Ultrafast dynamics

Since an electron is a quantum particle, it is described as a “wave of matter” or a “matter wave packet”, of which the maximum amplitude – the “heart” - is localized in space and in time, respectively at the typical Angström and attosecond scales. This is the case of the bound electrons in an atom or a molecule. Moreover, in matter, an electron with an energy of 1 electron-Volt moves over a distance of 1 Angström, the typical distance between two atoms in a molecule or in a solid lattice, within a time interval of typically 100 attoseconds. Here the electron motion refers to that of the localized “heart” of the wavepacket. In the case of a stringent localization, the electron motion may be assimilated to that of a classical point-like particle.

The same is true of the much heavier nuclei (the proton is 1835 times heavier than the electron), which move at the Angström scale at the typical time scale of femtoseconds.

The steady-state equilibrium structure of isolated molecules and solids corresponds to the sometimes very complex conditions which minimize the energy of the system in the electromagnetic interaction between the charged particles, or the charged particles and light (usually radiated by moving charges, gravity plays a minor role). Similarly, the electron and nuclear motions, which depart from equilibrium conditions, take place in the innumerable conditions in nature of energy exchange between charged particles or charged particles and light. The motions of, respectively, electrons and nuclei are strongly coupled : the lightest electrons are usually the first to respond to an external force, e.g., on the attosecond timescale; their displacement then push into motion the heavier nuclei; in turn, the moving nuclei drag the electron cloud which adiabatically adapts the nuclear backbone on femto to picosecond timescale.

Steady-state structure and dynamical processes in atomic systems are the two universal faces of matter around us, from the gas phase, the condensed phase, the highly organized living matter or disorganized plasmas, that physics, chemistry and biology are exploring.

The objective of ultrafast dynamics is to track, understand and possibly control the motion of electrons and nuclei – or electronic and nuclear wavepackets - on the shortest time and space scales [Agostini04, Bucksbaum07, Krausz09]. Ultrafast dynamics, its theoretical concepts and experimental techniques are relevant and transverse to several fields of physics and chemistry, from atomic and molecular physics in the gas phase, femtochemistry and femtobiology, charge carriers dynamics and magnetization dynamics in the solid state, to coherent motion of electrons in dense plasmas.

[Agostini04]	The physics of attosecond light pulses, Agostini P., DiMauro L. F., Reports on Progress in Physics 67, 813 (2004)
[Krausz09]	Attosecond physics, Ferenc Krausz and Misha Ivanov, Rev Mod Phys 81, 163 (2009), DOI: 10.1103/RevModPhys.81.163
[Bucksbaum07]	The Future of Attosecond Spectroscopy, P. H. Bucksbaum, Science 317, 766 (2007)