Bound-free $e^{+}{e}^{-}$ pair creation with a linearly polarized laser field and a nuclear field

The process of bound-free pair production of electrons and positrons in combined laser and Coulomb fields is investigated. It is assumed that an ion collides at relativistic speed with an intense x-ray laser beam of linear polarization. The process proceeds nonlinearly due to simultaneous absorption of a few laser photons. The capture of the electron into the ground state and the $L$ shell is considered. The scaling of the total rate, the angular distributions of the emitted positrons, and a comparison to the competing free-free channel are surveyed. Numerical results of pair production rates for parameters for the planned x-ray free-electron lasers at DESY and SLAC are presented. We find that pair production with these laser facilities can become observable in the near future.

Figure 1

Comparison of different calculations for the frequency dependence of the total pair production rate in the case of the absorption of a single photon (here, $\xi ={10}^{-4}$, $Z=1$, $\gamma =50$).

Figure 2

Dependence on frequency for various nuclear charge numbers $Z$. The lines are fits over calculated data points (here, $\xi ={10}^{-4}$, $\gamma =50$, $n=2$).

Figure 3

Dependence of pair production rate on atomic number $Z$ for various frequencies. The lines represent fits over calculated data points [here, $\hslash \omega =6\mathrm{keV}$ (dotted line), $\hslash \omega =9\mathrm{keV}$ (dashed line), $\hslash \omega =13\mathrm{keV}$ (solid line), $\xi ={10}^{-4}$, $\gamma =50$].

Figure 4

Polar angular distribution of created positrons in the nuclear rest frame (here, $\hslash \omega =9\mathrm{keV}$, $\xi ={10}^{-4}$, $\gamma =50$, $Z=50$) for the absorption of two (solid line), three (dashed line), and four (dotted line) photons.

Figure 5

Azimuth angular distribution of the created positron in the nuclear rest frame (here, $\hslash \omega =9\mathrm{keV}$, $\xi ={10}^{-4}$, $\gamma =50$, $Z=50$) for the absorption of two (solid line), three (dashed line), and four (dotted line) photons.

Figure 6

Rate dependence on the energy in the laboratory frame for the bound-free (solid line) and free-free (dashed line) channels (here, $\hslash \omega =9\mathrm{keV}$, $\xi ={10}^{-4}$, $\gamma =50$, $Z=50$, $n=2$).

Figure 7

Rate dependence on the polar angle in the laboratory frame for the bound-free (solid line) and free-free (dashed line) channel (here, $\hslash \omega =9\mathrm{keV}$, $\xi ={10}^{-4}$, $\gamma =50$, $Z=50$, $n=2$).

Figure 8

Bound-free pair production rates in the $\xi \sim 1$ domain for different photon orders. Also shown is the sum of the simultaneous absorption of nine photons (open squares) and an estimate for the sum over all photon orders (solid squares) (here, $\hslash \omega =9\mathrm{keV}$, $\gamma =50$, $Z=50$).

Figure 9

Polar angular distribution of the created positron for the capture in the $2s$ state in the nuclear rest frame (here, $\hslash \omega =9\mathrm{keV}$, $\xi ={10}^{-4}$, $\gamma =50$, $Z=50$) for the absorption of two (solid line), three (dashed line), and four (dotted line) photons.

Figure 10

Comparison of the (scaled) polar angular distributions of the emitted positrons for capture into different atomic states.