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Infrared spectra of N2O trimers are studied using a tunable diode laser to probe a pulsed supersonic slit-jet expansion. A previous observation by Miller and Pedersen [J. Chem. Phys. 108, 436 (1998)] in the N2O ν1+ν3 combination band region (∼3480 cm-1) showed the trimer structure to be noncyclic, with three inequivalent N2O monomer units which could be thought of as an N2O dimer (slipped antiparallel configuration) plus a third monomer unit lying above the dimer plane. The present observations cover the N2O fundamental band regions ν3 (∼1280 cm-1) and ν1 (∼2230 cm-1). In the ν3 region, two trimer bands are assigned with vibrational shifts and other characteristics similar to those in the ν1+ν3 region, but in the ν1 region all three possible trimer bands are observed. Relationships among the various bands are considered with reference to their rotational intensity patterns, their vibrational shifts, and the properties of the related N2O dimer, with results that generally support the conclusions of Miller and Pedersen. Three trimer bands are also observed for the fully 15N-substituted species in the ν1 region, and these results should aid in the detection of the as-yet-unobserved pure rotational microwave spectrum of the trimer. [ABSTRACT FROM AUTHOR]

The spectra of the weakly bound complex CO2–N2O are studied in the regions of the ν1 and ν3 fundamentals of N2O and the ν3 fundamental of CO2 using a rapid-scan tunable diode laser spectrometer to probe a pulsed supersonic jet expansion. Five bands are measured and analyzed, of which four have not been previously studied. Two bands at 2225.75 and 1279.58 cm-1, with a/b-type rotational structure, are assigned to the ν1 and ν3 fundamentals of the N2O component of the complex. Small perturbations are noted in the ν3 band, similar to those observed previously for the nonpolar dimer (N2O)2 in the ν3 and ν1+ν3 regions. The previously known band at 2348.86 cm-1 in the region of the ν3 CO2 stretch is analyzed in improved detail. Weaker c-type bands at 2251.46 and 2374.67 cm-1 are assigned as combinations of the intermolecular torsional (out-of-plane) vibration plus the N2O ν1 and CO2 ν3 stretches, respectively. The resulting torsional frequency of CO2–N2O is experimentally determined for the first time to be 25.8 cm-1. [ABSTRACT FROM AUTHOR]

A new parallel (ΔK=0) band of the cyclic CO2 trimer is observed at 2364 cm-1. The trimers are generated in a pulsed supersonic expansion from a slit-jet nozzle and probed with a tunable infrared diode laser. The band is assigned as a combination of an intramolecular CO2 monomer ν3 stretch and an intermolecular out-of-plane torsion, giving a torsional frequency of 12–13 cm-1. The band is surprisingly strong and completely unperturbed, providing a rare and near perfect example for a parallel band of a symmetric top molecule with C3h symmetry and zero nuclear spins. [ABSTRACT FROM AUTHOR]

Infrared spectra of the barrel-shaped trimer (OCS)3, previously known from its microwave spectrum, are reported for the first time. The observations are carried out in a supersonic slit-jet expansion of a He+OCS gas mixture which is probed with a tunable diode laser. Three rotationally resolved bands associated with the ν1 fundamental vibration of OCS (2062.20 cm-1) are observed, at about 2047, 2053, and 2077 cm-1. Small perturbations are noted in the 2077 cm-1 band and may also be present in the 2053 cm-1 band, which is weak and hence more difficult to analyze precisely. Employing a variety of evidence, we suggest a plausible assignment for the nature of the OCS vibrations in each of the three bands. [ABSTRACT FROM AUTHOR]

Spectra of the nitrous oxide dimer (N2O)2 are studied in the region of the N2O ν1 fundamental band around 2230 cm-1 using a rapid-scan tunable diode laser spectrometer to probe a pulsed supersonic jet expansion. The previously known band of the centrosymmetric nonpolar dimer is analyzed in improved detail, and a new band is observed and assigned to a polar isomer of (N2O)2. This polar form of the dimer has a slipped parallel structure, rather similar to the slipped antiparallel structure of the nonpolar form but with a slightly larger intermolecular distance. The accurate rotational parameters determined here should enable a microwave observation of the polar N2O dimer. The need for a modern ab initio investigation of the N2O–N2O intermolecular potential energy surface is emphasized. [ABSTRACT FROM AUTHOR]

Infrared spectra of the OCS–He van der Waals complex and of OCS–HeN clusters have been studied in the region of the OCS ν1 fundamental band using a tunable diode laser to probe a pulsed supersonic slit jet. For the complex, the spectrum of the normal isotope, 16O12C32S–4He, has been considerably extended and the 34S- and 13C-substituted forms have been recorded for the first time. The data could be analyzed satisfactorily using a conventional asymmetric rotor Hamiltonian with sextic centrifugal distortion terms. For the clusters, the 34S- and 13C-substituted forms have been observed and assigned for N=2–7, including some transitions with higher J values than previously reported for the normal isotope, e.g., R(5). The observed vibrational shifts, relative to the free OCS molecule, were very similar to those of the normal isotope, and most of the difference could be explained by simple scaling. These results constitute a subtle and precise probe of intermolecular forces and dynamical effects in a system which is of current interest for cluster studies. [ABSTRACT FROM AUTHOR]

Rovibrational spectra of the He2–N2O complex in the ν1 fundamental band of N2O (2224 cm-1) have been observed using a tunable infrared laser to probe a pulsed supersonic jet expansion, and calculated using five coordinates that specify the positions of the He atoms with respect to the NNO molecule, a product basis, and a Lanczos eigensolver. Vibrational dynamics of the complex are dominated by the torsional motion of the two He atoms on a ring encircling the N2O molecule. The resulting torsional states could be readily identified, and they are relatively uncoupled to other He motions up to at least υt=7. Good agreement between experiment and theory was obtained with only one adjustable parameter, the band origin. The calculated results were crucial in assigning many weaker observed transitions because the effective rotational constants depend strongly on the torsional state. The observed spectra had effective temperatures around 0.7 K and involved transitions with J≤3, with υt=0 and 1, and (with one possible exception) with Δυt=0. Mixing of the torsion-rotation states is small but significant: some transitions with Δυt≠0 were predicted to have appreciable intensity even assuming that the dipole transition moment coincides perfectly with the NNO axis. One such transition was tentatively assigned in the observed spectra, but confirmation will require further work. [ABSTRACT FROM AUTHOR]

