Unexpected product fine-structure distributions in (3+1)-photon ionization of xenon.

Abstract

Three-photon resonance enhanced four-photon ionization of xenon shows preferential formation of ${\mathrm{Xe}}^{+}$${(}^{2}$${\mathit{P}}_{1/2}$) when intermediate ns[3/2${]}_{1}$ Rydberg states are excited in the three-photon step. Here, the nl[K${]}_{\mathit{J}}$ notation refers to ${\mathit{j}}_{\mathit{c}}$l coupling, in which the angular momentum of the ion core, ${\mathit{j}}_{\mathit{c}}$, is coupled to the orbital angular momentum of the Rydberg electron l to give K, which is then coupled to the Rydberg electron's spin to give J. The primes following l denote states belonging to the $^{2}$${\mathit{P}}_{1/2}$ core. Multichannel-quantum-defect theory identifies these intermediates as nearly pure Rydberg states belonging to the ${\mathrm{Xe}}^{+}$${(}^{2}$${\mathit{P}}_{3/2}$) core. An equivalent behavior is found for the 7s'[1/2${]}_{1}$ intermediate state which preferentially produces ${\mathrm{Xe}}^{+}$${(}^{2}$${\mathit{P}}_{3/2}$). On the other hand, for nd and nd' resonances we observe final-state distributions which closely mirror the core character of the intermediate state. The suppression of the parent core channel for s-type Rydberg states is shown to be due to a Cooper minimum in the ns\ensuremath{\rightarrow}\ensuremath{\varepsilon}p transitio n amplitudes. As a consequence, minor components of the resonance state wave functions play a major role in the selection of the final ionization channel, in particular via the much larger amplitudes of nd'\ensuremath{\rightarrow}\ensuremath{\varepsilon}f' transitions.