Controlling ionization avalanching in clusters using XUV-driven seed electrons for ignition

The interaction of atomic clusters with intense near-infrared (NIR) laser pulses triggers highly complex dynamics, which evolve on attosecond to nanosecond timescales. It is experimentally very challenging to disentangle the various processes that were predicted to be important by theoretical models. Recently, we made progress in understanding the relaxation dynamics of clusters on a picosecond to nanosecond timescale using time-resolved photoelectron and -ion spectroscopy, where we studied the temporal evolution of electron-ion recombination [1]. Furthermore, we discovered a correlated electronic decay mechanism, in which electronically excited states decay via energy transfer to nearby electrons [2].

Novel experimental approaches are required in order to temporally resolve the ionization dynamics of clusters on a femtosecond timescale. Here we report on a concept, in which cluster ionization is ignited by generating a few seed electrons using an ultrashort extreme-ultraviolet (XUV) pulse from a high-harmonic generation source. Following ignition of the clusters, they become strongly ionized by an NIR laser pulse at an intensity far below the threshold, where tunnel ionization plays a role. In the experiment, we observe charge states up to Ar⁴⁺ at an NIR intensity of 3 x 10₁₂~W/cm², corresponding to a ponderomotive potential of only 170 meV, i.e. 2 orders of magnitude below the first ionization potential of atomic Ar. These results could be reproduced by calculations, demonstrating that inverse bremsstrahlung, electron impact ionization and avalanching lead to an exponential increase of the cluster charge, which is strongly enhanced at the plasmon resonance [3].

A major advantage of our experimental scheme is that focal volume averaging over different NIR intensities is avoided, allowing for precise comparisons of the results with the results from theoretical models. We will describe how this concept can be used in the future in order to control and investigate cluster ionization on an attosecond timescale. Furthermore, we will discuss application possibilities of the 2-color ionization scheme for efficient XUV pulse generation and for laser-based material processing.

[1] B. Schütte et al., Phys. Rev. Lett. 112, 253401 (2014).
[2] B. Schütte et al., Nature Commun., in press.
[3] B. Schütte et al., submitted to Phys. Rev. Lett. (arXiv:1509.03250 (2015)).