Femtochemistry

Femtochemistry is a branch of chemistry, the processes on the femtosecond time scale describes ( 1 fs = 10-15 s ).

The dynamic measurable processes that take place in this time range, are nuclear motion (vibrations ). The typical speed of moving nuclei is about 1 km / s In order for this move in the femtosecond range by a few angstroms ( 1 Å = 10-10 m); a molecular vibration takes about 10 to a few 100 fs. Since vibrations in molecules - in particular bond formation and bond breaking - are the basis of chemical reactions, this field of research is seen as a separate part of the field of chemistry and referred to as " femtosecond chemistry" or short Femtochemistry.

History

With the invention of "phase- locked" laser pulses the mid / late 1980s, the femtosecond range was experimentally accessible. Special spectroscopic methods, such as the pump-probe technique, make it possible to measure snapshots of the core movements directly. In his work on the NaI and ICN (among other molecules) Ahmed Zewail was able to produce such snapshots and measure, among other things, by which time break molecular bonds. For his work he was awarded the Nobel Prize for Chemistry in 1999.

Background

A typical femtosecond experiment consists of a pulse train of two pulses: a pumping pulse ( excitation pulse ), which enables the molecule to an excited ( dynamic ) state and a delayed sample pulse ( interrogation pulse ), the dynamic information of the system at different time points queries. Typically, the interrogation pulse is a pulse of ionizing, and the requested information is measured in the form of photo-electrons or fragments. The time interval between the two pulses is varied by a pulse has a detour over a distance run with mirrors. This detour is very small: 100 fs time difference mean 0.03 mm detour. The requested data returns were, a fingerprint of the system at the time of the query ( analogy: stopwatch).

In theory, such femtosecond experiments are typically treated mathematically by means of time-dependent perturbation theory. The interaction of the system in the ground state with the first pulse is described in perturbation theory of the first order, and the interaction with the sample pulses in the second order.

After it has been possible to measure these processes has been studied both in theory and in experimenter side, as these processes can be manipulated so as to increase, for example, the yield of chemical reactions. This area is referred to as quantum control.

Currently (2008) can be generated laser pulses with less than 5 fs pulse duration and peak intensities well above 1018 W/m2. For as generated fields, the phase of the field is under the envelope function is no longer negligible. With these and even shorter pulses now, the electron dynamics are observed and affected. The ultra-short, strong and phase-stabilized laser pulses find especially in the Attophysik and in the generation of high harmonics application.