Negative ion photodetachment and the electron effective mass in liquids

The electron photodetachment cross section for a negative ion in the gas phase is compared with the photodetachment cross section for the same ion when it is dissolved in a liquid supporting ‘‘free’’ electron propagation. The ratio of the amplitudes of these two cross sections near threshold is found to depend upon the effective mass m* of an electron in the conduction band of the liquid. We apply this result to electron photodetachment from O−2. In terms of the electron’s free mass m, we find for liquid argon at 87 K (m*=0.26 m), 2,2-dimethylbutane at 296 K (m*=0.27 m), 2,2,4-trimethylpentane at 296 K (m*=0.27 m), and tetramethylsilane at 296 K (m*=0.27 m). At 200 K, the effective mass in tetramethylsilane decreases to m*=0.21 m. In the case of liquid argon, the effective mass calculated herein agrees qualitatively with values which can be estimated from measurements of the electron mobility and exciton spectrum.