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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 weakly bound Kr-CO molecular complex formed in a pulsed molecular jet has been measured in the frequency range 112–150 GHz on an intracavity orotron-based spectrometer. The measured b-type transitions include the R branch of the band K = 1-0 with the rotational numbers J from 16 to 25 and the P branch of the band K = 2-1 with the rotational numbers J from 13 to 17. For the K = 1-0 band, we succeeded in recording transitions in complexes that consist of all stable Kr isotopes, i.e., 84Kr-CO, 86Kr-CO, 82Kr-CO, 83Kr-CO, and 80Kr-CO. For the K = 2-1 band, we observed transitions in the two most abundant isotopologues, 84Kr-CO and 86Kr-CO. The obtained data were used to determine the rotational and centrifugal constants of the Kr-CO complex.

A second-order perturbation theory, developed for calculating the energy levels of the He-CO binary complex, is applied to small He N -CO clusters with N = 2−4, the helium atoms being considered as a single bound object. The interaction potential between the CO molecule and HeN is represented as a linear expansion in Legendre polynomials, in which the free rotation limit is chosen as the zero approximation and the angular dependence of the interaction is considered as a small perturbation. By fitting calculated rotational transitions to experimental values it was possible to determine the optimal parameters of the potential and to achieve good agreement (to within less than 1%) between calculated and experimental energy levels. As a result, the shape of the angular anisotropy of the interaction potential is obtained for various clusters. It turns out that the minimum of the potential energy is smoothly shifted from an angle between the axes of the CO molecule and the cluster of θ = 100° in He-CO to θ = 180° (the oxygen end) in He3-CO and He4-CO clusters. Under the assumption that the distribution of helium atoms with respect to the cluster axis is cylindrically symmetric, the structure of the cluster can be represented as a pyramid with the CO molecule at the vertex.

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.

The rotational spectrum of the van der Waals complex NH3-CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 112-139 GHz. Newly observed and assigned transitions belong to the K = 0-0, K = 1-1, K = 1-0, and K = 2-1 subbands correlating with the rotationless (jk)NH3 = 00 ground state of free ortho-NH3 and the K = 0-1 and K = 2-1 subbands correlating with the (jk)NH3 = 11 ground state of free para-NH3. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. Some of these transitions are continuations to higher J values of transition series observed previously [C. Xia et al., Mol. Phys. 99, 643 (2001)], the other transitions constitute newly detected subbands. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the ortho-NH3-CO and para-NH3-CO complexes. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of NH3-CO has been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations and an augmented correlation-consistent triple zeta basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the N atom closest to the CO subunit and binding energy De = 359.21 cm(-1). The bound rovibrational levels of the NH3-CO complex were calculated for total angular momentum J = 0-6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 210.43 and 218.66 cm(-1) for ortho-NH3-CO and para-NH3-CO, respectively.

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 .

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)].

Analysis of the rotational spectrum of the molecular dimer (CO)2 measured in the millimeter wave range has been performed and four new rotational states are revealed. Three of these states are characterized by almost free rotations of both monomers in the dimer. These states have approximately the same first term σ in the expansion of the rotational energy in powers of the rotational angular momentum J for various values of the momentum projections on the dimer axis (K=0, 1, 2) and various rotational constants B. The intrinsic rotational angular momenta of CO dimers, j1=j2=1, are determined from the σ value. In addition, a state with K=2 is found which corresponds to one of the known shape isomers of (CO)2. The values of the tunneling splitting for each of the new states are determined. The results indicate that previous data on the suppressed tunneling are determined by the asymmetry of internal rotations in the CO monomers rather than by the K value.

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.

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.

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 ...

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.

Microwave and millimeter wave transitions of the CO–N2 complex were investigated using three different instruments, namely, a pulsed molecular beam Fourier transform microwave spectrometer in the frequency region from 4 to 26 GHz, a microwave-millimeter wave double resonance spectrometer in the frequency regions from 8 to 18 GHz for the microwave and 107–118 GHz for the millimeter wave range, and an OROTRON spectrometer in the frequency range from 107 to 132 GHz. Both a- and b-type transitions associated with the ground-state K=0 levels and the lower K=1 levels of the ortho-N2 states, and with rotational quantum number J up to 19, were measured and analyzed. Nuclear quadrupole hyperfine splittings due to the presence of two equivalent 14N nuclei were resolved and analyzed to give additional information about the angular anisotropy of the interaction potential. The nuclear quadrupole coupling constants obtained are χaa=0.196 41(52) MHz for K=0 levels, and χaa=−1.0391(17) MHz, χbb=0.0633(17) MHz for the low...

