WING-WING DOUBLE-RESONANCE STUDY ON
THE Hg--Ar SECOND-ORDER OPTICAL COLLISION

Kenji Ohmori, Masami Shoji, Kazuhiro Amano, Hisashi Chiba, 
Misaki Okunishi, and Yukinori Sato

Research Institute for Scientific Measurements, 
Tohoku University, Sendai, Japan

   We have applied the ``wing-wing double resonance'' technique 
to the thermal Hg--Ar collision 
and observed an ultrafast ``second-order optical collision'', 
where two-color, two-photon absorption takes place during a single atomic
collision, 
using a pair of nanosecond laser pulses.