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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. 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., Comment: 23 pages, 5 figures, 2 tables

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 nu3 fundamental region (~2350 cm-1), D2O-CO2 is also observed in the D2O nu3 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., Comment: 19 pages, 6 figure and 5 tables

Spectra of rare gas atom clusters containing a single carbon dioxide molecule are observed using a tunable mid-infrared (4.3 micron) 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-Arn with n = 3, 4, 6, 9, 10, 11, 12, 15, and 17, as well as CO2-Krn and -Xen with n = 3, 4, and 5. Each spectrum has (at least) partially resolved rotational structure, and each yields precise values for the shift of the CO2 vibrational frequency (nu3) 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-Arn species tend to be those with symmetric structures, and CO2-Ar17 represents completion of a highly symmetric (D5h) solvation shell. Those not assigned (e.g. n = 7, 13) are probably also present in the observed spectra, but with band structures which are not well-resolved and thus not recognizable. The spectra of CO2-Ar9, -Ar15, and -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)., Comment: 34 pages, 12 figures, 9 tables

Weakly-bound CO2-Rg2 trimers are studied by high resolution (0.002 cm-1) infrared spectroscopy in the region of the CO2 nu3 fundamental band (~2350 cm-1), using a tunable OPO 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 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 gives 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., Comment: 64 pages, 6 figure, 9 tables

High resolution spectra of (CO$_2$)$_2$-Rg trimers (Rg = Ne, Ar, Kr, and Xe) in the region of the CO2 nu3 fundamental (~2350 cm$^{-1}$) are reported, using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. These (CO$_2$)$_2$-Rg transitions tend to be hidden among stronger spectra due to other species, such as CO$_2$-Rg and (CO$_2$)$_2$. Each trimer consists of a (CO$_2$)$_2$ unit which is similar to the free carbon dioxide dimer (planar parallel staggered) plus an Rg atom located out-of-plane on the dimer C$_2$ symmetry axis, but the (CO$_2$)$_2$ unit may not remain exactly planar in the dimer. Experimental structures show that the C-Rg bond lengths in the trimers are similar to those in the corresponding CO$_2$-Rg dimers. As well, the vibrational band origin shifts, relative to (CO$_2$)$_2$ itself, are similar to those of CO$_2$-Rg relative to CO$_2$., Comment: 14 pages, 4 figures and 3 tables

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 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., Comment: 14 pages, 2 figures, 2 tables

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 $\nu_3$ fundamental of CO$_2$ (~2350 cm$^{-1}$), 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$_2$ have structures in which the CO or N$_2$ 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$_2$)$_3$, 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 N2 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., Comment: 35 pages, 7 figures and 4 tables

The infrared spectrum of the weakly-bound CO2-Ne complex is studied in the region of the carbon dioxide nu3 fundamental vibration (~2350 cm-1), using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. For the fundamental CO2 transition (v1, v2l2, v3) = (0001) <-- (0000), both CO2-20Ne and CO2-22Ne are assigned and analyzed in combination with available microwave data to obtain the best currently available molecular parameters. In addition, combination bands involving the intermolecular bending mode are observed for both isotopologues, leading to the determination of the bending frequency in the CO2 excited state, which is 17.717 cm-1 for CO2-20Ne and 17.726 cm-1 for CO2-22Ne. For the hot band CO2 transition, (0111) <-- (0110), detection of the weak CO2-Ne spectrum reveals the symmetry breaking of the CO2 nu2 bending mode induced by the Ne atom, with the out-of-plane component determined to lie 0.057 cm-1 higher in energy than the in-plane component., Comment: 4 figures, 2 tables, 35 pages

Extensive infrared spectra of the weakly-bound CO$_2$-CO dimer are observed in the carbon dioxide $\nu_3$ asymmetric stretch region (~2350 cm-1) using a tunable infrared OPO laser source to probe a pulsed slit jet supersonic expansion. Both C-bonded and O-bonded isomers are analyzed for the normal isotopologue as well as for 13CO$_2$-CO and $^{16}$O$^{13}$C$^{18}$O-CO, the latter being the first observation of an asymmetrically substituted form for which all values of Ka are allowed. Combination bands involving the lowest in-plane intermolecular bending modes are also studied for both isomers. Weak bands near 2337 cm-1 are assigned to CO$_2$ hot band transitions (v1, v2l2, v3) = (0111) <-- (0110), yielding the splitting of the degenerate CO$_2$ $\nu_2$ bend into in-plane and out-of-plane components due to the presence of the CO. This splitting has rather different values for the C- and O- bonded isomers, 4.56 and 1.59 cm-1, respectively, with the out-of-plane mode higher in energy than the in-plane for both cases., Comment: 26 pages, 5 figures and 5 tables

