Page 17 - Visions of the Future Chemistry and Life Science

Laser snapshots of molecular motions 7

application of such schemes offers continuous tunability from the near
ultraviolet, through the visible, into the infrared regions of the spectrum.
An important point is that these advances have been complemented
by the concomitant development of innovative pulse-characterisation pro-
cedures such that all the features of femtosecond optical pulses – their
energy, shape, duration and phase – can be subject to quantitative in situ
scrutiny during the course of experiments. Taken together, these resources
enable femtosecond lasers to be applied to a whole range of ultrafast pro-
cesses, from the various stages of plasma formation and nuclear fusion,
through molecular fragmentation and collision processes to the crucial,
individual events of photosynthesis.

1.4 Femtosecond spectroscopy of molecular dynamics

1.4.1 Ultrafast molecular fragmentation
To determine molecular motions in real time necessitates the application
of a time-ordered sequence of (at least) two ultrafast laser pulses to a molec-
ular sample: the first pulse provides the starting trigger to initiate a partic-
ular process, the break-up of a molecule, for example; whilst the second
pulse, time-delayed with respect to the first, probes the molecular evolu-
tion as a function of time. For isolated molecules in the gas phase, this
approach was pioneered by the 1999 Nobel Laureate, A. H. Zewail of the
California Institute of Technology. The nature of what is involved is most
readily appreciated through an application, illustrated here for the photo-
fragmentation of iodine bromide (IBr).
The forces between atoms in a molecule are most conveniently respre-
sented by a surface of potential energy plotted as a function of the inter-
atomic dimensions measured in ångströms (Å) (10Å are equivalent to a
millionth of a millimetre). For the IBr molecule in the gas phase, the elec-
tronic ground state in which the molecule resides at equilibrium is char-
acterized by a bound potential energy curve, labelled V in Figure 1.3. The
0
dissociative process is governed by two, interacting potential energy curves
V and V for different excited states, which enable the molecule to break
1 1
up along a coordinate leading to ground-state atoms (I Br) or along a
higher energy route which leads to excited bromine (I Br*). Typical separ-
1
ation velocities are in the range 1500–2500ms . The same figure illus-
trates how femtosecond laser pulses configured in a pump-probe sequence
can be applied to monitor the time-evolution of the photodissociation.