Formation of Print-Out Images on Large Crystals of Lead Chloride

Abstract

The presence of oxygen during exposure influences the formation of print-out images obtained on large crystals of lead chloride. The photolysis of these crystals can be divided into three different steps as a function of the exposure time:

1) Short exposure times corresponding to the latent image,
2) longer exposures lead to a print-out image if they are made in the absence of oxygen, and to an invisible one in the presence of oxygen,
3) very long exposures give a print-out image (in the presence as well as in the absence of oxygen).

For the same crystal and equally long exposure times, the visible image obtained in the presence of oxygen penetrates deeper into the crystal bulk than in the absence of oxygen. If an exposure in air follows an exposure in the absence of oxygen, a bleaching of the print-out occurs. These phenomena are interpreted as resulting from the photo-oxidation of lead nuclei by the oxygen molecules adsorbed at the crystal surface. As shown by diffuse reflectance measurements, the reflectance of exposed crystal surfaces of PbCl₂ varies with the square root of the exposure time up to certain saturation limit.