene were observed in the 4080-4530 Angstrom range by resonant 2-color 2-photon ionization spectroscopy. The strongest feature at 4264 Angstrom is estimated to have an oscillator strength of f=1.4x10^-3, placing an upper limit on the interstellar abundance of this polycyclic aromatic hydrocarbon at 4x10^12 cm^-2, accounting for a maximum of ~0.02% of interstellar carbon. This study opens up the possibility to rigorously test neutral polycyclic aromatic hydrocarbons as carriers of the diffuse interstellar bands in the near future.Comment: 9 pages, 1 figure. Fixed a typo on the frequency of the 'b' ban

ACCURATE FAR-INFRARED FREQUENCIES FOR PURE ROTATIONAL AND FINE-STRUCTURE TRANSITIONS IN THE 2Π^{2}\Pi ELECTRONIC GROUND STATE OF NITRIC OXIDE, NO

$^{a}$T.D.VARBERGE AND K.M. EVENSON, AP.J., 385, 768-765 (1992). $^{b}$J.M. BROWN, M. KAISE, C.M.L.KERR, AND D.J.MILTON, Mol {Phys}. 36, 553-582(1978) $^{c}$A.H. SALECK, G. WINNEWISSER, AND K.M.T. YAMADA, {Mol Phys}., 76 1443-55(1992).Author Institution: Time and Frequency Division, NIST; Department of Chemistry, Macalester CollegeThe far-infrared spectrum of the NO radical in its electronic ground state exhibits both pure rotational transitions and magnetic dipole transitions in {P}-, {Q}-and R-branches between the two spin states $^{2}\Pi_{1/2}$ and $^{2}\Pi_{3/2}$. We have measured 150 transitions using tunable far-infrared spectroscopy to supply accurate far-infrared frequencies for the stronger pure rotational and magnetic dipole transitions as well as to check on intensity predictions for perturbation allowed electric dipole transitions between the spin states. Two stabilized $CO_{2}$ laser frequencies were mixed with a tunable microwave frequency in a metal-insulator-metal (MIM) diode to generate far-infrared radiation. To obtain accurate center frequencies Voigt profiles were adjusted to the observed absorption spectra by a least-squares procedure. This technique was shown to supply center frequencies accurate to better than 10kHz for the CO $molecule^{a}$. Fits of the obtained transition frequencies to an effective Hamiltonian of the type introduced by Brown $et al^{b}$. have been performed. Standard deviations somewhat higher than 10kHz have been found for these fits up to now. The reason for that is under investigation and will be discussed at the meeting. The preliminary value for the spin-orbit coupling constant is 3691813.743(21) MHz, which is a factor of 50 more accurate than the one determined from the most recent Fourier transform $measurement^{c}$

The ultraviolet spectrum of the CoCl2 radical, studied at vibrational and rotational resolution

The laser excitation spectrum of the 327 nm band system of CoCl2, formed in a free-jet expansion, has been recorded at a rotational temperature of approximately 10 K. The spectrum is congested and suffers extensive perturbations. A progression in the excited state symmetric stretching vibration has been identified. The decrease in the symmetric stretching vibrational wave number on excitation is considerable [v'1=195.7(12), v''1=358.1(17) cmˉ¹]. Despite widespread perturbations in the rotational structure of these vibronic bands, they can be confidently assigned to a parallel Ω=7/2-7/2 transition, consistent with an inverted 4Δg ground electronic state. The rotational constant for Co35Cl2 in the ground state is determined to be 0.056 65(11) cmˉ¹, which corresponds to a value for the zero-point averaged Co-Cl bond length ro of 2.062 8(40)Å. The perturbations are found to be strongly isotopomer dependent

High Resolution Spectroscopy of CoH and CoD

Author Institution: Department of Chemistry, Massachusetts Institute of TechnologyFive new bands in the spectrum of gas phase CoH, with band heads near 5650, 5710, 6093, 6446 and 6586 {\AA}, as well as three bands in CoD, have been observed via laser excitation spectroscopy with a cw dye laser. Two of the CoH bands, at 5710 and 6093 A, have been rotationally analyzed and identified as $\Omega =4 \leftarrow \Omega =4$, where the lower state for both transitions is the previously characterized $X \Omega =4 (v=0)$ ground state. The two upper states, with approximate molecular constants $T_{v} = 18360 cm^{-1}, Bv = 4,7 cm^{-1}$ and $T_{v} = 17260 cm^{-1}, Bv = 4.9 cm^{-1}$, display strong perturbations characteristic of open-shell, transition-metal containing diatomics. A preliminary analysis of these perturbations will be presented