Strong Fields

Molecules in Strong Fields

Extending the strong field onto molecules brings another dimension of fundamental interest and also leads to more applications. Analytical mass spectroscopy is one of the most important means to identify an analyte composition, monitor technological processes in situ, to detect traces of hazardous chemicals etc. From a mass spectrometry perspective, the generation of parent or structurally characteristic fragment ions is a key feature in the technique’s analytical ability. However, rapid molecular dissociation of the analyte resulting in fragmentation to non-specific low mass ions often precludes unambiguous identification of the component in the mass spectra. Often the fragmentation is caused by an excess of vibrational energy the ion receives in result of brute ionization with a too energetic photon (or electron). While not suppressing fragmentation completely (some molecules are simply unstable in their ionic form), the ultrafast strong field ionization introduces another mean to ionize an analyte, likely bringing fragmentation pattern complementary to those resulting from electron impact, VUV and other ionization strategies, thus facilitating (/aiding) the reconstruction of the parent molecule mass and structure.

With respect to tunneling ionization, small molecules behave pretty much like atoms. However, upon increasing molecular size new aspects pop up – some effects cannot longer be neglected. Two main approximations underlie most atomic tunneling ionization theories. The first one is the assumption that only one electron actively participates in the ionization process, while others stay unaffected by a strong laser field. Second, an electron is assumed to instantaneously follow the time-varying electric field of the laser (adiabatic approximation). Both these may not be reasonable predicates for a large conjugated molecule where several equivalent electrons can swim along delocalized pi-system back and forth for nanometer distances. To add complexity, molecules have a certain shape and ionization can be different along each molecular axes. Molecules may align in the applied laser field and this can affect the ionization drastically. Molecules may vibrate on the timescale comparable with laser period (period of a C-H stretch vibration (3000cm-1) is about 10 fs). This should influence the ionization dynamics and will most likely lead to more fragmentation.