Ultraviolet-visible fluorescence of $2p$-excited argon

Ultraviolet-visible fluorescence $(300\mathrm{}\hspace{0.5em}\mathrm{nm}<~\lambda <~500\hspace{0.5em}\mathrm{nm})$ of argon is investigated in the Ar $2p$ excitation regime $(240\mathrm{}\hspace{0.5em}\mathrm{eV}<~E<~260\hspace{0.5em}\mathrm{eV})$ using monochromatic synchrotron radiation for primary excitation. Fluorescence excitation spectra and dispersed fluorescence spectra at constant photon energy are reported as well as time-resolved dispersed fluorescence profiles. Various radiative processes are assigned, indicating that these processes are primarily due to the relaxation of ${\mathrm{Ar}}^{+}.$ A distinct state-selective behavior is found for some fluorescence excitation spectra, whereas others show no state selectivity at all. Time-resolved experiments give further insight into this state-selectivity and excited-state lifetimes. The results indicate that excited cation states are either directly formed as a result of primary Ar $2p$ excitation and subsequent electronic relaxation or via subsequent radiative cascades. The present results are consistent with earlier work on resonant Auger spectroscopy, extending the dynamic range of processes occurring in the Ar $2p$ regime into the nanosecond time regime.