Precise Access to the Molecular-Frame Complex Recombination Dipole through High-Harmonic Spectroscopy

We report on spectral intensity and group delay measurements of the highest-occupied molecular-orbital (HOMO) recombination dipole moment of ${\mathrm{N}}_2$ in the molecular-frame using high harmonic spectroscopy. We take advantage of the long-wavelength $1.3\mu \mathrm{m}$ driving laser to isolate the HOMO in the near threshold region, 19–67 eV. The precision of our group delay measurements reveals previously unseen angle-resolved spectral features associated with autoionizing resonances, and allows quantitative comparison with cutting-edge correlated 8-channel photoionization dipole moment calculations.

Figure 1

HHG spectral intensity at 32 eV from ${\mathrm{N}}_2$ as a function of the pump-probe delay. The inset shows the nested interferometer which creates the four-kicker pulse train used to populate a coherent rotational wave packet in the molecular ensemble. Each arrow indicates the delay of one kick. At 0 delay, the huge enhancement is due to the pump-probe temporal overlap. At 8.4 ps delay (respectively 16.8 ps), the trace was recorded in the absence of the second (third) kick.

Figure 2

${\mathrm{N}}_2$ harmonic spectral intensity in the cutoff region. Alignment angles are shown from 0° (light blue) to 90° (light red). Each harmonic order is integrated for clarity.

Figure 3

Complex dipole of ${\mathrm{N}}_2$. Harmonic spectral intensity normalized to the isotropic yield for different alignment angles from 0° (light blue) to 90° (light red) as measured experimentally (a) and as predicted by theory using three- (c) and eight- (e) channel angle-convolved photoionization dipole moments. Dipole group delay as measured experimentally (b) and as predicted by theory using three (d) and eight (f) channels. In (b) only the largest and smallest error bars are shown and offset for clarity. The standard error of the experimental group delay is estimated from the fit to the RABBITT sideband oscillations.

Figure 4

Complex dipole of ${\mathrm{N}}_2$ along the internuclear axis. Direct comparison between experiment and the three- and eight-channel model of the dipole normalized intensity (a) and group delay (b) for the ensemble of ${\mathrm{N}}_2$ aligned at 0°.