Thursday, April 16, 2015
FW: TODAY ! AT 10:45 AM ! : Physics Colloquium - Physics Faculty Candidate Dr. Luca Argenti, TODAY 4/16, 10:45, PSB 161 & TOMORROW: Teaching Seminar, 4/17, 1:30-2:15, MSB 350
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