Circular dichroism in laser-assisted proton-hydrogen collisions

We investigate the effects of a strong laser field on the dynamics of electron capture and emission in ion-atom collisions within a reduced dimensionality model of the scattering system in which the motion of the active electron and the laser electric field vector are confined to the scattering plane. We examine the probabilities for electron capture and ionization as a function of the laser intensity, the projectile impact parameter $b$, and the laser phase $\varphi$ that determines the orientation of the laser electric field with respect to the internuclear axis at the time of closest approach between target and projectile. Our results for the $b$-dependent ionization and capture probabilities show a strong dependence on both $\varphi$ and the helicity of the circularly polarized laser light. For intensities above $5\times {10}^{12}\mathrm{W}∕{\mathrm{cm}}^2$ our model predicts a noticeable circular dichroism in the capture probability for slow proton-hydrogen collisions, which persists after averaging over $\varphi$. Capture and electron emission probabilities defer significantly from results for laser-unassisted collisions. Furthermore, we find evidence for a charge-resonance-enhanced ionization mechanism that may enable the measurement of the absolute laser phase $\varphi$.

Figure 1

(Color online) Collision scenario for a proton on a straight-line trajectory with impact parameter $b$ and velocity $v$ colliding with an atomic hydrogen target. The rotating laser electric field breaks the azimuthal symmetry: For positive impact parameters, the projectile follows the rotating laser field (corotating case); for negative impact parameters, the projectile moves against the rotating electric field (counterrotating case).

Figure 2

(Color online) Snapshot of the electronic potential. For negative helicity, the laser electric field causes a clockwise rotation of the inclined potential plane about the target while the projectile moves toward the right-rear end along a straight line.

Figure 3

(Color online) Capture probability as a function of the impact parameter for field-free collisions of $2\mathrm{keV}$ protons with hydrogen atoms. Results from independent two-dimensional wave function propagation calculations: Lein and Rost [16] (solid curve), present results (dots).

Figure 4

(Color online) Capture and ionization probability as a function of the laser phase $\varphi$ at the time of closest approach between projectile and target for $1.21\text{‐}\mathrm{keV}$ $p$-H collisions. The impact parameter is $b=\pm 4\mathrm{a.u.}$ and the laser intensity $5\times {10}^{13}\mathrm{W}∕{\mathrm{cm}}^2$. Phase-averaged results for the capture probability differ significantly for corotating and counterrotating laser-assisted collisions.

Figure 5

(Color online) Electron capture (a), ionization (b), and target electron loss probability (c) in laser-assisted $1.21\text{‐}\mathrm{keV}$ $p\text{‐}\mathrm{H}$ collisions for a laser intensity of $I=5\times {10}^{13}\mathrm{W}∕{\mathrm{cm}}^2$. The contour plots show the probabilities as a function of the impact parameter $b$ and the laser phase $\varphi$. The probability difference between consecutive contour lines is 0.125. The top panels in (a), (b), and (c) show phase averaged results. The side panel in (c) shows the impact-parameter average as a function of the laser phase. Capture, ionization, and loss probabilities as a function of the impact parameter and the laser phase for the case of a static electric field, corresponding in the direction and magnitude to the laser electric field at the distance of closest approach in (a), (b), and (c), are shown in (d) for comparison.

Figure 6

(Color online) $b{P}_{\mathit{cap}}$, averaged over the laser phase, at different laser intensities for corotating (positive impact parameter) and counterrotating (negative impact parameters) collisions.

Figure 7

(Color online) Total electron capture cross sections as a function of the laser intensity for corotating and counterrotating collisions. Also shown is the relative difference $\Delta$, which is largest at a laser intensity of $5\times {10}^{13}\mathrm{W}∕{\mathrm{cm}}^2$.