Igor Pak

Detection of 13C17O and Observations of Rare CO Isotopomers toward the ρ Ophiuchi Molecular Cloud

We report the detection of 13C17O and present observations of three additional rare CO isotopomers made toward core C of the ρ Ophiuchi molecular cloud. The data encompass single-pointed observations of the J = 1 → 0 and J = 2 → 1 rotational transitions for C18O, C17O, 13C17O, and the J = 1 → 0 transition for 13C18O. Emission is detected for all isotopes and transitions observed. For the two 17O-containing species, the interstellar spectra display a partially resolved hyperfine spectrum due to the I = 5/2 spin of the 17O nucleus. The isotope abundance ratios derived from these measurements are [12C]/[13C] = 65.0 ± 6.3(stat) ± 9.2(syst) and [18O]/[17O] = 4.15 ± 0.52(stat) ± 0.59(syst), consistent with recent measurements for nearby molecular clouds. Thus, our measurements support the idea that both ratios are smaller in the local interstellar medium (ISM) than their terrestrial values of 89 and 5.5.

Discovery of the rotational spectrum of the weakly bound complex CO–H2

Rotational transitions of the CO–H2 van der Waals complex have been measured between 108 and 125 GHz in a supersonic jet. Three absorption lines were recorded and assigned as belonging to CO–paraH2 with a new highly sensitive intracavity spectrometer based on the millimeter wave generator, called OROTRON. The results provide confirmation of the recent infrared data of McKellar at 4.7 μm, enabling a more precise determination of the energies of the rotational levels with different parity in the vibrational ground state. The measured millimeter wave transitions provide precise frequencies for an astronomical search of CO–paraH2.

The CO dimer millimeter wave spectrum: Detection of tunneling transitions

A portion of the CO dimer millimeter wave absorption spectrum has been studied by using our highly sensitive intracavity-jet OROTRON spectrometer in the frequency range from 131 to 174 GHz. By varying the CO concentration in the Ne/CO gas mixture feeding the supersonic jet expansion, the effective temperature of the beam could be changed, revealing a correlation between the observed line intensity and the relative energy of the respective lower state energy levels. Using this temperature dependence and the technique of combination differences together with the data from the infrared study of Brookes and McKellar [J. Chem. Phys. 111, 7321 (1999)], out of over 200 observed transitions, a total of 19 lines could be assigned. All assigned millimeter-wave transitions are tunneling transitions. They belong to four subbands, which connect seven lower energy levels with A+ symmetry to ten previously unknown upper energy levels with A− symmetry. The A+ and A− separation signifies the tunneling splitting of the CO ...

Unequivocal laboratory detection of CO dimer transitions in the millimeter wave region

Abstract CO dimer transitions were found for the first time in the millimeter wave region, i.e. in the frequency range from 142 to 172 GHz. An isotopic test was performed to authenticate the origin of the transition to the CO dimer. At present the millimeter wave data are not yet sufficient to secure a firm assignment. A brief overview of the history of this complex is given.

Laboratory Precision Measurements of the Rotational Spectrum of 12C17O and 13C17O

High-precision millimeter and submillimeter wave measurements were performed on two 17O isotopically substituted carbon monoxide species, i.e., 12C17O and 13C17O. Covering the frequency region from 100 GHz to 1 THz, the accuracy achievable is estimated to be ±5 kHz in the Doppler-limited mode and ±1 kHz for sub-Doppler-resolution measurements. From a weighted least-squares fit, the following molecular rotational parameters for 12C17O and 13C17O were obtained: for 12C17O, and for 13C17O, in both instances, the H0 values were kept fixed to IR data. The oxygen 17O nucleus exhibits a sizeable electric nuclear quadrupole moment, which has been measured for both isotopomers, i.e., eQq(12C17O) = 4.298(44) MHz and eQq(13C17O) = 4.355(182) MHz. The high precision of the Lamb dip measurements allowed us to observe additional small hyperfine effects caused by the magnetic moment of the 17O nucleus. These precision measurements allowed the determination of the nuclear spin-rotation constant CI(17O) = -31.60(72) Hz for 12C17O, solely from the Cologne data set. The highly precise transition frequencies reported here should warrant deep interstellar searches for the two molecules 12C17O and 13C17O. The latter has not been detected in space until very recently. On the basis of our laboratory data, we were able to report the discovery of 13C17O (by Bensch and coworkers) along with the observations of two additionalrare CO isotopomers including 12C17O and 12C18O toward core C of the ρ Ophiucus molecular cloud.

The weakly bound complex CO–orthoD2: Detection of millimeter-wave transitions

For the first time two rotational transitions of the CO–orthoD2 van der Waals molecular complex have been recorded in a supersonic jet and assigned to the two R-branch transitions, R(1) (J,j,l)=2,1,1←1,0,1 112 806.515(50) MHz, R(2)=3,1,2,←2,0,2 119 746.484(50) MHz. These transitions were detected with the highly sensitive intracavity spectrometer based on the millimeter wave generator, called OROTRON. Their measured frequencies are in close agreement with the frequency positions predicted from the infrared spectrum, which accompanies the fundamental band of CO in the 4.7 μm region, and which has been published recently by McKellar [J. Chem. Phys. 112, 9282 (2000)]. The millimeter wave transitions will allow a precise determination of the rotational levels with different parity in the vibrational vCO=0 ground state.

Millimeter wave spectroscopy in a pulsed supersonic slit nozzle discharge

Abstract A Doppler-free technique for direct absorption spectroscopy of rotationally cold molecular ions in the millimeter wave range is presented. The method uses a double modulation technique, based on a simultaneous modulation of a frequency doubled microwave sweeper and a pulsed slit nozzle, incorporating a discharge in a high-pressure supersonic expansion. The performance is demonstrated with the observation of the hyperfine structure of the J =2←1 pure rotational transition of N 2 H + . Further perspectives of the method are discussed.