Space-charge ion detection of multiphoton absorption phenomena in lithium vapor

The method of space-charge ion detection has been extended to the case of lithium. This was accomplished by the insertion of a space-charge-limited diode into a heat-pipe oven. Lithium vapor was probed with a N₂ laser-pumped tunable dye laser from 7150 to 4400 {} and with a Xe arc lamp from 7500 to 2300 {}. One-through four-photon events leading to ionization were observed. The atomic transitions observed were the principal series to $n=10$ and the $2s$ to $3d$, $4s$, and $4d$ two-photon transitions. The two-photon absorption signals depended upon the third and fourth powers of the light intensity, indicating a new ionization process not heretofore considered important in diode signals. This new process appears to be excitation transfer from excited-state atoms to other excited-state atoms or molecules. For the high excited-state densities achieved in this experiment, this process appears to predominate over photoionization of excited-state atoms and other collisional processes involving excited-state and ground-state species. The B←X, C←X, and D←X molecular transitions, investigated by previous workers, were observed here.