Femtochemistry

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

The dynamically measurable processes that take place in this time range are core movements (vibrations). The typical speed at which nuclei move is around 1 km / s. This means that in the femtosecond range they move a few Ångström (1 Å = 10 ⁻¹⁰  m); a molecular oscillation lasts approx. 10 to a few 100 fs. Since vibrations in molecules - in particular bond formation and bond breaking - represent the basis of chemical reactions, this research area is understood as a separate sub-area of ​​chemistry and is referred to as "femtosecond chemistry" or femtochemistry for short.

history

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

background

A typical femtosecond experiment consists of a pulse train of 2 pulses: a pump pulse (excitation pulse), which puts the molecule in an excited (dynamic) state, and a time-delayed test pulse (query pulse), which provides the dynamic information of the system at different times queries. Typically the interrogation pulse is an ionizing pulse and the interrogated information is measured in the form of photoelectrons or fragments. The time interval between the two pulses is varied in that a pulse has to run a detour over a path with mirrors. This detour is very small: 100 fs time difference means 0.03 mm detour. The requested information provides, so to speak, a fingerprint of the system at the time of the request (analogy: stopwatch ).

In theory, such femtosecond experiments are typically treated computationally using time-dependent perturbation theory . The interaction of the system in the ground state with the first pulse is described in first order perturbation theory, and the interaction with the probe pulse in second order.

After it was possible to measure these processes, both theory and experimentation research was carried out into how such processes can be manipulated, for example to increase the yield of chemical reactions. This area is known as quantum control .

Currently (2008) laser pulses with a pulse duration of less than 5 fs and peak intensities well above 10 ¹⁸  W / m ² can be generated. For thus generated fields is the phase of the field under the enveloping function , is no longer negligible. With such and even shorter pulses, the electron dynamics can now be observed and influenced. The ultra-short, strong and phase-stabilized laser pulses are used particularly in attosecond physics and in the generation of high harmonics . ${\displaystyle E(t)}$ ${\displaystyle \phi }$ ${\displaystyle f(t)}$ ${\displaystyle E(t)=f(t)\cdot \cos(\omega t+\phi )}$

See also

• Femtosecond laser

literature

• Ahmed H. Zewail: Femtochemistry. Past, present, and future. In: Pure and Applied Chemistry. 72, 2000, pp. 2219-2231, doi : 10.1351 / pac200072122219 .

Individual evidence

1. ↑ "Femtosecond Chemistry" Volume I and II, VCH Weinheim (1995).
2. ↑ M. Shapiro, P. Brumer, “Principles of Quantum Control of Molecular Processes,” Wiley, New York (2003); SA Rice, M. Zhao, "Optical Control of Molecular Dynamics", Wiley, New York (2000).