Page 26 - Visions of the Future Chemistry and Life Science

16 G. ROBERTS

atoms requires sophisticated computer simulations to map out the
motions of the individual atoms and to elucidate the structures of the tran-
sient molecular configurations that control the flow of energy between
atoms and molecules over a femtosecond time scale. For clusters contain-
ing, say, a diatomic molecule bound to one or two atoms, with computa-
tional facilities available today it is possible to carry out calculations in
which the dissociative evolution along every degree of freedom is treated
by quantum dynamics theory.
An example of this type of calculation is shown in Figure 1.6, which
portrays a snapshot of the wavepacket motion of iodine bromide attached
to Ar initiated by a 100fs laser pulse. The early-time ( 150fs) motions of
the complex, which is almost T-shaped, comprise a simultaneous length-
ening of the I–Br distance and a slower transfer of vibrational energy from
the intramolecular mode to the IBr–Ar coordinate. Just as was found for the
isolated IBr molecule (Section 1.4.1), a fraction of the wavepacket ampli-
tude along the I–Br direction proceeds to dissociation by curve crossing
whilst the remainder becomes trapped in the quasi-bound potential well.
By 840fs, bursts of vibrational energy transfer to the atom–molecule degree
of freedom give rise to a stream of population which eventually leads to
expulsion of argon from the complex. To connect this dynamical picture
with information available from experiments, calculations of the vibra-
tional spectra of the cluster as a function of time after the femtosecond
pump pulse show that relaxation of the nascent IBr vibrational content is
at first sequential but at times longer than about 500fs becomes quasi-con-
tinuous as a result of a complex interplay between intermode vibrational
energy redistribution and molecular dissociation.

1.5 What else and what next? A speculative prognosis
Ultrafast laser spectroscopy is very much a science that, by its very nature,
is driven by improvements in laser and optical technology. Dangerous
though it is to make forecasts of scientific advances, what is clear at the
time of writing (early 2000) is that at the cutting edge of this research field
is the progress towards even faster laser pulses and the ability to design
femtosecond laser pulses of a specified shape for optical control of individ-
ual molecular motions.