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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.

Pure rotational transitions of the deuterated NH3–H2 weakly bound complexes formed by NH3/ND3 and H2/D2, namely ND3–H2, NH3–D2 and ND3–D2, have been detected. Three different techniques, a millimeter-wave intracavity jet OROTRON spectrometer, molecular beam Fourier transform microwave (FTMW) cavity spectrometer and the broadband chirped pulse (CP) FTMW spectrometer were used to record the spectra in the frequency range from 20 to 140 GHz. The double resonance method was additionally applied to extend the frequency range of the OROTRON spectrometer to lower and of the FTMW cavity spectrometer to higher frequencies. A search for the transitions of deuterated isotopologues and their assignments were based on the recent study of the main isotopic species NH3–H2 [L.A. Surin et al., Astrophys. J., 838, 27 (2017)] and further confirmed by the bound state calculations presented here. The measured line positions including hyperfine splitting due to the 14N nuclear spin of NH3/ND3 and the D nuclear spins of D2 were analysed in order to determine the molecular parameters and structure of the deuterated NH3–H2 complexes. This study provides an effective new probe of the intermolecular interaction between ammonia and dihydrogen, knowledge of which is important for numerical modelling of astrophysical environments.

We present a five-dimensional intermolecular potential energy surface (PES) of the NH3-N2 complex, bound state calculations, and new microwave (MW) measurements that provide information on the structure of this complex and a critical test of the potential. Ab initio calculations were carried out using the explicitly correlated coupled cluster [CCSD(T)-F12a] approach with the augmented correlation-consistent aug-cc-pVTZ basis set. The global minimum of the PES corresponds to a configuration in which the angle between the NH3 symmetry axis and the intermolecular axis is 58.7° with the N atom of the NH3 unit closest to the N2 unit, which is nearly parallel to the NH3 symmetry axis. The intermolecular distance is 7.01 a0, and the binding energy De is 250.6 cm-1. The bound rovibrational levels of the four nuclear spin isomers of the complex, which are formed when ortho/para (o/p)-NH3 combines with (o/p)-N2, were calculated on this intermolecular potential surface. The computed dissociation energies D0 are 144.91 cm-1, 146.50 cm-1, 152.29 cm-1, and 154.64 cm-1 for (o)-NH3-(o)-N2, (o)-NH3-(p)-N2, (p)-NH3-(o)-N2, and (p)-NH3-(p)-N2, respectively. Guided by these calculations, the pure rotational transitions of the NH3-N2 van der Waals complex were observed in the frequency range of 13-27 GHz using the chirped-pulse Fourier-transform MW technique. A complicated hyperfine structure due to three quadrupole 14N nuclei was partly resolved and examined for all four nuclear spin isomers of the complex. Newly obtained data definitively established the K values (the projection of the angular momentum J on the intermolecular axis) for the lowest states of the different NH3-N2 nuclear spin isomers.

We report the first high resolution spectroscopic study of the NH3–H2 van der Waals molecular complex. Three different experimental techniques, a molecular beam Fourier transform microwave spectrometer, a millimeter-wave intracavity jet OROTRON spectrometer, and a submillimeter-wave jet spectrometer with multipass cell, were used to detect pure rotational transitions of NH3–H2 in the wide frequency range from 39 to 230 GHz. Two nuclear spin species, (o)-NH3–(o)-H2 and (p)-NH3–(o)-H2, have been assigned as carriers of the observed lines on the basis of accompanying rovibrational calculations performed using the ab initio intermolecular potential energy surface (PES) of Maret et al. The experimental spectra were compared with the theoretical bound state results, thus providing a critical test of the quality of the NH3–H2 PES, which is a key issue for reliable computations of the collisional excitation and de-excitation of ammonia in the dense interstellar medium.

The ground state of He–HCO+ is investigated using a recently developed double resonance technique, consisting of a rotational transition followed by a vibrational transition into a dissociative state. In order to derive precise predictions for the rotational states, the high resolution infrared predissociation spectroscopy of the v1 C–H stretching mode is revisited. Eleven pure rotational transitions are measured via the double resonance method. A least squares fit of these transitions to a standard linear rotor Hamiltonian reveals that the semirigid rotor model cannot fully describe the loosely bound He–HCO+ complex. The novel double resonance technique is compared with other action spectroscopic schemes, and some potential future applications are presented.

