Nils Byrial Andersen

Propensity rules for orientation by atom impact. II: Many-state description of direct transitions

P state orientation and alignment created in direct, collision-inducedS ↔P transitions of a quasi-one-electron atom are analyzed in the natural coordinate frame. In the velocity region of maximum transition probability, propensity rules for the orientation are derived, and their range of validity in impact parameter and velocity is discussed. The predictions are tested and illustrated by three-state calculations for Na(3s ↔ 3p) transitions in Na — He collisions.

Orientation and alignment of Mg+ (3p) states excited in 1–60 keV collisions with He and Ar

The orientation and alignment of Mg+ (3p) states created in 1–60 keV Mg+ (3s)-He, Ar collisions have been studied by the polarized photon-scattered particle coincidence technique at scattering angles corresponding to impact-parameter ranges of 0.5–1.0 a.u. (He) and 1.2–2.0 a.u. (Ar). Referring to the standard geometry, in the region of maximum excitation probability (∼ 35 keV), a strong propensity for population of the Mg+ (3p−1) state is observed. The propensity breaks down when going towards lower energies, and for collisions with He even a transient reversal of the angular momentum occurs. The alignment angle varies little in the entire range of impact parameters and energies investigated. These observations compare well with recent general predictions of Andersen and Nielsen.

Excitation in Simple Collision Systems

The paper reviews the present understanding of the mechanisms and collision dynamics responsible for excitation of outer shell electrons in atomic collisions. Experimental and theoretical results at increasing levels of sophistication are presented for selected model systems, culminating in so-called perfect scattering experiments which determine the quantum-mechanical state of the system completely. When theory and experiment agree at this level, one can go no further.

Orientation of Na(3p) states excited in Na-He collisions

The orientation of Na(3p) states created in 3–13 keV Na(3s)-He collisions has been studied by the polarised photon-scattered particle coincidence technique at scattering angles corresponding to the impact-parameter range 1–2 a.u. In the standard geometry, at large impact parameters the excitation process exhibits a very high degree of orientation with about 90% of the Na(3p) states havingm1=−1. Strong reduction in this propensity is observed at impact parameters smaller than about 1.3 a.u., where a molecular curve crossing causes simultaneous He(n=2) excitation. In this region, also ionisation becomes important.

Confirmation of Propensity Rule for Charge-Cloud Orientation: Mg+(3s → 3p±1) Excitation in (1 ÷ 60) keV Mg+-He, Ar Collisions

A recently proposed propensity rule for charge-cloud orientation in heavy-atom collisions is investigated experimentally. The scattered particle-polarized photon coincidence technique is used to study the excitation probability and electronic angular momentum of Mg+ ions undergoing 3s → 3p excitation in single collisions with He or Ar and impact parameters of typically (1 ÷ 2) a.u. The (1 ÷ 60) keV impact-energy range includes the region of maximum probability. The propensity rule is here strikingly confirmed and its range of validity illuminated.

COLLISION-PRODUCED ATOMIC STATES

The last 10–15 years have witnessed the development of a new, powerful class of experimental techniques for atomic collision studies, allowing partial or complete determination of the state of the atoms after a collision event, i.e. the full set of quantum-mechanical scattering amplitudes or — more generally — the density matrix describing the system. Evidently, such studies, involving determination of alignment and orientation parameters, provide much more severe tests of state-of-the-art scattering theories than do total or differential cross section measurements which depend on diagonal elements of the density matrix.1 The off-diagonal elements give us detailed information about the shape and dynamics of the atomic states. Therefore, close studies of collision-produced atomic states are currently leading to deeper insights into the fundamental physical mechanisms governing the dynamics of atomic collision events.