Control of two-photon double ionization of helium with intense chirped attosecond laser pulses

We study the two-photon double-ionization process of the helium atom by solving numerically the nonrelativistic, time-dependent Schrödinger equation in its full dimensionality. We investigate with intense chirped attosecond laser pulses of 23.5-nm wavelength the two-photon absorption near and above the sequential threshold. We show how it is possible by adjusting the chirp parameter to control the electronic transitions inside the atom, thereby reinforcing or weakening the ionization process. Attosecond chirped laser pulses offer a promising way to probe and control the two-photon double ionization of helium when compared with attosecond transform-limited pulses.

Figure 1

Schematic He energy-level diagram showing the direct and the sequential ionization processes from $1s^2$.

Figure 2

Dependence of laser pulse vector potential $A(t,\xi )$, spectral profile, and instantaneous frequency $\omega (t,\xi )$ on the chirp parameter $\xi$ for three cases $\xi =0,\pm 1.75$. Each laser pulse has a central frequency ${\omega }_0=1.938$ a.u. (52.7 eV), peak intensity ${10}^{15}$ W/cm${}^2$, and FWHM duration ${\tau }_0=460$ asec.

Figure 3

Total two-photon double-ionization probability $P$($2\omega ,2e)$ as a function of the chirp parameter $\xi$.

Figure 4

(Right) Single electron energy distribution ($dP/dE$) as a function of ejected electron energy. (Left) Contour plots of the electron energy distribution of both ejected electrons of energies $E_1$, $E_2$ in the dominant channel ($L=2,l_1=1,l_2=1$) for $\xi =0$ and $\pm 1.75$.