A potential energy surface is retrieved for the Ar-CO complex by carrying out a global analysis of its high-resolution spectroscopic data. The data set consists of already published microwave and infrared data and of new microwave transitions which are presented in the paper. The theoretical approach used to reproduce the spectrum is based on a model Hamiltonian which accounts simultaneously for the two large amplitude van der Waals modes and for the overall rotation of the complex. Only the vCO = 0 state is considered. The root-mean-square deviation of the analysis is 18 MHz for the microwave data and 1.4 x 10(-3) cm(-1) for the infrared energy difference data. Fifteen parameters corresponding to the potential energy function are determined in addition to two kinetic energy parameters and two distortion-type parameters. The potential energy surface derived is in good agreement with the one obtained by Shin, Shin, and Tao [J. Chem. Phys. 104, 183 (1996)].

The ν4 fundamental vibration spectrum of the weakly bound complex Ar–CH4 in the 7μm region was discovered, analyzed, and compared with a synthesized spectrum, derived from ab initio calculations. The measurements were made by probing a supersonic gas expansion with a tunable diode laser (TDL) spectrometer. Several bands of Ar–CH4 associated with different rovibrational transitions of the ν4 vibration (asymmetric bending vibration) of CH4 were recorded and analyzed in a spectral region from 1295 to 1330cm−1. In particular the following transitions were studied: j=1←0 at 1311cm−1 reported in Z. Naturforsch. 53 (1998) 725, j=0←1 at 1301cm−1, j=1←1 at 1306cm−1, j=2←1 at 1316cm−1, and j=3←2 transitions at 1322cm−1. Here, j denotes the angular momentum of the methane unit inside the complex. The densest part of the experimental spectrum located in the region between 1300 and 1315cm−1 is compared in depth with the synthesized ab initio spectra. The close agreement between observed and theoretical ab initio spectra is convincingly demonstrated with respect to the gross spectral features, including also many details of the spectra. The ab initio spectra were computed as described in J. Chem. Phys. 110 (1999) 5639 for the ν3 excited Ar–CH4 complex. Briefly, a potential was obtained from symmetry adapted perturbation theory and used as input for a variational solution of the rovibrational problem. After the levels were calculated the line intensities were computed with the ν4 transition dipole as unit of dipole strength. A Boltzmann distribution was assumed at 7K.

We present observations of the J=1-->0 and J=2-->1 transitions for (CO)-O-18, (CO)-O-17 and (CO)-C-13-O-17,and the J=1-->0 transition for (CO)-C-13-O-18, made toward core C of the rhoOph molecular cloud.(1) Using these measurements we derive isotopic abundance ratios of [C-12]/[C-13] = 65.0 6.3(stat) +/-9.2(syst) and [O-18]/[O-17] = 4.14 +/- 0.52(stat) +/- 0.59(syst). They are consistent with recent results from observations of CO rotational transitions, and absorption measurements of CO,(CO)-C-13 and CH+, (CH+)-C-13 at visible/UV wavelengths. Our results support the idea that these isotope ratios are smaller in the local interstellar medium than the terrestrial values of 89 and 5.5.

The bending vibration of the CO–N2 complex has been investigated in the millimeter wave range from 130 to 155GHz using an intracavity OROTRON jet spectrometer. Six transitions, P(2), P(1), R(0), R(1), R(2), and R(3) from the K=0 ground state to the K=0 bending state of the orthoN2 spin modification were measured and analyzed. Nuclear quadrupole structure due to the presence of two equivalent 14N nuclei was partly resolved and analyzed to give information about the angular anisotropy of the interaction potential. The frequency of the bending vibration was determined to be 139892.459(35)MHz. The nuclear quadrupole coupling constant for the K=0 bending state of CO–orthoN2 was obtained for the first time to be χaa=−0.768(43)MHz. The different value and sign of this constant from the one in the K=0 ground state, (χaa=+0.19641MHz) suggests that the orientation and motion of the N2 subunit are very different in these two states.

The spectrum of the weakly bound complex Ar–CH4 in the 7 μm region was discovered, analysed, and compared with a spectrum, predicted from ab initio calculations. The measurements were made by probing a supersonic gas expansion with a tunable diode laser (TDL). Several bands of Ar–CH4 associated with different ro-vibrational transitions of the ν4 vibration of CH4 were recorded and analysed in a spectral region from 1295 to 1330 cm−1. In particular the following transitions were studied: j=1←0 (at 1311 cm−1) reported in Pak et al. [Z. Naturforsch. 53 (1998) 725], j=0←1 (at 1301 cm−1), j=2←1 (at 1316 cm−1), and j=3←2 transitions (at 1322 cm−1). Here, j denotes the angular momentum of the methane unit inside the complex. Analysis of the recently recorded j=1←1 transitions at about 1306 cm−1 in the region of methane Q(1) is in progress. The experimental results are compared with ab initio calculations. The close agreement between observed and ab initio spectra is convincingly demonstrated with respect to the gross spectral features, including many details of the spectra.