The structures and rotational dynamics of clusters of a single carbon monoxide molecule solvated in para-hydrogen, (paraH2)N–CO, have been simulated for sizes up to N=17 using the reptation Monte Carlo technique. The calculations indicate the presence of two series of R(0) rotational transitions with J=1←0 for cold clusters, similar to those predicted and observed in the case of HeN–CO. Infrared spectra of these clusters have been observed in the region of the C–O stretch (≈2143 cm-1) in a pulsed supersonic jet expansion using a tunable diode laser probe. With the help of the calculations, the observed R(0) rotational transitions have been assigned up to N=9 for the b-type series and N=14 for the a-type series. Theory and experiment agree rather well, except that theory tends to overestimate the b-type energies. The (paraH2)12–CO cluster is calculated to be particularly stable and (relatively) rigid, corresponding to completion of the first solvation shell, and it is observed to have the strongest a-type transition. [ABSTRACT FROM AUTHOR]

High-resolution Fourier transform spectra of trans-acrolein, H2C=C(H)—C(H)=O, have been recorded in the 10 μm region at both room and cooled temperatures on the modified Bomem DA3.002 at the National Research Council of Canada and the Bruker IFS 125HR spectrometer at the far infrared beam line of the Canadian Light Source in Saskatoon. Vibrational fundamentals analyzed so far include the ν11, ν16 and ν14 bands centered at 911.3, 958.7 and 992.7 cm−1 corresponding respectively to the A′ in-plane=CH2-rocking mode, the A″ out-of-plane=CH2-wagging mode, and the A″ wagging mode highly mixed between the ⩾C—H vinyl and ⩾C—H formyl groups [Vibrational mode descriptions are based on Y.N. Panchenko, P. Pulay, F. Török, J. Mol. Spectrosc. 34 (1976) 283–289.] As well, the ν16 + ν18 − ν18 hot band centred at 957.6 cm−1 has been analyzed, where ν18 is the low-frequency (157.9 cm−1) A″ ⩾C—C⪕ torsional mode. The ν11 band is a/b type while the ν16, ν14 and ν16 + ν18 − ν18 bands are c-type. The assigned transitions of each band have been fitted to a Watson asymmetric rotor Hamiltonian, with ground state parameters fixed to values obtained from rotational analyses in the literature. As well, a combined 3-state fit for ν11, ν16 and ν14 was carried out including Coriolis and Z1 constants which account for J and ΔK interactions. Transition dipole moments have been calculated for each of the fundamentals using the ab initio B3LYP method and 6-311++G∗∗ basis set. For the A′ vibrational modes, we have also evaluated transition dipole a- and b-components in the principal axis system from vibrational displacements and dipole moment derivatives. Our ab initio results predict that the ν11 in-plane=CH2 rocking mode has an a-type transition strength about three times greater than the b-type, which is consistent with our observations. Our ab initio force field analysis gives vibrational mode descriptions consistent with previously published work for all 13 A′ modes. However, for the five A″ vibrational modes, our ab initio results disagree with two of the descriptions of Panchenko et al. and three of the descriptions of Hamada et al. [ABSTRACT FROM AUTHOR]

We triply ionize the van der Waals bound carbon monoxide dimer with intense ultrashort pulses and study the breakup channel (CO)23+→C++O++CO+. The fragments are recorded in a cold target recoil ion momentum spectrometer. We observe a fast CO2+ dissociation channel in the dimer, which does not exist for the monomer. We found that a nearby charge breaks the symmetry of a X³Π state of CO2+ and induces an avoided crossing that allows a fast dissociation. Calculation on the full dimer complex shows the coupling of different charge states, as predicted from excimer theory, gives rise to electronic state components not present in the monomer, thereby enabling fast dissociation with higher kinetic energy release. These results demonstrate that the electronic structure of molecular cluster complexes can give rise to dynamics that is qualitatively different from that observed in the component monomers. [ABSTRACT FROM AUTHOR]

Infrared spectra of acetylene-water complexes are studied in the regions of the H 2 O ν 2 bend (1600 cm -1 ) and the D 2 O ν 1 /ν 3 stretches (2670-2808 cm -1 ), using tunable infrared sources to probe a pulsed supersonic slit jet expansion. In the H 2 O bend region, there is a puzzling absence of K a = 0 transitions for C 2 H 2 -H 2 O, while both K a = 0 and 1 are observed for C 2 O. This continues a pattern of "missing" 2 -H 2 O. This continues a pattern of "missing" K a states noted in previous infrared studies of acetylene-water. Noticeable line broadening gives estimates of upper state predissociation lifetimes of about 1.6 ns for C 2 D 2 -H 2 O with K a ' = 1, 0.8 ns for C 2 H 2 -H 2 O with K a ' = 1, and 0.8 ns for C 2 D 2 -H 2 O with K = 0 ← 1, 1 ← 0, and 2 ← 1 are observed which enable a good determination of the a ' = 0. The lifetime for C 2 H 2 -H 2 ), a first for acetylene-water dimer and larger than previously expected.K a ' = 0 must be much shorter (<0.05 ns) since rapid predissociation seems to be the only explanation for its absence in the spectrum. In the D 2 O ν 3 region, clear sub-bands with K a = 0 ← 1, 1 ← 0, and 2 ← 1 are observed which enable a good determination of the A rotational constant of C 2 H 2 -D 2 O (7.313 ± 0.007 cm -1 ), a first for acetylene-water dimer and larger than previously expected.

