Attosecond Delays in Molecular Photoionization

We report measurements of energy-dependent photoionization delays between the two outermost valence shells of ${\mathrm{N}}_2\mathrm{O}$ and ${\mathrm{H}}_2\mathrm{O}$. The combination of single-shot signal referencing with the use of different metal foils to filter the attosecond pulse train enables us to extract delays from congested spectra. Remarkably large delays up to 160 as are observed in ${\mathrm{N}}_2\mathrm{O}$, whereas the delays in ${\mathrm{H}}_2\mathrm{O}$ are all smaller than 50 as in the photon-energy range of 20–40 eV. These results are interpreted by developing a theory of molecular photoionization delays. The long delays measured in ${\mathrm{N}}_2\mathrm{O}$ are shown to reflect the population of molecular shape resonances that trap the photoelectron for a duration of up to $\sim 110$ as. The unstructured continua of ${\mathrm{H}}_2\mathrm{O}$ result in much smaller delays at the same photon energies. Our experimental and theoretical methods make the study of molecular attosecond photoionization dynamics accessible.

Figure 1

(a) Photoelectron spectrum of ${\mathrm{N}}_2\mathrm{O}$ generated by an attosecond pulse train transmitted through a Sn filter (black line) and in the presence of the dressing IR field (orange line). Difference spectra, obtained by subtracting XUV only from $\mathrm{XUV}+\mathrm{IR}$ photoelectron spectra and vice versa, are shown in red and blue, respectively. (b) Difference spectrum as a function of the IR-XUV delay.

Figure 2

(a) Photoelectron spectrum of ${\mathrm{H}}_2\mathrm{O}$ and (b) difference spectrum as a function of the IR-XUV delay.

Figure 3

(a) Measured and calculated delays between photoelectrons leaving ${\mathrm{N}}_2{\mathrm{O}}^{+}$ in its ${\tilde{A}}^{+}$ or ${\tilde{X}}^{+}$ states, (b) same as (a) for ${\mathrm{H}}_2\mathrm{O}$, (c),(d) calculated molecular delays for the two species as defined by Eq. (8), (e) shape resonance of $\sigma$ symmetry in the photon-energy range of 25–30 eV associated with the ${\tilde{A}}^{+}$ state of ${\mathrm{N}}_2{\mathrm{O}}^{+}$. The lower surface shows the numerically calculated molecular potential containing electrostatic and exchange interactions. The upper surface shows the total potential, i.e., the sum of the molecular and centrifugal potentials for $\ell =5$. The wave functions of the bound orbital and the shape-resonant state are illustrated by isosurfaces with color-coded signs. The gray arrows represent tunneling of the photoelectron through the barrier.