Simulating the photoionisation of Rubidium atoms

Mihály András Pocsai, Imre Ferenc Barna, Gábor Bíró, Gergely Gábor Barnaföldi (2021.10.01 - 2022.01.31)
Wigner Research Centre for Physics

Abstract: In the concept of plasma based particle acceleration, the particles are accelerated in the wakefield generated by a driver pulse, which may be either a beam of charged particles or a short, intense laser pulse, instead of guiding,collimating and them in vacuum with strong electromagnetic fields [1]. The witness bunch is usually injected in the plasma from an external source, but in case of electron acceleration using a laser pulse as a driver pulse, at sufficiently high laser intensities, some of the plasma electrons are being trapped in the wakefield generated by the laser pulse. This phenomenon is referred as self-injection. In the schemes mentioned above, the driver bunches transfer their energy to the witness bunches through the plasma waves they generate. In the CERN–AWAKE experiment the wakefield is generated by a train of proton microbunches, produced from the SPS proton beam via the self-modulation instability [2]. In this experiment it is essential for the plasma to be ultrahomogeneous, furthemore, at prescribed points of the plasma, the plasma density has to follow the prescribed density ramps accurately. The plasma itself is produced by photoionising the rubidium vapour with a 120 fs long, intense, infra-red laser pulse. Therefore studying the corresponding photoionisation phenomena is a relevant sub-topic of the CERN–AWAKE experiment.

The processes in question have been already studied earlier via quantum mechanical simulations [3]. In our approach, we expanded the solution of the time-dependent Schrödinger-equaion (TDSE) on the basis of the eigenfunctions of the free Hamiltonian-operator of the Rubidium atom. The expansion coeffitiens are timedependent. Substituting this Ansatz into the TDSE, one obtains a first order, linear ODE system, referred as Coupled Channel Equations. Every channel, i. e. every time-dependent expansion coeffitient gives the occupation amplitude of the corresponding bound or continuum state of the rubidium atom. From the final state wave function, the total photoinisation probabilities, the photoelectron energy spectra, angular distributions and energy-and-angle resolved spectra can be obtained.

References:
[1] T. Tajima, J.M. Dawson: „Laser electron accelerator". Phys. Rev. Lett 43, 267–270 (1979).
[2] C. Petit-Jean-Genaz, G. Arduini, P. Michel, V. R. W. Schaa, (eds.), Proceedings, 5th International Particle Accelerator Conference (IPAC2014): Dresden, Germany, June 15–20, 2014, JACoW Conferences (CERN, Geneva, Switzerland, 2014).
[3] M.A. Pocsai, I.F. Barna and K. Tőkési: „Photoionisation of Rubidium in strong laser fields". Eur. Phys. J. D 73, 74 (2019).

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