Effects of gases on photoionization of caesium by line absorption

Relative measurements of the photoionization by line absorption in caesium vapor with and without foreign gases were made by the space-charge method. If the effect of a collision with a gas molecule is to change the probability of ionization from E to Ef, then the probability E v at a gas pressure p is given by E -Ev E -EfV^ Aa*T'pJ where A is a known constant, a is the distance between atom centers at collision and t' is the life of the excited state. Nitrogen re'duces the ionization of states 4 P to 8 P by about the same amounts. <r 2 t' is of the order of 10~21 cm2 sec. and Ef/E o= 0.1. Hydrogen gives about the same result. Helium increases the ionization of 4 P (Ef/E for 4 Pi is 3) and decreases the effect of other lines to 60 or 70 per cent at high pressure. <r2 r' is 10 for 5 P and less for other states. Argon gives very little effect. The effectiveness of nitrogen increases about threefold when the caesium pressure is changed from 0.012 to 0.002 mm, and the computed change in r' accounts for this variation. Results indicate a value of a between 1.4 and 4X10 for all foreign gases, while the value for the Cs-Cs collision giving ionization was ten times this.