Spectra of the D2O dimer in the O-D fundamental stretch region: The acceptor symmetric stretch fundamental and new combination bands

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.

Infrared Spectra of the Water-CO2 Complex in the 4.3-3.6 μm Region and Determination of the Ground State Tunneling Splitting for HDO-CO2

Spectra of water─CO₂ dimers are studied using a tunable mid-infrared source to probe a pulsed slit jet supersonic expansion. H₂O-CO₂ and D₂O-CO₂ are observed in the CO₂ ν₃ fundamental region (≈2350 cm^-1), D₂O-CO₂ is also observed in the D₂O ν₃ fundamental region (≈2790 cm^-1), and HDO-CO₂ 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 D₂O-CO₂ (0.003 cm^-1) and HDO-CO₂ (0.0234 cm^-1). The latter value is a direct determination made possible by the reduced symmetry of HDO-CO₂. These results provide stringent and easily interpreted tests for theoretical water-CO₂ potential energy surface calculations.

Doped rare gas clusters up to completion of first solvation shell, CO2-(Rg)n, n = 3-17, Rg = Ar, Kr, Xe

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 CO₂-Ar_n with n = 3, 4, 6, 9, 10, 11, 12, 15, and 17, and CO₂-Kr_n and CO₂-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 CO₂ vibrational frequency (ν₃) 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 CO₂-Ar_n species tend to be those with symmetric structures, and CO₂-Ar₁₇ 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 CO₂-Ar₉, CO₂-Ar₁₅, and CO₂-Ar₁₇ 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).

Spectra of CO2-Rg2 and CO2-Rg-He trimers (Rg = Ne, Ar, Kr, and Xe): Intermolecular CO2 rock, vibrational shifts and three-body effects

Weakly bound CO2-Rg2 trimers are studied by high-resolution (0.002 cm−1) infrared spectroscopy in the region of the CO2 ν3 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. CO2–Ar2 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 CO2-Rg2 trimers are compared to those of CO2-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 CO2 rock) and give values of about 32.2, 33.8, and 34.7 cm−1 for CO2–Ar2, –Kr2, and –Xe2. Structural parameters derived for CO2-Rg2 trimers are compared with those of CO2-Rg and Rg2 dimers. Spectra of the mixed trimers CO2-Rg-He are also reported.

Observing the Completion of the First Solvation Shell of Carbon Dioxide in Argon from Rotationally Resolved Spectra

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, CO₂-Ar₁₅ and CO₂-Ar₁₇. These mark the completion of the first solvation shell for CO₂ 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 CO₂ 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.

Weakly-bound clusters of atmospheric molecules: infrared spectra and structural calculations of (CO2)n-(CO)m-(N2)p, (n,m,p) = (2,1,0), (2,0,1), (1,2,0), (1,0,2), (1,1,1), (1,3,0), (1,0,3), (1,2,1), (1,1,2)

Structural calculations and high-resolution infrared spectra are reported for trimers and tetramers containing CO₂ together with CO and/or N₂. Among the 9 clusters studied here, only (CO₂)₂-CO was previously observed by high-resolution spectroscopy. The spectra, which occur in the region of the ν₃ fundamental of CO₂ (≈2350 cm^-1), were recorded using a tunable optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. The trimers (CO₂)₂-CO and (CO₂)₂-N₂ have structures in which the CO or N₂ is aligned along the symmetry axis of a staggered side-by-side CO₂ dimer unit. The observation of two fundamental bands for (CO₂)₂-CO and (CO₂)₂-N₂ shows that this CO₂ dimer unit is non-planar, unlike (CO₂)₂ itself. For the trimers CO₂-(CO)₂ and CO₂-(N₂)₂, the CO or N₂ monomers occupy equivalent positions in the 'equatorial plane' of the CO₂, pointing toward its C atom. To form the tetramers CO₂-(CO)₃ and CO₂-(N₂)₃, a third CO or N₂ monomer is then added off to the 'side' of the first two. In the mixed tetramers CO₂-(CO)₂-N₂ and CO₂-CO-(N₂)₂, this 'side' position is taken by N₂ and not CO. In addition to the fundamental bands, combination bands are also observed for (CO₂)₂-CO, CO₂-(CO)₂, and CO₂-(N₂)₂, yielding some information about their low-frequency intermolecular vibrations.