The journal of chemical physics 149(22), 224305 (2018). doi:10.1063/1.5063346, The microwave spectrum of the NH$_3$–N$_2$ van der Waals complex has been observed in a supersonic molecular jet expansion via broadband (2-8 GHz) chirped-pulse Fourier-transform microwave spectroscopy. Two pure rotational R(0) transitions (J = 1 − 0) with different hyperfine structure patterns were detected. One transition belongs to the (ortho)-NH$_3$–(ortho)-N$_2$ nuclear spin isomer in the ground K = 0 state reported earlier [G. T. Fraser et al., J. Chem. Phys. 84, 2472 (1986)], while another one is assigned to the (para)-NH$_3$–(para)-N$_2$ spin isomer in the K = 0 state not reported before (K is the projection of the total angular momentum J on the intermolecular axis). The complicated hyperfine structure arising from three quadrupole $^{14}$N nuclei of NH$_3$–N$_2$ was resolved for both transitions, and the quadrupole coupling constants associated with the NH$_3$ and N$_2$ subunits were precisely determined for the first time. These constants provided the dynamical information about the angular orientation of ammonia and nitrogen indicating that the average angle between the C$_3$ axis of NH$_3$ and the N$_2$ axis is about 66°. The average van der Waals bond lengths are slightly different for (ortho)-NH$_3$–(ortho)-N$_2$ and (para)-NH$_3$–(para)-N$_2$ and amount to 3.678 Å and 3.732 Å, respectively. Similar results for the deuterated isotopologues, ND$_3$–N$_2$, NHD$_2$–N$_2$, and NH$_2$D–N$_2$, and their nuclear spin isomers were also obtained thus confirming and extending the analysis for the parent NH$_3$–N$_2$ complex., Published by American Institute of Physics, Melville, NY

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 studies of small helium clusters (up to 100 atoms) and molecular-hydrogen clusters (up to 20 molecules) that are formed in a supersonic gas jet and are coupled by the weak van der Waals interaction with a linear chromophore molecule are reviewed The shift of the frequency of the fundamental vibration of the chromophore, as well as a change in the moment of inertia of a cluster with its growth, has been detected by their rotational and vibrational-rotational spectra. A nonclassical behavior of the moment of inertia manifested in its decrease beginning with a certain number of attached He atoms (H2 molecules) has been revealed. This behavior indicates that a part of a cluster is decoupled from the rotational motion of a molecule. The key question of these studies is whether such behavior of the moment of inertia is the manifestation of the super-fluidity of helium and hydrogen at microscopic level. The results are compared to the spectroscopy of molecules and hydrogen clusters in liquid-helium nanodroplets.

The rotational spectrum of the van der Waals NH3-N2 complex is studied in the frequency range of 112–130 GHz. The transitions are measured in a cold molecular beam with an intracavity spectrometer based on an orotron. A total of six new transitions of different spin modifications of the complex are recorded. Molecular parameters of the K = 0 ground state are determined for the orthoNH3-orthoN2 modification.

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.

The rotational spectrum of the CO-para-H2 van der Waals complex, produced using a molecular jet expansion, was observed with two different techniques: OROTRON intracavity millimeter-wave spectroscopy and pulsed Fourier transform microwave spectroscopy. Thirteen transitions in the frequency range from 80 to 130 GHz and two transitions in the 14 GHz region were measured and assigned, allowing for a precise determination of the corresponding energy level positions of CO-para-H2. The data obtained enable further radio astronomical searches for this molecular complex and provide a sensitive test of the currently best available intermolecular potential energy surface for the CO-H2 system.

A van der Waals complex that consists of a deuterium molecule in the ortho state and a carbon monoxide molecule, CO-ortho-D2, was studied in the frequency range 85–130 GHz with the help of an intracavity orotron-based spectrometer. Nine new lines, which correspond to rotational transitions and belong to the R and Q branches, were measured and identified. The positions of the rotational energy levels of CO-ortho-D2 were refined by comparative analysis of the transition frequencies measured in this work and previously reported microwave and infrared data.

