Photo-ionization of caesium vapor by absorption between the series lines

Measurements by the space-charge method have been made on photo-ionization resulting from a small continuous absorption between the series lines. Results are expressed in terms of relative sensitivity I (X)/I (3,200). At 0.064 mm pres- sure I (X)/I (3,200) increases from 0.0003 at 3,750 A to 0.07 at 3,250 A. I (X)/I (3,200) increases roughly as the square root of the pressure throughout this wave-length range. On the short wave length side of 3,500 A the effect decreases with increasing temperature of the vapor, the effect being reduced to half for a 70rise in temperature.