Coherent Attosecond Ionization Dynamics in Laser-Dressed Atomic and Molecular Systems

The University Of Central Florida Board Of Trustees, Orlando FL

Investigators

Abstract

Many chemical transformations, such as combustion, photosynthesis, and radiational damage, are driven by the motion of electric charge. Observing and steering the motion of charge at the molecular level so to efficiently harness energy, transfer information, and control chemical reactions, however, is complicated by the extremely short timescale at which electrons move. An electron can travel across tens of atoms in less than a millionth of a billionth of a second (one femtosecond, fs). This motion can only be affected by using probes that operate at a similar speed. Such a probe, an extreme ultraviolet light pulse with sub-femtosecond duration, was reported for the first time only at the turn of this century. In the two decades elapsed since, attosecond (1 as = 0.001 fs) laser and detection technology has advanced to a level that allows us to follow and to partially alter the natural course of transformations triggered by ionizing light. Many aspects of ultrafast transformations, however, are still unknown. In this project, the PI and his group will theoretically study the statistical properties and the time evolution of localized charges created in organic molecules by the absorption of short pulses of ionizing radiation using novel numerical techniques complementary to those employed by other groups. In particular, the PI's group will explore how infrared pulses, such as those used in pulsed-laser surgery, can be used to increase the purity of the quantum states produced. The project will serve the national interest through the advancement of ultrafast science, through the development of researchers at the undergraduate, graduate, and post-graduate level, through synergistic collaborations with US research groups, and through outreach programs that include research internships of high-school students as well as hands-on workshops on molecular structure at local minority-serving high schools. The PI's group has developed wave-function-based ab initio correlated methods for the time-resolved study of multiphoton ionization of polyelectronic atoms and, more recently, of small molecules. This project tackles three open challenges: the stabilization, conversion, and fragmentation control of autoionizing polaritons in atomic and molecular systems; the creation, propagation, and monitoring of localized electron holes in molecular photoions; and the calculation of the photoelectron distribution from the ionization of laser-dressed helium atoms by free-electron-laser pulses, mediated by autoionizing states. The project has multiple methodological components: the implementation of a non-Hermitian Floquet solver in a basis of Siegert states, to determine the complex energy surfaces of autoionizing polaritons and to identify exceptional points for topologically robust conversions; the calculation, within a wave-function based approach, of the ensemble of molecular ions emerging from a photoionization event, to characterize hole localization; and the segmentation of the wavefunction, to reconstruct the photoelectron distribution from the ionization of laser-dressed helium atoms by long XUV FEL pulses. On the phenomenological side, these advancements will allow the group to identify and characterize exceptional points between autoionizing states in laser dressed atoms and molecules, thus extending coherent control above the ionization threshold to include topologically robust conversion protocols; to characterize hole localization and state purity in the ions generated in the photoionization of small molecules, their subsequent correlated dynamics and the optical and photoelectron observables able to probe such dynamics; and to explain coincidence measurements in the ionization of laser-dressed helium by FEL pulses. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.