NOTE TIME
CHANGE! : Physics Colloquium - Physics Faculty Candidate Dr. Luca Argenti,
4/16, 10:45, PSB 161
Dr. Luca Argenti
Universidad Autónoma de Madrid
How to Win Electrons’ Friendship and
Make Them Dance for You
In the realm of electrons, in atoms and molecules, things happen fast.
According to Bohr’s planetary model for the hydrogen atom, for example, the “sidereal
year’’, i.e., the time it takes to the electron to complete the shortest orbit
around the nucleus, has the fantastically small duration of 152 attoseconds
(as; 1as = 10-18 s). This estimate remains true even when
formulated in the language of quantum mechanics. Such ultrafast motion could
not be directly confirmed until, at the turn of this century, groundbreaking
advancements in laser technology led to the production of flashes of light with
sufficiently short duration (the world record is 67 as) to take clear snapshots
of it.
In systems bigger than hydrogen, the same Coulomb force that binds an
electron to the nucleus also acts repulsively between electrons. The main
effect of such repulsion is to screen the nuclear charge, thus reducing the
binding energy of each individual electron. To a first
approximation, therefore, in several complex systems, electrons can act as if
they were independent. As a result, their motion may appear not much more
complicated than the one observed in the hydrogen atom. In fact, most of
the experiments carried out so far to follow the electronic movement could set
in motion only one electron at a time, thus confirming this picture.
Electrostatic repulsion, however, has also a secondary, subtler
effect. In the same way as a bus passenger avoids not only to sit next to
the other passengers, if she can, but also to bump into them as she walks down
the isle, electrons try to avoid each other as they move across an
atom or a molecule: the motion of the electrons is correlated. Keeping each
other at arm’s length, electrons minimize their mutual repulsion and, as a
consequence, they stabilize the ground state of the atom or molecule to which
they belong. Such stabilization influences the energy balance of all natural
processes, and is key to our understanding and control of matter, from energy
transfer in photosynthetic systems, to the inner workings of futuristic quantum
computers. Correlation alters even more dramatically the dynamics in
excited states, where pairs of electrons move in unison. Until recently, such
concerted motion eluded direct experimental observation. By combining flashes of
extreme ultraviolet and visible laser light with a duration of only a few hundred
attoseconds and timed very accurately with respect to each other, however, it
was eventually shown that monitoring and controlling this motion is in fact
possible.
In this colloquium I will
illustrate with examples taken from my research on the helium atom, a prototype
of poly-electronic systems, how attosecond spectroscopies have opened the road that leads to
the direct observation and control of correlated electronic motion in matter.
Teaching Seminar, 4/17, 1:30-2:15, MSB 350
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