Optical Transmission Measurements on Phase Transitions of O2 and CO in Mesoporous Glass

The optical transmission of O₂ and CO condensates embedded in porous Vycor glass has been studied as function of the filling fraction and of the thermal history of the samples. The freezing transition as well as the solid-solid transitions (β-α of CO and γ-β of O₂) induce a coarsening of the separation into empty and filled regions which results from the hysteretic behavior of the transitions in the presence of a pore size distribution. In the birefringent β phase of O₂ domains with a fixed crystallographic orientation extend over distances much larger than the pore diameter (10 nm).