Session T8: Invited Session: Non-Born-Oppenheimer Dynamics

Electron Dynamics during Strong Field Molecular Ionization

Strong Field Ionization plays a central role in the study of ultrafast electron dynamics, both as the first step in attosecond pulse generation and in the launch of electron wave packets in atoms and molecules. This talk will focus on studies of strong field molecular ionization with shaped laser pulses, where the pulse shape dependence yields insight into the electron dynamics during ionization. Coincidence velocity map imaging and close collaboration with theory enable us to examine the role of both neutral and ionic resonances as well as electron correlation.   

Ab initio quantum dynamics on conically intersecting potential energy surfaces: general considerations and application to SO2

Conical intersections of potential energy surfaces are the typical signatures of strong nonadiabatic coupling effects, whereby the systems undergoes nonradiative transitions on the femtosecond time scale with far-reaching consequences on the spectral intensity distribution, photoreactivity etc. In the talk, after a brief introduction into the field, our specific quantum dynamical approach to describe such phenomena theoretically is outlined. Some emphasis is given on a suitable construction scheme for quasidiabatic electronic states. The general considerations are applied to and exemplified for low-energy singlet and triplet excited states of SO2. These feature several conical intersections and are also mutually coupled through spin-orbit couplings. The relevant electronic structure data are obtained from extensive ab initio MRCI calculations, while the treatment of the nuclear motion relies on wavepacket propagation techniques. The experimental UV spectrum of SO2 in the 290 nm wavelength range is well reproduced. A novel coupling mechanism is proposed which provides for the spectral intensities in the long wavelength (380 nm) regime, and an experimental setup for its verification is suggested. We also demonstrate how these interactions affect the conventional HeI photoelectron spectrum as well as the time-resolved photoelectron-photoion coincidence spectrum measured recently.   

Non-Born-Oppenheimer Dynamics in Ring-Opening and Biological Processes

We discuss simulations of nonadiabatic dynamics in molecular systems, highlighting the role of conical intersections. The influence of conical intersection shape and energy is explored. We summarize new methods for calculating the electronic structure and dynamics on graphical processing units, which are leveraged in order to enable simulation of excited state molecular dynamics in systems containing hundreds of atoms. Specific systems studied encompass photoinduced isomerization about carbon double bonds and ring-opening reactions. We discuss the basic mechanisms of these processes in isolated molecules, including direct comparison of theoretical and experimental results. We further discuss the role of solvent and/or protein environments in directing and modifying these processes.   

Vibrational- and Laser-Driven Electronic Dynamics in the Molecules

Electronic dynamics within molecules can be driven by both motions of the atoms, via non-Born-Oppenheimer coupling, and by applied laser fields, driving electron motions on sub-cycle time scales. The challenging but most general case of Molecular Dynamics is where electronic and vibrational motions are fully coupled, the making and breaking of chemical bonds being the most prominent example. Time-Resolved Coincidence Imaging Spectroscopy (TRCIS) is a ultrafast photoelectron probe of Molecular Frame dynamics in polyatomic molecules. It makes use of full 3D recoil momentum vector determination of coincident photoions and photoelectrons as a function of time, permitting observations of coupled electronic-vibrational dynamics from the Molecular Frame rather than the Lab Frame point of view. Methods in non-resonant quantum control, based on the dynamic Stark effect, have also emerged as important tools for enhancing molecular dynamics studies. In particular, molecular alignment can fix the Molecular Frame within the Lab Frame, avoiding loss of information due to orientational averaging. Provided that the molecular dynamics are fast compared to rotational dephasing, this method also permits time-resolved Molecular Frame observations. As laser fields get stronger, a sub-cycle (attosecond) physics emerges, leading to new probes of driven multi-electron dynamics in polyatomic molecules. Understanding driven multi-electron responses will be central to advancing attosecond science towards polyatomic molecules and complex systems.