The weakly bound dimer water-carbon disulfide is studied by ab initio structure theory and high-resolution infrared spectroscopy. The calculations yield three stable minima in the potential energy surface, all planar. The most stable, isomer 1, was observed previously by microwave spectroscopy. It has C symmetry, an S-O bond, and a linear O-S-C-S backbone. Isomer 2, the next most stable, has not been considered previously by theory or experiment. It also has 2v symmetry, but with a side-by-side structure. Isomer 3, which is slightly less stable, is side-by-side with one O-H bond pointing toward an S atom. The first gas phase water-CS C 2v symmetry, but with a side-by-side structure. Isomer 3, which is slightly less stable, is side-by-side with one O-H bond pointing toward an S atom. The first gas phase water-CS 2 infrared spectra are reported. For isomer 1, H 2 O-, HDO-, and D 2 O-CS 2 are observed in the CS 2 ν 1 + ν 3 region, H 2 O-CS 2 in the H 2 O ν 2 bend region, and D 2 rotational parameter, which turns out to be larger than expected. The new isomer 2 is observed by means of a band in the H 2 in the D 2 O ν 1 and ν 3 stretch regions. The latter ν 3 spectrum enables an experimental determination of the A rotational parameter, which turns out to be larger than expected. The new isomer 2 is observed by means of a band in the H 2 O ν 2 region, confirming its C 2v symmetry and calculated structure.

Spectra of mixed H/D water dimers are studied in the mid-infrared region of the O-D stretch fundamental (2630-2800 cm-1) using a pulsed supersonic slit jet and a tunable optical parametric oscillator infrared source. Over 30 bands, belonging to nine of the ten possible isotopologues (only H2O-HOD is missed), are observed and analyzed. The implications for excited state tunneling splittings, lifetime effects, and vibrational shifts are discussed. These are the first significant new experimental results on (gas phase) mixed water dimers in over 25 years, and they are valuable for testing water dimer theoretical calculations, a field which continues to be of lively current interest., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

The O-D stretch fundamental region of the deuterated water dimer, (D2O)2, is further studied using a pulsed supersonic slit jet and a tunable optical parametric oscillator infrared source. The previously unobserved acceptor symmetric O-D stretch fundamental vibration is detected, with Ka = 0 ← 0 and 1 ← 0 sub-bands at about 2669 and 2674 cm-1, respectively. The analysis indicates that the various water dimer tunneling splittings generally decrease in the excited vibrational state, similar to the three other previously observed O-D stretch fundamentals. Two new (D2O)2 combination bands are observed, giving information on intermolecular vibrations in the excited O-D stretch states. The likely vibrational assignments for these and a previously observed combination band are discussed., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

Spectra of water─CO 2 dimers are studied using a tunable mid-infrared source to probe a pulsed slit jet supersonic expansion. H 2 O-CO 2 and D 2 O-CO 2 are observed in the CO 2 ν 3 fundamental region (≈2350 cm -1 ), D 2 O-CO 2 is also observed in the D 2 O ν 3 fundamental region (≈2790 cm -1 ), and HDO-CO 2 is observed in the HDO O-D stretch fundamental region (≈2720 cm -1 ), all for the first time in these regions. Analysis of the spectra yields excited state rotational parameters and vibrational shifts. They also yield the first experimental values of the ground state internal rotation tunneling splittings for D 2 O-CO 2 (0.003 cm -1 ) and HDO-CO 2 (0.0234 cm -1 ). The latter value is a direct determination made possible by the reduced symmetry of HDO-CO 2 . These results provide stringent and easily interpreted tests for theoretical water-CO 2 potential energy surface calculations.

Spectra of rare gas atom clusters containing a single carbon dioxide molecule are observed using a tunable mid-infrared (4.3 µm) source to probe a pulsed slit jet supersonic expansion. There are relatively few previous detailed experimental results on such clusters. The assigned clusters include CO 2 -Ar n with n = 3, 4, 6, 9, 10, 11, 12, 15, and 17, and CO 2 -Kr n and CO 2 -Xe n with n = 3, 4, and 5. Each spectrum has (at least) a partially resolved rotational structure, and each yields precise values for the shift of the CO 2 vibrational frequency (ν 3 ) induced by the nearby rare gas atoms, together with one or more rotational constants. These results are compared with theoretical predictions. The more readily assigned CO 2 -Ar n species tend to be those with symmetric structures, and CO 2 -Ar 17 represents completion of a highly symmetric (D 5h ) solvation shell. Those not assigned (e.g., n = 7 and 13) are probably also present in the observed spectra but with band structures that are not well-resolved and, thus, are not recognizable. The spectra of CO 2 -Ar 9 , CO 2 -Ar 15 , and CO 2 -Ar 17 suggest the presence of sequences involving very low frequency (≈2 cm -1 ) cluster vibrational modes, an interpretation which should be amenable to theoretical confirmation (or rejection).

Weakly bound CO 2 -Rg 2 trimers are studied by high-resolution (0.002 cm -1 ) infrared spectroscopy in the region of the CO 2 ν 3 fundamental band (≈2350 cm -1 ), using a tunable optical parametric oscillator to probe a pulsed supersonic slit jet expansion with an effective rotational temperature of about 2 K. CO 2 -Ar 2 spectra have been reported previously, but they are extended here to include Rg = Ne, Kr, and Xe as well as new combination and hot bands. For Kr and Xe, a unified scaled parameter scheme is used to account for the many possible isotopic species. Vibrational shifts of CO 2 -Rg 2 trimers are compared to those of CO 2 -Rg dimers, and in all cases the trimer shifts are slightly more positive (blue-shifted) than expected on the basis of linear extrapolation from the dimer. Combination bands directly measure an intermolecular vibrational mode (the CO 2 rock) and give values of about 32.2, 33.8, and 34.7 cm -1 for CO 2 -Ar 2 , -Kr 2 , and -Xe 2 . Structural parameters derived for CO 2 -Rg 2 trimers are compared with those of CO 2 -Rg and Rg 2 dimers. Spectra of the mixed trimers CO 2 -Rg-He are also reported.

