Two-photon Ionization of 3,4-Benzpyrene in Boric Acid Glass

A STUDY of the photoionization of 3,4-benzpyrene in boric acid glass has shown that two-photon ionization occurs in this system. The glass was prepared in the usual way1, formed into a strip and placed in the focal plane of a quartz spectrograph. The glass was irradiated with light from a high-pressure mercury lamp for periods, of several hours. The variation of ionization density with exciting wave-length was determined by measuring the optical absorption in a region where the ion alone absorbs (≍ 18,000 cm−1 (ref, 2)) and correcting for the intensity distribution of the exciting light3. The expected short-wave-length region of ionization was found with a low-energy cut off of 4.9 eV, together with a second region of considerably lower density extending from 4.1 to 3.0 eV. In the region 4.9–4.1 eV the density was too low to be measured and corresponds to a region of low exciting intensity.