[MOS] May 8, 2007

[Zina Queen]

Seminar on
Modern Optics and Spectroscopy


Adam Steeves, MIT

Acetylene:  What happens when a well-behaved molecule gets bent out of shape?

May 8, 2007

12:00 - 1:00 p.m.
Grier Room


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.
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