Measuring autoionization decay lifetimes of optically forbidden inner valence excited states in neon atoms with attosecond noncollinear four-wave-mixing spectroscopy

Abstract

Attosecond noncollinear four-wave-mixing spectroscopy with one attosecond extreme ultraviolet (XUV) pulse and two few-cycle near-infrared (NIR) pulses was used to measure the autoionization decay lifetimes of inner valence electronic excitations in neon atoms. After a 43--48-eV XUV photon excites a $2s$ electron into the $2s2{p}^{6}\phantom{\rule{0.16em}{0ex}}(np)$ Rydberg series, broadband NIR pulses couple the $2s2{p}^{6}3p$ XUV-bright state to neighboring $2s2{p}^{6}3s$ and $2s2{p}^{6}3d$ XUV-dark states. Controllable delays of one or both NIR pulses with respect to the attosecond XUV pulse reveal the temporal evolution of either the dark or bright states, respectively. Experimental lifetimes for the $3s, 3p$, and $3d$ states are measured to be $7\ifmmode\pm\else\textpm\fi{}2, 48\ifmmode\pm\else\textpm\fi{}8$, and $427\ifmmode\pm\else\textpm\fi{}40$ fs, respectively, with 95% confidence. Accompanying calculations with two independent ab initio theoretical methods, newstock and astra, verify the findings. The results support the expected trend that the autoionization lifetime should be longer for states that have a smaller penetration in the radial region of the $2s$ core hole, which in this case is for the higher angular momentum Rydberg orbitals. The underlying theory thus links the lifetime results to electron correlation and provides an assessment of the direct and exchange terms in the autoionization process.