by
R. Walsworth
Harvard-Smithsonian C.F.A.
C. G. Koay
R. Peterson
M. Crescimanno
Physics Department, Berea College
$ by
National Science Foundation EPS-9874764 and PHY 94-07194
Research Corporation Cottrell Science Award CC3943
Berea College
Accepted for publication in Phys. Rev. A.