Strong-Field Excitation of Helium: Bound State Distribution and Spin Effects

Using field ionization combined with the direct detection of excited neutral atoms we measured the distribution of principal quantum number $n$ of excited He Rydberg states after strong-field excitation at laser intensities well in the tunneling regime. Our results confirm theoretical predictions from semiclassical and quantum mechanical calculations and simultaneously underpin the validity of the semiclassical frustrated tunneling ionization model. Moreover, since our experimental detection scheme is spin sensitive in the case of He atoms, we show that strong-field excitation leads to strong population of triplet states. The origin of it lies in the fact that high angular momentum states are accessible in strong-field excitation. Thus, singlet-triplet transitions become possible due to the increased importance of spin-orbit interaction rather than due to direct laser induced spin-flip processes.

Figure 1

Sketch of the experimental setup, which is explained in the text. The panel illustrates the radiative decay of the initial laser excited distribution (red) into the detectable metastable states (green) and the nondetectable ground state (black). The thickness of the arrows roughly depicts the probability of population.

Figure 2

Yield of excited He* atoms as a function of the field strength (a) of an electric field pulse applied shortly after the laser pulse for laser intensities $1.8\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ (blue dots), $2.2\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ (green triangles), and $2.9\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ (red squares), and (b) of a static electric field at a laser intensity of $2.2\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ (black open squares). The full orange curve in (b) is the result of a calculation, described in the Supplemental Material [27].

Figure 3

He* yield as a function of the electric field strength at a laser intensity of $2.3\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$. See text for more information.

Figure 4

Measured $n$ distributions for a laser intensity $1.8\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ (blue dots) and $2.9\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ (red squares). Corrected calculated He* yield: classical FTI model at field strengths of ${10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ (open diamonds) and $1.4\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ (open squares) and quantum mechanical SAE calculations for laser intensities $1.8\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ (open circles) and $2.9\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ (open triangles). The inset shows for the FTI calculation (laser intensity ${10}^{15}\mathrm{W}/{\mathrm{cm}}^2$) the difference between the distribution corrected for the state detection efficiency (open triangles) and the uncorrected one (filled triangles).