On the origin of the photoluminescence of mercurous chloride

The well known orange emission (γmax = 645 nm of Hg2Cl2 at room temperature is apparently caused by the cluster cation Hg32+ which is formed by the disproportionation of trace amounts of (Hg2Cl2 → Hg + HgCl2) and the subsequent addition of a mercury atom to Hg2Cl2. The green emission of Hg2Cl2 at 77 K is composed of two components which are attributed to Hg32+ (γmax = 505 nm) and Hg34+ (γmax = 735 nm). The latter cluster cation is assumed to be formed by the addition of HgCl2 to Hg2Cl2.