a Experimental slices through the optical frequency and linear polarization parameter space, with photoemission peaks at (795 nm, 90°) and (860 nm, 15°). At either spectral peak, the laser polarization dependence shows minimal signature of the other resonance, while between resonances (835 nm) the four-lobed behavior shows equal contributions from both resonance modes. The peak polarization axes are indicated in the correlated SEM insets for each resonance, as determined via nonlinear cosine fits shown in the polarization plots (50 nm scale bar). b Theoretical surface multiphoton photoemission rates for the correlated nanostar geometry. Aside from a small blue shift, peaks at (785 nm, 90°) and (830 nm, 15°) are in good agreement with the experimental results. All four tips are modeled with the same radius (3.4 nm), but photocurrent density maps demonstrate single-tip excitations at the resonance conditions due to the specific plasmonic mode structure. Measurements and calculations are performed at 1 × 108 W cm−2 peak input intensity.