Routes to multiphoton double ionization in combined extreme ultraviolet and infrared laser pulses

Xenon multiphoton double ionization pathways are studied in a reaction microscope using a pump-probe arrangement of extreme ultraviolet high harmonic and infrared laser radiation. The momentum of photoelectrons is recorded in coincidence with singly or doubly charged ions. Among all possible routes to multiphoton double ionization, sequential processes using ionic excited states as intermediate steps are clearly identified.

Figure 1

(Color online) Pathways to multiphoton DI: (a) nonsequential “direct,” or “indirect” through an $A^{+*}$ Rydberg state (thick dashed line). (b) Sequential: via an $A^{+*}$ state below the DI continuum threshold. Corresponding photoelectron spectra are schematized on the right.

Figure 2

Photoelectron spectra from SI by the 21st harmonic alone (dashed curve) and DI when superposed with the fundamental radiation (full curve). Xe $5s^{1}5p^6$ correlation satellites are indicated by straight lines.

Figure 3

The dependence of the ion yield ratio $\left[{\mathrm{Xe}}^{2+}\right]∕\left[{\mathrm{Xe}}^{+}\right]$ on the delay of the infrared with respect to the 21st harmonic pulse.

Figure 4

Same as Fig. 2 with the 17th harmonic. The solid lines labeled (i)–(v) indicate photoelectron energies for multiphoton ionization of the ${\mathrm{Xe}}^{+}$ $\left({}^{1}D\right)6s\left({}^{2}D_{5∕2}\right)$ state via the pathways: $+5\hslash {\omega }_{IR}\to {\mathrm{Xe}}^{2+}$ $\left({}^{3}P_2\right)$ (i); $+6\hslash {\omega }_{IR}\to {\mathrm{Xe}}^{2+}$ $\left({}^{3}P_1\right)$ (ii), and $\left({}^{3}P_0\right)$ (iii); $+7\hslash {\omega }_{IR}\to {\mathrm{Xe}}^{2+}$ $\left({}^{1}D_2\right)$ (iv); $+8\hslash {\omega }_{IR}\to {\mathrm{Xe}}^{2+}$ $\left({}^{1}S_0\right)$ (v).

Figure 5

Photoelectron angular distributions for DI (open symbols) and SI (filled symbols) by the 17th harmonic in the kinetic energy ranges $0.08–0.3\mathrm{eV}$ (triangles) and $0.45–0.75\mathrm{eV}$ (circles).