Widespread interest in weakly bound molecular clusters of medium size (5-50 molecules) is motivated by their complicated energy landscapes, which lead to hundreds or thousands of distinct isomers. But most studies are theoretical in nature, and there are no experimental results which provide definitive structural information on completion of the first solvation shell. Here we assign rotationally resolved mid-infrared spectra to argon clusters containing a single carbon dioxide molecule, CO 2 -Ar 15 and CO 2 -Ar 17 . These mark the completion of the first solvation shell for CO 2 in argon. The assignments are confirmed by nuclear spin intensity alternation in the spectra, a marker of highly symmetric structures for these clusters. Precise values are determined for rotational parameters and for shifts of the CO 2 vibrational frequency induced by the argon atoms. The spectra indicate possible low-frequency (∼2 cm -1 ) vibrational modes in these clusters, posing a challenge for future cluster theory.

Structural calculations and high-resolution infrared spectra are reported for trimers and tetramers containing CO 2 together with CO and/or N 2 . Among the 9 clusters studied here, only (CO 2 ) 2 -CO was previously observed by high-resolution spectroscopy. The spectra, which occur in the region of the ν 3 fundamental of CO 2 ), were recorded using a tunable optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. The trimers (CO -1 ), were recorded using a tunable optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. The trimers (CO 2 ) 2 -CO and (CO 2 ) 2 -N 2 is aligned along the symmetry axis of a staggered side-by-side CO 2 is aligned along the symmetry axis of a staggered side-by-side CO 2 dimer unit. The observation of two fundamental bands for (CO 2 ) 2 -CO and (CO 2 ) 2 -N 2 shows that this CO 2 dimer unit is non-planar, unlike (CO 2 ) 2 itself. For the trimers CO 2 -(CO) 2 and CO 2 -(N 2 ) 2 , the CO or N 2 monomers occupy equivalent positions in the 'equatorial plane' of the CO 2 , pointing toward its C atom. To form the tetramers CO 2 -(CO) 3 and CO 2 -(N 2 , a third CO or N 3 , a third CO or N 2 monomer is then added off to the 'side' of the first two. In the mixed tetramers CO 2 -(CO) 2 -N 2 and CO 2 -CO-(N 2 ) 2 , this 'side' position is taken by N 2 and not CO. In addition to the fundamental bands, combination bands are also observed for (CO 2 ) 2 -CO, CO 2 -(CO) 2 , and CO 2 -(N 2 ) 2 , yielding some information about their low-frequency intermolecular vibrations.

We extend studies of micro-solvation of carbon monoxide by a combination of high-resolution IR spectroscopy and ab initio calculations. Spectra of the (H 2 O) 4 -CO and (D 2 O) 4 -CO pentamers are observed in the C-O stretch fundamental region (≈2150 cm -1 ). The H 2 O containing spectrum is broadened by predissociation, but that of D 2 O is sharp, enabling detailed analysis that gives a precise band origin and rotational parameters. Ab initio calculations are employed to confirm the assignment to (water) 4 -CO and to determine the structure in which the geometry of the (water) 4 fragment is a cyclic ring very similar to the isolated water tetramer. The CO fragment is located "above" the ring plane, with a partial hydrogen bond between the C atom and one of the "free" protons (deuterons) of the water tetramer. Together with the previous results on D 2 O-CO, (D 2 O) 2 -CO, and (D 2 O) 3 -CO, this represents a probe of the four initial steps in the solvation of carbon monoxide at a high resolution.

The weak infrared spectrum of CO2-Ar corresponding to the (0111) ← (0110) hot band of CO2 is detected in the region of the carbon dioxide ν3 fundamental vibration (≈2340 cm-1), using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. While this method was previously thought to cool clusters to the lowest rotational states of the ground vibrational state, here we show that under suitable jet expansion conditions, sufficient population remains in the first excited bending mode of CO2 (1-2%) to enable observation of vibrationally hot CO2-Ar, and thus to investigate the symmetry breaking of the intramolecular bending mode of CO2 in the presence of Ar. The bending mode of the CO2 monomer splits into an in-plane and an out-of-plane mode, strongly linked by a Coriolis interaction. Analysis of the spectrum yields a direct measurement of the in-plane/out-of-plane splitting measured to be 0.8770 cm-1. Calculations were carried out to determine if key features of our results, i.e., the sign and magnitude of the shift in the energy for the two intramolecular bending modes, are consistent with a quantum chemical potential energy surface. This aspect of intramolecular interactions has received little previous experimental and theoretical consideration. Therefore, we provide an additional avenue by which to study the intramolecular dynamics of this simplest dimer in its bending modes. Similar results should be possible for other weakly-bound complexes.

Infrared spectra of the CO 2 -N 2 dimer are observed in the carbon dioxide ν 3 asymmetric stretch region (≈2350 cm -1 ) using a tunable infrared optical parametric oscillator to probe a pulsed slit jet supersonic expansion. Previous results for the b-type fundamental band are extended to higher values of K a . An a-type combination band involving the lowest in-plane intermolecular bending mode is observed. This yields a value of 21.4 cm -1 and represents the first experimental determination of an intermolecular mode for CO 2 -N 2 . This intermolecular frequency is at odds with the value of 45.9 cm -1 obtained from a recent 4D intermolecular potential energy surface. In addition, two weak bands near 2337 cm -1 are assigned to the CO 2 hot band transition (v 1 , v 2 l2 , v 3 ) = (01 1 0). They yield a value of 2.307 cm 1 0). They yield a value of 2.307 cm -1 for the splitting of the degenerate CO 2 ν 2 . The in-plane mode lies at a lower energy relative to the out-of-plane mode.2 . The in-plane mode lies at a lower energy relative to the out-of-plane mode.

