L.F. Dimauro, P. Agostini, in breakthroughs In Atomic, Molecular, and Optical Physics, 1995


Helium atoms (Fittinghoff et al.

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, 1992) exposed come an extreme pulse the 100–fs, 0.8–μm light exhibit a behavior qualitatively similar to the of xenon yield curves: a premature manufacturing of double ionized helium which is strong polarization dependent (Walker et al., 1993). The apparent question is, walk such comparable behavior imply a typical mechanism i m sorry in the instance of xenon does not lead to straight two-electron ionization. The major difference, as debated earlier, is that helium ionizes nonperturbatively (tunneling) when xenon is perturbative (multiphoton). This indicates that the initial problems determined at the time the electron is being supported to the continuous should result in an extremely different nonsequential dynamics. In fact, in the intensity regime provided in the experiment, helium has no bound excited states. Thus, the “resonant” scenario provided to define the Xe2 + nonsequential manufacturing becomes physically nonapplicable to helium. Fittinghoff et al. (1992) said that helium then should proceed via straight ionization entailing a mechanism analogous come the two-electron shake-off process observed through high-frequency photons (Wehlitz et al., 1991; Mittelman, 1966). In this scenario the suddenly loss the screening by the instantaneous remove of the very first electron results in part probability that diabatic two- electron ejection. For synchrotron studies using high-energy photons (keV) this problem is met by the high kinetic power imparted to the first electron and also produces ~ 3% dual ionization in helium. In the strong-field script the sudden loss that screening is detailed by the instantaneous displacement the the electron native the main point via tunneling. A calculation (Fittinghoff et al., 1992) making use of a kinetic summary resulted in great agreement with speculative results. However, the polarization dependence of the nonsequential procedure remained not a priori clean in a strong-field shake-off model (Fig. 17b).

A 2nd scenario (Corkum, 1993) based on the two-step model results in a clear intuitive explanation of the nonsequential production in helium as an e–2e scattering occasion (Fig. 17c). Because the electron’s rescattering after half a cycle is well supported by other evidence (plateaus and also rings in ATI), the is natural to envision the this scattering could liberate a 2nd electron. Making use of the well-known e–2e cross-sections, Corkum (1993) can fit the observed helium yields with great agreement. Likewise, the sharp dependence that the nonsequential price on the polarization is automatically interpreted in this model as the perpendicular drift movement of the electron. As mentioned above, in circular polarization the timeless motion has actually a continuous (ℰ/ω) transverse drift velocity which is independent the the initial phase. The drift prevents the electron from returning to the core and also therefore quenches the nonsequential process, continual with the experiments.

The presence of the two different models prompted more precise measurements of both the rates and their ellipticity dependence. The ion productivity curves (Walker et al., 1994) because that linearly polarized excitation are shown in Fig. 4; the result nonsequential (NS) He2 + manufacturing is clearly evident, saturating in ~ 0.8PW/cm2. This measurement was created using a kilohertz titanium-sapphire laser capable of developing 10PW/cm2. The laser’s high repetition rate made monitoring of the helium manufacturing over an extraordinary 12 order of magnitude in dynamic range. These data exceed previous measurements by 5 orders of size and carry out the an initial view right into the exact nature of the NS process. The relationship between the the + and the He2 + NS manufacturing was experimentally developed by systematically differing the confocal spot size of the laser focus with respect to the ion spectrometer’s detection image. Together the spot dimension exceeded the detection picture a reduction from the I3/2 gaussian scaling was observed over saturation. This completely spatial effect clearly demonstrates that the NS He2 + manufacturing is exactly attached to the saturation of the He+ yield and verifies a connection with the depletion of neutral helium atoms.

A sensitive measure of the NS dynamics is shown in Fig. 18 i m sorry plots the He2 + (NS)/He+ ratio as a role of intensity. The close up door circles are the experimentally measured ratio which at saturation is 0.0020(3). This worth is end 10 times smaller sized than the single photon ratio (Wehlitz et al., 1991). Back the ion curves in Fig. 4 show a solid intensity dependence, changing by 7 orders that magnitude between 0.15 and also 8.0PW/cm2, the ratio exhibits a tenderness slope, scaling as I1.3. The solid heat in Fig. 18 is the ratio in between the calculation AC-tunneling and the SAE (total) rates displayed in Fig. 4 normalized to the experimentally saturated value. The striking covenant implies the the NS two-electron procedure depends exclusively on tunneling, also when the multiphoton procedure dominates the full rate. Back this walk not define the mechanism responsible because that NS production, the does provide an essential insight right into the underlying physics.


Fig. 18. Intensity dependence of the He2 + (NS)/He2 + proportion for 0.78–μm excitation. Error bars show one typical deviation. The solid heat is calculated; view the text for details. The flower area is calculated using the two-step model; watch the message for details.

Finally, researches of the nonsequential manufacturing ellipticity dependence have been lugged out for neon (Dietrich et al., 1994) and helium (Walker et al., 1994). It is clear in Fig. 19 that this dependency is lot stronger for the nonsequential than for the sequential process. The dependence has actually no a priori explanation in the shake-off model. However, a quantitative to compare (Dietrich et al., 1994) through the forecast of the two-step version yields excellent commitment for the polarization dependence of the high harmonic and nonsequential returns for neon. However, the covenant with the absolute nonsequential yield in neon and helium is poor and also will be questioned in the following section.

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Fig. 19. Ellipticity dependency for (a) He+ and also (b) He2 + ion returns at various 0.78–μm intensities. The worths of 1,0, and also − 1 exchange mail to right, linear, and left circularly polarized light, respectively. The dotted lines exchange mail to the intensities at which nonsequential He2 + manufacturing dominates.