f-electron Rydberg series of triatomic hydrogen

Photoexcitation from the lowest metastable level of ${\mathrm{H}}_3$ is used to prepare d-electron Rydberg states with total angular momentum N=1 that are built on ${\mathrm{H}}_3^{+}$ cores with different rotational quantum numbers. These states are then photoexcited to high-n members of f-electron Rydberg series. Excitation is detected by monitoring ${\mathrm{H}}_3^{+}$ ions that are formed in field ionization and autoionization. The photoexcitation of the f-electron series allows a sensitive analysis of the molecular-ion-core configuration of the 3d states. We have analyzed the f-electron Rydberg series converging to the two lowest rotational levels of ${\mathrm{H}}_3^{+}$ and we have determined their quantum defects, ionization potentials, and their excitation intensity.