The infrared spectrum of the weakly bound C 2 H 4 -CO 2 complex is investigated in the region of the ν 3 fundamental band of CO 2 (≈2350 cm -1 ), using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. The spacing of the various K -subbands in this perpendicular (Δ K = ±1) spectrum is very irregular, and the pattern of irregularity is quite different from that observed previously in another C 2 H 4 -CO 2 band by Bemish et al. [ J. Chem. Phys. 1995 , 103 , 7788 ]. But by allowing for the different symmetry of the ν 3 (CO 2 ) upper vibrational state, both results can be strikingly well explained using the "double internal rotor" model as described by Bemish et al.

The weakly-bound molecular clusters (D2O)2-CO and (D2O)3-CO are observed in the C-O stretch fundamental region (≈2150 cm-1), and their rotationally-resolved infrared spectra yield precise rotational parameters. The corresponding H2O clusters are also observed, but their bands are broadened by predissociation, preventing detailed analysis. The rotational parameters are insufficient in themselves to determine cluster structures, so ab initio calculations are employed, and good agreement between the experiment and theory is found for the most stable cluster isomers, yielding the basic cluster geometries as well as confirming the assignments to (D2O)2-CO and (D2O)3-CO. The trimer, (D2O)2-CO, has a near-planar geometry with one D atom from each D2O slightly out of the plane. The tetramer, (D2O)3-CO, has the water molecules arranged in a cyclic quasi-planar ring similar to the water trimer, with the carbon monoxide located 'above' the ring and roughly parallel to its plane. The tunneling effects observed in the free water dimer and trimer are quenched by the presence of CO. The previously observed water-CO dimer together with the trimer and tetramer reported here represent the first three steps in the solvation of carbon monoxide.

Bound state rovibrational energy level calculations using a high-level intermolecular potential surface are reported for H2O-CO and D2O-CO. They predict the ground K = 1 levels to lie about 20 (12) cm-1 above K = 0 for H2O-CO (D2O-CO) in good agreement with past experiments. But the first excited K = 1 levels are predicted to lie about 3 cm-1 below their K = 0 counterparts in both cases. Line strength calculations also indicate that mid-infrared transitions from the K = 0 ground state to this seemingly anomalous excited K = 1 state should be observable. These predictions are strikingly verified by new spectroscopic measurements covering the C-O stretch region around 2200 cm-1 for H2O-CO, D2O-CO, and HOD-CO, and the O-D stretch region around 2700 cm-1 for D2O-CO, HOD-CO, and DOH-CO. The experiments probe a pulsed supersonic slit jet expansion using tunable infrared quantum cascade laser or optical parametric oscillator sources. Discrete perturbations in the O-D stretch region give an experimental lower limit to the binding energy D0 of about 340 cm-1 for D2O-CO, as compared to our calculated value of 368 cm-1. Wavefunction plots are presented to help understand the intermolecular dynamics of H2O-CO. Coriolis interactions are invoked to explain the seemingly anomalous energies of the first excited K = 1 levels.

The fundamental O-D stretch region (2600-2800 cm -1 ) of the fully deuterated water dimer (D 2 O) 2 is studied using a pulsed supersonic slit jet source and a tunable optical parametric oscillator source. Relatively high spectral resolution (0.002 cm -1 ) enables all six dimer tunneling components to be observed, in most cases, for the acceptor asymmetric O-D stretch, the donor free O-D stretch, and the donor bound O-D stretch vibrations. The dominant acceptor switching tunneling splittings are observed to decrease moderately in the excited O-D stretch states, to roughly 75% of their ground state values, whereas the smaller donor-acceptor interchange splittings show more dramatic and irregular decreases. Excited state predissociation lifetimes, as determined from the observed line broadening, show large variations (0.2 ≤ τ ≤ 5 ns) depending on the vibrational state, K-value, and tunneling symmetry. Another very weak band is tentatively assigned to a combination mode involving an intramolecular O-D stretch plus an intermolecular twist overtone. Asymmetric O-D stretch bands of the mixed isotopologue dimers D 2 O-DOH and D 2 O-HOD are also observed and analyzed.

We report observation of new infrared bands of (CS 2 ) 2 and (CS 2 ) 3 in the region of the CS 2 ν 1 + ν 3 combination band (at 4.5 µm) using a quantum cascade laser. The complexes are formed in a pulsed supersonic slit-jet expansion of a gas mixture of carbon disulfide in helium. We have previously shown that the most stable isomer of (CS 2 ) 2 is a cross-shaped structure with D 2d symmetry and that for (CS 2 ) 3 is a barrel-shaped structure with D 3 symmetry. The dimer has one doubly degenerate infrared-active band in the ν 1 + ν 3 region of the CS 2 monomer. This band is observed to have a rather small vibrational shift of -0.844 cm -1 . We expect one parallel and one perpendicular infrared-active band for the trimer but observe two parallel bands and one perpendicular band. Much larger vibrational shifts of -8.953 cm -1 for the perpendicular band and -8.845 cm -1 and +16.681 cm -1 for the parallel bands are observed. Vibrational shifts and possible vibrational assignments, in the case of the parallel bands of the trimer, are discussed using group theoretical arguments.

Only a few weakly-bound complexes containing the O2 molecule have been characterized by high resolution spectroscopy, no doubt due to the complications added by the oxygen molecule's unpaired electron spin. Here we report an extensive infrared spectrum of CO-O2, observed in the CO fundamental band region using a tunable quantum cascade laser to probe a pulsed supersonic jet expansion. The rotational energy level pattern derived from the spectrum consists of stacks of levels characterized by the total angular momentum, J, and its projection on the intermolecular axis, K. Five such stacks are observed in the ground vibrational state, and ten in the excited state (ν(CO) = 1). They are divided into two groups, with no observed transitions between groups. The groups correspond to different projections of the O2 electron spin, and correlate with the two lowest fine structure states of O2, (N, J) = (1, 0) and (1, 2). The rotational constant of the lowest K = 0 stack implies an effective intermolecular separation of 3.82 Å, but this should be interpreted with caution since it ignores possible effects of electron spin. A new high-level 4-dimensional potential energy surface is developed for CO-O2, and rotational energy levels are calculated for this surface, ignoring electron spin. By comparing calculated and observed levels, it is possible to assign detailed quantum labels to the observed level stacks.

