Laser-peak-intensity calibration using recoil-ion momentum imaging

We exploit a method to precisely calibrate the peak intensity of intense laser pulses by measuring the momentum transferred to the ion in single ionization by circularly polarized light. Using this approach, we show how the experimental data can be used to get information about the absolute ionization rate at a specific local laser-peak intensity within the focal volume. The results of this method are compared to other results obtained with different methods of laser-peak-intensity calibration described in the literature.

Figure 1

Time-of-fight spectra for ${\mathrm{H}}_2$ in the case of linearly and circularly polarized light. In addition to the single ionization peak, other fragmentation channels are shown in the spectra. The double-peak structure of the single ionization by the circularly polarized light is an indication of the drift momentum gained by the ion in the laser field.

Figure 2

(Color online) $dN∕d{p}_x$ for ${\mathrm{H}}_2$ and He targets obtained using Eq. (10). The value of the momentum at which the yield maximizes is different for the two targets due to the difference in their ionization potentials.

Figure 3

(Color online) A comparison between the measured momentum spectra (points) and the calculations (curve) in circularly polarized light which is used to find the laser-peak intensity.

Figure 4

$\mathrm{H}_2{}^{+}$ ion yield as a function of peak intensity. By fitting the experimental data with molecular ADK predictions, an independent intensity calibration scale can be obtained.

Figure 5

Time-of-flight spectra of ${\mathrm{H}}_2$ at different laser peak intensities in linearly polarized light. The branching ratios of the different fragmentation channels can be used as a guide to assign the laser peak intensity.