Vibron band structure in chlorinated benzene crystals: lattice dynamics calculations and Raman spectra of 1,4-dichlorobenzene

Abstract

Abstract. Lattice dynamics calculations of the lattice modes (phonons) and the internal modes (vibrons) and Raman spectra at liquid helium temperatures are presented for the / 3 a-and y-phase of 1,4-dichlorobenzene (d c b ). It follows from the calculations and it is confirmed by the Raman spectra that these three phases are characteristically different with respect to the following properties: vibron band dispersion, Davydov splitting, 35CI/37C1 isotopic effects on the bandstructure and vibron-phonon mixing. Only the low temperature y-phase shows a nice one-dimensional band structure for some vibrations. The vibron band dispersion in all phases of d cb is caused by the repulsive exponential terms in the intermolecular potential. This is in contrast to 1,2,4,5-tetrachlorobenzene, where the elec trostatic interactions are responsible for the dispersion. Some of the ungerade vibron modes become visible in the Raman spectra of dcb due to the (random) occurrence of molecules with different distributions of 35C1/37C1 isotopes. A few of the Raman lines observed could not be uniquely assigned; they are probably caused by Fermi resonances.

High-resolution vibronic spectroscopy of a single molecule embedded in a crystal

“…Bulk pDCB crystals at liquid Helium temperatures are rather expected to be found the γ-phase if they are cooled down slowly. 53,[86][87][88] In our experiments, we cooled pDCB crystals enclosed in nanochannels from room temperature to liquid helium temperature at pressures < 10 −3 mbar. While we are not certain in which phase the pDCB crystal forms under these conditions, we find that besides the prominent features analyzed in the experimental data, there also exist less intense peaks that are predicted by the simulations (e.g.…”