[MOS] March 23, 2010

[Zina Queen]

Seminar on

Modern Optics and Spectroscopy

Dipole-dipole interactions of cold
Rydberg atoms

Tom Gallagher,
University of Virginia,

Tuesday, March 23, 2010

12:00 - 1:00 p.m.





Dipole-dipole energy transfer collisions between 
Rydberg atoms can be tuned into resonance with 
modest electric fields, at which point they have 
cross sections _=n4/v and duration _=n2/v3/2, in 
atomic units. Here n and v are the principal 
quantum number and the velocity of the colliding 
atoms. Reducing the temperature of the colliding 
atoms from 400K to 1K results in much larger 
cross sections and sharper energy transfer 
resonances. Further reduction of the temperature 
to 300µK results in the atoms being effectively 
frozen in place on the time scale of the 
experiments, and it is no longer obvious that 
only binary interactions occur; simultaneous 
interactions with several atoms by several 
different dipole-dipole interactions are 
possible. An alternative, potentially more 
illuminating, way of probing the dipole-dipole 
interactions is by microwave spectroscopy, and 
this approach has been used to study both 
dipole-dipole and van der Waals interactions. The 
van der Waals experiments show clearly that 
microwave spectroscopy in a cold dense Rydberg 
gas is not atomic, but molecular spectroscopy. 
One of the surprising aspects of a cold Rydberg 
gas it that it spontaneously ionizes, and while 
this process is not completely understood, 
dipole-dipole processes play obvious roles. 
Initially stationary atoms which have an 
attractive dipole-dipole interaction can collide 
and ionize, on a time scale of microseconds. At 
high density and high n ionization happens on a 
time scale of tens of ns, too fast for motion of 
the ions, and we attribute it to a many atom form 
of autoioization enabled by the dipole-dipole 
interaction.


Grier Room, MIT Bldg 34-401
Refreshments served after the lecture
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