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Librational motion of CO in solid Ar: Raman and IR spectra and quantum simulations
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Image of FIG. 1.
FIG. 1.

The fundamental region of the CO Raman (lower panel) and IR (upper panel) spectra in solid Ar for a sample with 1/750 dilution. The 9 K spectra at the bottom were measured after deposition, and the vertical order of the curves corresponds to the subsequent annealing cycle that extends to 30 K. The peaks are located at 2136.5, 2138.3, 2139.9, and 2149 cm−1. See the text for the A-D band assignments, respectively.

Image of FIG. 2.
FIG. 2.

The fixed (a) and adiabatic (b) approaches for computing the angular potential energy surfaces for a single-substitution, ground state CO in a fcc Ar lattice. (a) Energy minimum is at the θ = 0 orientation and rotational barrier reaches 755 cm−1. (b) The adiabatic surface shows preferential orientations along the crystal x, y, and z-axis directions. Barrier height is 43.2 cm−1 and the peak is at 53.3 cm−1.

Image of FIG. 3.
FIG. 3.

A rovibrational level scheme illustrating the allowed transitions in Raman and IR experiments when the adiabatic, tunneling rotor approach is used to explain the high-temperature spectra. The Raman A 1 g A 1 g component at the 2141.6 cm−1 band origin provides the sharpest signal, while the split transitions are subject to libration-phonon coupling and broadening.

Image of FIG. 4.
FIG. 4.

Simulated infrared (panels (a) and (b)) and Raman (panels (c) and (d)) spectra. Left panels refer to the fixed lattice results and right to the fully adiabatic simulations. Temperature variation is shown to affect the line shapes only in the latter case. The vibrational band origin at is indicated by dashed line. The rotational constants are 1.92 and 1.24 cm−1 for the fixed and adiabatic cases, respectively.

Image of FIG. 5.
FIG. 5.

Panel (a): A potential energy contour plot for CO rotation in a adiabatically responsive, double-substitution cage. The global minimum at (θ,ϕ) = (90,225) corresponds to CO oriented with its carbon end towards the vacancy (OC-×). The local minimum at (90,45) with opposite orientation (CO-×) is 10 cm−1 higher. Panels (b) and (c) show simulated Raman and IR spectra that exhibit a doublet structure. The band origins for the global and local minima are indicated by the broken lines and labeled as OC and CO, respectively.


Generic image for table
Table I.

Lowest energy levels (in cm−1) for different computational models.

Generic image for table
Table II.

Vibrational data (cm−1) obtained by different computational models. The gas phase result refers to that given by the potential V CO, and ab initio results are shifted overall by −15.8 cm−1.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Librational motion of CO in solid Ar: Raman and IR spectra and quantum simulations