Rescattering Theory of Strong Field Ionization in Atoms

Marwah, Risheek and Sharma, Vandana (2019) Rescattering Theory of Strong Field Ionization in Atoms. Masters thesis, Indian institute of technology Hyderabad.

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Abstract

In extreme nonlinear regimes, a laser cannot be simply regarded as a weak perturbation but as a strong field which can ionize the electrons from the gas atoms. In our experiment, 25-fs laser pulses from a Titanium: sapphire laser oscillator were focused using spherical mirror on the atoms of argon gas ejected out through a gas valve in to vacuum. The optical field intensities were high enough of the order of 1016Wcm2 to ionize the electrons which were detected by the MCP. Spectral measurements reveal above-threshold photoemission with clearly resolved multiphoton peaks. At increasing intensities we observe the increase in the number of electrons ionized with higher average kinetic energies. These effects indicate that the strong-field photoemission regime has been reached in our system. Furthermore, photoelectelectron angular distribution from atomic targets has also been studied. The theoretical model known as simple man model is used to explain what has been analysed from the experimental data collected. This model depicts that photoelectrons are driven back to the atomic target by the laser field and recollide there leading to very high kinetic energies and resulting in a laser induced above threshold ionization plateau structure in photoelectron spectra. The high energy electrons can be used in ultrafast microscopes to investigate structural processes on a femtosecond time scale with nanometer resolution.

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