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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2019
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Massey University
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Abstract
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.
Description
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).
Keywords
Dichloroethylene, Analysis, Spectrum analysis, Infrared spectroscopy