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Abstract

A promising new method of attosecond spectroscopy, the attoclock, offers attosecond resolution without requiring attosecond laser pulses. However, it requires knowledge of the ionization time, opening up a long standing conceptual problem in physics (“how much time does tunnelling take?”). In this work, the time delay in the tunnel ionization process is considered. It is shown that a delay of the peak of the tunnelling wavefunction exists as a matter of principle, and is caused by the interference of transmitted and reflected components of the wavefunction. If sub-barrier reflections are neglected from the wavefunction, the delay in the peak vanishes and tunnelling is seen to be instantaneous. This is shown by considering a series of models of increasing complexity: the square barrier, an adiabatically tunnelling electron, and a wavefunction based on the Strong Field Approximation. This work has implications on the interpretation of attoclock experiments: treatments based on instantaneous tunnelling should be adjusted in order to achieve appropriate calibration of the attoclock.