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A millimeter-wave intracavity OROTRON spectrometer in combination with a pulsed supersonic pin-hole jet expansion has been used to record a new subband of the CO–N 2 weakly bound van der Waals complex in the 100–150 GHz region. Seven lines were assigned to the K = 0–0, ( j CO , j N2 ) = (2, 0)–(1, 0) “hot band” transitions to a thus far unobserved K = 0 state at 9.336 cm −1 above the ground state. This is the highest in energy van der Waals mode detected so far, and this mode may be interpreted as a doubly excited bending vibration of the CO– ortho N 2 nuclear spin isomer. Hyperfine structure caused by two 14 N nuclei was partly resolved for the measured lines, and the quadrupole coupling constant, associated with the N 2 subunit, together with rotational and centrifugal distortion parameters were precisely determined for the new K = 0, ( j CO , j N2 ) = (2, 0) state. The high lying energy levels of this state provided a sensitive test for the recent ab initio potential energy surfaces.

New millimeter-wave transitions of the CO–N 2 van der Waals complex have been observed using the intracavity OROTRON jet spectrometer in the frequency range of 103–159 GHz. For the less abundant form, CO– para N 2 , a total of 37 rotational transitions were assigned to three K = 0–0, 0–1, 2–1 subbands connecting the ( j CO , j N2 ) = (1, 1) and ( j CO , j N2 ) = (0, 1) internal rotor states. The upper K = 0 and K = 2 “stacks” of rotational levels were probed for the first time here by millimeter-wave spectroscopy following a recent infrared study by Rezaei et al. (2013). The observation of new subbands fixes with higher precision not only these upper K = 0 and K = 2 but also lower K = 1( f ) levels, not linked with other stacks in earlier rotational studies. For the more abundant form, CO– ortho N 2 , five new P -branch rotational transitions of the K = 0–0 “CO bending” subband are reported, thus extending previous measurements. Nuclear quadrupole hyperfine structure due to the presence of two equivalent 14 N nuclei was partly resolved and analyzed to give additional information about the angular orientation of the N 2 molecule in the complex.

The pure rotational spectrum of the Ne–CO van der Waals complex has been measured in the frequency range of 208–230 GHz by using the Cologne supersonic jet spectrometer for terahertz applications (SuJeSTA). Eleven new transitions were assigned as belonging to two R- and two Q-branches of the K = 2–1 subband detected for the first time in the ground vibrational state of CO (νCO = 0). Improved molecular parameters of the Ne–CO complex were obtained which allowed for a sensitive test of the available intermolecular potential energy surfaces of the Ne–CO system.

Rotational transitions of the CO–N 2 van der Waals complex have been measured using the intracavity OROTRON jet spectrometer in the frequency range of 83–133 GHz. For the CO– ortho N 2 spin modification, the observed and assigned 21 rotational transitions belong to the Q - and P -branches of the K =1–0 subband. They complete the previous observation of the corresponding R -branch transitions. In the case of the less abundant form, CO– para N 2 , a new K =1 state, representing simultaneous rotation of both monomers, was detected for the first time. A total of 31 rotational transitions of CO– para N 2 were assigned to two subbands connecting this new K =1 state with already known lower K =0 and 1 states. The assignments relied in part on energy level differences obtained from precise microwave data. The origin of the newly observed state was determined to be 3.41 cm −1 relative to the K =0 ground state of CO– para N 2 . The analysis yielded rotational parameters, centrifugal distortion parameters and nuclear quadrupole coupling constants due to the presence of two equivalent 14 N nuclei for both CO– ortho N 2 and CO– para N 2 .

The b -type rotational transitions of the van der Waals complex, Ne–CO have been measured using the intracavity OROTRON jet spectrometer in the frequency range of 80–115 GHz. The high sensitivity of this technique enabled us to detect all three Ne isotopic modifications of the complex, 20 Ne–CO, 22 Ne–CO, and 21 Ne–CO in natural abundance. The observed and assigned transitions belong to the Q -branch of the K = 1–0 subband and include also R (0) and P (2) lines. The newly obtained data were analysed together with previously observed millimeter-wave b -type and microwave a -type rotational transitions.

The millimeter wave spectrum of the isotopically substituted CO dimer, (13C16O)2, has been studied for the first time, confirming and extending a recent infrared study. Eighty-seven transitions in the 77-180?GHz region have been assigned and analyzed in terms of a model-independent term value scheme involving 57 rotational levels with J=0-8. The levels can be classified into 7 "stacks" which have symmetry classifications of either A-/B+ or A+/B- and K-values of either 0 or 1. For the normal isotope, symmetry and nuclear spin statistics cause alternate rotational levels to be missing, but for (13C16O)2 all levels are present with an intensity alternation of 1:3 between A and B symmetries. The four A-/B+ stacks have not previously been observed, and the lowest of them establishes the tunneling splitting of (13C16O)2 to be 3.769?cm-1, slightly larger than the (12C16O)2 value of 3.731?cm-1. A large amount of precise experimental data is now available for the CO dimer, which should lead to greater theoretical insight into its structure and tunneling dynamics.

