Wavelength dependent high-energy ion emission from intense mid-IR laser-cluster interaction

We present the first measurements on the wavelength dependence from the near-infrared to mid-infrared of inert gas clusters interacting with an intense, ultrafast pulse. In the experiments, ion energy distributions have been recorded with various wavelength (0.8-2.2 $\mu$m), while all other conditions are fixed. It is found that the wavelength plays a significant role in electron-plasma heating and thus energetic ion production. The maximum energy of the detected ion, E$_{\mathrm{max}}$, decreases with increasing wavelength, reaches a minimum, then increases. We attribute this result to two different electron-heating mechanisms depending on the wavelength- volume (Inverse Bremsstrahlung: IB) and surface (Brunel) heating. In the short wavelength regime (0.8-1.5 $\mu$m), IB heating dominates the production of multiply charged ions, since the electrons are resonantly heated near plasma frequency. As the wavelength is increased, IB heating is progressively suppressed, resulting in a smaller value of E$_{\mathrm{max}}$. Brunel heating, on the other hand, increases due to a quadratic increase of the electrons ponderomotive energy, and becomes dominant in the long wavelength regime (1.7-2.2 $\mu$m). The lowest E$_{\mathrm{max}}$ values would thus occur at the wavelength where the dominant heating mechanism switches from volume to surface.