<!doctype html public "-//W3C//DTD W3 HTML//EN">
<html><head><style type="text/css"><!--
blockquote, dl, ul, ol, li { padding-top: 0 ; padding-bottom: 0 }
--></style><title>March 23, 2010</title></head><body>
<div align="center"><font face="Times New Roman"
color="#000000"><b>Seminar on</b></font></div>
<div align="center"><font face="Times New Roman"
color="#000000"><b><br></b></font></div>
<div align="center"><font face="Times New Roman"
color="#000000"><b>Modern Optics and Spectroscopy</b></font></div>
<div align="center"><font face="Times New Roman"
color="#000000"><b><br></b></font></div>
<div align="center"><font face="Times New Roman"
color="#000000"><i><b>Dipole-dipole interactions of
cold</b></i></font></div>
<div align="center"><font face="Times New Roman"
color="#000000"><i><b>Rydberg atoms</b></i></font></div>
<div align="center"><font face="Times New Roman"
color="#000000"><i><b><br></b></i></font></div>
<div align="center"><font face="Times New Roman"
color="#000000"><b>Tom Gallagher</b>,</font></div>
<div align="center"><font face="Times New Roman"
color="#000000">University of Virginia,</font></div>
<div align="center"><font face="Times New Roman"
color="#000000"><br></font></div>
<div align="center"><font face="Times New Roman"
color="#000000">Tuesday, March 23, 2010</font></div>
<div align="center"><font face="Times New Roman"
color="#000000"><br></font></div>
<div align="center"><font face="Times New Roman" color="#000000">12:00
- 1:00 p.m.</font></div>
<div align="center"><font face="Times New Roman"
color="#000000"><br></font></div>
<div align="center"><font face="Times"
color="#000000"><x-tab>
</x-tab></font></div>
<div align="center"><font face="Times"
color="#000000"><br></font></div>
<div align="center"><font face="Times"
color="#000000"><br></font></div>
<div align="center"><font face="Times"
color="#000000"><br></font></div>
<div align="center"><font face="Times New Roman"
color="#000000">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 300ľK 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.</font></div>
<div align="center"><font face="Times New Roman"
color="#000000"><br></font></div>
<div align="center"><font face="Times New Roman"
color="#000000"><br></font></div>
<div align="center"><font face="Times New Roman" color="#000000">Grier
Room, MIT Bldg 34-401</font></div>
<div align="center"><font face="Times New Roman"
color="#000000">Refreshments served after the lecture</font></div>
</body>
</html>