In 2009 ASML announced that sources would be needed at 6.x nm for future lithography beyond 13.5 nm predicated on the availability of mirrors, this time La/B4C multilayers with a reflectivity of in a 0.06 nm bandwidth near 6.7 nm.33 The theoretical reflectivity is close to 70%.34 Both Gd and Tb have transitions near this wavelength and emit an intense UTA. The emission processes are more efficient than in Sn because the 4f wavefunction is completely contracted in the relevant ions, thereby maximizing the oscillator strength and leading to a better overlap of emission in adjacent stages than was obtained with Sn. In addition, transitions of the type , not present in Sn, may also contribute to the in-band emission. Thus many ion stages can contribute and strong emission is expected over a wide range of plasma temperatures. The spectra of vacuum spark and laser plasma emission from laser-produced plasmas of Gd and Tb were first compared by Churilov and co-workers.35 In modeling the optimum conditions, the presence of these open subshell configurations make calculation of spectra much more complex than in the case of Sn, because of the greatly increased number of transitions possible and the close proximity of configurations containing variable numbers of , , and electrons in lower ion stages.36 Indeed, it is virtually impossible to make complete detailed calculations for excitation and radiative processes for these elements using existing atomic structure codes. As a result, despite the similarity in many instances to Sn, because of the difficulties posed by the presence of open subshell ions, no full hydrocode simulation has been reported to date and thus no predictions exist for the variation of CE with electron temperature. However, even in the absence of such a calculation, it is possible to make some reasonable assumptions. The spectra from plasmas where the , UTA contains contributions from both open and open subshell ions has been studied in detail for tungsten because of its importance as a wall material for fusion devices.37 In all reported studies the emission is dominated by Ag- and Pd-like lines, i.e., the spectra containing fewest lines where the emission is not divided among many transitions. Thus, for Gd, for example, the strongest lines should result from lines in the spectra of Ag-like Gd XVIII, Pd-like Gd XIX, and Rh-like Gd XX and the optimum electron temperature in a Gd plasma should, most likely, correspond to that which maximizes the population of these ions. The strongest transitions from Ag- and Pd-like ions have been identified38–40 and are listed in Table 1. Results calculated in the present study with the Cowan suite of codes41 as well as those from other theoretical calculations are also included for comparison. The configurations chosen in the present case are given in Table 2. The Slater Condon , , and parameters were reduced by 15% while the spin orbit integrals were left unchanged. The transitions from Ag-, Pd-, and Rh-like ions around 6.7 nm are presented in Tables 3004005006007–8 for Gd and Tb, respectively. Since the optimum center wavelength is yet to be decided, a glance at this table would suggest 67.65 as a suitable choice.