[Paper Review] Attosecond angular streaking and tunnelling time in atomic hydrogen

U. Satya Sainadh, Han Xu+10 more|arXiv (Cornell University)|Jul 18, 2017

Laser-Matter Interactions and Applications14 citations

TL;DR

This study performs the first attosecond angular streaking experiment on atomic hydrogen, using precise 3D-TDSE simulations with experimental laser parameters. It finds excellent agreement between measured offset angles and simulations, concluding that tunnelling in hydrogen is instantaneous (upper bound: 1.8 as), with the observed angular offset arising solely from Coulomb scattering, not tunnelling time delay.

ABSTRACT

Tunnelling, one of the key features of quantum mechanics, ignited an ongoing debate about the value, meaning and interpretation of 'tunnelling time'. Until recently the debate was purely theoretical, with the process considered to be instantaneous for all practical purposes. This changed with the development of ultrafast lasers and in particular, the 'attoclock' technique that is used to probe the attosecond dynamics of electrons. Although the initial attoclock measurements hinted at instantaneous tunnelling, later experiments contradicted those findings, claiming to have measured finite tunnelling times. In each case these measurements were performed with multi-electron atoms. Atomic hydrogen (H), the simplest atomic system with a single electron, can be 'exactly' (subject only to numerical limitations) modelled using numerical solutions of the 3D-TDSE with measured experimental parameters and acts as a convenient benchmark for both accurate experimental measurements and calculations. Here we report the first attoclock experiment performed on H and find that our experimentally determined offset angles are in excellent agreement with accurate 3D-TDSE simulations performed using our experimental pulse parameters. The same simulations with a short-range Yukawa potential result in zero offset angles for all intensities. We conclude that the offset angle measured in the attoclock experiments originates entirely from electron scattering by the long-range Coulomb potential with no contribution from tunnelling time delay. That conclusion is supported by empirical observation that the electron offset angles follow closely the simple formula for the deflection angle of electrons undergoing classical Rutherford scattering by the Coulomb potential. Thus we confirm that, in H, tunnelling is instantaneous (with an upperbound of 1.8 as) within our experimental and numerical uncertainty.

Motivation & Objective

• To resolve the long-standing debate on tunnelling time in quantum mechanics using a benchmark system.
• To perform the first attosecond angular streaking experiment on atomic hydrogen, the simplest single-electron atom.
• To test whether tunnelling time delays contribute to observed electron angular offsets in attoclock measurements.
• To validate theoretical models by comparing experimental data with accurate 3D-TDSE simulations using measured laser parameters.
• To determine the origin of electron offset angles in attoclock experiments—tunnelling delay or Coulomb scattering.

Proposed method

• Conducted attosecond angular streaking experiments on atomic hydrogen using few-cycle laser pulses.
• Performed 3D-TDSE simulations with experimentally measured laser parameters (intensity, polarization, duration).
• Used a long-range Coulomb potential to model electron dynamics in hydrogen.
• Replaced the Coulomb potential with a short-range Yukawa potential in simulations to isolate the role of long-range interactions.
• Compared experimental offset angles with simulations to assess agreement and infer the origin of angular shifts.
• Applied classical Rutherford scattering formula to predict deflection angles for comparison with experimental and simulated data.

Experimental results

Research questions

• RQ1 Does tunnelling in atomic hydrogen exhibit a measurable time delay, or is it instantaneous?
• RQ2 What is the origin of the electron offset angle observed in attoclock experiments—tunnelling time delay or Coulomb scattering?
• RQ3 Can 3D-TDSE simulations with experimental laser parameters accurately reproduce experimental attoclock data for hydrogen?
• RQ4 How does the inclusion of a short-range potential (Yukawa) affect the predicted offset angles compared to the Coulomb potential?
• RQ5 To what extent do classical Rutherford scattering predictions match the observed electron deflection angles in the experiment?

Key findings

• Experimentally measured offset angles in atomic hydrogen show excellent agreement with 3D-TDSE simulations using the Coulomb potential.
• Simulations with a short-range Yukawa potential predict zero offset angles at all intensities, indicating that long-range Coulomb interaction is essential for the observed angular shift.
• The measured offset angles closely follow the classical Rutherford scattering formula, confirming that Coulomb scattering is the dominant origin of the angular offset.
• The data rule out any significant contribution from tunnelling time delay in hydrogen, with an upper bound of 1.8 as for any such delay.
• The results support the conclusion that tunnelling in atomic hydrogen is effectively instantaneous within experimental and numerical uncertainty.