Page 13 - Visions of the Future Chemistry and Life Science

4 G. ROBERTS

principles in terms of the wave description of matter and light due to
quantum theory.
Following a description of femtosecond lasers, the remainder of this
chapter concentrates on the nuclear dynamics of molecules exposed to
ultrafast laser radiation rather than electronic effects, in order to try to
understand how molecules fragment and collide on a femtosecond time
scale. Of special interest in molecular physics are the critical, intermedi-
ate stages of the overall time evolution, where the rapidly changing forces
within ephemeral molecular configurations govern the flow of energy and
matter.

1.3 Femtosecond lasers
To carry out a spectroscopy, that is the structural and dynamical determi-
nation, of elementary processes in real time at a molecular level necessi-
tates the application of laser pulses with durations of tens, or at most
hundreds, of femtoseconds to resolve in time the molecular motions. Sub-
100fs laser pulses were realised for the first time from a colliding-pulse
mode-locked dye laser in the early 1980s at AT&T Bell Laboratories by
Shank and coworkers: by 1987 these researchers had succeeded in produc-
ing record-breaking pulses as short as 6fs by optical pulse compression of
the output of mode-locked dye laser. In the decade since 1987 there has
only been a slight improvement in the minimum possible pulse width, but
there have been truly major developments in the ease of generating and
characterising ultrashort laser pulses.
The major technical driving force behind this progress was the discov-
ery by Sibbett and coworkers in 1990 of a new category of ultrafast laser
operation in solid-state materials, the most important of which is sapphire
impregnated with titanium (others are the chromium-doped colquiriite
minerals). These devices rely upon the intensity dependence of the refrac-
tive index of the gain medium to generate powerful, ultrashort laser pulses
in a single ‘locked’ mode: a photograph of a commercial titanium:sapphire
laser is shown in Figure 1.2.
Titanium:sapphire lasers typically deliver pulses with durations
between 4.5 and 100 fs, and can achieve a peak power of some 0.8watts,
but this is not high enough to obtain adequate signal-to-noise ratio in
experiments where the number of molecules that absorb light is low. To
overcome this limitation, the peak power of a femtosecond laser can be dra-