Figure 3: Ion sum- and relative-momentum distributions of Ar⁺+Xe⁺ breakup.

(a,c) Ion sum-momentum distributions. These reflect the recoil of two sequentially emitted electrons. The red squares and blue circles are events where Ar⁺ fly in the +y and –y directions, and the grey squares are the sum of them. To guide the eye, four possible sum momenta of the sequentially ionized electrons are indicated by the pink bars in (a) under the grey curve. (b,d) Ion relative-momentum distributions. These reflect the ArXe orientation. Only the events when the instantaneous field vector (violet arrow) at the release of the second electron pointing along (b) +y, or (d) –y, are selected by choosing pz_sum>1.0 a.u. (see text for details). In agreement with the intuitive picture of the enhanced ionization, the second electron from ArXe is favoured to be freed when the instantaneous laser field is pointing from Xe⁺ towards the Ar atom. As sketched by the inset red ellipses at the top-right corners of (a) and (c), the sense of rotation of the electric field vector of our laser pulse is adjusted to be (a,b) clockwise, and (c,d) anticlockwise, respectively. The unit for the colour scale is counts.