In Fig. , a spectral trace of the wave-mixing emission shown in Fig. is obtained by integrating over the negative absorbance features in the angular range of 2.7 -5.8 mrad. The most prominent features arise at energies associated with members of the Henning sharp series with principal quantum numbers from n = 5 to n = 8. Weaker features mirroring these strong wavemixing signals at slightly lower photon energies appear to emit at the energies of the Henning diffuse series. While emission from the lowest member of the Henning diffuse series (n = 6) can be distinguished from the nearby Henning sharp series feature (n = 5), the spectral resolution of the apparatus is insufficient to resolve the majority of adjacent Henning diffuse and sharp emission features, particularly as the states become more finely spaced in the approach to the ionization threshold (IP). Consequently, the wave-mixing emission features reported here exhibit broader spectral linewidths than would be expected for members of the Henning sharp series alone given previous synchrotron measurements .