A highly sensitive intracavity millimeter-wave spectrometer was developed for the investigation of the absorption spectra of van der Waals complexes in a supersonic jet. The key element of the spectrometer is a tunable oscillator, called OROTRON, which generates the millimeter-wave radiation through the interaction of an electron beam with the electromagnetic field of a high quality (Q≈104) Fabry–Perot resonant cavity. This cavity consists of a movable spherical mirror and a fixed planar mirror with the periodic structure imprinted on its surface. The electron beam moves along the periodic structure of the plane mirror. This part separated from the rest of the resonator by a mica foil is kept under ultrahigh vacuum conditions. The molecular jet is injected by a pulsed valve into the other part of the resonator. The absorption in the jet is sensitively detected by measuring the electric current in a special collector circuit of the OROTRON. The spectral purity of the OROTRON radiation is 10–15 kHz providing the capability of sub-Doppler spectral resolution without phase locking. An increase in sensitivity of a factor of about 100 in comparison with the usual single pass arrangement was evaluated from the measurements of the absorption lines of the CO rare isotopomers, the Ar–CO and Ne–CO van der Waals complexes. The high sensitivity, wide spectral range, and simple tunability of the spectrometer make it a very efficient tool for the searching of weakly absorbing species in a jet.

Doppler-free two-photon rotational transitions J = 13<--<--11 and J = 12<--<--10 of OCS and J = 8<--<--6 and J = 7<--<--5 of CHF (3) were detected in the frequency range 134-156 GHz, using a novel, highly sensitive intracavity-jet technique. The sub-Doppler narrowing of the observed peaks (down to 40 kHz full width at half maximum as compared to 300 kHz of the Doppler width) demonstrates the potential of this new technique for high precision millimeter wave spectroscopy. The possibilities of the further reduction of the two-photon absorption line widths are considered.

The infrared absorption spectrum of the weakly bound rare-gas–spherical-top complex Ne–CH4 was discovered and analyzed for the first time. Measurements were made with tunable diode laser spectrometers using a pulsed supersonic jet and a long-path low-temperature absorption cell. Close to the R(0) transition of the methane ν4 fundamental band at 1311.430 cm–1, the Ne–CH4 spectrum was recorded as a very compact absorption pattern. Within a total wave-number range of about 0.1 cm–1, P-, Q-, and R-branches are located. As the first step, the Ne–CH4 spectrum was recorded and analyzed in a supersonic jet at low rotational temperature of about 5 K. Three branches were identified, of which the P- and R-branches were partially resolved and the Q-branch remained unresolved. Compared with the previously measured spectra of Ar–CH4 and Kr–CH4 [Z. Naturforsch. A, 53, 725 (1998).], the absorption pattern in the spectrum of Ne–CH4 is much denser and considerably more compact. However, by analogy with the spectra of Ar–CH4 and Kr–CH4, assignment and analysis were carried out using a Hamiltonian model that incorporates a Coriolis interaction between the total angular momentum of the complex and the angular momentum of the methane monomer. This analysis then allowed us to assign the same spectrum as recorded in a long-path (160 m) cell at a higher temperature of 62 K. The observed rotational constant for Ne–CH4, B"= 0.129(9) cm–1, corresponds to an effective intermolecular separation of 3.8 Å. PACS Nos.: 33.20E, 34.25, 35.20P, 36.40

For the first time, millimeter wave absorption spectra of the van der Waals complexes 3He–CO and 4He–CO were detected in a supersonic jet. Altogether four rotational transitions of 3He–CO and six rotational transitions of 4He–CO were recorded between 110 GHz and 127 GHz with an intracavity spectrometer based on the millimeter wave generator, called OROTRON. The obtained results were included in a global fit together with previously known data enabling a more precise determination of the energy levels of the 3He–CO and 4He–CO complexes. In extremely cold, dark, and dense interstellar clouds the He–CO complex may have astrophysical relevance.

Abstract The pure rotational b-type spectrum of the van der Waals complex Ne-CO has been measured using a pulsed jet, intracavity millimeter wave spectrometer. The millimeter wave generation is based on the OROTRON principle. The high sensitivity of the spectrometer allowed measurements of R(J), K = 1 ← 0 transitions between 108 and 150 GHz of the Ne isotopomers 20 Ne-CO and 22 Ne-CO. This new millimeter wave data set together with the microwave data in the literature, i.e. a-type microwave transitions, yield in a fit to an asymmetric rotor a reliable set of ground state constants. These are for 20 Ne-CO: A = 107127.021(14) MHz, B = 3479.6597(95) MHz, and C = 3039.5387(93) MHz. For both 20 Ne-CO and 22 Ne-CO, a global fit to a near-symmetric rotor was performed, taking into account the infrared and microwave transition frequencies from the literature and the millimeter wave measurements of the present work.

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 stretching ground state vCO=0 energy levels. The energies of all levels were determined to microwave accuracy. The discovered energy levels fall into two substates, corresponding to the projection K=0 and to K=1 of the total angular momentum J onto the intermolecular axis. The effective intermolecular CO–CO separation for these new K=0 and K=1 states is 4.26 and 4.17 Å, respectively.

With the Cologne submillimeter-wave supersonic jet spectrometer, we extended molecular jet spectroscopy with backward wave oscillators up to frequencies of about 600 GHz. For the first time, the van der Waals stretching vibration of the Ar–CO molecular complex was detected in direct absorption. We measured 13 ro–vibrational transitions (Kvstretch = 1 ← 0, Ka = 0 ← 0) in the frequency range from 528 to 600 GHz and additionally the two R(3) K doublet (Ka = 4 ← 3) pure rotational transitions at 447 GHz with an accuracy of about 200 kHz. The ro–vibrational transitions were assigned and fitted within experimental accuracy to a simple Hamiltonian taking into account the Coriolis interaction between the stretching and bending states, i.e., between vstretch = 1, Ka = 0, and vbend = 1, Ka = 1. The intensity of the transitions in the van der Waals stretching mode was estimated to be a factor of 5–10 less than that in the bending mode of Ar–CO.