Strong fields and ultrafast motions: How to generate and steer electrons
in liquid water

Date:
September 2, 2020
Source:
Forschungsverbund Berlin
Summary:
Water molecules undergo ultrafast dithering motions at room
temperature and generate extremely strong electric fields in their
environment. New experiments demonstrate how in presence of such
fields free electrons are generated and manipulated in the liquid
with the help of an external terahertz field.



FULL STORY ========================================================================== Water molecules undergo ultrafast dithering motions at room temperature
and generate extremely strong electric fields in their environment. New experiments demonstrate how in presence of such fields free electrons
are generated and manipulated in the liquid with the help of an external terahertz field.


==========================================================================
The water molecule H2O displays an electric dipole moment due to
the different electron densities on the oxygen (O) and hydrogen (H)
atoms. Such molecular dipoles generate an electric field in liquid
water. The strength of this field fluctuates on a femtosecond time scale
(1 femtosecond = 10^-15 seconds = one billionth of a millionth of a
second) and, for short periods, reaches peak values of up to 300 MV/cm
(300 million volts per cm). In such a high field, an electron can leave
its bound state, a molecular orbital and tunnel through a potential energy barrier into the neighboring liquid. This event represents a quantum
mechanical ionization process. In equilibrium, the electron returns very quickly to its initial state since the fluctuating electric field has
no preferential spatial direction and, thus, the electron does not move
away from the ionization site. Because of the highly efficient charge recombination, the number of unbound (free) electrons remains extremely
small, on average less than a billionth of the number of water molecules.

Researchers from the Max-Born-Institute in Berlin have now shown that
an external electric field with frequencies in the range of 1 terahertz
(1 THz = 10^12 Hz, approximately 500 times higher than a typical cell
phone frequency) enhances the number of free electrons by up to factor
of 1000. The THz field has a maximum strength of 2 MV/cm, that is less
than 1% of the strength of the fluctuating field in the liquid. However,
the THz field has a preferential spatial direction. Along this direction, electrons generated by the fluctuating field are being accelerated and
reach a kinetic energy of approximately 11 eV, the ionization potential of
a water molecule. This transport process suppresses charge recombination
at the ionization site. The electrons travel over a distance of many
nanometers (1 nm = 10^-9 m) before they localize at a different site in
the liquid. The latter process causes strong changes of the absorption
and the refractive index of the liquid by which the dynamic behavior
of the electrons can be followed with the method of two-dimensional
THz spectroscopy.

These surprising results reveal a new aspect of extremely strong
electric fields in liquid water, the occurrence of spontaneous events of tunneling ionization. Such events could play an important role in the self-dissociation of H2O molecules into OH -- und H3O+ ions. Moreover,
the experiments establish a novel method for the generation, transport,
and localization of charges in liquids with the help of strong THz
fields. This allows for manipulating the basic electric properties
of liquids.


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


========================================================================== Journal Reference:
1. Ahmed Ghalgaoui, Lisa-Marie Koll, Bernd Schu"tte, Benjamin
P. Fingerhut,
Klaus Reimann, Michael Woerner, Thomas Elsaesser. Field-Induced
Tunneling Ionization and Terahertz-Driven Electron Dynamics in
Liquid Water. The Journal of Physical Chemistry Letters, 2020;
7717 DOI: 10.1021/ acs.jpclett.0c02312 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200902114433.htm

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