Dynamics of Polyatomic Molecules in Laser Fields

Date

Nov 25, 2013

Time

4:30 PM - 5:30 PM

Speaker

Albert Stolow

Affiliation

National Research Council Canada

Series

MPI-PKS Kolloquium

Language

en

Main Topic

Physik

Other Topics

Physik

Host

Colloquium, atom2013

Description

The dynamics of polyatomic molecules ( ≥ 3 atoms) exhibit features absent in atoms and diatomic molecules. These are the (i) coupling of vibrational modes, leading to vibrational energy flow and structural rearrangements within molecules, and (ii) conical intersections which involve the ultrafast non-adiabatic (i.e. non-Born-Oppenheimer) coupling of electronic and vibrational dynamics, leading to charge rearrangements within molecules. 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]. The TRPES method, in both its energy- and angle-resolved implementations, has been applied to a broad range of problems [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. This permits direct Molecular Frame imaging of valence electronic dynamics during a chemical reaction [3]. In order to learn more about polyatomic dynamics, we make use of the second order (polarizability) Non-Resonant Dynamic Stark Effect (NRDSE), which uses the electric field intensity envelope of a laser pulse rather than the electric field oscillations directly. Importantly, NRDSE can 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 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]. The important new field of Attosecond Science, which offers direct observation of purely electronic dynamics, emerged out of the physics of Strong Field Ionization (SFI). In strong 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). For example, multiple electronic continua have been implicated in the HHG spectroscopy of polyatomic molecular dynamics [9]. We discuss an experimental method, Channel-Resolved Above Threshold Ionization (CRATI), which directly unveils the electronic continua participating in the attosecond molecular SFI response [10]. The implications for imaging molecular orbitals, excited state dynamics and attosecond chemistry will be discussed. [1] Nature 401, 52, (1999). [2] Chemical Reviews 104, 1719 (2004). [3] Science 311, 219 (2006). [4] Science 314, 278 (2006). [5] Physical Review Letters 94, 143002 (2005); 97, 173001 (2006). [6] Science 323, 1464 (2009). [7] Nature Physics 7, 612 (2011). [8] Physical Review Letters 86, 51 (2001); 93, 203402 (2004); 93, 213003 (2004). [9] Science 322, 1207 (2008). [10] Science 335, 1336 (2012); Physical Review Letters 110, 023004 (2013)

Last modified: Nov 25, 2013, 9:03:26 AM