Infrared induced dynamics of loosely bound molecules : the dynamical process of vibrational state to continuum coupling for trans-1, 2-dichloroethylene (DCE) using infrared spectroscopy : Master of Science in Physics at Massey University, Albany, New Zealand

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Massey University
The infrared absorption spectrum of crystalline trans-1, 2-dichloroethylene has been measured down to 50 K using a specially designed and constructed infrared (IR) cell, using a Nicolet iS50R FT-IR. The IR scans show the absorbance of trans-1, 2-dichloroethylene molecules at bu CH stretch (9), bu CH bend (10), au CH bend (6) and bu CCl stretch (11) are changed at different temperature conditions. In the solid phase, the absorption spectra of crystalline trans -1, 2-dichloroethylene at different temperatures demonstrates a dramatic change in some of the peak shapes for bu CH stretch (9), bu CH bend (10), au CH bend (6) and bu CCl stretch (11). Instead of a symmetric lineshape that would generally be expected for the excitations of these modes, the spectra clearly showed asymmetric lineshape in some of the peaks as result of the presence of Fano resonance which is not commonly seen in infrared spectroscopy. Although asymmetric peaks are often attributed to interference from reflected rays due to refractive index changes at boundary surfaces the observation, in some cases, of complete peak inversion allows for an unambiguous identification of Fano interference as the source of the asymmetry. The origin of this effect refers to quantum interference on the optical absorption through the coupling between the discrete and continuum states in which the excited eigenstates are mixtures of the discrete and continuum states, as the excitation of trans -1, 2-dichloroethylene occurring simultaneously with the continuum background transitions. In this thesis, we report IR spectroscopic observations that show interference between the vibrational states of the molecules of trans -1, 2- dichloroethylene with the continuum background states. The background continuum arises from the overlapping of many other resonances due to the overtone/ combination bands and coupling of other vibrational states, including the bulk optical phonons, providing essentially a broad continuum. Individual resonances are conspicuous at low temperature and continuously reduce with increasing temperature. Additionally, the IR bands for the fundamental modes have been observed and discussed in this work. Further, the overtone and combination bands are also assigned on the basis of (gas phase) anharmonic calculations as the analysis of anharmonic and harmonic frequencies shows that the anharmonic frequencies are close and correspond better with the experiment than those acquired using a harmonic approximation.
The following Figures have been removed for copyright reasons: Figures 1 (=Misochko et al., 2015 Fig 1), 5 (=ETH Zũrich, n.d. Fig 3.3) & 8 (=Shimanouchi, 1972 Table).
Dichloroethylene, Analysis, Spectrum analysis, Infrared spectroscopy