Time-Resolved Measurement of Interatomic Coulombic Decay Induced by Two-Photon Double Excitation of Ne2

Abstract

The hitherto unexplored two-photon doubly-excited states [Ne * (2p −1 3s)]2 were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD) which predominantly produces singly-ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne + 2 ions was recorded as a function of delay between the XUV pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly-excited states, 390 (-130/+450) fs, and of the short-lived ones, less than 150 fs, are in good agreement with ab initio quantum mechanical calculations.

Application of Matched-Filter Concepts to Unbiased Selection of Data in Pump-Probe Experiments with Free Electron Lasers

“…[5] to associate the position of the maximum of the cross-correlation function to the shift of the peak being sought. In our work [1] it was not necessary to include a shift; while Equation (5) could be used to estimate the signal amplitude α, it is preferable to use the autocorrelation instead:…”

“…S is the quantity reported in our work ( [1] note for the sake of exactness that in Figure 2a therein, the unity baseline was not subtracted); we want to determine which scans to include in the average, and their weights w k . From Equation (6) and Figure 1b we can estimate the signal-to-noise ratio as:…”

“…We show in Appendix A that the weight which maximizes the SNR of S is w k = α k /σ 2 k = SNR k /σ k ; in Ref. [1] we used w k = SNR k , which gives a slightly worse result, although the difference is not significant. We presume that the latter fact depends on the limited number of samples, the predominance of few of them (see Figure 2a), and the low dispersion of the σ k .…”

“…The integer p in Equation (9) is the normalization order; for the data presented in Ref. [1] we chose p = 0, i.e., no normalization. Let us also note that the intensity of the UV pulse was kept constant and is very stable, so it does not appear in the data processing.…”

“…Recently we described the results of a pump-probe experiment, in which the lifetimes of doubly excited states of neon dimers were measured [1]. The dimers were excited by absorption of two EUV photons from the Free Electron Laser (FEL) FERMI-1 [2] and probed via ionization by a UV laser pulse.…”