Page 25 - Visions of the Future Chemistry and Life Science

Laser snapshots of molecular motions 15

1.4.3 Many-body e≈ects on ultrafast dynamics
Over recent years, advances in high-vacuum technology and mass
spectrometry have enabled experimentalists to prepare clusters of selected
size and composition in the gas phase. A cluster is a smallish globule, com-
prising up to about 1000 atoms or molecules held together by weak attrac-
tive forces, that is supremely well-suited for the study of ultrafast
phenomena in which many-body effects dominate the collisional outcome.
The most important of these concern the fate of the energy initially depos-
ited in the cluster by the laser pulse as a result of intra- and intermolecu-
lar energy redistribution, coherence loss of the nascent wavepacket and
molecular fragmentation, and how these effects evolve with increasing
degrees of freedom. A popular choice for investigation has been the disso-
ciation of molecular halogens attached to one or more rare gas atoms.
A recent experimental study by the group of Neumark at UC Berkeley,
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USA, on the dissociation of the negatively charged diiodide (I ) ion in the
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presence of zero, six or 20 argon atoms exemplifies marvellously the way
in which the issues listed above can be successfully addressed by femtose-
cond spectroscopy. In these experiments, the dissociation of size-selected
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I ·Ar clusters was triggered using 100fs pulses from a Ti:sapphire laser and
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monitored by a second ultrafast pulse which detaches the excess electron
from the negatively charged molecule. Measurements of the kinetic energy
distribution of the photoejected electrons, called a photoelectron spec-
trum, as a function of pump-probe delay time turn out to be an extremely
sensitive probe of the rapidly changing local environment of the detached
electron, in that they reveal how the forces between the iodine atoms and
between the I molecule and its immediate surroundings evolve during the
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dissociative separation of the halogen atoms. The experiments show that,
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whereas in the absence of argon atoms the break-up of diiodide to I and I
evolves over a time scale of 250fs, it is effectively stopped and returned to
near its starting position when 20 argon atoms form a shell around the dis-
sociating molecule; subsequent to the caging process, vibrational cooling
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of the I molecule thereby regenerated takes an amazingly long 200ps to
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complete!
Experiments such as these provide an incomparable level of detail on
the temporal ordering of elementary processes in a multidimensional col-
lisional environment. To understand the dynamical evolution of many-
body systems in terms of the changing forces that act on the interacting