The line shape of laser-induced ionizing collisions of ultracold Kr gas is investigated near the ${1s}_5[4p{}_{}{}^5{}_{}{}^{}5s:{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}]$ and ${2p}_9[4p{}_{}{}^5{}_{}{}^{}5p:{}_{}{}^3{}_{}{}^{}D_3^{}{}_{}{}^{}]$ transition. The rate increases several times when the laser is red tuned by a few times of the natural width, while it sharply decreases for a slight blue detuning. This frequency dependence of the collision rate is due to the collision dynamics characteristic of ultracold atoms in a near-resonant light. Our experimental results on metastable Kr atoms show a good qualitative agreement with semiclassical Monte Carlo simulation of the cold collision dynamics.

• Received 8 April 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.2555