The experimental apparatus utilized to generate and measure XUV emission signals has been described previously and is depicted in Fig. . Briefly, a commercial multipass Ti:Sapphire laser system (Femtopower HE, Femtolasers) produces 2 mJ of 22 fs NIR pulses with a central wavelength of 780 nm at a 1 kHz repetition rate. These pulses are spectrally broadened in a stretched 2 m long, 500 µm inner diameter hollow core fiber (Few-Cycle Inc.) statically filled with 2 bar of neon gas. The broadband fiber output is then temporally recompressed by seven pairs of double-angle chirped mirrors (PC70, Ultrafast Innovations) and a 2 mm thick ammonium dihydrogen phosphate (ADP) crystal , resulting in 600 µJ, 6 fs pulses with a spectral profile spanning from 550 -950 nm (Fig. ). A 75:25 (T:R) beamsplitter divides the compressed NIR pulses into two arms. The transmitted portion of the beam is focused by a spherical silver mirror (f = 50 cm) through 300 µm holes drilled into a 1-mm pathlength cell contained within a vacuum chamber at 10 -6 Torr. The interaction of the focused NIR driver (~10 14 W cm -2 ) with xenon gas flowing through the cell produces XUV harmonics via high harmonic generation (HHG). The pressure of xenon gas (~5 Torr) flowing through the cell and its position relative to the few-cycle NIR driver pulse focus are optimized to generate a train of 2 -3 subfemtosecond XUV pulses with photon energies that cover the neutral Rydberg states of CO2 located between 17.3 -18.1 eV (Fig. ). A 0.15 µm thick Al foil (Lebow) filters out the co-propagating NIR driver before the transmitted XUV pulse train is focused by a gold-coated toroidal mirror (f = 50 cm, ARW Optical Corporation) through an annular mirror into a 1-mm pathlength target cell. Small 500 µm diameter holes drilled into the target cell permit the XUV light to interact with the CO2 gas flowing continuously though the cell. At the target cell, the XUV beam has an estimated waist of ~20 µm and a peak intensity of the order of 10 8 -10 10 W cm -2 due to the low conversion efficiency of HHG .