[Paper Review] Anisotropic dynamics of two-photon ionization: An attosecond movie of photoemission

Alice Autuori, Dominique Platzer+16 more|arXiv (Cornell University)|Jul 29, 2021

Laser-Matter Interactions and Applications54 references35 citations

TL;DR

The paper experimentally and numerically reconstructs the complete angular- and energy-resolved two-photon photoemission dynamics in helium, revealing anisotropic transition amplitudes and attosecond-like delays through resonant intermediate states.

ABSTRACT

Imaging in real time the complete dynamics of a process as fundamental as photoemission has long been out of reach due to the difficulty of combining attosecond temporal resolution with fine spectral and angular resolutions. Here, we achieve full decoding of the intricate angle-dependent dynamics of a photoemission process in helium, spectrally and anisotropically structured by twophoton transitions through intermediate bound states. Using spectrally- and angularly-resolved attosecond electron interferometry, we characterize the complex-valued transition probability amplitude towards the photoelectron quantum state. This allows reconstructing in space, time and energy the complete formation of the photoionized wavepacket.

Motivation & Objective

• Motivate the need for real-time, angle-resolved decoding of photoemission dynamics beyond energy-resolved treatments.
• Demonstrate spectrally and angularly resolved attosecond interferometry to access complex transition amplitudes M(E,θ).
• Decode the full formation of the photoionized wavepacket through resonant intermediate states in a two-photon process.
• Provide a calibration-free experimental path to extract M(E,θ) and compare with TDSE simulations.

Proposed method

• Use spectrally- and angularly-resolved attosecond electron interferometry combining high-order harmonic generation XUV with IR to create a two-photon (XUV+IR) transition.
• Measure the sideband I16(τ;E,θ) in a VMIS setup and perform a Fourier transform in delay τ to extract the complex two-photon transition amplitude M(E,θ).
• Decompose M(E,θ) into partial waves via M(E,θ)=M0(E)Y00(θ)+M2(E)Y20(θ) and relate Mℓ(E) to resonant 1snp intermediate states using a near-resonant two-photon model Eq. (3).
• Calibrate the amplitude and phase to obtain intrinsic transition amplitudes independent of the exciting pulses.
• Analyze the angular and spectral structure and compare experimental M(E,θ) with TDSE simulations for helium.
• Define transition delay τ_tran(E,θ)=∂arg M(E,θ)/∂E and interpret its sign and angular dependence in resonance and non-resonance regions.
• Perform a Gabor spectro-temporal analysis of M(E,θ) to visualize the build-up of the photoelectron wavepacket across energy and angle.

Experimental results

Research questions

• RQ1 What is the full angular and spectral structure of the two-photon transition amplitude M(E,θ) in resonant two-photon helium ionization?
• RQ2 How do intermediate bound-state resonances (1s3p, 1s4p) shape the modulus, phase, and delay of the emitted photoelectron across energy and emission angle?
• RQ3 Can spectrally and angularly resolved attosecond interferometry reveal the true photoemission delays and differentiate them from conventional atomic delays?
• RQ4 How well do TDSE simulations reproduce the measured anisotropic transition amplitudes and delays?

Key findings

• The experiment retrieves both the modulus and phase of M(E,θ) with high spectral resolution and angular sensitivity across ~0.8 eV of electron energy.
• The transition amplitudes show π-rad phase jumps and strong angular dependence tied to intermediate 1s3p and 1s4p resonances.
• Partial-wave analysis reveals distinct s- and d-wave contributions (ℓ=0,2) with energy-dependent cancellations and phase behavior.
• The measured angularly-resolved delays τ_tran(E,θ) are strongly resonance-biased (positive) and highly angle-dependent in resonance regions, and negative and highly angle-dependent between resonances.
• Gabor analysis of M(E,θ) highlights how spectral phase jumps drive destructive interferences shaping the temporal build-up of the wavepacket.
• TDSE simulations reproduce the main experimental features and confirm the interpretations of the angularly-resolved dynamics.