Speaker: Cosmin Blaga, Kansas State University
Program Description:
When atoms and molecules are ionized by intense, linearly polarized laser fields, a portion of the emitted photoelectron wavepacket returns and collides with to the parent ion. This three-step rescattering mechanism consisting of photoionization, field propagation and collision is a fundamental process in strong field physics, underpinning numerous phenomena such as high harmonic generation, nonsequential ionization, laser induced electron diffraction, electron holography, etc. During the photoionization step, resonances are known to play an important part during photoionization, as seen from the presence of Rydberg series in the low-energy part of the above threshold ionization (ATI) photoelectron spectra. On the other hand, it has long been suggested that resonant processes can play a role in the collisional step resulting in the production of high-energy electrons as well. Although there has been some evidence for this speculation, it faced two challenges. First, experiments result in spectra that are averaged over a wide intensity range throughout the laser focal volume. This makes correlating the observed features of any experiment to a given laser intensity problematic, rendering it difficult to test theoretical predictions in detail. The second, perhaps even more difficult obstacle is that the rescattering model of strong field processes provides a perfectly good non-resonant model of high energy photoelectron production, one that reproduces the most important observed features: a long plateau in the spectrum that ends at a cutoff that can be predicted via a simple classical trajectory model. Indeed, the so-called strong field approximation, which instantiates this non-resonant ionization plus classical trajectory model, has been the dominant method of interpreting strong field experiments for the last three decades. In this talk, I will present and discuss a series of wavelength-resolved ionization experiments and theoretical simulations in noble gases to prove that resonant processes dominate the production of high energy photoelectrons in strong field experiments.