We triply ionize the van der Waals bound carbon monoxide dimer with intense ultrashort pulses and study the breakup channel (CO)&#95;{2}^{3+}→C^{+}+O^{+}+CO^{+}. The fragments are recorded in a cold target recoil ion momentum spectrometer. We observe a fast CO^{2+} dissociation channel in the dimer, which does not exist for the monomer. We found that a nearby charge breaks the symmetry of a X^{3}Π state of CO^{2+} and induces an avoided crossing that allows a fast dissociation. Calculation on the full dimer complex shows the coupling of different charge states, as predicted from excimer theory, gives rise to electronic state components not present in the monomer, thereby enabling fast dissociation with higher kinetic energy release. These results demonstrate that the electronic structure of molecular cluster complexes can give rise to dynamics that is qualitatively different from that observed in the component monomers.

The weakly bound van der Waals dimer (CO 2 ) 2 has long been of considerable theoretical and experimental interest. Here, we study its low frequency intermolecular vibrations by means of combination bands in the region of the CO 2 monomer ν 3 fundamental (≈2350 cm -1 ), which are observed using a tunable infrared laser to probe a pulsed supersonic slit jet expansion. With the help of a recent high level ab initio calculation by Wang, Carrington, and Dawes, four intermolecular frequencies are assigned: the in-plane disrotatory bend (22.26 cm -1 ); the out-of-plane torsion (23.24 cm -1 ); twice the disrotatory bend (31.51 cm -1 ); and the in-plane conrotatory bend (92.25 cm -1 ). The disrotatory bend and torsion, separated by only 0.98 cm -1 , are strongly mixed by Coriolis interactions. The disrotatory bend overtone is well behaved, but the conrotatory bend is highly perturbed and could not be well fitted. The latter perturbations could be due to tunneling effects, which have not previously been observed experimentally for CO 2 dimer. A fifth combination band, located 1.3 cm -1 below the conrotatory bend, remains unassigned.

A broad-band (2135-2200 cm(-1)) infrared spectrum of the CO dimer is recorded using a tunable quantum cascade laser to probe a supersonic jet expansion with an effective rotational temperature of about 2.5 K. Analysis of the spectrum reveals the first known levels of the excited state (vCO = 1) with A(+) symmetry and establishes that resonant vibrational splittings are small (<0.2 cm(-1)) for both the C-bonded and O-bonded dimer isomers. The spectrum extends over a surprisingly large range, with somewhat reduced intensity above 2150 cm(-1). A total of 28 new "stacks" of rotational levels having A(-) symmetry are assigned for vCO = 1 on the basis of combination differences, adding to the 8 stacks previously known, and extending up to 51 cm(-1) above the vCO = 1 origin. Assignments are given for the first 13 stacks of vCO = 1 in terms of the very low frequency geared bending intermolecular vibration.

Rovibrational spectroscopy of molecules boasts extremely high precision, but its usefulness relies on the assignment of spectral features to corresponding quantum mechanical transitions. In the case of ortho-H(2)-CO, a weakly bound complex abundant in the interstellar medium (although not yet observed there), the rather complex spectrum has been unexplained for more than a decade. We assigned this spectrum by comparison with a purely ab initio calculation. For most lines, agreement to within 0.01 centimeter(-1) between experiment and theory was achieved. Our results show that the applicability of rovibrational spectroscopy can be extended with the assistance of high-accuracy quantum mechanical computations.

Clusters of para-H(2) and/or ortho-H(2) containing a single carbon dioxide molecule are studied by high resolution infrared spectroscopy in the 2300 cm(-1) region of the CO(2) ν(3) fundamental band. The (H(2))(N)-CO(2) clusters are formed in a pulsed supersonic jet expansion from a cooled nozzle and probed using a rapid scan tunable diode laser. Simple symmetric rotor type spectra are observed with little or no resolved K-structure, and prominent Q-branch features for ortho-H(2) but not para-H(2). Observed rotational constants and vibrational shifts are reported for ortho-H(2) up to N = 7 and para-H(2) up to N = 15, with the N > 7 assignments only made possible with the help of theoretical simulations. The para-H(2) cluster with N = 12 shows clear evidence for superfluid effects, in good agreement with theory. The presence of larger clusters with N > 15 is evident in the spectra, but specific assignments are not possible. Mixed para- + ortho-H(2) cluster transitions are well predicted by linear interpolation between corresponding pure cluster line positions., (© 2012 American Institute of Physics)

Spectra of solid para-H(2) doped with CH(3)F at 1.8 K are studied in the ν(3) region (~1040 cm(-1)) using a quantum cascade laser source. As shown previously, residual ortho-H(2) in the sample (~1000 ppm) gives rise to distinct spectral features due to clusters of the form CH(3)F-(ortho-H(2))(N), with N = 0, 1, 2, 3, etc. Brief annealing at 7 K is found to give narrower spectral lines (≥0.006 cm(-1)) than conventional (5 K) annealing, and causes the N = 3 and 4 lines to fragment into two or more components. The N = 3 line is observed to be particularly stable and persistent. The N = 0 line (no ortho-H(2) neighbors) is resolved into two closely spaced (≈0.007 cm(-1)) components which are assigned to the K = 0 and 1 states of CH(3)F rotating around its C(3v) symmetry axis (ortho- and para-CH(3)F, respectively). Similar K-structure is also evident for other lines. Weak but persistent features ("N = 1/2 lines") are observed mid way between N = 0 and 1., (© 2011 American Institute of Physics)

In spite of wide interest in CO(2) clusters, only dimers and trimers have previously been assigned to specific infrared bands. Here, transitions for clusters with 6-13 molecules are identified in the ν(3) region (∼2350 cm(-1)). Spectra are observed in a supersonic jet (T ∼ 2.5 K) using a tunable laser probe, and analyzed with the aid of cluster calculations based on a widely-used model potential. Vibrational origins show blue-shifts significantly larger than predicted by resonant dipole interactions.

