Strong Field

Non-Adiabatic Multi-Electron Dynamics

A pi-system of a large conjugated molecule is a box with particles-electrons in it. The wider the box the longer time it takes for electron to “traverse” it. When this time approaches the inverse laser frequency the electrons cannot catch up with the oscillating laser field and the adiabatic approximation fails. (Note that here the term adiabatic means that electron instantly follows driving electric field of the laser. That is analogous to but not the same with the term adiabatic in Born-Oppenheimer approximation context, where electrons instantly follow nuclei.) As a result, the laser will induce transitions between energy eigenstates of the box. Electron climbs up the “stairs” and eventually escapes, demonstrating classical multiphoton ionization (MPI) mechanism. At the same time, the tunneling contribution to the ionization gets effectively suppressed as the lagging electron moves out of phase with the laser field, at some point even away from the tunneling barrier.

In many-equivalent-electrons molecules it is important to realize that all delocalized electrons experience that nonresonant absorption. As the result a set of electronically excites states become populated yielding an electronic (not vibrational) wavepacket, which moves as a classical “particle” scattering of the edges of the potential well and gaining further energy to eventually escape nuclei’s attraction.

A nice visual representation of a large molecule in the strong field is a wide bath (potential well) half filled with water (delocalized electrons). The bath can be tilted left and right (oscillating laser field), and the water can be poured over the edge (ionization). If the bath is tilted reeeally slowly, the water can be neatly poured (adiabatic over-the-barrier ionization). If the bath is tilted quickly (looks more like shaking) then the water will not keep up with the bath. On the turning points of the oscillating tilt motion the walls will hit the lagging water (laser field doing work on electrons) and create the wave (electronic wavepacket excitation). The waves will continue moving inside the bath occasionally scattering of the edge and splashing some of the water off the bath (= nonadiabatic multielectron dynamics[1]).

1: JCP: 117, 1575 (2002)