High-harmonic spectroscopy of transient two-center interference calculated with time-dependent density-functional theory

Abstract

We demonstrate high-harmonic spectroscopy in many-electron molecules using time-dependent density-functional theory. We show that a weak attosecond-pulse-train ionization seed that is properly synchronized with the strong driving mid-infrared laser field can produce experimentally relevant high-harmonic generation (HHG) signals, from which we extract both the spectral amplitude and the target-specific phase (group delay). We also show that further processing of the HHG signal can be used to achieve molecular-frame resolution, i.e., to resolve the contributions from rescattering on different sides of an oriented molecule. In this framework, we investigate transient two-center interference in CO 2 and OCS, and how subcycle polarization effects shape the oriented/aligned angle-resolved spectra.

Orbital-resolved calculations of two-center interferences in linear triatomic molecules

“…To drive HHG, we use a linearly-polarized MIR laser field with an envelope which has a two-cycle sin 2 ramp-up and then a constant amplitude. For some of the calculations shown below, we add a weak APT ionization seed to select the short trajectory contribution to the spectrum [23,24]. The APT used here is comprised of odd harmonics, from the 9 th to the 17 th orders, and is synchronized such that its peak is 0.06 cycles after the peak of the MIR.…”

“…We systematically match the APT and the MIR polarizations, and scale the APT intensity to 2% of that of the MIR. Note that orbital-resolving HHG spectra allows us to somewhat relax the constraints on the ionization-seed energy components discussed in [23] and potentially have more than just the HOMO being near-ionized by the APT. In all TDDFT simulations we use OCTOPUS' approximated enforced time-reversal symmetry (AETRS) propagation scheme with a time step of 0.05 a.u.…”

“…In experiments, this can be achieved using the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) technique [3,16,[19][20][21]. In ab initio calculations, however, extracting phase information is complicated by the interference from multiple quantum-path contributions that tend to wash out the target-specific phase information [21][22][23], and which is not in general practical to remove through full multi-scale calculations including phase matching effects. In several previous experimenttheory collaborations [16,23], we have shown that seed-ing the ionization step in the HHG process by an attosecond pulse train (APT), timed so as to preferentially select the short-quantum-path contribution to the harmonic yield [24], vastly improves the ability of ab initio calculations to extract target-specific information from the spectral amplitude.…”

“…In ab initio calculations, however, extracting phase information is complicated by the interference from multiple quantum-path contributions that tend to wash out the target-specific phase information [21][22][23], and which is not in general practical to remove through full multi-scale calculations including phase matching effects. In several previous experimenttheory collaborations [16,23], we have shown that seed-ing the ionization step in the HHG process by an attosecond pulse train (APT), timed so as to preferentially select the short-quantum-path contribution to the harmonic yield [24], vastly improves the ability of ab initio calculations to extract target-specific information from the spectral amplitude. However, a drawback of substituting the mid-infrared (MIR) driven tunnel ionization step with APT-driven near-one-photon ionization is that orbitals below the HOMO are much more likely to contribute to the harmonic yield than is experimentally realistic, and this again complicates the extraction of spectral phase information [23].…”

“…In several previous experimenttheory collaborations [16,23], we have shown that seed-ing the ionization step in the HHG process by an attosecond pulse train (APT), timed so as to preferentially select the short-quantum-path contribution to the harmonic yield [24], vastly improves the ability of ab initio calculations to extract target-specific information from the spectral amplitude. However, a drawback of substituting the mid-infrared (MIR) driven tunnel ionization step with APT-driven near-one-photon ionization is that orbitals below the HOMO are much more likely to contribute to the harmonic yield than is experimentally realistic, and this again complicates the extraction of spectral phase information [23]. Furthermore, several studies have indicated that multiple orbitals are likely to contribute to the harmonic spectrum for driving wavelengths in the near-infrared as opposed to the MIR [25][26][27].…”