Clusters of para-hydrogen (pH₂) have been predicted to exhibit superfluid behavior, but direct observation of this phenomenon has been elusive. Combining experiments and theoretical simulations, we have determined the size evolution of the superfluid response of pH₂ clusters doped with carbon dioxide (CO₂). Reduction of the effective inertia is observed when the dopant is surrounded by the pH₂ solvent. This marks the onset of molecular superfluidity in pH₂. The fractional occupation of solvation rings around CO₂ correlates with enhanced superfluid response for certain cluster sizes.

High resolution spectra of (4)He(N)-CO(2) clusters are studied in the region of the CO(2) nu(3) fundamental band (approximately 2300 cm(-1)). The clusters are produced in a pulsed supersonic jet expansion from a cooled nozzle source and probed by direct absorption using a tunable diode laser operating in a rapid-scan mode. Four carbon dioxide isotopes ((16)O(12)C(16)O, (16)O(13)C(16)O, (18)O(13)C(18)O, and (16)O(13)C(18)O) are used to support the analysis, and because additional rotational transitions are allowed for the asymmetric one ((16)O(13)C(18)O). Resolved R(0) (J=1<--0) rotation-vibration transitions are observed for clusters up to N=60. A detailed rotational analysis is possible up to N approximately 20 and, with some assumptions, to N approximately 37 and beyond. The derived rotational constants (B values) vary smoothly with N and show evidence for broad oscillations similar to those already reported for He(N)-OCS and He(N)-N(2)O. Possible indications of a disruption are observed in the J=2 levels of larger clusters (N>22) which could be caused by interactions with a "dark" helium cluster modes.

High-resolution spectra of HeN-OCS clusters with N up to 39 in the microwave region and up to 72 in the infrared region were observed with apparatus-limited line widths of about 15 kHz and 0.001 cm(-1), respectively. The cold (approximately 0.2 K) clusters were produced in pulsed supersonic jet expansions of very dilute OCS + He mixtures and probed using a microwave Fourier transform spectrometer or a tunable infrared diode laser spectrometer. Consistent analyses of the microwave and infrared data yield band origins for the carbonyl stretching vibration, together with rotational parameters for the ground and excited vibrational states. The rotational constant, B, passes through a minimum at N = 9 and then rises as the He atoms uncouple from the OCS rotational motion as a result of superfluid effects. There are broad unexpected oscillations in B, with maxima at N = 24 and 47 and minima at N = 36 and 62. The change in B upon vibrational excitation, which is negative for the OCS molecule, converges to positive values for N > 15. These results help to bridge the gap between individual molecules and bulk matter with atom-by-atom resolution over a significant range of cluster sizes.

High resolution infrared spectra of HeN-N2O clusters are studied in the 2200 cm(-1) region of the N2O nu1 fundamental band. The clusters are produced in a pulsed supersonic jet expansion from a cooled nozzle source and probed using a tunable diode laser operating in a rapid-scan mode. Three isotopic forms are used (14N14N16O, 15N14N16O, and 15N15N16O) in order to support the spectral analyses. For clusters up to N approximately 24, the individual spectra are resolved, assigned, and analyzed together with complementary microwave data. Assignments for larger clusters are uncertain due to overlapping transitions, but an approximate analysis is still possible for N approximately 25-80. Compared to helium clusters containing the related CO2 or OCS molecules, the rotational dynamics of HeN-N2O clusters show similarities but also important differences. In particular, HeN-N2O has more irregular behavior in the range of N=6-17, indicating that conventional molecular structure plays a greater role. In general terms, these differences can be attributed to a greater degree of angular anisotropy in the He-N2O intermolecular potential.

A 4-D ab initio potential energy surface is calculated for the intermolecular interaction of hydrogen and carbon dioxide, using the CCSD(T) method with a large basis set. The surface has a global minimum with a well depth of 212 cm(-1) and an intermolecular distance of 2.98 A for a planar configuration with both the O-C-O and H-H axes perpendicular to the intermolecular axis. Bound state calculations are performed for the H(2)-CO(2) van der Waals complex with H(2) in both the para and ortho spin states, and the binding energy of paraH(2)-CO(2)(50.4 cm(-1)) is found to be significantly less than that of orthoH(2)-CO(2)(71.7 cm(-1)). The surface supports 7 bound intermolecular vibrational states for paraH(2)-CO(2) and 19 for orthoH(2)-CO(2), and the lower rotational levels with J< or = 4 follow an asymmetric rotor pattern. The calculated infrared spectrum of paraH(2)-CO(2) agrees well with experiment. For orthoH(2)-CO(2), the ground state rotational levels allowed by symmetry are found to have (K(a), K(c))=(even, odd) or (odd, even). This somewhat unexpected fact enables the previously observed experimental spectrum to be assigned for the first time, in good agreement with theory, and indicates that the orientation of hydrogen is perpendicular to the intermolecular axis in the ground state of the orthoH(2)-CO(2) complex.

A 2.3 microm near-room temperature tunable diode laser was tested for applications in high-resolution laboratory spectroscopy. It was mounted using a simple adaptor in a spectrometer usually used with lead-salt diode lasers, and was found to be electrically and optically compatible with the system. Good output power (several milliwatts) was observed, and a tuning range of 4460-4150 cm(-1) was achieved for laser temperatures of 210-310 K. Some spectra of N(2)O and NH(3) were recorded in the 4300-4430 cm(-1) region. However, the laser was not generally useful due to noisy and unstable output and high sensitivity to optical feedback.

