Two-color photoionization spectroscopy using vacuum ultraviolet synchrotron radiation and infrared optical parametric oscillator laser

A two-color, vacuum ultraviolet (VUV)-infrared (IR), photoionization study using monochromatized undulator VUV synchrotron radiation and high-repetition rate IR optical parametric oscillator (OPO) laser source has been demonstrated at the Chemical Dynamics Beamline of the Advanced Light Source. The OPO setup, optical alignment, and ion detection arrangement are described. The VUV-IR ionization spectrum of Ar has been recorded in the energy region between the ionization thresholds for ${\mathrm{Ar}}^{\mathrm{+}}{(}^2{P}_{\text{3/2}})$ and ${\mathrm{Ar}}^{\mathrm{+}}{(}^2{P}_{\text{1/2}})$ to illustrate the feasibility and efficacy of this scheme as a general approach for high-resolution two-color photoionization studies. The autoionization resonances of the $\mathrm{Ar}{\mathrm{(np}}^{\prime }$ and ${\mathrm{nf}}^{\prime })$ Rydberg series are resolved up to the principal quantum number of n=57, showing that the optical resolution of 0.3 ${\mathrm{cm}}^{\mathrm{-1}}$ (full width at half maximum) achieved in this two-color photoionization study is mostly limited by the bandwidth of the IR OPO laser and the Doppler width due to the random motion of the Ar sample.