ELECTRON-PHOTON COINCIDENCE STUDY OF HEAVY-NOBLE-GAS EXCITATION AT SMALL SCATTERING ANGLES

The electron-polarized-photon coincidence technique has been used to study the finer details of the excitation of the first excited states of the heavy noble gases neon, argon, and krypton by electron impact in the regime of large impact parameters (small scattering angles and intermediate impact energies). Measurements with higher statistical accuracy of the P1 coherence parameter in forward scattering for excitation of the (2P1/2)ns'[1/2]1(0)("1P1") state in neon (n = 3) and argon (n = 4) are reported along with P1 measurements for both the (2P1/2)5s'[1/2]1(0)("1P1") state and the (2P3/2)5s'[3/2]1(0)("3P1") state in krypton for incident electron energies from 30 to 100 eV. All measurements are consistent with a P1 value of +1, which is indicative of the dominance of direct excitation via transfer of orbital angular momentum. No evidence was found that exchange excitation via spin transfer plays a role in forward scattering at these energies, in agreement with theoretical predictions. A series of systematic measurements of the two linear coherence parameters P1 and P2 was carried out for excitation of the "1P1" state in argon and the "3P1" state in krypton at 50-eV impact energy and electron-scattering angles up to 25-degrees. A detailed comparison with the predictions of the distorted-wave Born approximation and the first-order many-body theory reveals a generally satisfactory agreement and indicates that the theories are capable of reproducing the general features of the measured parameters as a function of scattering angle. Two parameters characterizing the angular part of the collisionally induced P-state charge cloud, the alignment angle gamma, and the linear polarization P(lin), were extracted from the measured P1 and P2 values. The agreement between experiment and theory in the case of gamma is in general good, whereas it is somewhat poorer as far as P(lin) is concerned. This indicates that the theoretical models are quite good in predicting the alignment angle of the collisionally induced charge cloud in the scattering plane, but less capable of predicting the exact shape of the charge cloud.