Reflectance of mirrors exposed to a strontium beam

The chemical reactivity of strontium, which opacifies vacuum viewports exposed to strontium sources, is a concern for atomic physics experiments where a laser beam counter-propagates relative to a strontium beam. Some experiments use heated sapphire viewports to reduce strontium deposition. Here, we study another approach wherein the laser beam counter-propagates after first reflecting off an in-vacuum mirror at 45o exposed to the strontium flux. We show that an SiO2-protected reflective surface is a solution for strontium depositions up to 1.5μm. A reaction with SiO2 creates a transparent film, maintaining the back surface reflectivity. Deposition on inert sapphire results in much lower reflectance. Our results provide guidance for Zeeman-decelerated strontium experiments, and an alternative to heated sapphire viewports. We provide flux-dependent viability estimates.