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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We report a new Doppler-free rephasing effect, the tri-level echo, which we use to study Ar-Na collisional relaxation of several 3^Si/2-w^D3/2 superpositions in atomic Nao Three excitation pulses are required: The first resonantly excites a selected state, while the others resonantly couple this state with a higher-lying state. This sequence produces a delayed rephasing on the resonance transition which radiates strongly; the radiated intensity monitors relaxation in the higher-lying stepwise-excited state.It is well known that an echo, i. e. , a rephasing of superposition states, may be formed in a sample of inhomogeneously broadened two-level systemSo First observed by Hahn in spin systems (spin echoes)^ and later by Abella, Kurnit, and Hartmann in systems of electronic states (photon echoes),^ these two-level single-frequency echoes have proven useful in the study of homogeneous relaxation. This basic phenomenon has recently been extended by the observation^ of the Raman echo,^ which is similar to the photon echo except that the two states are connected by a two-photon interaction matrix element. In this Letter we present a new type of echo peculiar to multilevel (three levels or more) systems. We have discovered that for a sample of multilevel systems inhomogeneously broadened by the Doppler effect^ a rephasing of a coherent superposition between a particular pair of levels can be induced even if the constituent systems of the sample spend most of the time between their first excitation and the echo dephasing in a superposition between a different pair of levels. The echo signals produced are large since they arise on an allowed transition. Through them one can study the relaxation of a step-wise, resonantly excited superposition which would ordinarily be inaccessible. We term this effect tri-level echoes. Similar effects were predicted several years ago by Aihara and Inaba,^ who described them as anomalous photon echoes. Related effects were observed in three-level spin systems by Hatanaka, Terao, and Hashi. ' We have observed tri-level echoes in Na on the i^S^,^-3^^

Coherent radiation at the 1.28-/

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