Symmetry breaking of the bending mode of CO2 in the presence of Ar

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.

Spectra of CO2-N2 dimer in the 4.2 μm region: Symmetry breaking of the intramolecular CO2 bend, the intermolecular bend, and higher K-values for the fundamental

Infrared spectra of the CO₂-N₂ dimer are observed in the carbon dioxide ν₃ 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 CO₂-N₂. 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 CO₂ hot band transition (v₁, v₂ ^l2, v₃) = (01¹1) ← (01¹0). They yield a value of 2.307 cm^-1 for the splitting of the degenerate CO₂ ν₂ bend into in-plane and out-of-plane components due to the presence of the nearby N₂. The in-plane mode lies at a lower energy relative to the out-of-plane mode.

The Ethylene-Carbon Dioxide Complex and the Double Rotor Model

The infrared spectrum of the weakly bound C₂H₄-CO₂ complex is investigated in the region of the ν₃ fundamental band of CO₂ (≈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 (ΔK = ±1) spectrum is very irregular, and the pattern of irregularity is quite different from that observed previously in another C₂H₄-CO₂ band by Bemish et al. [ J. Chem. Phys. 1995 , 103 , 7788 ]. But by allowing for the different symmetry of the ν₃ (CO₂) upper vibrational state, both results can be strikingly well explained using the "double internal rotor" model as described by Bemish et al.

Spectra of the D2O dimer in the O-D fundamental stretch region: Vibrational dependence of tunneling splittings and lifetimes

The fundamental O-D stretch region (2600-2800 cm^-1) of the fully deuterated water dimer (D₂O)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 D₂O-DOH and D₂O-HOD are also observed and analyzed.

Infrared bands of CS2 dimer and trimer at 4.5 μm

We report observation of new infrared bands of (CS₂)₂ and (CS₂)₃ in the region of the CS₂ ν₁ + ν₃ 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 (CS₂)₂ is a cross-shaped structure with D_2d symmetry and that for (CS₂)₃ is a barrel-shaped structure with D₃ symmetry. The dimer has one doubly degenerate infrared-active band in the ν₁ + ν₃ region of the CS₂ 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.

The water–carbon monoxide dimer: new infrared spectra, ab initio rovibrational energy level calculations, and an interesting in-termolecular mode

Bound state rovibrational energy level calculations using a high-level intermolecular potential surface are reported for H₂O–CO and D₂O–CO.

Micro-solvation of CO in water: infrared spectra and structural calculations for (D2O)2–CO and (D2O)3–CO

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.

Infrared spectrum and intermolecular potential energy surface of the CO–O2 dimer

The spectrum of the weakly-bound radical complex CO–O₂ is studied for the first time.

Ultrafast Dissociation of Metastable CO^{2+} in a Dimer

We triply ionize the van der Waals bound carbon monoxide dimer with intense ultrashort pulses and study the breakup channel (CO)_{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.

Five intermolecular vibrations of the CO2 dimer observed via infrared combination bands

The weakly bound van der Waals dimer (CO₂)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 CO₂ monomer ν₃ 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 CO₂ dimer. A fifth combination band, located 1.3 cm^-1 below the conrotatory bend, remains unassigned.

Infrared spectra reveal box-like structures for a pentamer and hexamer of mixed carbon dioxide-acetylene clusters

Except for a few cases like water and carbon dioxide, identification and structural characterization of clusters with more than four monomers is rare. Here, we provide experimental and theoretical evidence for existence of box-like structures for a pentamer and a hexamer of mixed carbon dioxide-acetylene clusters. Two mid-infrared cluster absorption bands are observed in the CO2ν3 band region using a tunable diode laser to probe a pulsed supersonic jet. Each requires the presence of both carbon dioxide and acetylene in the jet, and (from observed rotational spacings) involves clusters containing about 4 to 7 molecules. Structures are predicted for mixed CO2 + C2H2 clusters using a distributed multipole model, and the bands are assigned to a specific pentamer, (CO2)3-(C2H2)2, and hexamer, (CO2)4-(C2H2)2. The hexamer has a box-like structure whose D2d symmetry is supported by observed intensity alternation in the spectrum. The pentamer has a closely related structure which is obtained by removing one CO2 molecule from the hexamer. These are among the largest mixed molecular clusters to be assigned by high-resolution spectroscopy.

