Momentum transfers in correlation-assisted tunneling

Abstract

We consider correlation-assisted tunnel ionization of a small molecule by an intense low-frequency laser pulse. In this mechanism, the departing electron excites the state of the ion via a Coulomb interaction. We show that the wavepackets emerging from this process can have nontrivial spatial structure and give a measurable indicator of correlated multielectron dynamics during the tunnelling step. We also show that the saddle-point approximation requires special attention in this geometric analysis.The strong correlations and interactions of electrons in close proximity are core concepts in atomic, molecular and solid-state physics. For example, in photoionization they feature in mechanisms like auto-ionization [7,8], and many others. These correlationdriven mechanisms can leave clear traces that identify them, such as the Fano line-shapes in autoionization, but the distinction between different mechanisms can also be blurry, as in the case of separating contributions of shakeup and post-ionization interaction (see e.g. Ref. 9).In contrast to one-photon ionization, analyses of strongfield ionization, often viewed as optical tunnelling, have been dominated by the single active electron approximation. The inclusion of multi-electron effects beyond selfconsistent field corrections [10] was triggered by the realization that molecular ions produced in strong laser fields are often electronically excited [11,12], and that these excitations affect all subsequent processes [13][14][15][16][17]. Recent ab initio simulations [18,19] and experiments [20,21] confirm that in molecules electronic excitations during the ionization process are a rule rather than an exception.Two main mechanisms are responsible for creating an ion in an excited electronic state after optical tunnelling. First, the laser pulse may remove electron from a low-lying orbital, leaving the ionic core excited [11,[13][14][15][16][17][22][23][24][25][26][27] (shown schematically in Fig. 1(a)). Alternatively, the electron may depart from the highest occupied molecular orbital (HOMO) and subsequently excite the core through a Coulomb interaction (shown in Figs. 1(b,c)). This can happen either inside the tunnelling barrier [28], shown in (b), or after the tunnelling step [11,12], shown in (c). We refer to both (b) and (c) as correlation-assisted tunnelling. A more formal description of these processes has recently been developed [29,30], which applies an analytical version of the R-matrix approach [31] to strong field ionization. It appears that correlation-inducing interactions (as opposed to mean-field interactions such as those studied in Ref. 10) are strong enough to influence and even dominate the ionization process. However, the calculations in Refs. [29,30] only present total ionization rates, and these do not readily yield direct, qualitative traces of the interactions that shape the tunnelling process.This work looks for such traces in the angular distribution of the photoelectron. We show that correlation-assisted tunnelling, as shown in Figs. 1(...

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