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.

A detailed description of a new ab initio interaction potential energy surfaces for the H2-CO complex computed on a six-dimensional grid (i.e., including the dependence on the H-H and C-O separations) is presented. The interaction energies were first calculated using the coupled-cluster method with single, double, and noniterative triple excitations and large basis sets, followed by an extrapolation procedure. Next, a contribution from iterative triple and noniterative quadruple excitations was added from calculations in smaller basis sets. The resulting interaction energies were then averaged over the ground-state and both ground- and first-excited-states vibrational wave functions of H2 and CO, respectively. The two resulting four-dimensional potential energy surfaces were fitted by analytic expressions. Theoretical infrared spectra calculated from these surfaces have already been shown [P. Jankowski, A. R. W. McKellar, and K. Szalewicz, Science 336, 1147 (2012)] to agree extremely well, to within a few hundredth of wavenumber, with the experimental spectra of the para and orthoH2-CO complex. In the latter case, this agreement enabled an assignment of the experimental spectrum, ten years after it had been measured. In the present paper, we provide details concerning the development of the surfaces and the process of spectral line assignment. Furthermore, we assign some transitions for paraH2-CO that have not been assigned earlier. A completely new element of the present work are experimental investigations of the orthoH2-CO complex using microwave spectroscopy. Vast parts of the measured spectrum have been interpreted by comparisons with the infrared experiments, including new low-temperature ones, and theoretical spectrum. Better understanding of the spectra of both para and orthoH2-CO complexes provides a solid foundation for a new search of the bound H2-CO complex in space.

Polycyclic Aromatic Hydrocarbons (PAHs) are widely accepted as the carriers of the Aromatic Infrared Bands (AIBs), but an unambiguous identification of any specific interstellar PAH is still missing. For polar PAHs, pure rotational transitions can be used as fingerprints for identification. Combining dedicated experiments, detailed simulations and observations, we explored the mm domain to search for specific rotational transitions of corannulene (C20H10). We performed high-resolution spectroscopic measurements and a simulation of the emission spectrum of UV-excited C20H10 in the environment of the Red Rectangle, calculating its synthetic rotational spectrum. Based on these results, we conducted a first observational campaign at the IRAM 30m telescope towards this source to search for several high-J rotational transitions of (C20H10). The laboratory detection of the J = 112 134 transition at 137.615 GHz. Comparing the noise level with the synthetic spectrum, we are able to estimate an upper limit to the fraction of carbon locked in corannulene of about 1.0x10(-5) relative to the total abundance of carbon in PAHs. The sensitivity achieved shows that radio spectroscopy can be a powerful tool to search for polar PAHs. We compare this upper limit with models for the PAH size distribution, emphasising that small PAHs are much less abundant than predicted. We show that this cannot be explained by destruction but is more likely related to the chemistry of their formation in the environment of the Red Rectangle., Comment: 8 pages, 7 figures, 2 tables, accepted for publication in MNRAS

High resolution microwave and millimeter-wave spectra of He N -CO clusters with N up to 10, produced in a molecular expansion, were observed. Two series of J = 1-0 transitions were detected, which correspond to the a-type and b-type J = 1-0 transitions of He 1 -CO. The B rotational constant initially decreases with N and reaches a minimum at N = 3. Its subsequent rise indicates the transition from a molecular complex to a quantum solvated system already for N = 4. For N≥6, the B value becomes larger than that of He 1 -CO, indicating an almost free rotation of CO within the helium environment.

Contains fulltext : 34677.pdf (Publisher’s version ) (Open Access) New extensive millimeter-wave measurements of the (CO)-C-12-O-16 dimer have been made, and more than 300 new spectral transitions have been observed in the frequency range 81-135 GHz. A joint analysis of these and previous millimeter-wave data yielded the precise location of 33 new energy levels of A(+) symmetry and 20 levels of A(-) symmetry. These energy levels are located at 8-18 cm(-1) above the zero-point level. Some of them belong to already known stacks, and others make up 9 new stacks of the dimer. Newly determined stacks have K = 0, 1, and, for the first time, 2, where K is the projection of the total angular momentum on the intermolecular axis. The energy levels from accompanying rovibrational calculations with the use of a recently developed hybrid CCSD(T)/DFT-SAPT potential are in very good agreement with experiment. Analysis of the calculated wave functions revealed that two new stacks of A(+) symmetry with K = 2 correspond to overall rotation of the dimer while the other newly observed stacks belong to the geared bend overtone modes. The ground vibrational states of the two "isomers" found are more or less localized at the two minima in the potential surface, whereas all the geared bend excited states show a considerable amount of delocalization.

