Long wave-length limit for photo-electric emission from caesium vapor.---Using a special quartz tube and taking precautions to avoid photo-electric emission from the electrodes, a narrow beam of nearly monochromatic light of gradually decreasing wave-length, was focused on the hot caesium vapor, and it was found that above 3220 A the emission was zero, between 3220 and 3145 A the emission changed linearly because of the width of the slit used, and below 3145 A the emission was practically constant. The mean, 3180 A, is taken as the critical wave-length. This is evidently identical with the convergence wave-length 3184.28 A, which is related to the ionization potential Vᵢ according to the equation Vᵢ=hceλᵢ. Therefore the separation of an electron from a caesium atom requires the same amount of work whether produced by an impinging electron or by absorption of light.
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Kunz et al. (1923) studied this question.