Page 30 - Visions of the Future Chemistry and Life Science

Laser snapshots of molecular motions 19

eration also offers the chance to develop coherent, ultrafast X-ray laser
devices.

1.5.2 Coherent control of molecular dynamics
When it was invented in 1960, the laser was considered by many to be the
ideal tool for controlling the dynamics of molecular dissociation and colli-
sions at the molecular level. The reasoning was that by choosing the fre-
quency of a monochromatic (long pulse duration) laser to match exactly
that of a local vibrational mode between atoms in a polyatomic molecule,
it ought to be possible to deposit sufficient energy in the mode in question
to bring about a massively enhanced collision probability, and thereby gen-
erate a selected set of target states. With the benefit of hindsight, it is clear
that the approach failed to take into account the immediate and rapid loss
of mode specificity due to intramolecular redistribution of energy over a
femtosecond time scale, as described above.
Eight years ago it was suggested by US researchers that in order to
arrive at a particular molecular destination state, the electric field asso-
ciated with an ultrafast laser pulse could be specially designed to guide a
molecule during a collision at different points along its trajectory in such
a way that the amplitudes of all possible pathways added up coherently just
along one, specific pathway at successive times after the initial photoab-
sorption event. To calculate the optimal pulse shapes required by this
scheme dictates the use of a so-called ‘evolutionary’ or ‘genetic’ computer
algorithm to optimise, by natural selection, the electric field pattern of the
laser applied to the colliding molecule at successive stages, or ‘genera-
tions’, during its dynamical progress from the original progenitor state
until the sought-after daughter state is maximally attained.
In order that this proposal can be made to work, what is required is a
device which can make rapid changes to the temporal pattern of the elec-
tric field associated with a femtosecond laser pulse. The recent develop-
ment of liquid-crystal spatial light modulators to act as pulse shapers
fulfils this task, and may open the gateway to a plethora of experimental
realisations of coherent control of molecular dynamics. There has been
much theoretical work on the types of laser pulse shapes required to bring
about specific molecular goals. In the laboratory, successful optical control
of molecular events has been demonstrated for strategic positioning of
wavepackets, enhancement of molecular ionisation probabilities, and
optimisation of different photodissociation pathways. With the advent of