Electric field ionization of excited Na atoms by microwave fields between 12 and 15 6Hz exhibits several interesting phenomena.The~m~=0 and 1 states may be ionized with a very low microwave field, -1/3n', corresponding to the point at which the n and n + 1 Stark manifolds of principal quantum numbers n and n+1 intersect. The ionization occurs by an atom's making a succession of Landau-Zener transitions to higher-lying states, finally reaching the classical ionization limit. The~m~=2 states, on the other hand, ionize at a very high microwave field, -1/9n . Both these observations and the more familiar static or low-frequency result of ionization by a field of -1/16n are all consistent with ionization in a completely classical fashion. The apparent differences are due to the dynamics of passing from zero field to a high ionizing field.
or holes broken (A6 = 0 for J^ = i"), so that our data should lie on a straight line. (The results deduced from our tables are consistent with these predictions.) Finally, let us note that these pseudo mirror pairs all satisfy the relations •within a few percent. This is an enlargement of the so-called De Shalit relation {gs''gi)p/(gs -gi)n = -1-20 deduced in the frame of the extreme single-particle model. ^See, for example, R. G. Sachs, Phys. Rev. ^, 611 (1946).A technique which employs heretofore unappreciated aspects of the optical stimulated echo was developed and used to measure the total scattering cross section for Na(3Siy5)-He and Na(3Pi/2)-He collisions. The repulsive-core-dominated Na(3Si/2)-He collision is found to be relatively soft. The cross section for degradation of the 3Siy5-3Pi/2 superposition, determined from photon-echo-dec ay measurements, is anomalous in that it is smaller than the total elastic-so altering cross section with Na in either state separately.The study of atomic scattering, traditionally carried out by atomic-beam measurements/ has recently been supplemented by laser techniques.^ In this Letter we report the first application of a coherent-transient laser technique to the study of velocity-changing collisions (VCC) in an atomic system.^ CXir technique utilizes unappreciated novel aspects of the three-pulse stimulated photon echo^ (SP echo) to provide relatively direct measurements of VCC. We predict and demonstrate experimentally (e.g., Fig. 1) that phase memory information stored in either the ground ^^^ •^' O^scilloscope trace showing scattered light from three excitation pulses (third row of the table in Fig. 3 with K'-^/|K'[ = -1) and the subsequent SP echo (fourth pulse) pronounced on the SS-SP1/2 transition of Na when the second-to-third pulse separation is = 17 times the 16-nsec lifetime of the SP^/z state. At the time of the third pulse a negligible ^ 10"^ of the intially excited population remains in the ^P^^ state. The echo persists because of the information stored in the ground state alone. (Horizontal: 50 nsec/div.)
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