A. R. W. McKellar

Infrared spectrum and potential energy surface of He–CO

For 3He–CO and 4He–CO van der Waals bimers, fully resolved infrared spectra in the 4.7 μm region near the fundamental band origin of the CO monomer have been measured for the first time. Only a small fraction of the observed lines could be assigned using conventional empirical spectroscopic techniques, and little additional insight was gained from synthetic spectra generated from a published ab initio potential for this system. However, a complete set of unique assignments was made on the basis of comparisons with synthetic spectra generated from a variety of trial potential energy surfaces, and least‐squares fits to the observed transition frequencies were used to determine a new anisotropic potential energy surface for this system. This new surface is much deeper and has a much stronger well depth anisotropy than the best previous one, and its predictions of very low temperature microwave line broadening cross sections raise serious questions regarding the methodology for calculating that property.

Laser magnetic resonance spectroscopy of the ν2 fundamental band of HCO at 9.25 μm

The ν2 bending fundamental (ν0=1080.76 cm−1) of the formyl radical, HCO, has been studied using CO2 laser magnetic resonance. Fluorine atoms from a discharge in CF4 were reacted with H2CO to form the short‐lived HCO, which then flowed through an absorption cell located between the pole faces of an electromagnet and within the optical cavity of a CO2 laser. By means of the Zeeman effect, HCO, vibration‐rotation transitions were tuned through resonance with the laser lines. Numerous resonances involving levels with 1⩽N⩽7 and 1⩽Ka⩽3 were assigned, and from analysis of the spectra accurate determinations of the band origin, rotational, centrifugal distortion, spin‐rotation, and Fermi interaction parameters for ν2 were made. Fairly large changes in the values of A, ΔK, eaa, and ηaaaa between the ground and v2=1 vibrational states were observed.

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.

Stark spectroscopy with the CO laser: The dipole moment of H2CO in the v2=2 state

Some transitions of the 2ν2←ν2 hot band of H2CO near 5.8 μm have been observed by the laser Stark technique using a 20 cm long intracavity absorption cell with about 4 mtorr total pressure at room temperature. The detection of this weak hot band (partial pressure in the v2=1 state ?8×10−7 torr) illustrates the high sensitivity of intracavity laser absorption spectroscipy. From the measured resonant fields of the transitions, the dipole moment of H2CO in the v2=2 state has been determined. The use of phase sensitive detection at twice the Stark modulation frequency is illustrated; this 2f detection effectively measures the strength of the transient nutation signal in the absorbing gas, and in some cases results in a better signal to noise ratio than does conventional 1f detection.

Spin–rotation and hyperfine parameters for the (001) excited vibrational state of NO2 from infrared–radiofrequency double resonance

Magnetic–dipole spin–rotation transitions within the ground and v3=1 excited vibrational states of 14N16O2 have been observed by the technique of infrared–radiofrequency double resonance, utilizing near coincidences between CO laser lines and NO2 ν3 band transitions around 6.2 μm. The ground state data contribute to the extensive body of microwave, EPR, and double resonance measurements now available for NO2. The excited state data are the first high resolution measurements within this state, and they are used here to determine a set of spin–rotation and hyperfine interaction parameters for NO2 in the v3=1 state.

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.