우리는 산을 정복하기 위해서는 산을 올라야 한다.
양자물리에서는 다른 길이 있다. 물체는
힘들게 언덕을 오르는 대신에 단순히 언덕을 터널링 하는
것만으로도 반대편에 다다를 수 있
다. the Max Planck Institute for
Quantum Optics 의 Ferenc Krausz교수의 국제연구팀은 이
터널링 과정에
있는 전자를 관측했다.
This effect is responsible
for the ionization of atoms under the influence of
strong
magnetic fields. The electrons overcome the
attraction of the atomic nucleus by
tunnelling
through a potential wall. The scientists used
ultra-short laser pulses to
show discrete stages of
ionization in this process, each of which lasts 100
attoseconds - a fraction of a billionth of a second.
The results make a significant
contribution to
understanding how electrons move around in atoms and
molecules.
In the same way as gravity brings a
body to a halt on the floor of a valley, the
nuclear
force (which binds protons and neutrons to form the
atomic nucleus) and the
electrical force (which
combines negatively charged electrons with the
positively
charged atomic nucleus to make an atom)
hold these particles within a tiny space. This
binding effect can also be depicted as a type of
valley, which is also called a
potential by
physicists.
In the world of quantum particles,
it is, to a certain extent, a normal event to
tunnel
through the wall surrounding the potential well. An
international team of
researchers working with
Ferenc Krausz has now caught the electrons in the act of
tunnelling through the binding potential of the atom
nucleus under the influence of
laser light. The
physicists used the new tools provided by attosecond
metrology. "For
the first time, our findings
confirmed in real time observation the theoretical
predictions of quantum mechanics," says Ferenc
Krausz, Director at the Max Planck
Institute for
Quantum Optics and head of the team of scientists.
The tunnelling effect can be explained by the
wave behaviour of each particle.
Macroscopic objects
are extremely unlikely to tunnel, which is why the
phenomenon has
never been observed in them. In
contrast, there is a significant probability that
particles from the microcosmos will tunnel through
areas where, according to the rules
of traditional
physics, they are not even supposed to be. The
tunnelling effect is
considered to be responsible
for processes as varied as atomic nuclei decay and the
switching process in electronic components. However,
since it only lasts for an
extremely short time, it
has not yet been observed in real time.
Krausz
and his colleagues have now followed this process live
with the aid of two
light pulses: an intense pulse
of just a few wave trains of red laser light and an
attosecond pulse of extreme ultraviolet light
perfectly synchronized with the red
pulse. The
electrical field of the laser pulses periodically exerts
strong forces on
the electrons. When the force is at
its strongest, the light force presses the
potential
wall downwards. For a short moment when the wave peaks,
the electron has the
opportunity to penetrate the
barrier and escape from the atom. This opportunity only
arises when the wave peaks, that is over an
extremely short interval of a fraction of
a
femtosecond, a trillionth of a second.
There is
no instrument that can directly resolve the tunnel
effect. It is only
possible to show the existence of
the end products, the atoms, which, following the
laser pulse, disintegrate into an electron and a
positively charged ion. The
researchers therefore
had to use the trick of experimenting with neon atoms.
In these,
the electrons are in a closed shell, and
therefore in particularly strong bonds, and
resist
the attempts of the laser pulse to release them from the
atom. Only electrons
hit by an attosecond flash of
UV manage to reach the periphery of the atom and can
extricate themselves from the atom by tunnelling.
Therefore, the physicists can only
ionize neon atoms
with a red laser pulse that they have first prepared
with this
flash.
"With a UV pulse lasting
just 250 attoseconds, which was synchronized exactly
with the
red laser pulse, we moved an electron at
any point in time during the laser wave with
attosecond precision to the periphery," explains
Krausz. Step by step, the researchers
shifted this
point in time and measured the number of atoms ionized
by the laser. This
allowed them to reconstruct the
chronology of the ionization process. As the theory
predicted, the electrons left the atoms in the
immediate vicinity of the most intense
wave peaks,
which can be seen clearly from the discrete stages of
ionization
coinciding exactly with the peaks in fig.
3 (the green line). The electrons remained
at this
stage for less than 400 attoseconds. Within such a short
period, the electrons
are released from the atom by
the light energy.
"The experiments not only
provide us with insight into the dynamics of electron
tunnelling for the first time," says Krausz. "We
have also shown that the movement of
electrons in
atoms or molecules can be observed in real time with the
aid of laser
field-induced tunnelling." Based on
this finding and the enabled control over electron
movement within the atom, in the future scientists
will be able to research how the
boundaries of
microelectronics can be shifted, or how to develop
sources of compact,
very bright X-rays. These will
in turn allow progress to be made in the imaging of
biological objects and in radiation therapy.
Citation: M. Uiberacker, Th. Uphues, M.
Schultze, A. J. Verhoef, V. Yakovlev, M. F.
Kling,
J. Rauschenberger, N. M. Kabachnik, H. Schröder, M.
Lezius, K. L. Kompa, H.-G.
Muller, M. J. J.
Vrakking, S. Hendel, U. Kleineberg, U. Heinzmann, M.
Drescher und F.
Krausz, Attosecond real-time
observation, Nature, 5 April 2007
Source:
Max-Planck-Gesellschaft
(right: Each
time a wave peak hits the atom, the probability that an
electron will be
progressively released within a few
100 attoseconds increases. This phenomenon, which
was predicted in theory, evaded direct observation
for more than four decades -- an
international team
of scientists has now demonstrated it for the first
time. Credit:
Max Planck Institute for Quantum
Optics)
(left: The electrical field of a laser
pulse exerts a strong force on an electron
located
at the edge of an atom (green cloud around the
nucleus).This force changes
over time. In
approximately only a femtosecond, a trillionth of a
second, it changes
direction -- at t1 it is
strongest towards the right, at t2 it is strongest
towards
the left and after another femtosecond, at
t3, again towards the right. Credit: Max
Planck
Institute for Quantum Optics)
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