The multiphoton ionization spectrum of xenon: Interatomic effects in multiphoton transitions

The multiphoton ionization spectrum of xenon was taken in the vicinity of the laser wavelengths corresponding to one-third the energy of transitions to the 5p5(2P3/2)6s, 5p5(2P1/2)6s, and 5p5(2P3/2)5d atomic levels. Many atomic resonances were observed, the most prominent being third photon resonances with atomic 5d levels and fourth photon resonances with atomic 4f levels. However, peaks corresponding to ionization through the 6s levels were weak or absent. Resonances in these regions showed extreme pressure dependence. At higher pressures (≳50 torr) a broadening of the atomic lines as well as the existence of continuum backgrounds and strong broad resonances peaking at the energy of transitions between excited states were seen. At low pressure (40 torr) the atomic lines were narrow and the broad peaks and background were no longer present. These high pressure features were interpreted to be a consequency of resonances with vibrational continua of excited state diatomic xenon molecules and illustrate the necessity of inclusion of molecular Franck–Condon factors into the multiphoton ionization probability of atomic gases at moderate pressures.