In this webcast, I'll
explain how a molecule uses
the energy absorbed from light
to promote a
photochemical reaction.
Upon excitation,
a reactant is in a new
electron configuration,
and in that new
electron configuration,
it's governed by a new
potential energy surface.
It can move along that surface
all the way to the product
still in the excited state.
In the excited state, that
product can emit a photon
and jump down to a
ground state surface
of a new product
that's shown here.
This process where only
the excited state
surface is involved
is what's known as the
adiabatic photoreaction.
To summarize then,
the adiabatic photoreaction
involves absorption of light
by the reactant
to generate the excited state
electron configuration.
And then by the normal
thermal motions
that are imparted to that
excited state molecule,
it travels along to the
excited state of the product,
emitting a photon and
generating the final product.
This process, the
adiabatic photoreaction,
is what's responsible for most
chemiluminescent ah processes.
A more common chemical
photochemical process
is what's known as the
diabatic photoreaction
in which both the excited
state potential energy surface
and the ground state potential
energy surface are involved.
Here's how that happens.
The reactant absorbs
a photon of light
and changes into that excited
state electron configuration.
It begins to travel along
the excited state surface
until the geometry matches
a geometry that
is also found
on the ground state
potential energy surface.
This position is what's known
as a conical intersection.
And at that point,
it's possible
for the molecule
to transfer over to the
ground state surface,
at which point it can continue
along in the normal way
using thermal motion
to promote the reaction
leading to the product.
So in the diabatic
photoreaction,
the molecule absorbs light,
changes to the excited
state configuration,
and then, at some point,
crosses back over
to the ground state
and generates the
product in its ground state.
What's important to realize
about the diabatic reaction
is that the molecule
uses the energy of light
in order to hop over this
initial thermal barrier
and set the wheels in motion
to carry out the subsequent
chemical reaction.
