Description:

Title: Rydberg atoms, loosely bound yet hard to ionize

Speaker: Vincent Carrat, Northwestern University

Abstract: The resilience of Rydberg states against ionization has fascinated physicists for a long time. One might expect that the loosely bound electron would be ionized by modest electromagnetic field. However, experiments show that a notable fraction of neutral atoms survive in Rydberg states when exposed to strong microwave or laser fields.
While strong field physics phenomena are explored with great success using powerful femto or attosecond lasers, we use an alternative approach taking advantage of the large electric dipole moment of higly excited Rydberg states (n>150) with moderate microwave power.
Energy transfer between the field and the photoelectron occurs when the electron is close to the ionic core and depends on the phase of the field. Using an amplitude modulated excitation laser with the modulation phase-locked to the microwave field, we control when we populate the Rydberg state and as a consequence the initial energy transfer from the field. By comparing the ionization signal for isolated single pulse and pulse train excitation we experimentally demonstrated the coherence of the energy transfer. After this first transfer if ionization did no occur, our classical calculation suggests that the population tends to accumulate in very high n states with orbital time that can be longer the the field pulse duration. So the electron will come back to the ionic core only a few time while the field is on which decreases the probability of gaining energy and consequently ionize. Furthermore, the phase when the electron is returning to the ionic core on the next orbit is chaotic. Statistically, the electron only has a 50% chance to gain energy. Though incomplete, this classical Monte¬Carlo simulation provides useful insights for understanding the experimental observations.

Host: Brian Odom

Keywords: Physics, Astronomy, colloquium