High-order harmonics of 248.6-nm KrF laser from helium and neon ions

We report high harmonic generation from a 248.6-nm KrF laser giving harmonic orders up to the 37th (67 Å) in a helium gas jet and the 35th (71 Å) in neon, for laser intensities up to 4 × ${10}^{17}$ W/${\mathrm{cm}}^2$ in 380-fs pulses. These observations are interpreted using theoretical modeling that identifies the ion species ${\mathrm{He}}^{+}$, ${\mathrm{Ne}}^{+}$, and ${\mathrm{Ne}}^{2+}$ as the sources of the highest harmonics.