Ultrafast laser-driven quantum dynamics in positronium chloride

Abstract

We present a computational study of the laser-driven quantum dynamics of positronium (Ps), PsH, and PsCl at the time-dependent Hartree-Fock level of theory. To eliminate finite-basis effects and to properly capture continuum dynamics, we use a spherical polar pseudospectral representation. The multicomponent theory and its implementation are described in detail. We find that while the presence of the positron delays electron ionization in PsH, a slight enhancement of electron ionization is observed in PsCl. In both cases, the positronic response is faster than that of the electrons. We propose that the formation of PsCl may be directly observed through photopositron spectra in the multiphoton regime, where PsCl peaks are expected at roughly twice the energy of Ps peaks, making PsCl clearly distuinguishable from Ps. In the tunelling regime, however, photopositron rescattering peaks may only be distuinguishable if the amount of Ps is sufficiently low.

Figures (9)

• Figure 1: Radial distribution functions for the positron orbital and the outermost electron orbitals of PsH (top) and PsCl (bottom).
• Figure 2: Position expectation value for electrons and positron in PsCl subject to a weak ($\mathcal{E}_0=0.0001\,\mathrm{a.u.}$) 8-cycle laser pulse with frequencies $0.01\,\mathrm{a.u.}$ (top), $0.034\,\mathrm{a.u.}$ (middle) and $0.24\,\mathrm{a.u.}$ (bottom). These correspond, in order, to a frequency in the transparent region, the first positron resonance, and the first electronic resonance. The units of the laser pulse are arbitrary, as its meant to show the phase between the electric field and the position expectation values. The grid parameters were $r_{\text{max}}=100$ and $l_{\text{max}}=9$.
• Figure 3: The external potential (excluding nucleus) seen by the electrons in a Cl$^-$ system with and without a positron.
• Figure 4: The relative phase angle, $\theta$, between the laser field and $\langle z \rangle$ for the positron and electrons in PsH (top) and PsCl (bottom) as a function of the field frequency. A weak ($\mathcal{E}_0=0.0001\,\mathrm{a.u.}$) 8-cycle laser pulse was applied.
• Figure 5: Electron and positron ionization densities for PsH (top, $\mathord{\sim}5.35 \times 10^{12}\ \mathrm{W/cm^2}$) and PsCl (bottom, $\mathord{\sim}4.05 \times 10^{12}\ \mathrm{W/cm^2}$). We used a 12 cycle pulse with frequency 2280 nm. The grid parameters were $r_{\text{max}}=175$ and $l_{\text{max}}=23$, and we used time step $\mathrm{dt}=0.05$ a.u.