The acceleration of electrons generated during ionization of low density gases has been studied using seventh order correction fields of a laser pulse for linearly and circularly polarized laser pulse. The spectrum of energy and angle of emittance of the electrons generated and accelerated during ionization of nitrogen ions N5+, oxygen ions O6+, neon ions Ne8+, krypton ions Kr32+, and argon ions Ar16+ has been obtained for normalized laser intensity parameter a0=4, 6, 12, 20, and 75, respectively, for normalized laser spot size r0=60, 90, and 120. Energy and scattering spectrum for nitrogen, oxygen, and neon ions show two peaks and may generate quasimonoenergetic beams for small laser spot sizes. The energy spectrum is wide and peak lies at low energy for krypton than that for argon. The energy peaks are at higher energy for circularly polarized laser pulse than that for linearly polarized laser pulse. The paraxial approximation may fail to yield accurate results at low values of laser spot size and high laser intensity. The energy associated with spectrum peak tends to saturate with laser intensity.

A scheme is proposed to enhance the energy of the electrons generated during the ionization of low-density krypton ions Kr 32+ and argon ions Ar 16+ by a radially polarized laser pulse using a negative frequency chirp. If a suitable frequency chirp is introduced then the energy of the electrons increases significantly and scattering decreases. The optimum value of the frequency chirp decreases with laser intensity and as well as spot size. The laser spot size also has an optimum value. The electron energy shows strong initial phase dependence. The scheme can be used to obtain quasi-monoenergetic collimated MeV GeV / electrons using the right choice of parameters. The chirped radially polarized laser pulse is more efficient than a chirped circularly polarized laser pulse to enhance energy and obtain quasi-monoenergetic electron beams.

We have investigated the effect of initial phase on error in electron energy obtained using paraxial approximation to study electron acceleration by a focused laser pulse in vacuum using a three dimensional test-particle simulation code. The error is obtained by comparing the energy of the electron for paraxial approximation and seventh-order correction description of the fields of Gaussian laser. The paraxial approximation predicts wrong laser divergence and wrong electron escape time from the pulse which leads to prediction of higher energy. The error shows strong phase dependence for the electrons lying along the axis of the laser for linearly polarized laser pulse. The relative error may be significant for some specific values of initial phase even at moderate values of laser spot sizes. The error does not show initial phase dependence for a circularly laser pulse.

The spectrum of energy and angle of emittance of the electrons generated during ionization of neon ions Ne8+, krypton ions Kr32+, and argon ions Ar16+ by a laser pulse have been obtained for different values of laser frequency chirp and normalized laser pulse duration. The energy of the electron beam shifts to higher energy with the introduction of frequency chirp. The energy peak shifts towards lower energy with an increase in frequency chirp, and the electron beam becomes more quasi-monoenergetic. The energy peak shifts to higher energy with decreasing laser pulse duration due to increase in asymmetry of the pulse, however, the quasi-monoenergetic property of the electron beam decreases. We can obtain MeV, MeV/GeV, and GeV electron beams using neon, krypton, and argon gases as target. The scattering of the electrons decreases with decreasing laser pulse duration and increasing laser intensity. The energy peak is sharper and at higher energy for the ions located after laser focus than that for the ions located before laser focus for a tightly focused laser pulse.