The infrared spectrum of the weakly-bound CO2-Xe complex is studied in the region of the carbon dioxide nu3 fundamental vibration (~2350 cm-1), using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. The Xe isotope dependence of the spectrum is modeled by scaling the vibrational and rotational parameters, with the help of previous microwave data. The scaling model provides a good simulation of the observed broadening and (partial) splitting of transitions in the fundamental band, and it is essential for understanding the intermolecular bending combination band where some transitions are completely split by isotope effects. The combination band is influenced by a significant bend-stretch Coriolis interaction and by the relatively large Xe isotope dependence of the intermolecular stretch frequency. The weak CO2-Xe spectrum corresponding to the (0111) <-- (0110) hot band of CO2 is also detected and analyzed, providing a measurement of the symmetry breaking of the CO2 bending mode induced by the nearby Xe atom. This in-plane / out-of-plane splitting is determined to be 2.14 cm-1., Comment: 55 pages, 5 figures and 4 tables

Infrared combination bands of the polar isomer of the N2O dimer are observed for the first time, using a tunable infrared laser source to probe a pulsed slit-jet supersonic expansion in the N2O nu1 region (~2240 cm-1). One band involves the torsional (out-of-plane) intermolecular mode and yields a torsional frequency of 19.83 cm-1 if associated with the out-of-phase fundamental (N2O nu1) vibration of the N2O monomers in the dimer. The other band, which is highly perturbed, yields an intermolecular in-plane geared bend frequency of 22.74 cm-1. The results are compared with high level ab initio calculations. The less likely alternate assignment to the in-phase fundamental would give torsional and geared bend frequencies of 17.25 and 20.16 cm-1, respectively., Comment: 15 page, 2 figures

We extend studies of micro-solvation of carbon monoxide by a combination of high-resolution IR spectroscopy and ab initio calculations. Spectra of the (H2O)4-CO and (D2O)4-CO pentamers are observed in the C-O stretch fundamental region (~2150 cm-1). The H2O containing spectrum is broadened by predissociation, but that of D2O is sharp, enabling detailed analysis which 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 previous results on D2O-CO, (D2O)2-CO, and (D2O)3-CO, this represents a probe of the four initial steps in the solvation of carbon monoxide at high resolution., Comment: 27 page, 3 figures

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 nu3 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 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 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., Comment: 19 pages, 1 table, and 3 figures

Infrared spectra of the CO2-N2 dimer are observed in the carbon dioxide nu3 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 Ka. 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 nu2 bend into in-plane and out-of-plane components due to the presence of the nearby N2. The in-plane mode lies at lower energy relative to the out-of-plane mode., Comment: 16 pages, 3 figures

The infrared spectrum of the weakly-bound C2H4-CO2 complex is investigated in the region of the nu3 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 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. 103, 7788 (1995)]. But by allowing for the different symmetry of the nu3 (CO2) upper vibrational state, both results can be strikingly well explained using the 'double internal rotor' model as described by Bemish et al., Comment: 27 page and 6 figures

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 experimental 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., Comment: 3 figures and 25 pages

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 experiment. But the first excited K = 1 levels are predicted to lie about 3 cm-1 below their K = 0 counterparts in both cases. Intensity 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 of about 340 cm-1 for D2O-CO, as compared to our calculated binding energy 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., Comment: 46 pages, 8 figures, and 9 tables

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 observed line broadening, show large variations (0.2 to 5 nanoseconds) depending on 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., Comment: 35 pages, 5 tables and 8 figures

We report observation of new infrared bands of (CS2)2 and (CS2)3 in the region of the CS2 {\nu}1+ {\nu}3 combination band (at 4.5 cm-1) 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 D2d 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 {\nu}1+ {\nu}3 region of the CS2 monomer. This band is observed to have a rather small vibrational shift of -0.846 cm-1. We expect one parallel and one perpendicular infrared-active band for the trimer but observe two parallel and one perpendicular bands. 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., Comment: 24 pages, 5 figures and 2 tables

