After passing through the target cell, the XUV harmonics and any generated nonlinear XUV emission signals transit through a slit that blocks the angled NIR beams. Residual NIR contamination is further attenuated by a second 0.15 µm Al filter before the transmitted XUV light is spectrally dispersed by a gold-coated flat-field grating (001-0639, Hitachi). An X-ray CCD camera (Pixis XO 400B, Princeton Instruments) records the target-attenuated harmonics and wavemixing emission signals on a 1340 × 400 pixel array as a function of XUV photon energy and phase-matched divergence angle relative to the harmonic axis. The horizontal energy axis is calibrated by comparing wave-mixing emission from the 2s 2 2p 5 nl excitations of neon gas to atomic line transition data available from the National Institute of Standards and Technology (NIST). In these experiments, a homebuilt mount translates the camera vertically such that the harmonics no longer impinge on the CCD chip and only wave-mixing signals that emit at non-zero divergence angles are recorded. This spatial masking procedure is performed to reduce interference from artifacts induced by strong absorption features, allowing the weaker wave-mixing emission signals to be imaged cleanly.