Experiments in the gas phase show that vibrational control of both unimolecular
photodissociation and bimolecular reaction is possible. These experiments
rely on the molecule being isolated from its surroundings (in a gas cell
or a molecular beam) so that the vibrational energy remains in the molecule
for a long time relative to the timescale of the reaction. One of our
goals is to now extend vibrational control of reactions into the liquid
phase, where the time evolution of the vibrational energy is important.
A vibrationally excited molecule in solution rapidly loses its energy
because it is constantly interacting with the solvent. The vibrational
energy typically drains out of a molecule within 1-100 ps (1 ps = 10-12
s, or 1 trillionth of a second!), so the vibrationally controlled reaction
must occur faster than this. In fact, this requires that we use ultrafast
laser pulses that are ~100 fs (1 fs = 1/1000 ps) in duration to impulsively
excite vibrational motions and induce the reaction before the energy is
lost.
Vibrational relaxation in solution:
The first step toward vibrational control of reactions in solution is
understanding the processes that cause a molecule to lose energy to the
solvent. Because we use an ultrafast laser pulse to excite the vibration,
we create a non-stationary state, which means that the vibrational energy
will also move around within the molecule. Both the intramolecular
vibrational relaxation (IVR) and the intermolecular energy transfer (IET)
potentially compete with our ability to control the outcome of a reaction
in solution. To read more about vibrational relaxation click
here.
Excited-state dynamics in solution:
The second step towards realizing vibrationally-mediated control is understanding
the overall dynamics of the reaction. As many molecules cannot undergo isomerization
on the ground state, we use a pair of ultrafast laser pulses to electronically
excite a molecule and probe its motion along the excited state. By monitoring
the progress of this photoisomerization, we have a standard by which to gauge the
success of the vibrationally-controlled reaction.
Vibrationally mediated photoisomerization in solution:
The ultimate goal in this lab is to use vibrational excitation to control
the outcome of unimolecular photoisomerization reactions. We can acheive this through
a combination of the above two steps;  we use a sequence of laser pulses to give a molecule vibrational energy,
excited it electronically, and probe its isomerization dynamics on the excited state.
By varying the amount of vibrational energy or the time delay between these events, we
hope to affect the product, or, in the case of multiple pathways, the branching ratio of
the reaction. |