Van der Waals complexes consisting of helium atoms and carbon monoxide molecules, He-12C16O, He-13C16O, He-12C18O, and He-13C18O are studied in the frequency range 110–140 GHz on an intracavity orotron-based spectrometer. Ten new lines, which correspond to rotational transitions and transitions to the bending vibration of the complex, are detected and identified as lines belonging to the R and P branches. The positions of the energy levels of isotopologue of the He-CO complex are refined by joint analysis of both the transition frequencies measured in this study and previously published microwave and infrared data. Isotopic dependences of the positions of the He-CO energy levels are obtained. These results can serve as the starting point for studying small helium clusters with embedded CO molecules.

This paper reviews research on helium clusters (of up to 20 atoms) and molecular hydrogen clusters (of up to 17 molecules) weakly bound by van der Waals forces to a light chromophore molecule such as OCS, N2O, CO2, or CO. Such clusters form in supersonic gas jets and are studied through the spectrum of the particular chromophore used. It is found that as the cluster grows, its rotational frequency increases (the effective moment of inertia decreases) starting from a certain number of atoms (molecules) attached. Also, in CO-based clusters a nearly free rotation of the chromophore molecule was observed. Experimental studies of such clusters are reviewed, as are those of the N2–CO and CO–CO complexes in which both monomers nearly freely rotate. The relation of these rotations to the superfluidity of helium and hydrogen is discussed, and comparisons are made with spectroscopic experiments on chromophores and hydrogen clusters in liquid helium nanodroplets.

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.

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 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 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 providin...

A review of recent results on the millimetre-wave spectroscopy of weakly bound van der Waals complexes, mostly those which contain H 2 and He, is presented. In our work, we compared the experimental spectra to the theoretical bound state results, thus providing a critical test of the quality of the M–H 2 and M–He potential energy surfaces (PESs) which are a key issue for reliable computations of the collisional excitation and de-excitation of molecules (M = CO, NH 3 , H 2 O) in the dense interstellar medium. The intermolecular interactions with He and H 2 play also an important role for high resolution spectroscopy of helium or para -hydrogen clusters doped by a probe molecule (CO, HCN). Such experiments are directed on the detection of superfluid response of molecular rotation in the He and p -H 2 clusters.

The interaction between ammonia and helium has attracted considerable interest over many years, partly because of the observation of interstellar ammonia. The rate coefficients of NH3 –He scattering are an important ingredient for numerical modeling of astrochemical environments. Another, though quite different application in which the NH3 –He interaction can play an important role is the doping of helium clusters with NH3 molecules to perform high-resolution spectroscopy. Such experiments are directed on the detection of non-classical response of molecular rotation in helium clusters addressing fundamental questions related to the microscopic nature of superfluidity. High-resolution spectroscopy on the NH3–He complex is an important tool for increasing our understanding of intermolecular forces between NH3 and He.

The rotational spectra of the NH 3 –D 2 and ND 3 –H 2 van der Waals complexes have been observed in a supersonic molecular jet for the first time. The OROTRON intracavity spectrometer was used to detect pure rotational transitions in the frequency range from 75 to 120 GHz. The search of the spectral lines was based on the initial observation of the main isotopic species of the NH3−H2. The measured transition frequencies including hyperfine splitting were analyzed in order to determine the energy levels, molecular parameters and structure of the ortho ND 3 − ortho H 2 and ortho NH 3 − para D 2 complexes.

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.

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.

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.

The design and operation of a gas millimeter-wave spectrometer based on a tunable generator orotron are presented. The absorption cell occupies a part of the high quality (Q approximately equals 10 4 ) orotron Fabry-Perot cavity. The limit of sensitivity (gamma) min approximately equals (3- 5)X 10 -10 cm -1 has been achieved with source frequency modulation at the output band of a receiver of 1 Hz. The narrow spectral width of the orotron radiation (10 - 15 kHz without locking) provides the Doppler resolution of spectral lines.

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.