Cluster size resolving analysis of CH₃F-(ortho-H ₂)_n in solid para-hydrogen using FTIR absorption spectroscopy at 3 μm region

Infrared absorption spectra of methyl fluoride with ortho-hydrogen (ortho-H₂) clusters in a solid para-hydrogen (para-H₂) crystal at 3.6 K were studied in the C-H stretching fundamental region (∼3000 cm^-1) using an FTIR spectrometer. As shown previously, the ν₃ C-F stretching fundamental band of CH₃F-(ortho- H₂)_n (n 0, 1, 2, ⋯) clusters at 1040 cm ^-1 shows a series of n discrete absorption lines, which correspond to different-sized clusters. We observed three unresolved broad peaks in the C-H stretching region and applied this cluster model to them assuming the same intensity distribution function as the ν₃ band. A fitting analysis successfully gave us the linewidth and lineshift of the components in each vibrational band. It was found that the separately determined linewidth, matrix shift of the band origin, and cluster shift are dependent on the vibrational mode. From the transition intensities of the monomer component derived from the fitting analysis, we discuss the mixing ratio of the vibrational modes due to Fermi resonance.