Multidimensional high-harmonic echo spectroscopy: Resolving coherent electron dynamics in the EUV regime

We theoretically propose a multidimensional high -harmonic echo spectroscopy technique which utilizes strong optical fields to resolve coherent electron dynamics spanning an energy range of multiple electronvolts. Using our recently developed semiperturbative approach, we can describe the coherent valence electron dynamics driven by a sequence of phase -matched and well -separated short few -cycle strong infrared laser pulses. The recombination of tunnel -ionized electrons by each pulse coherently populates the valence states of a molecule, which allows for a direct observation of its dynamics via the high harmonic echo signal. The broad bandwidth of the effective dipole between valence states originated from the strong -field excitation results in nontrivial ultra -delayed partial rephasing echo, which is not observed in standard twodimensional optical spectroscopic techniques in a two -level molecular systems. We demonstrate the results of simulations for the anionic molecular system and show that the ultrafast valence electron dynamics can be well captured with femtosecond resolution. Significance To detect coherent dynamics spanning an energy range of multiple electronvolts, broadband EUV is usually necessary. In this work, we proposed an all -optical two-dimensional (2D) high harmonic echo (HHE) spectroscopy in strong -field regime to detect both the population transfer and coherent dephasing dynamics. We are able to selectively produce a delayed harmonic echo originated from partial rephasing of a given pair of bound states which is not possible in traditional echo experiments that rely exclusively on full rephasing of electronic coherence. Strong field spectroscopy usually deals with small molecules and requires alignment to eliminate inhomogeneous broadening. The proposed 2D technique based on HHE can be used to study large molecules without the requirement of molecular alignment.