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Register a new userExperimental Evidence for Selection Rules in Multiphoton Double Ionization of Helium
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K. Henrichs
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We report on the observation of phase space modulations in the correlated electron emission after strong field double ionization of helium using laser pulses with a wavelength of 394~nm and an intensity of $3cdot10^{14}$W/cm$^2$. Those modulations are identified as direct results of quantum mechanical selection rules predicted by many theoretical calculations. They only occur for an odd number of absorbed photons. By that we attribute this effect to the parity of the continuum wave function.
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We report on a kinematically complete experiment on strong field double ionization of helium using laser pulses with a wavelength of 394,nm and intensities of $3.5-5.7times10^{14},W/cm^2$. Our experiment reaches the most complete level of detail which previously has only been reached for single photon double ionization. We give an overview over the observables on many levels of integration, starting from the ratio of double to single ionization, the individual electron and ion momentum distributions over joint momentum and energy distributions to fully differential cross sections showing the correlated angular momentum distributions. Within the studied intensity range the ratio of double to single ionization changes from $2times 10^{-4}$ to $1.5times 10^{-3}$. We find the momentum distributions of the $rm{He}^{2+}$ ions and the correlated two electron momentum distributions to vary substantially. Only at the highest intensity both electrons are emitted to the same direction while at the lowest intensity back-to-back emission dominates. The joint energy distribution of the electrons shows discrete structures from the energy quantization of the photon field which allows us to count the number of absorbed photons and thus access the parity of the final state. We find the energy of the individual electron to show a peak structure indicating a quantized sharing of the overall energy absorbed from the field. The joint angular momentum distributions of the two electrons show a highly directed emission of both electrons along the polarization axis as well as clear imprints of electron repulsion. They strongly change with the energy sharing between the electrons. The aspect of selection rules in double ionization which are also visible in the presented dataset has been subject to a preceding publication [1].
We present accurate time-dependent ab initio calculations on fully differential and total integrated (generalized) cross sections for the nonsequential two-photon double ionization of helium at photon energies from 40 to 54 eV. Our computational method is based on the solution of the time-dependent Schroedinger equation and subsequent projection of the wave function onto Coulomb waves. We compare our results with other recent calculations and discuss the emerging similarities and differences. We investigate the role of electronic correlation in the representation of the two-electron continuum states, which are used to extract the ionization yields from the fully correlated final wave function. In addition, we study the influence of the pulse length and shape on the cross sections in time-dependent calculations and address convergence issues.
Multiphoton ionization of helium is investigated in the superintense field regime, with particular emphasis on the role of the electron-electron interaction in the ionization and stabilization dynamics. To accomplish this, we solve ab initio the time-dependent Schrodinger equation with the full electron-electron interaction included. By comparing the ionization yields obtained from the full calculations with corresponding results of an independent-electron model, we come to the somewhat counterintuitive conclusion that the single-particle picture breaks down at superstrong field strengths. We explain this finding from the perspective of the so-called Kramers-Henneberger frame, the reference frame of a free (classical) electron moving in the field. The breakdown is tied to the fact that shake-up and shake-off processes cannot be properly accounted for in commonly used independent-electron models. In addition, we see evidence of a change from the multiphoton to the shake-off ionization regime in the energy distributions of the electrons. From the angular distribution it is apparent that correlation is an important factor even in this regime.
We analyze two-photon double ionization of helium in both the nonsequential and sequential regime. We show that the energy spacing between the two emitted electrons provides the key parameter that controls both the energy and the angular distribution and reveals the universal features present in both the nonsequential and sequential regime. This universality, i.e., independence of photon energy, is a manifestation of the continuity across the threshold for sequential double ionization. For all photon energies, the energy distribution can be described by a universal shape function that contains only the spectral and temporal information entering second-order time-dependent perturbation theory. Angular correlations and distributions are found to be more sensitive to the photon energy. In particular, shake-up interferences have a large effect on the angular distribution. Energy spectra, angular distributions parameterized by the anisotropy parameters, and total cross sections presented in this paper are obtained by fully correlated time-dependent ab initio calculations.
We investigate the role of electron correlation in the two-photon double ionization of helium for ultrashort XUV pulses with durations ranging from a hundred attoseconds to a few femtoseconds. We perform time-dependent ab initio calculations for pulses with mean frequencies in the so-called sequential regime (photon energy above 54.4 eV). Electron correlation induced by the time correlation between emission events manifests itself in the angular distribution of the ejected electrons, which strongly depends on the energy sharing between them. We show that for ultrashort pulses two-photon double ionization probabilities scale non-uniformly with pulse duration depending on the energy sharing between the electrons. Most interestingly we find evidence for an interference between direct (nonsequential) and indirect (sequential) double photo-ionization with intermediate shake-up states, the strength of which is controlled by the pulse duration. This observation may provide a route toward measuring the pulse duration of FEL pulses.
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