The theory component of this UXSL is focused on two distinct problems: (1) The theoretical description of ultrafast probes of electron dynamics in atoms and molecules, and (2) scalable methods for the computation of molecular electronic excited state potential surfaces.

 Powerpoint presentation

Electron Dynamics

Attosecond pulses open the possibility of probing electron dynamics, and therefore electron correlation, on its own timescale, but the design and interpretation of such experiments is still a major theoretical challenge. Detailed new experiments and theory on He and H2 (and other systems with two active electrons) over the next few years will determine the scope and power of new ultrafast techniques to produce a deeper understanding of electron correlation in atoms and molecules. UXSL theory effort is extending time-independent methods developed by the AMO theory group at LBNL to the exact time-dependent description of attosecond pump probe experiments of two electron systems, and applying them to design pump probe experiments that create and then probe wave packets of excited states in processes that result in the ejection of both electrons,. Ultrafast double photoionization is a sensitive probe of electron correlation, but for molecular systems the simultaneous inclusion of nuclear motion in its theoretical description is essential. The pump pulses in new experiments will place the molecule a wave packet spanning many excited bound or autoionizing electronic states with multiple avoided crossings. The key challenge to theory is to describe these experiments accurately by treating electronic and nuclear motion on the same footing, leaving behind completely the Born-Oppenheimer separation of their time scales for the first time in multiple ionization processes.

Molecular Electronic Excited States

The crucial photochemical events in processes ranging from vision to light-harvesting to light emission involve ultrafast processes that involve multiple excited states. While there are established methods to calculate the potential surfaces for small molecules, it is an open challenge to develop appropriate methods for larger molecules and interface them to appropriate tools for finding reaction pathways and performing direct dynamics. The work on scaleable methods for molecular excited states is directed at this goal, and is specifically focused on new theories that are capable of describing the conical intersections that are believed to often play crucial roles in ultrafast photochemistry. Over the past two years we have made some significant progress towards this goal, with the development of an accurate yet low-scaling quasi-degenerate excited state theory including electron correlation effects that correctly describes conical intersections between one-electron excited states. However the problem of correctly describing conical intersections with the ground state, and the treatment of low-lying multi-electron excited states are challenges that we are planning to make progress on in the future.