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Laser snapshots of molecular motions 11
recorded as a function of pump-probe time delay: the decrease in signal
intensity with increasing pump-probe time delay monitors the loss of
initial IBr* to form separated I and Br over the potential V ; and the oscil-
1
lations superimposed upon the decay reflect the quantized nature of vibra-
tional motion of the quasi-bound [I . . . Br] molecules at intermediate
configurations within the bound V curve.
1
A series of measurements in which the pump wavelength is varied
reveal that at some energies the oscillations predominate for times beyond
10ps, whilst at others the decay of population by curve-crossing wins out
within 400fs or so. The time resolution of the experiment is in this
example is determined by the convolution of the two laser pulse widths,
here about 125fs.
These attributes can be accounted for by theoretical calculations of the
motion of the wavepacket over the repulsive potential, which aim to deter-
mine the time-resolved ionisation signal from fundamental theory. These
are performed by solving the time-dependent Schrödinger equation for the
dissociation, which expresses the temporal development of the quantum
wavefunction prepared by the laser pulse subject to all the forces that act
on the nuclei as it progresses from starting to final states. Figure 1.4(c) dis-
plays a calculated pump-probe ionisation trace that corresponds to the
same initial conditions of Figure 1.4(b). A mathematical analysis of these
data using the technique of Fourier transformation reveals how quantised
vibrational motion of the molecule along the dissociation coordinate is
transformed into kinetic energy of separation as the I and Br atoms fly
apart.
1.4.2 Ultrafast molecular collisions
Unfortunately, femtosecond laser pulses are not so readily predisposed to
study collisions between atoms and molecules by the pump-probe
approach. The reason is that, typically, the time between collisions in the
gas phase is on the order of nanoseconds. So, with laser pulses of sub-100fs
duration, there is only about one chance in ten thousand of an ultrashort
laser pulse interacting with the colliding molecules at the instant when the
transfer of atoms is taking place; in other words, it is not possible to
perform an accurate determination of the zero of time.
An ingeneous method to circumvent this problem was first devised by
Zewail and colleagues, who took advantage of the vibrational and rota-
tional cooling properties and collision-free conditions of the supersonic