Strong field physics at attosecond timescales: possible chances and pitfalls

Recent attosecond experiments have been based on precise GV/cm electric field synthesis (see e.g.[1]). Accurate control over the carrier-envelope offset phase in combination with advanced spectral broadening and mirror compression techniques have opened the door to few-cycle fields, and single events of strong field tunneling have been timed with sub-cycle accuracy [2]. This attractive type of sudden ionization has been applied by laboratories in two different ways: rescattering of the generated attosecond electron wavepacket onto the parent molecule, as well as attosecond XUV pulse generation via HHG, a typical daughter process of rescattering [3]. Both have been used for attosecond time domain spectroscopy, but up to now only to a limited number of text-book cases (see e.g. [4, 5]). The talk will address some of the reasons for our present difficulties to investigate slightly more complex situations with the present means of attosecond spectroscopy. In particular, attosecond spectroscopy of complex systems appears to be affected from a breakdown of the single active electron approximation, as soon as one enters the strong field regime (see e.g. [6, 7]). We observe that even weakly coupled systems like VdW clusters appear to show an SAE breakdown at low photoelectron energies [8]. Another important difficulty is the present low intensity of attosecond light pulses, which can hopefully be addressed by an increase of target densities, e.g. by the use of solid targets. High target densities are, however, often incompatible to strong laser field photoelectron spectroscopy, due to strong field plasma generation[9]. References