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J. K. G. (Jim) Watson was a world-leading authority on the basic theory of molecular rotation and vibration, especially renowned for his simplification of the rotational and rotation–vibration Hamiltonians (energy expressions). Other significant scientific achievements include the treatment of ‘forbidden’ rotational transitions in symmetric molecules and analysis of the spectrum of the H+3 molecular ion, both of which have important astrophysical implications. Jim was a reticent individual who carried out much of his most important research solo, while at the same time he had a wide circle of collaborators who were often experimentalists seeking theoretical guidance. Although his most profound contributions were highly mathematical, he always endeavoured to present the ultimate results in a widely useful form.

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., 64 pages, 6 figure, 9 tables

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 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 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 a high resolution.

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 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 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 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, v₂ˡ², 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.

Extensive infrared spectra of the weakly-bound CO₂-CO dimer are observed in the carbon dioxide ν₃ 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 analysed for the normal isotopologue as well as for ¹³CO₂-CO and ¹⁶O¹³C¹⁸O-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 CO₂ hot band transitions (v1, vˡ²₂, v₃) = (01¹1) ← (01¹0), yielding the splitting of the degenerate CO₂ ν₂ 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, 4.57 and 1.59 cm⁻¹, respectively, with the out-of-plane mode higher in energy than the in-plane for both cases.

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), Comment: 26 pages, 5 figures and 5 tables

Extensive infrared spectra of the weakly-bound CO$_2$-CO dimer are observed in the carbon dioxide $��_3$ 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), 26 pages, 5 figures and 5 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 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.

Spectra of dimers and trimers containing C₆D₆ and He, Ne, Ar, or H₂ are observed in the region of the C₆D₆ ν₁₂ fundamental using a pulsed supersonic jet expansion and a tunable optical parametric oscillator laser source. These are the first reported infrared spectra of hydrogen – benzene complexes. Two bands coupled by anharmonic resonance are studied for each complex: the ν₁₂ fundamental itself (≈2289 cm⁻¹) and the ν₂ + ν₁₃ combination band (≈2276 cm⁻¹). Vibrational shifts for the trimers are found to be approximately twice those of the corresponding dimer. Effective intermolecular separations for (H₂)₁,₂-C₆D₆ are about 3.46 Å, similar to those determined previously from electronic spectra of (H₂)₁,₂-C₆H₆ and also similar to (Ne)₁,₂-C₆D₆.

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.

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 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 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.

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

We triply ionize the van der Waals bound carbon monoxide dimer with intense ultrashort pulses and study the breakup channel (CO)3+2→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.

Infrared spectra of the CO₂─CO dimer are observed in the carbon monoxide CO stretch region (≈2150 cm⁻¹). Combination bands yield the first experimental determinations of intermolecular mode frequencies for the planar T-shaped C-bonded form (‘isomer 1’): 24.34 cm ⁻¹ for the in-plane CO rock and 43.96 cm ⁻¹ for the out-of-plane CO rock. These values are significantly higher than the analogous modes of the O-bonded form (‘isomer 2’), previously determined to be 14.19 and 22.68 cm ⁻¹, respectively. New results are also reported for both isomers of the¹²C¹⁸O₂-substituted form of the dimer. Weak ‘satellite bands’ observed for both isomers are tentatively assigned to the trimer He─CO₂─CO.

Infrared spectra in the carbon monoxide CO stretch region (≈2150 cm-1) are assigned to the previously unobserved O-bonded form of the CO2-CO dimer ("isomer 2"), which has a planar T-shaped structure like that of the previously observed C-bonded form ("isomer 1"), but with the CO rotated by 180°. The effective center of mass intermolecular distances are 3.58 Å for isomer 2 as compared to 3.91 Å for isomer 1. In addition to the fundamental band, two combination bands are observed for isomer 2, yielding values for two intermolecular vibrational modes: 14.19 cm-1 for the in-plane CO bend and 22.68 cm-1 for the out-of-plane bend.

