Using Circular Dichroism to Control Energy Transfer in Multiphoton Ionization

Chirality causes symmetry breaks in a large variety of natural phenomena ranging from particle physics to biochemistry. We investigate one of the simplest conceivable chiral systems, a laser-excited, oriented, effective one-electron Li target. Prepared in a polarized $p$ state with $|m|=1$ in an optical trap, the atoms are exposed to co- and counterrotating circularly polarized femtosecond laser pulses. For a field frequency near the excitation energy of the oriented initial state, a strong circular dichroism is observed and the photoelectron energies are significantly affected by the helicity-dependent Autler-Townes splitting. Besides its fundamental relevance, this system is suited to create spin-polarized electron pulses with a reversible switch on a femtosecond timescale at an energy resolution of a few meV.

Figure 1

Scheme of few-photon ionization of $\mathrm{Li}(2p)$ and $\mathrm{Li}(2s)$ in lowest-order perturbation theory. The magnetic quantum number $m$ is denoted with respect to the direction of the photon spin. Atomic levels undergoing Autler-Townes splitting are shown as double lines in the graph (see text). Measured PADs and fitted spherical harmonics are shown on the top for the counter- (left) and corotating case (right).

Figure 2

Measured (top row) and calculated (bottom row) electron momentum distributions in the $xy$ plane integrated over the $z$ component for a laser field peak intensity of $0.68\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. The initial target states are $2s$ (left column) and $2p$ for corotating (center) and counterrotating (right) circular field polarizations.

Figure 3

Measured (left) and calculated (right) electron energy spectra for $2s$ and $2p$ ionization for both relative helicities of the circularly polarized laser field. Peak intensities $I$ and $CD$ values are given in the graphs.