A method has been developed to determine the fraction of excited ions present in an ion beam formed using various energies E8of the electrons in the ion source. The determination is made ∼20 μsec after formation of the ions so that only long-lived states remain in the beam, as is usual in beam experiments. The method consists of attenuating the ion beam in a gas-filled reaction chamber, where different states of the ions suffer different attenuations. Results have been obtained for O2+ and O+ ion beams. In each case only one excited state appears to be important: For O2+ the fraction f of this excited state was found to be 0.22, 0.30, and 0.33 for E8 values of 25 eV, 50 eV, and 100 eV, respectively. For O2+, f is 0.27 for E8 = 50 eV and 0.30 for E8 = 100 eV. The use of these results is illustrated by combining them with crossed-beam measurements of charge transfer to determine separately the cross sections for the ground state and the excited-state ions of O+ and O2+.

The photoconductivity of anthracene has been studied using three types of crystals: large scintillation crystals, crystals grown from dimethylformamide, and sublimation flakes. The existence of a true bulk conductivity as well as a surface conductivity has been demonstrated. This bulk conductivity is not affected by oxygen. The space charge found by previous workers has been shown to be located at the electrodes and to be due to poor contacts. The dependence of both the surface and the bulk photocurrent on temperature, voltage, and wavelength and intensity of light, have been studied and found to be markedly different from each other. The surface photocurrent shows a non-ohmic voltage dependence. It is concluded that the formation of charge carriers is not a primary process of light absorption, but a secondary process, competing with fluorescence and thermal degradation of the absorbed light energy, which is free to move through the crystal in the form of excitons. A mechanism for the migration of charge carriers has been put forward to explain the main features of the photoconduction. A much higher effective carrier mobility on the surface than in the bulk of the crystal is postulated. The behavior of the surface photocurrent is interpreted in terms of the formation of carriers in the bulk, those that diffuse to the surface playing the greater part in carrying the current. The bulk current is considered as a recombination-limited photocurrent. In agreement with previous workers, it is concluded that most of the current is carried by positive holes.

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