Most of the pair-potentials (in cm−1) used in this work. (a) H2–Ar pair-potential. Global minimum corresponds to linear configuration H–H–Ar. (b) The CO–Ar intermolecular potential. The minimum energy configuration is a near T-shape with argon slightly closer to oxygen end. The CO bond length was 1.1324 Å. (c) CO–H2 pair-potential, collinear configuration where either the oxygen (CO–H–H), or the carbon (OC–H–H) end points to hydrogen, and T-shape where the CO molecular axis is perpendicular to H2 molecular axis (open circles). (d) CO–H2 pair-potential, perpendicular configuration. In (c) and (d), the dihedral angle was set to zero. (e) CO–H2 potential, where hydrogen molecule is considered as a spherical species, exhibits a T-shaped global minimum.
Rotational Raman spectra for the two matrix-isolated hydrogen species at precursor dilution ratios 1/750 (bottom) and 1/150 (top). Dissociation of formaldehyde is induced at 9 K and the sample is then probed (broken line). Subsequent measurements were then performed at 30 K, 20 K, and 10 K in that order. Raman shifts correspond to J = 2 ← 0 and J = 3 ← 1 rotational excitations for pH2 and oH2, respectively. The spectral features are identified as a stable central peak and a temperature dependent broad band structure.
Measured Raman signals in the fundamental frequency region of CO for dilution ratios 1/150 (upper) and 1/750 (lower). The sample conditions are the same as for H2 in Fig. 2. The spectra exhibit a structure (indicated by the dotted lines) due to two trapping sites and a complex formation analyzed before for a pure CO/Ar sample.25
Rotational energy V(θ, ϕ) in cm−1 for the pH2 in substitution (S) and interstitial (O) sites. Low barrier heights indicate that hydrogen is a nearly free rotor in argon.
Simulated rotational Raman spectra for the two H2 species in Ar. The spectra consist of two calculations, one where the hydrogen is located in the substitution site (S) (broken blue line) and another where the molecule resides in an interstitial site (O) (red broken line). The final observed spectrum is then understood as a sum of the two signals for both hydrogen species (not shown). The experimental spectrum from measurements at 10 K is reproduced for comparison (solid blue line). Also shown is a Lorentzian line shape fit (circles) to the experimental results. See text for the discussion regarding the fit parameters.
Simulated rotational Raman spectra for the hydrogens when CO is located at the origin. The H2 coordinates are marked according to Table I. The individual spectra are normalised.
The adiabatic rotational potential energy surfaces (in cm−1) for isolated CO in DS (left) and SS (right) trapping geometries.
Fully adiabatic rotational potentials V(θ, ϕ) (in cm−1) for rotating CO in different complex geometries. See text for details.
Simulated rovibrational Raman spectra for complexed CO using potentials shown in Fig. 8. The rotational constant was set to 1.92 cm−1. Transitions to next librational manifold, i.e., the Δn = 1 transitions, are not shown. Boltzmann distribution corresponds to experiments in 10 K.
The trapping coordinates for hydrogen molecule site labels S1, S2, S3, and O in solid argon. For the H2–CO complexes studied, the CO resides at monosubstitution site at origin. Lattice constant a is 5.31 Å.
Rotational manifold energies in cm−1 pertaining to rotational transitions in para- and orthohydrogen molecules trapped in substitution (S) and interstitial (O) sites in solid argon. Symmetry labels are in parentheses.
Lowest energy levels (in cm−1) for isolated CO in SS and DS configuration. Row a: adiabatic lattice response is assumed for all CO orientations. Level degeneracies are indicated in parentheses. In the double-substitution site, only OC orientation is reproduced. Rotational constant was 1.24 cm−1 and 1.92 cm−1 for SS and DS sites, respectively.
Selected lowest states for adiabatically rotating CO in argon lattice that includes a H2 molecule in the same unit cell. Rotational constant was B 0 = 1.92 cm−1 and the values are in wavenumbers. See Table III for rotational levels without the perturbance of hydrogen.
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