Page 12 - Visions of the Future Chemistry and Life Science

Laser snapshots of molecular motions 3

grammes in its own right. Figure 1.1 offers a simplified portrayal of some
of these events, showing the ionisation of an electron from the warped
potential energy structure an atom by an intense laser pulse, the path sub-
sequently followed by the electron in repsonse to the oscillating electric
field of the laser pulse, and the emission of a high-frequency harmonic
photon which occurs when the electron scatters off the ion core (high-har-
monic emission can be exploited to generate attosecond laser pulses, dis-
cussed in Section 1.4.1). A similar series of events, with due alteration of
the details, occurs in molecules exposed to intense laser light.
From careful measurements of such processs, it is possible to develop
quantitative models to describe the molecular dynamical response to
impulsive laser excitation. These enable the fundamental interaction of
intense, ultrafast laser light with molecules to be understood from first

Laser-distorted
atomic electron Ponderomotive
potential
electron trajectory

High harmonic
photon emission

Tunnel ionisation
Figure 1.1. A sequence of events following the interaction of an intense, ultrafast
laser pulse with an atom. The potential energy structure of the electron, which
would otherwise be symmetric either side of a minimum, thereby confining the
electron to the vicinty of the atomic nucleus, is distorted by the incident laser
radiation. The electron first escapes (ionises) from the atom by tunnelling through
the barrier on the side of lower potential energy and then executes an oscillatory
trajectory determined by its kinetic (or ponderomotive) energy in the electric field
of the laser pulse. If the electron follows a trajectory that brings it back close to
the nucleus of the ionised atom, emission of a high-frequency photon can occur as
the negatively charged electron is accelerated by the positively charged ion. This
high-frequency photon is found to be an exact harmonic (overtone) of the laser
frequency originally absorbed by the atom.