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An introduction is presented of vector correlations in collision experiments involving atomic orbital alignment or orientation. At present, aligned or oriented species can be prepared (or probed) with "relative" ease using polarized laser beams. First, an extensive expos6 of the necessary mathematical formalisms for two-and three-vector correlations is given, and then experimental examples for atomic Ca are discussed to elucidate the theory. It is demonstrated both theoretically and experimentally that azimuthal structure about the initial relative velocity vector (called coherence) becomes important when three or more vector quantities are controlled in the collision process. I. Ioboduction: Vector CorrelationsCollision processes are very sensitive to vector quantities. The most obvious vector quantity in a collision is the initial relative velocity, vi. With the availability of crossed-beam apparatuses it is possible to have pr& control over the direction of the relative velocity vector. If we consider elastic conditions of structureless species, there is only one additional vector quantity that may be resolved experimentally, the final relative velocity vf. The correlation between the initial and final relative velocity vectors can be determined in a differential scattering experiment, where both velocities are resolved. This correlation can select collision processes within certain well-defined impact parameter ranges. By analyzing the scattering as a function of impact parameter range, one can obtain accurate potential information.I4Atomic collision processes that attract recent interest involve structured states, characterized by the quantum state b,m) for the total electronic angular momentum, j , and magnetic quantum number, m. The potential curves governing the colliiion dynamics are now characterized by a magnetic substate dependence. In most early experimental situations this m dependence is not resolved, because one encounters a statistical average of the varidus substates. The availability of polarized laser beams has made it possible to manipulate the alignment (symmetric m distribution) or orientation (asymmetric m distribution) of these structured species and to study their role in collision dynamics.s-21 For example, a p orbital can be prepared asymptotically parallel (I; state) or perpendicular (n state) in a collision. Interesting magnetic substate dependences can thus be obtained for the collision process, which may reveal important features of the potential cur~es.2~-~ Further possibilities of polarized laser beams may be utilized by preparing two collision partners in an aligned (oriented) state prior to the collision event. Furthermore, it is possible to probe the alignment (orientation) distribution after the collision for either one or both of the structured species. Therefore, it is evident that the number n of relevant vector quantities can easily range from n = 3 to n = 6, or higher if we consider molecular targets.In a Ycompleten scattering experiment all the relevant vector quantitie...

A laser pump-probe experiment is used to study the orbital alignment effects, orientation effects and vector correlations of collisional transfer of the Ca (4s)'(4p)', 3P, state to the Ca (4s)'(4p)', 3P,,0 levels. The experiment is configured in a singlecollision crossed-beam arrangement between Ca and He, and multi-structure crosssections are determined using appropriate combinations of linear and circular laser light for the pump/probe steps. Real and imaginary parts of coherence cross-sections are obtained along with the conventional population cross-sections for the m , + m 2 magnetic sublevel transitions into the 3P, level. The total relative crosssection ratio for the perpendicular (m, = f 1 ) to parallel (m, = 0) polarization preparation of 3P, transferring to 3P, is 1.46 f 0.15. For initial 3P, preparation with laser light linearly polarized perpendicular to the initial relative velocity vector, the transfer into the m,-sublevels of the 3P, state show a distinct preference for the signchanging m , = + 1 -+ m, = -1 transition. Preparation of Ca 3P, with laser light linearly polarized parallel to the initial relative velocity vector produces population transfer into the 3P, level that is completely aligned in the f 1 and & 2 sublevels, consistent with symmetry considerations. The magnitudes of the coherence cross-sections range from a few percent to greater than 100°/o of some of the population transfer conventional cross-sections. Study of the alignment effect into the final 3P0 state found a very large observed effect (atmt'l/alml~o) of 23 f 0.9. Interpretation of the energy transfer results indicates that the energy transfer obeys symmetry rules and follows predictions of curve crossings between the Z and l l potentials, where for the transfer into 3P, only

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We describe the design, operation, and calibration ofa crossed-beam experiment for the study ofintramultiplet mixing collisions of short-lived electronically excited Ne{(2p)S(3p)}={a} atoms with ground-state atoms/molecules. The excellent performance of almost 1 kHz/ A 2 (number of counts per unit of inelastic cross section) enables us to measure, with good accuracy, absolute total Ne··-X cross sections Q l~~, for the {ah --{a} I transition. Here Mk is the magnetic quantum number of the electronic angular momentum J of the initial {ah state with respect to the asymptotic relative velocity. The polarized {ah state is produced with a polarized laser. Narrow-band interference filters are used to detect the fluorescence radiation from the short-lived {ah and {a}1 states. An extensive series of measurements has been undertaken to calibrate the experiment. These are related to, e.g., beam properties, the optical-pumping process, and the optical detection system. The basic principles of the collision experiment itself have been thoroughly examined as well. We discuss the kinds of experiments it is possible to perform. These have yielded absolute (within 30%) cross sections between 0.05 and 50 A2. Very strong polarization effects have been observed, with 0.1 :S Q l~k/Q l~k :S 10.The average collision energy has been varied between 50 and 250 meV (depending to some extent on the collision partner), by using a seeded primary beam and by manipulating the Newton diagram of primary-and secondary-beam velocity vectors. Time-of-flight measurements with a laser chopper have been performed as well. The wide range of Ne··-collision partners offers the option of studying intramultiplet mixing pure (He, Ne), and in conjunction with Penning ionization (noble gas atoms Ar, Kr, Xe) or even angularmomentum coupling and anisotropy effects (molecules, from H2 to CO 2 , N 2 0).