Self-imaging of a molecule by its own electrons
Mapping the atomic motion during a molecular vibration

Date:
September 17, 2020
Source:
Forschungsverbund Berlin
Summary:
Researchers have shown that high-resolution movies of molecular
dynamics can be recorded using electrons ejected from the molecule
by an intense laser field.



FULL STORY ==========================================================================
One of the long-standing goals of research on the light-induced dynamics
of molecules is to observe time-dependent changes in the structure of molecules, which result from the absorption of light, as directly and unambiguously as possible. To this end, researchers have developed and
applied a plethora of approaches. Of particular promise among these
approaches are several methods developed in the last years that rely on diffraction (of light or electrons) as means of encoding the internuclear spacings between the atoms that together form the molecule.


==========================================================================
In a recent paper, researchers at the Max Born Institute (MBI) led by Dr.

Arnaud Rouze'e have shown that high-resolution movies of molecular
dynamics can be recorded using electrons ejected from the molecule by an intense laser field. Following strong field ionization, the electrons
that are set free are generally accelerated away from the molecule
under the influence of the laser electric field. However, due to the oscillating nature of this field, a fraction of the electrons are driven
back to their parent molecular ion. This sets the stage for a so-called re-collision process, in which the electron can be reabsorbed in the
molecule (and where the absorbed energy is released in the form of high
energy photons) or scatters off the molecular ion. Depending on the
kinetic energy of the electron, it can be transiently trapped inside a centrifugal potential barrier. This is a well-known process in electron scattering and in single photon ionization experiments, and is referred
to as a shape resonance. The smoking gun for the occurrence of a shape resonance is a large increase of the scattering cross-section. As its
name implies, the kinetic energy for which the shape resonance occurs is
highly sensitive to the shape of the molecular potential, and consequently
to the molecular structure.

Therefore, shape resonances can be used to make a movie of a molecule
that is undergoing ultrafast nuclear rearrangement.

To demonstrate this effect, the team at MBI recorded a movie of the
ultrafast vibrational dynamics of photo-excited I2 molecules. A first
laser pulse, with a wavelength in the visible part of the wavelength
spectrum, was used to prepare a vibrational wavepacket in the electronic B-state of the molecule. This laser pulse was followed by a second, very intense, time-delayed laser pulse, with a wavelength in the infrared part
of the wavelength spectrum. Electron momentum distributions following
strong field ionization by the second laser pulse were recorded at
various time delays between the two pulses, corresponding to different
bond distances between the two iodine atoms. A strong variation of the laser-driven electron rescattering cross-section was observed with delay,
which could unambiguously be assigned to a change of the shape resonance
energy position induced by the vibrational wavepacket motion. As such,
this work introduces new opportunities for investigating photo-induced molecular dynamics with both high temporal and spatial resolution.


========================================================================== Story Source: Materials provided by Forschungsverbund_Berlin. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Felix Brausse, Florian Bach, Faruk Krečinić, Marc
J. J.

Vrakking, Arnaud Rouze'e. Evolution of a Molecular Shape Resonance
Along a Stretching Chemical Bond. Physical Review Letters, 2020;
125 (12) DOI: 10.1103/PhysRevLett.125.123001 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200917104619.htm

--- up 3 weeks, 3 days, 6 hours, 50 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)