Novel applications of femtosecond electron-ion coincidence imaging: from coherent control to mass-selective discrimination of chiral molecules

Insight into the properties of isolated atoms and molecules is achieved by looking at their interactions with light. Upon absorption of one or more photons, the atoms or molecules are in an excited state and often undergo complex dynamics. The absorbed energy can be channeled for instance into a (i) radiative relaxation (ii) dissociation of molecular bonds or (iii) even ionization of the atom/molecule releasing an electron. For the latter process, in addition to the electron, a positively charged particle (the ion) is produced. Both the electron and the ion contain information about the excited state and the ionization process. The most complete information on the molecular dynamics is obtained, when both the angular and momentum distribution of the electron as well as the ion are detected in coincidence in the ionization experiment. The timescale of the atomic and molecular motion is in the range of a few femtoseconds (10 s) to hundreds of picoseconds (10 s). In the case of molecules, the motion during the dissociation of a chemical bond can be studied in a time-resolved manner by the use of femtosecond laser pulses. These studies involve a pump-probe configuration, where the molecule absorbs one or more photons from a first laser pulse (pump) and subsequently the dynamics on the excited state is probed by a second laser pulse (probe) at various time steps. These time-resolved studies have proven to be invaluable in unraveling complex chemical reactions, which occur in chemical, physical and biological systems. The molecular dynamics can be studied and even be manipulated by changing the characteristics of the laser pulses such as wavelength, intensity, phase, amplitude, frequency etc. The technique of manipulating the molecular dynamics in a well-controlled manner is called ’coherent control’. Shaped laser pulses generated by altering the order and the intensity of the colors (wavelengths) in a femtosecond laser pulse can lead to coherent control of molecular dynamics.