By integrating a narrow slice of the kinetic energy region, a 9d level is also revealed. The beat frequencies reveal these states as the 6-8d Rydberg energy levels. These beat frequencies clearly identify states that are shifted into resonance and populated coherently in the strong field excitation step. In this case the temporal dependence of the peaks is readily determined after energy-slicing selected electron time-of-flight regions, which are denoted, for example, in sections a-d in the bottom panel of Figure 1A. The photoelectron spectra become more complicated as more energy regions are shifted into resonance. This is reflected in the time-dependent dynamics arising from excitation, for example, with 51 TW/cm 2 pulses. The inset shows the π-phase shift of the dynamics.Īs noted, the excitation with higher intensity pulses produces even more significant level shifts. Time-integrated photoelectron signal of the n = 8 and 9 features. Only the energy slices belonging to the n = 8 and 9 states, that reflect the vibrational wave packets, are highlighted. Electron kinetic energy spectrum truncated to show only low kinetic energy electrons. REMPI scheme employed to generate and detect vibrational wave packets generated in the 4d-Rydberg state. This shows that even at the more modest photon densities, typical for three-photon excitation, the state levels can be perturbed quite significantly during the laser pulse.įigure 3A. The 8d origin is located approximately 600 cm −1 lower than the 9d state. Rather they track well with what would be expected from the lower 8d band. The beat frequencies determined from the 9.3 TW/cm 2 data are not consistent with those anticipated for the 9d Rydberg state. It is also clear that different wave packet states are created with stronger excitation fields. The beat frequencies recovered from excitation pulse energies of 9.3 and 18 TW/cm 2 are shown in Figure 1C. By applying a Flanning window and taking the Fourier transform, the beat frequencies of the states involved are extracted. Oscillations in the temporal profile are clearly evident, indicating the presence of wave packet recurrences. The pump-probe delay is extended to longer than 100 ps however the delay is only shown out to 12 ps. The temporal dependence of the intermediate state reached with pump powers of 9.3 TW/cm 2 following ionization to the 2P 3/2 threshold is shown in Figure IB. Experiments employing mass detection alone would not reveal the effect of the ac-Stark shift, which is clear with electron kinetic energy resolution. It is important to note that these intermediate states are only discerned when the REMPI method is combined with photoelectron kinetic energy analysis. Thus coherent superpositions are formed during the strong transient Stark shifting of the energy levels during the high field of the pump pulse, and these superpositions remain intact after the pump pulse is over. The temporal analysis of the quantum beat recurrences shows that these intermediate resonances are from lower-lying states that are shifted by as much as 3000 cm −1 to lower energy. This is shown directly in going from the top panel where only one peak is observed for ionization to the 2P 3/2 core of the ion to the bottom panel where three peaks appear. As is evident in Figure 1A, with increasing pump power an increasing number of states are accessed by the pump pulse. Many of the individual states of interest in this study display a preferential kinetic energy in the 2P 3/2 component, and the attention is mainly focused there. This simplifies the peak assignment process as has been discussed elsewhere. The 2P 1/2 and 2P 3/2 spin-orbit components of the krypton ion state are separated by 5370 cm −1. The photoelectron spectra shown in Figure 1A have been truncated to include only the electron energy range corresponding to the pump-probe REMPI response when the probe pulse is delayed from the pump by more than 1 ps. The photoelectron time-of-flight spectra presented in Figure 1A shows the influence of increasing the pump photon density from 9.3 to 51 TW/cm 2. Photoelectron spectra are collected with pump laser powers spanning 9.3-65 TW/cm 2. By varying the pump laser power, while maintaining a low probe pulse power, the extent to which ac-Stark shifting may be used to coherently excite otherwise non-resonant Rydberg levels is demonstrated. In this scheme absorption of three 270 nm photons results in a resonance with the 9d Rydberg level, which converges to the 3/2 spin-orbit pair of the ion core. A two color (3+1′) pump-probe REMPI scheme is combined with photoelectron spectroscopy to study Rydberg wave packets created in krypton atoms.
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