Observation of Polarized Optical Radiation following Electron Capture into Slow, Highly Ionized Neon

In charge-transfer collisions of 4-keV ${\mathrm{Ne}}^{8+}$ recoil ions with Na, we have observed 434-nm photons emitted in the $n=9\mathrm{} \mathrm{to} 8$ transition in ${\mathrm{Ne}}^{7+}$. We find this radiation to be strongly polarized, with a degree of polarization $P=0.32\mathrm{}\pm 0.02$. This result is in accord with a recent calculation of Salin indicating that electron capture into slow, highly charged ions preferentially populates the $m=0, \pm 1$ magnetic substates.