Double photoelectron momentum spectra of helium at infrared wavelength

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Double photoelectron momentum spectra of the helium atom are calculated ab initio at extreme ultraviolet and near-infrared wavelengths. At short wavelengths two-photon double-ionization yields, two-electron energy spectra, and triply differential cross sections agree with results from recent literature. At the near-infrared wavelength of 7800.28em0exnm the experimental single-to-double-ionization ratio is reproduced up to intensities of 4×10¹⁴0.28em0exW/cm², and two-electron energy spectra and joint angular distributions are presented. The time-dependent surface flux approach is extended to full $3+3$ spatial dimensions and systematic error control is demonstrated. We analyze our differential spectra in terms of an experimentally accessible quantitative measure of correlation.