Resolving the time when an electron exits a tunnelling barrier

Abstract

The tunnelling of a particle through a barrier is one of the most fundamental and ubiquitous quantum processes. When induced by an intense laser field, electron tunnelling from atoms and molecules initiates a broad range of phenomena such as the generation of attosecond pulses, laser-induced electron diffraction and holography. These processes evolve on the attosecond timescale (1 attosecond ≡ 1 as = 10(-18) seconds) and are well suited to the investigation of a general issue much debated since the early days of quantum mechanics--the link between the tunnelling of an electron through a barrier and its dynamics outside the barrier. Previous experiments have measured tunnelling rates with attosecond time resolution and tunnelling delay times. Here we study laser-induced tunnelling by using a weak probe field to steer the tunnelled electron in the lateral direction and then monitor the effect on the attosecond light bursts emitted when the liberated electron re-encounters the parent ion. We show that this approach allows us to measure the time at which the electron exits from the tunnelling barrier. We demonstrate the high sensitivity of the measurement by detecting subtle delays in ionization times from two orbitals of a carbon dioxide molecule. Measurement of the tunnelling process is essential for all attosecond experiments where strong-field ionization initiates ultrafast dynamics. Our approach provides a general tool for time-resolving multi-electron rearrangements in atoms and molecules--one of the key challenges in ultrafast science.

Axis-dependence of molecular high harmonic emission in three dimensions

“…Most molecules are asymmetric tops and even reactions with linear molecules generally proceed via an asymmetric-top transition state. Thus, an alignment decomposition into three dimensions is a solution that could be employed by much of the ultrafast molecular science community, enhancing or augmenting techniques such as time-resolved X-ray 13,14 and electron diffraction 15,16 , photoelectron spectroscopy 17,18 , as well as HHG 19,20 .…”

“…In the absence of external phase information, only the relative phase of the three coefficients is meaningful. The phase information may be particularly useful because it suggests the underlying HHG processes of field emission and recombination 20 .…”

Revealing quantum path details in high-field physics

“…The aim of the present work is to experimentally reveal the intricacies which are associated with the high-orderharmonic generation process occurring during the interaction of noble gases with intense infrared (IR) laser pulses. The tunneling of an electron through a suppressed atomic potential, followed by its motion in the continuum, is the fundamental mechanism underlying strong-field laser-atom interactions [16][17][18][19][20][21]. In the spirit of the "three-step model" [18,20,21], the IR field suppresses the atomic potential and allows the valence electron to tunnel through.…”

Numerical Detector Theory for the Longitudinal Momentum Distribution of the Electron in Strong Field Ionization

“…However, in recent years, the adiabatic tunneling theory has been challenged [9][10][11][12][13]. These advanced studies raise the question: if the tunneling process is nonadiabatic and significantly time-dependent, how do we describe an electron's momentum distribution near the tunneling exit point, especially its momentum component longitudinal to the laser field's major polarization axis?…”