Efficient Autoionization Following Intense Laser-Cluster Interactions

Electron emission as a result of the interaction of clusters with intense laser pulses is commonly understood in terms of direct and evaporative ionization processes. In contrast, we provide evidence here of an important role played by autoionization in intense field ionization of molecular oxygen clusters. Superexcited states are populated during the cluster expansion, and their autoionization is observed on a ns time scale. Decay processes on fs to ps time scales are obscured by energy exchange of the emitted electrons with the environment.

Figure 1

Experimental scheme for the NIR-NIR pump-probe measurement. See text for details.

Figure 2

(a) 2D momentum map of ${\mathrm{O}}_2$ molecules at an NIR intensity of $9\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ and (b) the corresponding angle-integrated photoelectron spectrum. (c) 2D momentum map of ${\mathrm{O}}_2$ clusters with an average size of $⟨N⟩=2400$ molecules at the same NIR intensity. (d) Peaks in the angle-integrated kinetic energy spectrum are attributed to autoionization processes of superexcited oxygen atoms formed during the cluster expansion. The $2s^{2}2p^3({}^2{P}^o)3s({}^3{P}^o)$ state and the first states of the $2s^{2}2p^3({}^2{D^o}_{3/2})nd({}^3{P}^o)$ series are marked.

Figure 3

(a) Electron kinetic energy spectra from ${\mathrm{O}}_2$ clusters taken at different time delays between two NIR pulses with intensities of $9\times {10}^{13}$ and $5\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. The pump and probe pulses have a duration of 32 fs and 1 ps, respectively. The spectra show the emission in perpendicular direction to the laser polarization and were normalized with respect to the signal obtained in the range between 4 and 5 eV, where no influence from the probe pulse is expected. (b) Difference spectra of the spectra in (a), where the signal from the pump pulse only was subtracted from the signal obtained in the $\text{pump}+\text{probe}$ experiment. The negative signal at the $A2$ and $A5$ peaks is attributed to the suppression of autoionization by the probe pulse. The inset shows the autoionization signal integrated between 1.65 and 1.85 eV for different time delays. From the exponential fit, an autoionization lifetime of about 1 ns is obtained.

Figure 4

Electron kinetic energy spectra from ${\mathrm{O}}_2$ clusters at different NIR intensities of (a) $7\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$, (b) $9\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$, (c) $1.4\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$, and (d) $2.3\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. The arrow in (a) marks the low-kinetic energy distribution attributed to frustrated recombination. The insets show the corresponding ion TOF spectra for each intensity.