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<div><font face="Arial" size="+1" color="#000000">Seminar on<br>
<b>Modern Optics and Spectroscopy<br>
<br>
<br>
Adam Steeves</b>, MIT<br>
<br>
<i><b>Acetylene: What happens when a well-behaved molecule gets
bent out of shape?<br>
<br>
</b></i>May 8, 2007<br>
<br>
12:00 - 1:00 p.m.</font></div>
<div><font face="Arial" size="+1" color="#000000">Grier Room<br>
</font><font face="Times" size="+1"
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</x-tab></font><br>
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<div><font face="Arial" size="+1" color="#000000">At energies
approaching those required for chemical transformation, analysis of
the vibrational spectrum of an isolated polyatomic molecule often
becomes complicated by a rapidly increasing density of states,
destruction of the utility of normal mode quantum numbers as labels,
and the coexistence of classically chaotic and regular dynamic
regimes. Assignment and interpretation of a vibrational spectrum
depend, in such a region, on our ability to utilize unconventional
information to isolate and assign the states most critical to the
dynamics, those that sample chemical reaction barriers. We discuss two
cases in which the recognition of novel spectroscopic patterns have
aided our understanding of large amplitude vibrational dynamics in
acetylene. In the first electronically excited singlet state of
acetylene, purely vibrational (anharmonic) and vibration-rotation
(Coriolis) interactions between the normal mode basis states involving
the two low frequency vibrations spoil the traditional
vibration-rotation energy level patterns upon which vibrational
assignments are typically based. However, these strong interactions
drive the system to a new, but still recongnizable pattern of energy
levels, which can be described by an approximately conserved polyad
quantum number. Due to distortion of the electronic wavefunction as
the molecule approaches barriers to chemical rearrangement, the
identification of special large amplitude motions can also be aided by
the measurement of electronic properties of the vibrationally excited
molecule. We demonstrate how measurement of a electronic property, the
electric dipole moment, can identify local-bending, barrier-proximal
vibrational states in the ground electronic state of
acetyelene.</font></div>
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