The infrared spectrum of the Ne-C2D2 complex

Infrared spectra of Ne-C2D2 are observed in the region of the ν3 fundamental band (asymmetric C-D stretch, ≈2440 cm(-1)) using a tunable optical parametric oscillator to probe a pulsed supersonic slit jet expansion from a cooled nozzle. Like helium-acetylene, this system lies close to the free rotor limit, making analysis tricky because stronger transitions tend to pile up close to monomer (C2D2) rotation-vibration transitions. Assignments are aided by predicted rotational energies calculated from a published ab initio intermolecular potential energy surface. The analysis extends up to the j = 3←2 band, where j labels C2D2 rotation within the dimer, and is much more complete than the limited infrared assignments previously reported for Ne-C2H2 and Ne-C2HD. Two previous microwave transitions within the j = 1 state of Ne-C2D2 are reassigned. Coriolis model fits to the theoretical levels and to the spectrum are compared. Since the variations observed as a function of C2D2 vibrational excitation are comparable to those noted between theory and experiment, it is evident that more detailed testing of theory will require vibrational averaging over the acetylene intramolecular modes.

Theory vs. experiment for molecular clusters: Spectra of OCS trimers and tetramers

All singly substituted (13)C, (18)O, and (34)S isotopomers of the previously known OCS trimer are observed in natural abundance in a broad-band spectrum measured with a chirped-pulse Fourier transform microwave spectrometer. The complete substitution structure thus obtained critically tests (and confirms) the common assumption that monomers tend to retain their free structure in a weakly bound cluster. A new OCS trimer isomer is also observed, and its structure is determined to be barrel-shaped but with the monomers all approximately aligned, in contrast to the original trimer which is barrel-shaped with two monomers aligned and one anti-aligned. An OCS tetramer spectrum is assigned for the first time, and the tetramer structure resembles an original trimer with an OCS monomer added at the end with two sulfur atoms. Infrared spectra observed in the region of the OCS ν1 fundamental (≈2060 cm(-1)) are assigned to the same OCS tetramer, and another infrared band is tentatively assigned to a different tetramer isomer. The experimental results are compared and contrasted with theoretical predictions from the literature and from new cluster calculations which use an accurate OCS pair potential and assume pairwise additivity.

The infrared spectrum of the He-C2D2 complex

Spectra of the helium-acetylene complex are elusive because this weakly bound system lies close to the free rotor limit. Previously, limited assignments of He-C2D2 transitions in the R(0) region of the ν3 fundamental band (≈2440 cm(-1)) were published. Here, new He-C2D2 infrared spectra of this band are obtained using a tunable optical parametric oscillator laser source to probe a pulsed supersonic slit jet expansion from a cooled nozzle, and the analysis is extended to the weaker and more difficult P(1) and R(1) regions. A term value approach is used to obtain a consistent set of "experimental" energy levels. These are compared directly with calculations using two recently reported ab initio intermolecular potential energy surfaces, which exhibit small but significant differences. Rovibrational energies for the He-C2H2 complex are also calculated using both surfaces. A Coriolis model, useful for predicting spectral intensities, is used to interpret the energy level patterns, and a comparison with the isoelectronic complex He-CO is made.

Infrared spectra of ethylene clusters: (C2D4)2 and (C2D4)3

Spectra of ethylene dimers and trimers are studied in the ν(11) fundamental band region of C(2)D(4) (≈2200 cm(-1)) using a tuneable quantum cascade laser to probe a pulsed supersonic slit jet expansion. The dimer spectrum is that of a prolate symmetric top perpendicular band, with a distinctive appearance because the A rotational constant is almost exactly equal to six times the B constant. The analysis supports the previously determined cross-shaped dimer structure with D(2d) symmetry. An ethylene trimer has not previously been observed with rotational resolution. The spectrum is that of an oblate symmetric top parallel band. It leads to a proposed trimer structure which is barrel shaped and has C(3h) or C(3) symmetry, with the ethylene monomer C-C axes approximately aligned along the trimer symmetry axis.