The millimeter wave spectrum of the isotopically substituted CO dimer, (12C18O)2, was studied with the Orotron jet spectrometer, confirming and extending a previous infrared study [A. R. W. McKellar, J. Mol. Spectrosc. 226, 190 (2004)]. A very dilute gas mixture of CO in Ne was used, which resulted in small consumption of 12C18O sample gas and produced cold and simple spectra. Using the technique of combination differences together with the data from the infrared work, six transitions in the 84-127 GHz region have been assigned. They belong to two branches, which connect four low levels of A+ symmetry to three previously unknown levels of A- symmetry. The discovery of the lowest state of A- symmetry, which corresponds to the projection K=0 of the total angular momentum J onto the intermolecular axis, identifies the geared bending mode of the 12C18O dimer at 3.607 cm(-1). Accompanying rovibrational calculations using a recently developed hybrid potential from ab initio coupled cluster [CCSD(T)] and symmetry-adapted perturbation theory calculations [G. W. M. Vissers et al., J. Chem. Phys. 122, 054306 (2005)] gave very good agreement with experiment. The isotopic dependence of the A+/A- energy splitting, the intermolecular separation R, and the energy difference of two ground state isomers, which change significantly when 18O or 13C are substituted into the normal (12C16O)2 isotopolog [L. A. Surin et al., J. Mol. Spectrosc. 223, 132 (2004)], was explained by these calculations. It turns out that the change in anisotropy of the intermolecular potential with respect to the shifted monomer centers of mass is particularly significant.

Contains fulltext : 83553.pdf (Publisher’s version ) (Closed access) 2 p.

High resolution microwave (a-type) and millimetre-wave (b-type) spectra of He(N)-(13)C(16)O, He(N)-(12)C(18)O, and He(N)-(13)C(18)O clusters (Nor= 10) were observed, significantly extending the initial spectroscopic observations on He(N)-(12)C(16)O by Surin et al. [Phys. Rev. Lett., 2008, 101, 233401]. The frequencies of the a-type series, which evolves from the end-over-end rotation of the dimer, decrease from N = 1 to 3, then increase smoothly to at least N = 9. This turnaround indicates a rapid evolution of the solvation character from classical to quantum. The b-type series, which evolves from the free molecule rotation of CO, increases from N = 0 to 6 and then decreases to at least N = 10. This is consistent with an initially increasing anisotropy of the helium environment, followed by a tendency of the solvation shell to become more isotropic. The shift of the vibrational frequency of CO as determined from the infrared [A. R. W. McKellar, J. Chem. Phys., 2004, 121, 6868; ibid., 2006, 125, 164328] and microwave data reveals an approximately linear decrease from N = 1 to at least 9. If the linear shift were to continue until completion of the first solvation shell (N approximately 14), the estimated helium nanodroplet shift will be well undershot [K. von Haeften, S. Rudolph, I. Simanovski, M. Havenith, R. E. Zillich and K. B. Whaley, Phys. Rev. B: Condens. Matter Mater. Phys., 2006, 73, 054502]. In this case, there must be an upturn in the vibrational shift beyond N = 14, which is not predicted by theory [T. Skrbić, S. Moroni and S. Baroni, J. Phys. Chem. A, 2007, 111, 7640]. By extrapolating the a-type series to N = 14 (assuming a linear vibrational shift), we estimate the rotational constant, B, of CO in the helium nanodroplet to be approximately 74% of its gas phase value. This is in reasonable agreement with simulations (76% at N = 14), which predict the limiting value to be approximately reached upon completion of the first solvation shell (73% at N = 100) [T. Skrbić, S. Moroni and S. Baroni, J. Phys. Chem. A, 2007, 111, 7640]. However, this value is significantly larger than that inferred from helium nanodroplet experiments (63%).

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.