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.