High resolution quantum cascade laser studies of the ν3 band of methyl fluoride in solid para-hydrogen

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.

High resolution infrared spectroscopy of carbon dioxide clusters up to (CO2)13

Thirteen specific infrared bands in the 2350 cm(-1) region are assigned to carbon dioxide clusters, (CO(2))(N), with N = 6, 7, 9, 10, 11, 12 and 13. The spectra are observed in direct absorption using a tuneable infrared laser to probe a pulsed supersonic jet expansion of a dilute mixture of CO(2) in He carrier gas. Assignments are aided by cluster structure calculations made using two reliable CO(2) intermolecular potential functions. For (CO(2))(6), two highly symmetric isomers are observed, one with S(6) symmetry (probably the more stable form), and the other with S(4) symmetry. (CO(2))(13) is also symmetric (S(6)), but the remaining clusters are asymmetric tops with no symmetry elements. The observed rotational constants tend to be slightly (≈2%) smaller than those from the predicted structures. The bands have increasing vibrational blueshifts with increasing cluster size, similar to those predicted by the resonant dipole-dipole interaction model but significantly larger in magnitude.

Theoretical and experimental study of weakly bound CO2-(pH2)2 trimers

The infrared spectrum of CO(2)-(pH(2))(2) trimers is predicted by performing exact basis-set calculations on a global potential energy surface defined as the sum of accurately known two-body pH(2)-CO(2) (J. Chem. Phys. 2010, 132, 214309) and pH(2)-pH(2) potentials (J. Chem. Phys. 2008, 129, 094304). These results are compared with new spectroscopic measurements for this species, for which 13 transitions are now assigned. A reduced-dimension treatment of the pH(2) rotation has been employed by applying the hindered-rotor averaging technique of Li, Roy, and Le Roy (J. Chem. Phys. 2010, 133, 104305). Three-body effects and the quality of the potential are discussed. A new technique for displaying the three-dimensional pH(2) density in the body-fixed frame is used, and shows that in the ground state the two pH(2) molecules are localized much more closely together than is the case for the two He atoms in the analogous CO(2)-(He)(2) species. A clear tunneling splitting is evident for the torsional motion of the two pH(2) molecules on a ring about the CO(2) molecular axis, in contrast to the case of CO(2)-(He)(2) where a more regular progression of vibrational levels reflects the much lower torsional barrier.

Infrared photochromism in a quantum crystal: laser pumping and probing in solid para-H2 doped with CH3F

Solid para-H(2) doped with CH(3)F at 1.8 K is studied in the ν(3) region (∼1040 cm(-1)) using a quantum cascade laser source. Residual ortho-H(2) gives rise to distinct spectral features due to CH(3)F-(ortho-H(2))(N) clusters with N = 0, 1, 2, etc. The laser power (∼30 mW) is sufficient to significantly affect the sample, enabling novel photochromism experiments to be performed on a solid para-H(2) matrix-isolated sample. It is found that population can be reversibly transferred between the N = 1 line and satellite features close to the N = 0 line.

Spectroscopic identification of carbon dioxide clusters: (CO2)6 to (CO2)13

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.

Molecular superfluid: nonclassical rotations in doped para-hydrogen clusters

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.

Theoretical and experimental study of the rovibrational spectrum of He(2)-CO

We report calculated microwave and infrared ro-vibrational transitions of the van der Waals complex He(2)-CO. The calculations were done using a product basis and a Lanczos eigensolver, together with He-CO and He-He potential functions taken from the literature. The results are found to be in good agreement with previously reported experimental results, and they enable the experimental assignments to be clarified, augmented, and (in one case) corrected. Unlike some other van der Waals complexes with two He atoms such as He(2)-N(2)O, it is not possible to associate a set of energy levels with the "torsional" motion of the two He atoms on a ring encircling the dopant (in this case CO). Although we assume that the dipole moment is along the CO axis we find nonetheless that many transitions have appreciable intensity due to ro-vibrational coupling.