High-resolution infrared spectra of clusters containing para-H2 and/or ortho-H2 and a single nitrous oxide molecule are studied in the 2225-cm(-1) region of the upsilon1 fundamental band of N2O. The clusters are formed in pulsed supersonic jet expansions from a cooled nozzle and probed using a tunable infrared diode laser spectrometer. The simple symmetric rotor-type spectra generally show no resolved K structure, with prominent Q-branch features for ortho-H2 but not para-H2 clusters. The observed vibrational shifts and rotational constants are reported. There is no obvious indication of superfluid effects for para-H2 clusters up to N=13. Sharp transitions due to even larger clusters are observed, but no definite assignments are possible. Mixed (para-H2)N-(ortho-H2)M-N2O cluster line positions can be well predicted by linear interpolation between the corresponding transitions of the pure clusters.

The design and operation of an apparatus for studying infrared spectra of weakly-bound complexes is described in detail. A pulsed supersonic jet expansion is probed using a tunable Pb-salt diode laser spectrometer operated in a rapid-scan mode. The jet may be fitted with either pinhole or slit shaped nozzles, the former giving lower effective rotational temperatures, and the latter giving sharper spectral lines. Notable features of the apparatus include use of a toroidal multi-pass mirror system to give over 100 passes of the laser through the supersonic jet, use of the normal laser controller for laser sweeping during both setup and data acquisition, and use of a simple semi-automated wavenumber calibration procedure. Performance of the apparatus is illustrated with observed spectra of the van der Waals complex He-OCS, and the seeded helium clusters He(N)-OCS and He(N)-CO.

Infrared spectra of hydrogen-carbonyl sulfide clusters containing paraH2, orthoH2, or HD have been studied in the 2060 cm(-1) region of the C-O stretching vibration. The clusters were formed in pulsed supersonic jet expansions and probed using a tunable infrared diode laser spectrometer. Simple symmetric rotor type spectra were observed and assigned for clusters containing up to N = 7 hydrogen molecules. There was no resolved K structure, and Q-branch features were present for orthoH2 and HD but absent for paraH2. These characteristics can be rationalized in terms of near symmetric rotor structures, very low effective rotational temperatures (0.15 to 0.6 K), and nuclear spin statistics. The observed vibrational shifts were compared with those from recent observations on the same clusters embedded in helium nanodroplets. The observed rotational constants for the paraH2 clusters are in good agreement with a recent quantum Monte Carlo simulation. Some mixed clusters were also observed, such as HD-HD-He-OCS and paraH2 - orthoH2 - OCS., ((c) 2004 American Institute of Physics.)

Infrared spectra of He(N)-CO(2) clusters with N up to about 20 have been studied in the region of the CO(2) nu(3) fundamental band ( approximately 2350 cm(-1)) using a tunable diode laser spectrometer and pulsed supersonic jet source with cooled (>-150 degrees C) pinhole or slit nozzles and high backing pressures (<40 atm). Compared to previous studies of He(N)-OCS and -N(2)O clusters, the higher symmetry of CO(2) results in simpler spectra but less information content. Discrete rotation-vibration transitions have been assigned for N=3-17, and their analysis yields the variation of the vibrational band origin and B rotational constant over this size range. The band origin variation is similar to He(N)-OCS, with an initial blueshift up to N=5, followed by a monotonic redshift, consistent with a model where the first five He atoms fill a ring around the equator of the molecule, forcing subsequent He atom density to locate closer to the ends. The B value initially drops as expected for a normal molecule, reaching a minimum for N=5. Its subsequent rise for N=6 to 11 can be interpreted as the transition from a normal (though floppy) molecule to a quantum solvation regime, where the CO(2) molecule starts to rotate separately from the He atoms. For N>13, the B value becomes approximately constant with a value about 17% larger than that measured in much larger helium nanodroplets., ((c) 2004 American Institute of Physics.)

High resolution infrared spectra of He(N)-CO2 clusters with N up to 17 have been studied in the region of the CO2 nu(3) fundamental band. The B rotational constant initially drops as expected for a normal molecule, reaching a minimum for N=5. Its subsequent rise for N=6 to 11 can be interpreted as the transition from a normal (though floppy) molecule to a quantum solvation regime. For N>13, the B value becomes approximately constant with a value about 17% larger than that measured in much larger helium nanodroplets. Quantum Monte Carlo calculations of pure rotational spectra are in excellent agreement with the measured B in this size range and complement the experimental study with detailed structural information. For a larger cluster size (N=30-50) the simulations show a clear sign of convergence towards the nanodroplet B value.

High resolution microwave and infrared spectra of He(N)-N2O clusters were studied in the range N=3 to 12. The apparent cluster moments of inertia increase from N=3 to 6, but then decrease, showing oscillatory behavior for N=7 to 12. This provides direct experimental evidence for the decoupling of helium atoms from the rotation of the dopant molecule in this size regime, signaling the transition from a molecular complex to a quantum solvated system and directly exploring the microscopic evolution of "molecular superfluidity."

High-resolution infrared and microwave spectra of He(N)-carbonyl sulfide (He(N)-OCS) clusters with N ranging from 2 to 8 have been detected and unambiguously assigned. The spectra show the formation of a solvation layer beginning with an equatorial "donut" of five helium atoms around the OCS molecule. The cluster moment of inertia increases as a function of N and overshoots the liquid droplet limit for N > 5, implying that even atoms in the first solvation shell are decoupled from the OCS rotation in helium nanodroplets. To the extent that this is due to superfluidity, the results directly explore the microscopic evolution of a phenomenon that is formally macroscopic in nature.

A new pulsed supersonic jet millimeter-wave spectrometer with coaxial propagation of the molecular jet and millimeter-waves has been constructed. The coaxial configuration provides greater sensitivity which has been demonstrated by observing b-type transitions of five isotopomers of Kr-(12)C(16)O and seven isotopomers of Xe-(12)C(16)O. These results were analyzed together with a-type transitions obtained by FTMW spectroscopy to determine improved rotational parameters for each isotopomer. The ground vibrational state K = 1 <-- 0 intervals have thus been precisely determined for Kr-CO and Xe-CO. Copyright 2001 Academic Press.