Temporal coherence effects in multiple ionization of $N_2$ via XUV pump-probe autocorrelation

Using a split-mirror stage combined with a reaction microscope, nonlinear autocorrelation traces of XUV pulses from the Free-electron LASer at Hamburg were recorded for ${\mathrm{N}}_2$ multiphoton-induced fragmentation into noncoincident ${\mathrm{N}}^{2+}$ and coincident ${\mathrm{N}}^{2+}+{\mathrm{N}}^{2+}$ channels. We find a pulse duration of $40\pm 10\hspace{0.5em}\mathrm{fs}$ along with a sharp spike pointing to a coherence time of $4\pm 1\hspace{0.5em}\mathrm{fs}$, almost twice as short as in previous observations. Both are well reproduced by a simulation based on the partial-coherence model that includes the molecular dynamics leading to an $~12-\mathrm{fs}$ substructure in the trace.

Figure 1

Schematic of the experimental setup with split-mirror stage and ion-detection part of the reaction microscope.

Figure 2

(a) Potential curves of ${\mathrm{N}}_2^{3+}({}^2{\Pi }_u)$ [16] and ${\mathrm{N}}_2^{4+}$ (pure $4/R$ Coulomb potential) together with the experimental KER distribution for ${\mathrm{N}}^{2+}+{\mathrm{N}}^{2+}$ production for pump-probe delay times $\tau <\pm 15\hspace{0.5em}\mathrm{fs}$. The vertical arrows indicate possible transitions at different $R$. Vertical dashed lines mark the Franck-Condon regime. (b) Intensity dependence of ion yields for the ${\mathrm{N}}^{2+}+{\mathrm{N}}^{+}$ and ${\mathrm{N}}^{2+}+{\mathrm{N}}^{2+}$ dissociation channels. The number $n$ of absorbed photons is extracted from exponential fits (lines).

Figure 3

(a) Experimental autocorrelation traces for uncorrelated ${\mathrm{N}}^{2+}$ fragments with $22<E_{\mathrm{KER}}<40\hspace{0.5em}\mathrm{eV}$ and restricted emission angles of ${45}^{°}<\theta <{135}^{°}$ with respect to the laser polarization axis. Solid cyan line: sum of two Gaussians with FWHM of 40 and 4 fs, respectively. (b) Same as in (a) but for coincident ${\mathrm{N}}^{2+}+{\mathrm{N}}^{2+}$ data integrated over all emission angles. Solid cyan line: Gaussian fit with 4-fs FWHM. (c) Same experimental data as in (b) but compared with PCM result (full green line) normalized to the same maximum value as the experiment.

Figure 4

Measured average frequency spectrum. Solid line: Gaussian fit. Dashed line: result of PCM.