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VERSION:2.0
CALSCALE:GREGORIAN
PRODID:UW-Madison-Physics-Events
BEGIN:VEVENT
SEQUENCE:0
UID:UW-Physics-Event-2241
DTSTART:20110930T203000Z
DURATION:PT1H0M0S
DTSTAMP:20240319T032930Z
LAST-MODIFIED:20110923T133924Z
LOCATION:2241 Chamberlin Hall (coffee at 4:30 pm)
SUMMARY:Vibrations\, Conical Intersections\, and Reaction Dynamics in
Gases and Liquids\, Physics Department Colloquium\, Fleming Crim\, Uni
versity of Wisconsin-Madison
DESCRIPTION:Conical intersections between potential energy surfaces oc
cur when the two electronic states become degenerate and\, consequentl
y\, the Born-Oppenheimer approximation fails. Although long regarded a
s interesting curiosities\, it is now clear that they are central to a
large variety of molecular transformations. Photodissociation and pho
toisomerization often occur by passage of electronically excited molec
ules through a conical intersection\, and the details of that passage
control the partitioning of the products among competing pathways.
\n
\nAmmonia is a prototypical molecule in which a conical inters
ection is important in excited-state dissociation. High resolution stu
dies of the dissociation of isolated ammonia molecules show that excit
ation of selected vibrations changes the course of the dissociation dr
amatically\, leading to the formation of electronically excited produc
ts at the expense of ground-state products. The key to this behavior i
s the influence of vibrational motion on passage of the system through
the conical intersection.
\n
\nSimilar studies in solution r
equire high time resolution rather than high spectral resolution\, and
it is possible to prepare vibrationally excited molecules in solution
and monitor their evolution using 100-fs laser pulses. In these exper
iments an infrared pulse excites a vibration\, an ultraviolet pulse tr
ansfers molecules to an excited state\, and broadband continuum absorp
tion monitors their evolution in the excited state. The competition be
tween excited state vibrational relaxation and barrier crossing is the
feature that determines the influence of vibrational excitation on th
e isomerization rate.
\n
URL:https://www.physics.wisc.edu/events/?id=2241
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