Physical Society Colloquium
Molecules in Laser Fields:
From Wavepacket Dynamics
to Attosecond Physics
Department of Chemistry and Biomolecular Sciences
University of Ottawa
& Molecular Photonics Group
National
Research Council Canada
Molecules, solids and materials are all held together by electric forces.
Ultrashort laser fields allow us to apply electric forces to matter with the
unusual combination of extreme power and extreme precision. There are many
interesting consequences of this. Perhaps most obvious is the ability to
measure some of the fastest processes in Nature - the motions of electrons
and atoms within a molecule or material. A most fundamental processes is the
non-adiabatic coupling of electronic and vibrational motions - the breakdown
of the Born-Oppenheimer approximation which underpins our Quantum Mechanical
understanding of structure. This is the basic physics that underlies Molecular
Electronics, Photovoltaics, Vision and Photosynthesis.
We will present examples of how such electronic-vibrational dynamics can
be observed in real time using the technique of Time-Resolved Photoelectron
Spectroscopy [1]. The most information obtains by observing
such dynamics from the molecule's point of view - the Molecular Frame of
Reference. Borrowing techniques from particle physics, we can achieve this
via kinematically complete momentum vectors measurements of both fragments
and electrons - in coincidence and as a function of time. This essentially
allows us to watch internal electronic motions from the molecule's perspective
during an ultrafast dynamical process [2].
As laser fields become stronger, the applied electric force is no longer a
small perturbation and we enter the regime of the Dynamic Stark Effect. The
laser's electric field, through polarizability interactions, allows us to
reshape or twist a molecule, almost as if we could put our hands on it. It is,
in other words, a form of Dynamic Quantum Control. We will give examples of
how Quantum Control can be used to align molecules [3,
4] or to control their dissociation [5].
As laser fields get stronger still, a new laser-matter physics emerges wherein
the applied electric forces are stronger than those that bind matter itself. A
new driven multi-electron dynamics emerges [6] with important
consequences for all strong field processes, including High Harmonic Generation
[7] and attosecond spectroscopy. New experimental methods
can directly unveil the attosecond electronic timescale strong field response
[8,9].
A new frontier in measurement is now emerging: ultrafast X-ray science. The
combination of X-ray spectroscopy and diffraction with ultrafast techniques
present new opportunities for tracking electronic and vibrational dynamics in
molecules and materials with atom-specific resolution. We will give a recent
example from time-resolved soft X-ray spectroscopy [10].
[1] Nature 401, 52, (1999).
[2] Science 311, 219 (2006).
[3] Science 323, 1464 (2009).
[4] Phys.Rev.Lett. 97, 173001 (2006).
[5] Science 314, 278 (2006).
[6] Phys.Rev.Lett. 86, 51 (2001); 93, 203402
(2004); 93, 213003 (2004).
[7]. Science 322, 1207 (2008).
[8] Science 335, 1336 (2012).
[9] Phys.Rev.Lett. 110, 023004 (2013).
[10] Faraday Discussions 194, 117 (2016).
Friday, November 17th 2017, 15:30
Ernest Rutherford Physics Building, Keys Auditorium (room 112)
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