Earle K. Plyler Prize Lecture: The Three Pillars of Ultrafast Molecular Science - Time, Phase, Intensity*

We discuss the probing and control of molecular wavepacket dynamics in the context of three main `pillars' of light-matter interaction: time, phase, intensity. Time: Using short, coherent laser pulses and perturbative matter-field interactions, we study molecular wavepackets with a focus on the ultrafast non-Born-Oppenheimer dynamics, that is, the coupling of electronic and nuclear motions. Time-Resolved Photoelectron Spectroscopy (TRPES) is a powerful ultrafast probe of these processes in polyatomic molecules because it is sensitive both electronic and vibrational dynamics [1, 2]. Ideally, one would like to observe these ultrafast processes from the molecule's point of view -- the Molecular Frame -- thereby avoiding loss of information due to orientational averaging. This can be achieved by Time-Resolved Coincidence Imaging Spectroscopy (TRCIS) which images 3D recoil vectors of both photofragments and photoelectrons, in coincidence and as a function of time, permitting direct Molecular Frame imaging of valence electronic dynamics during a molecular dynamics [3]. Phase: Using intermediate strength non-perturbative interactions, we apply the second order (polarizability) Non-Resonant Dynamic Stark Effect (NRDSE) to control molecular dynamics without any net absorption of light [4]. NRDSE is also the interaction underlying molecular alignment and applies to field-free 1D of linear molecules and field-free 3D alignment of general (asymmetric) molecules [5]. Using laser alignment, we can transiently fix a molecule in space, yielding a more general approach to direct Molecular Frame imaging of valence electronic dynamics during a chemical reaction [6, 7]. Intensity: In strong (ionizing) laser fields, a new laser-matter physics emerges for polyatomic systems [8] wherein both the single active electron picture and the adiabatic electron response, both implicit in the standard 3-step models, can fail dramatically. This has important consequences for all attosecond strong field spectroscopies of polyatomic molecules, including high harmonic generation (HHG) [9]. We discuss an experimental method, Channel-Resolved Above Threshold Ionization (CRATI), which directly unveils the electronic channels participating in the attosecond molecular strong field ionization response [10]. \textbf{[1]} Nature \underline {401}, 52, (1999). \textbf{[2]} Chemical Reviews \underline {104}, 1719 (2004). \textbf{[3]} Science \underline {311}, 219 (2006). \textbf{[4]} Science \underline {314}, 278 (2006). \textbf{[5] }Physical Review Letters \underline {94}, 143002 (2005); \underline {97}, 173001 (2006). \textbf{[6]} Science \underline {323}, 1464 (2009). \textbf{[7] }Nature Physics \underline {7}, 612 (2011). \textbf{[8]} Physical Review Letters \underline {86}, 51 (2001); \underline {93}, 203402 (2004); \underline {93}, 213003 (2004). \textbf{[9]} Science \underline {322}, 1207 (2008). \textbf{[10]} Science \underline {335}, 1336 (2012); Physical Review Letters \underline {110}, 023004 (2013)