Low-frequency Raman spectra of paradichlorobenzene thin films

The low-frequency Raman spectra of paradichlorobenzene nanofilms with a thickness of about 30 μm, 1 μm, and 400 nm are experimentally measured. With decreasing film thickness, the spectral lines shift to lower frequencies and broaden and some additional lines appear, the intensity of which increases with decreasing film thickness. The film structure is modeled using the method of molecular dynamics, and the histograms of lattice vibration spectra are calculated by the Dean method. It is found that a decrease in the film thickness leads to an increase in the lattice parameters and to a higher orientational disorder both inside the film and, especially, at the film surface. The film structure is similar to the structure of the α-paradichlorobenzene single crystal. To correctly interpret the spectra of lattice vibrations, it is necessary to take into account surface vibrations and existence of vacancies in the structure.