Nonlinear dynamics applied to atomic, molecular, and optical science

How electrons move inside matter regulates chemical reactions, macro- and bio-molecular (re)activity, and photovoltaic energy transfer. At their fastest, these electronic dynamics unfold on femtosecond (one millionth of one billionth of a second) timescales or shorter, and are commonly called charge migration. We introduce a novel way to understand charge migration in organic molecules with the help of nonlinear dynamics. We reveal how multiple interacting electrons arrange their individual dynamics to collectively form charge-migration modes that are spatially localized while periodically traveling back and forth across the molecule. Our results provide an intuitive and predictive model that explains the regularities seen in large scale quantum mechanical simulations and pave the way for chemical control of charge-migration modes in organic molecules.

Picture from: Physical Review Research 4, 013073 (2022)

What do asteroid capture and double ionization have in common? A great deal, it turns out:  A circularly polarized (CP) laser field hurls ionized electrons back at the core in the same way that comets and interplanetary debris make their way to planets. According to conventional wisdom, a CP field suppresses collision-induced double ionization since ionized electrons spiral away and therefore cannot revisit the core. A few experiments carried out with rare gas atoms in the past confirmed this belief, and the matter would rest there if it weren't for conflicting experiments showing the signature of electron-electron correlation in the double ionization of magnesium. We reconcile these seemingly contradictory results by finding the conditions for an ionized electron to revisit the core to ionize more electrons and show that the so-called recollision model of strong-field physics can also be valid in CP.

Picture from: Physical Review Letters 105, 083002 (2010)

It is now understood that correlated double ionization occurs through the recollision mechanism -- an electron previously ionized by a  strong, linearly-polarized, laser is brough back to its parent ion upon reversal of the electric-field direction where It can exchange some of its excess energy to ionize a second electron. This picture focuses on the firstly ionized electron and leaves mostly out the second one before the recollision event. We have shown that the standard recollision picture must be complemented by the dynamics of the other electron, which is subjected to a competition between the Coulomb attraction from the nucleus and the excitation of the laser field. When combined, these two reduced models yield quantitative predictions on the double ionization signal as a function of the laser intensity, a commonly investigated experimental datum.

Picture from: Physical Review Letters 102, 173002 (2009)