The present state of research on the CO dimer is reviewed. In recent years, both infrared and millimeter-wave spectra have been measured and partially assigned by the use of combination differences. A microwave-millimeter wave double resonance experiment, reported here for the first time, provides independent confirmation of these assignments and the resulting (CO)2 energy level scheme. In the double resonance experiment, the OROTRON spectrometer functions as a supersensitive intra-cavity millimeter-wave detector. We update the continuing, but difficult, experimental efforts in recording the spectra, the quest for secure assignments, and the construction of a consistent and reliable energy level scheme. Although at present we have only limited knowledge of some aspects of the CO dimer, such as its geometrical structure, we have succeeded in characterizing unambiguously nine “stacks” of ground state energy levels with “microwave accuracy” (∼0.1 MHz). Every energy level within a given stack exhibits the same symmetry: either A− or A+. Only transitions between A+ and A− levels are allowed, and consequently ordinary pure rotational transitions within a stack are forbidden. Transitions between stacks can be thought of as tunneling transitions, and the separation of the lowest energy A+ and A− states corresponds to a value of 3.73 cm 1 for the effective “tunneling splitting” of the CO dimer. The stacks tend to fall into two groups, corresponding to “isomers” with effective inter-molecular separations of either 4.0 or 4.4 A. The larger inter-molecular separation of the true ground state (4.4 A) likely corresponds to a C-bonded configuration, while the low-lying (0.88 cm −1 ) excited state with the smaller separation (4.0 A) likely displays an O-bonded geometry.

The bending vibration of the CO–N 2 complex has been investigated in the millimeter wave range from 130 to 155 GHz 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 ortho N 2 spin modification were measured and analyzed. Nuclear quadrupole structure due to the presence of two equivalent 14 N 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– ortho N 2 was obtained for the first time to be χ aa =−0.768(43) MHz . The different value and sign of this constant from the one in the K =0 ground state, (χ aa =+0.19641 MHz ) suggests that the orientation and motion of the N 2 subunit are very different in these two states.

The pure rotational spectrum of the van der Waals complex, CH 4 –CO, has been measured using the intracavity OROTRON jet spectrometer in the frequency range of 116–146 GHz. The newly observed and assigned transitions belong to the R -branch with J numbers from 9 to 16 of the K = 1–0 sub-band. This series is a continuation to higher J -values of transitions correlating with the rotationless j M = 0 state ( A symmetry state) of free methane observed previously at lower frequencies [C. Xia, K.A. Walker, A.R.W. Mckellar, J. Chem. Phys. 114 (2001) 4824]. Both data sets were analyzed together with known infrared and microwave transitions in order to determine the molecular parameters of the of CH 4 –CO complex in the A state. The observed millimeter-wave spectrum shows numerous additional transitions which very likely belong to CH 4 –CO composed of methane in the F ( j M = 1) and E ( j M = 2) symmetry states, but these have not yet been assigned.

A recently developed highly sensitive intracavity millimeter wave (MMW) spectrometer has been applied for spectroscopy of molecules in excited vibrational states. The spectrometer utilizes an Orotron oscillator as the tunable, coherent source of radiation from 90 to 160 GHz. A gas cell is placed into a high quality ( Q ≈10 4 ) Fabry–Perot resonator of Orotron, and the absorption signal is detected from the variation of the Orotron electron current in a collector circuit. The spectrometer combines a wide spectral range with Doppler spectral resolution without any phase or frequency lock techniques and achieves the sensitivity in absorption of (3–5)×10 −10 cm −1 . The measurements are presented for rotational and ro-vibrational transitions of SiH 4 in the ν 2 / ν 4 vibrational dyad and high- J pure inversion spectrum of ND 3 in the v 2 =1 vibrational state. Qualitative description of MMW ν 2 / ν 4 dyad spectrum of SiH 4 and analysis of available microwave (MW) data on the pure inversion spectrum of ND 3 in v 2 =1 state are also given.

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. Copyright 1999 Academic Press.

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.

Highly sensitive millimeter-wave spectrometer based on tunable coherent source—orotron is proposed for detection of molecular compounds in the air and water vapor by observation of their rotational spectra in frequency region 90–150 GHz. High sensitivity of measurements (2–3)×10−10 cm−1 which gives the detection limit better than 1 ppm for wide class of molecules is achieved by the placement of investigated gas into a high quality (Q≈104) orotron Fabry–Perot resonator. The absorption signal is measured simply from variation of orotron current in the collector circuit. The narrow spectral width of orotron radiation (15 kHz) provides Doppler resolution of spectral lines and high selectivity of analysis respectively without any locking by external frequency standard. The spectrometer has relatively simple operation principles taking account its universality. At the first step four compounds, namely methanol, acetone, carbonyl sulfide and carbon monoxide, have been measured in mixtures with water vapor or air.

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