Photoionization study using few attosecond pulses and coincidence imaging techniques

Abstract: Photoionization is one of the most fundamental photoreactions that leads to the ejection of an electron and the formation of an ion. With the help of modern 3D momentum imaging spectrometers, the interplay between the time and frequency domain upon ionization is studied. In this work, single photoionization of helium is investigated in the few-cycle pulse regime of the driving infrared laser pulses with ultrashort attosecond pulses in a dressing infrared field. By varying the carrier-to-envelope phase of the infrared field to modify the spectro-temporal structure of the attosecond pulses, we analyze how the infrared field impacts the measured angularly-resolved photoelectron energy distribution. To this end, a newly built coincidence 3D momentum spectrometer was used. A thorough understanding of the experiment is achieved by using simulations within the strong field approximation. This analysis allows us to separate our observation into two scenarios, one at two attosecond pulses when the carrier-to-envelope phase is $\pi/2$ and the second one at three attosecond pulses when the carrier-to-envelope phase is 0 or $\pi$. When the dressing infrared field is added, the photoelectron spectrum changes in comparison to photoionization with attosecond pulses only. While two attosecond pulses in the dressing infrared field modify the photoelectron spectrum in a continuous way, we observe sidebands appearing in the spectrum generated by three pulses in the dressing infrared field. Finally, the resolution of the spectrometer is studied before and after an upgrade of the spectrometer was performed in the context of this work. This study allows us to understand the limiting factors of the energy resolution. In particular, it shows that the resolution is essentially limited by the source volume of the emitted particles, while the magnetic field does not limit the resolution as it was expected.