Spectra of Ne─C₂D₄ and Ar─C₂D₄ are studied in the ν11 fundamental band region of C₂D₄ (≈2200 cm⁻¹) using a tuneable quantum cascade laser to probe a pulsed supersonic slit jet expansion. The spectra are those of perpendicular bands of near-prolate asymmetric rotors, but with a doubling of transitions due to tunneling. These tunneling splittings are due to hindered internal rotation of C₂D₄ around its C─C axis, and are more prominent for Ne─C₂D₄ than for Ar─C₂D₄. The analysis yields the first direct measurement of the A rotational constants for these complexes, and shows that the vibrational shifts are very small (

The ν18 fundamental band (~158 cm⁻¹) of acrolein is studied at high resolution (0.0015 cm⁻¹) using synchrotron radiation from the Canadian Light Source facility and a Bruker IFS 125HR Fourier transform spectrometer. By fitting this band, together with some pure rotational transitions, molecular parameters are obtained to accurately determine the energies of the ν18 = 1 state levels for values of (J,Ka) up to at least (45,24). These parameters should be useful for future high resolution studies of acrolein hot bands. This is demonstrated here by a detailed analysis of the (ν17 + ν18) - ν18 hot band at ~589 cm⁻¹. The upper state (ν17 + ν18) of this band is found to be perturbed by Coriolis interactions analogous to those affecting the ν17 state.

Infrared spectra of helium clusters seeded with doubly substituted carbon monoxide molecules, ¹³C¹⁸O, have been studied in order to complement recent helium nanocluster results and to determine whether additional isotopic data would help to separate vibrational and rotational contributions to the observed transitions. The experiments were made by direct infrared absorption in pulsed supersonic jet expansions using a tunable diode laser probe in the region of the fundamental band (~2045 cm⁻¹ for ¹³C¹⁸O). Even with data on the R(0) transitions from four CO isotopomers, it was found that a clear and consistent separation of vibration and rotation could not be achieved for HeN–CO clusters in the size range N≈10–20. Isotope shifts observed for clusters with ¹³C¹⁸O (relative to ¹²C¹⁶O) were found to be close to the sums of the shifts previously determined for ¹³C¹⁶O and ¹²C¹⁸O. The new measurements generally supported previous assignments of cluster size, but some modifications for the range N=14–16 are suggested here. New measurements for HeN–¹²C¹⁶O under conditions favoring larger clusters (high backing pressure and low jet temperature) showed that individual transitions could be resolved even at N ≈ 50. For larger clusters, a partly resolved "lump" of transitions was observed to approach the nanodroplet limit.

Infrared spectra of three isotopic forms of the weakly bound CO₂–He van der Waals complex have been studied in the region of the CO₂ v₃ fundamental band around 2300 cm⁻¹, using a tunable diode laser to probe a pulsed supersonic expansion. The complex is a T-shaped near-oblate asymmetric rotor, and it is found that ¹⁸O isotopic substitution is sufficient to interchange the a and b inertial axes. For the symmetric isotopes, such as the normal species and the ¹⁶O¹³C¹⁶O and ¹⁸O¹³C¹⁸O forms studied here, half of the normal rotational levels of the complex are missing due to the effects of ¹⁶O (or ¹⁸O) interchange symmetry. However, for asymmetrically substituted ones, such as ¹⁶O¹³C¹⁸O, all rotational levels are present. Moreover, for the asymmetric isotope, both a- and b-type transitions were observed, so that the spectrum was much richer. The CO₂–He system is of interest both as a benchmark for intermolecular potential energy surface calculations, and because CO₂ is a valuable probe molecule for helium cluster spectroscopy.

Infrared spectra of the weakly bound complexes N₂O-⁴He, N₂O-³He, and OCS-³He have been observed using a tunable diode laser to probe a pulsed supersonic jet expansion. The rotational structure of the bands was analyzed using a conventional asymmetric rotor Hamiltonian. The N₂O-³He and OCS-³He spectra are mostly a type (ΔKa=0) in structure, with very weak b-type (ΔKa = ±I ) transitions, but for N₂O-⁴He the a- and b-type components are both prominent. The fitted rotational parameters are consistent with roughly T-shaped structures with intermolecular separations around 3.4-3.5 Å for N₂O-He and 3.8-3.9 Å for OCS-He. The angle between the N₂O or OCS axis and the He position is about 80° for N₂O-He and 65° for OCS-He. The vibrational band origins are slightly blueshifted from those of the free molecule, with the N₂O-He shifts (+0.2 cm⁻¹) being about twice the magnitude of the OCS-He shifts (+0.1 cm⁻¹). The results are of particular interest since N₂O and (especially) OCS have both been used as probes in experiments on ultracold helium nanodroplets.