Infrared spectra of CH_3F (ortho-H_2)_n clusters in solid parahydrogen

摘要

The formation of CH_3F(ortho-H_2)_n clusters in rapid vapor deposited solid molecular hydrogen containing low concentrations of CH_3F and ortho-H_2 has been investigated using high-resolution Fourier transform infrared spectroscopy of the C-F stretching mode (v_3) of the CH_3F chromophore. Distributions of CH_3F(ortho-H_2)_n clusters ranging in size from n=0 to n=12 are synthesized in para-H_2 crystals by systematically varying the ortho-H_2 concentration between 100 to 20 000 parts per million. The rotational motion of CH_3F is quenched in solid para-H_2; this simplifies the spectrum such that a single sharp transition is observed for each cluster. In the fundamental v_3 region, the spectrum consists of a series of well-separated peaks shifted toward lower energy with increasing numbers of nearest neighbor orthohydrogen molecules. The CH_3F(ortho-H_2)_n clusters are also investigated in the v_3 overtone region and for analogous transitions of the CD_3F(ortho-H_2)_n. Theoretical calculations based on an electrostatic interactions potential qualitatively reproduce the measured vibrational and isotopomer dependence of the spectra.