Formation ofNcenters in pure NaCl

Sodium chloride on the surface of Europa

“…Some NaCl irradiation experiments, performed under different conditions, have observed only F-center production ( 28 , 29 ). This behavior suggests that M-centers would likely never form under the low radiation flux at Europa, reflecting competition between their slow growth and the contributions of decay processes, such as photobleaching [e.g., ( 30 , 31 )], which can influence the relative abundance of NaCl color centers. Last, this behavior may also explain the band center of the observed 450-nm feature.…”

Spectral Behavior of Irradiated Sodium Chloride Crystals Under Europa‐Like Conditions

“…Other works have examined decay of color center features, often referring to the process as "bleaching," due to the loss of material color as the centers are lost (e.g., Georgiou & Pollock, 1989;Mador et al, 1954). Thermal bleaching and resulting luminescence have been documented, but the rates were only significant on laboratory timescales for temperatures above 300 K (Mador et al, 1954).…”

“…While the 520 nm feature may be an experimental artifact, it could also be due to F 0 (negatively charged F center) and/or R 1 centers. Both centers are known to form when F centers absorb light (e.g., Georgiou & Pollock, 1989). F 0 centers are unstable at 290 K, but under constant illumination of F centers, F 0 centers are still possible.…”

“…Thermal bleaching and resulting luminescence have been documented, but the rates were only significant on laboratory timescales for temperatures above 300 K (Mador et al, 1954). Photobleaching and resulting luminescence have also been examined (Georgiou & Pollock, 1989); however, these works limited the light to a single color center at a time. In fact, differing behavior was observed when the color centers were Journal of Geophysical Research: Planets 10.1002/2017JE005429 stimulated in different orders, showing this to be a complicated system of defect aggregation and energy transfer from center to center.…”

VIS spectroscopy of NaCl – water ice mixtures irradiated with 1 and 5 keV electrons under Europa's conditions: Formation of colour centres and Na colloids

“…Finally, a combination of all these parameters in various proportions would be reasonable to assume. Photobleaching has been proposed as a possible explanation to prevent the formation of M-centres, and possibly other crystal defects, by resorbing the F-centres (Georgiou and Pollock, 1989;Mador et al, 1954;Trumbo et al, 2019), and even on a daily basis at the surface of Europa (Denman et al, 2022). This hypothesis is complicated to assess with our dataset as our illumination conditions during hyperspectral cube acquisition were drastically shorter and with monochromatic light.…”