The infrared spectrum of the cross-shaped van der Waals complex N2O-CS2 is observed in the region of the N2O nu1 fundamental band (~2220 cm-1) using a tuneable diode laser to probe a pulsed supersonic slit jet expansion. Both 14N- and 15N-substituted species are studied. Analysis of their spectra establishes that this dimer has a cross-shaped structure, similar to its isoelectronic cousin CO2-CS2. This is the first spectroscopic observation of N2O-CS2, and the molecular parameters determined here should be useful for detection of its pure rotational microwave spectrum., Comment: 10 Pages, 3 figures

Infrared spectra of Rg1,2 - C6H6 complexes (Rg = He, Ne, Ar) are observed in the region of the nu12 fundamental of C6H6 using a pulsed supersonic jet expansion and a tunable optical parametric oscillator laser source. The mixed trimer He - Ne - C6H6 is also detected. Four bands are analyzed for each complex, namely nu12 itself (~3048 cm-1) and three linked combination bands (~3079, 3100, and 3102 cm-1). The results are consistent with previous ultraviolet and microwave results, with Ne2 - C6H6 and He - Ne - C6H6 being analyzed spectroscopically here for the first time., Comment: 22 pages, 3 figures

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 (v(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 rotational 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 Angstroms, 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., Comment: 35 pages and 8 figures

Spectra of ethylene dimers and trimers are studied in the nu11 and (for the dimer) nu9 fundamental band regions of C2H4 (~2990 and 3100 cm-1) using a tunable optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. The deuterated trimer has been observed previously, but this represents the first rotationally resolved spectrum of (C2H4)3. The results support the previously determined cross-shaped (D2d) dimer and barrel-shaped (C3h or C3) trimer structures. However, the dimer spectrum in the nu9 fundamental region of C2H4 is apparently very perturbed and a previous rotational analysis is not well verified., Comment: 21 pages, 4 figures

OCS is an ideal probe for quantum solvation effects in cold helium clusters. He2-OCS is the "second step" in going from a single OCS molecule to a large doped superfluid helium cluster. Here assignment of the spectrum of He2-OCS is significantly extended to higher values of J, K, and vt (the low frequency torsional vibration). The observation of a new infrared band, OCS nu1 + nu3, taken together with the known nu1 band, enables assignments to be verified by comparing ground state combination differences. Relatively straightforward scaling of previously calculated theoretical energy levels gives a remarkably good fit to experiment, Comment: 34 pages, 2 figures

The fundamental band for the OC-C2H2 dimer and two combination bands involving the intermolecular bending modes nu9 and nu8 in the carbon monoxide CO stretch region are re-examined. Spectra are obtained using a pulsed supersonic slit jet expansion probed with a mode-hop free tunable infrared quantum cascade laser. Analogous bands for OC-C2D2 and the fundamental for OC-DCCH as an impurity are also observed and analysed. A much weaker band in the same spectral region is assigned to a new mixed trimer, CO-(C2H2)2. The trimer band is composed uniquely of a-type transitions, establishing that the CO monomer is nearly aligned with the a-inertial axis. The observed rotational constants agree well with ab initio calculations and a small inertial defect value indicates that the trimer is planar. The structure is a compromise between the T-shaped structure of free acetylene dimer and the linear geometry of free OC-C2H2. A similar band for the fully deuterated isotopologue CO-(C2D2)2 confirms our assignment., Comment: 35 pages and 5 figures

Three new infrared bands of the weakly-bound He-OCS complex are studied, using tunable lasers to probe a pulsed supersonic slit jet expansion. They correspond to the (0400) <-- (0000), (1001)<-- (0000), and (0401) <-- (0000) transitions of OCS at 2105, 2918, and 2937 cm-1, respectively. The latter band is about 7900 times weaker than the previously studied OCS nu1 fundamental. Vibrational shifts relative to the free OCS monomer are found to be additive. Since carbonyl sulfide has previously been shown to be a valuable probe of superfluid quantum solvation effects in helium clusters and droplets, the present results could be useful for future studies of vibrational effects in such systems., Comment: 16 pages, 1 figure, 4 tables

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.