Double-Quantum External Photoelectric Effect in Organic Crystals

Illumination of anthracene, tetracene, and perylene single crystals with strongly absorbed light of energy several electron volts smaller than the reported threshold for electron emission, produces electron emission at a rate proportional to the square of the light intensity, indicating the occurrence of a biphotonic process. Evidence is presented in the case of anthracene indicating that a singlet exciton—exciton interaction is involved. The efficiency of photoemission in anthracene is 10−25 I0, where I0 is the intensity of 3650-Å light. The escape depth of the electrons is estimated as equal to or greater than 40 Å.