Photoemission from atoms is assumed to occur instantly in response to incident radiation and provides the basis for setting the zero of time in clocking atomic-scale electron motion. We used attosecond metrology to reveal a delay of 21 +/- 5 attoseconds in the emission of electrons liberated from the 2p orbitals of neon atoms with respect to those released from the 2s orbital by the same 100-electron volt light pulse. Small differences in the timing of photoemission from different quantum states provide a probe for modeling many-electron dynamics. Theoretical models refined with the help of attosecond timing metrology may provide insight into electron correlations and allow the setting of the zero of time in atomic-scale chronoscopy with a precision of a few attoseconds.

The detailed time dependence of the decay of a three-electron autoionizing state close to threshold has been obtained ab initio by solving the time-dependent Schrödinger equation (TDSE). The theory allows the definition and computation of energy-dependent matrix elements in terms of the appropriate N-electron wavefunctions, representing the localized initial state, 0 , the stationary scattering states of the continuous spectrum, U(ε), and the localized excited states, n , of the effective Hamiltonian QH Q, where Q ≡ | 0 0 |. The time-dependent wavefunction is expanded over these states and the resulting coupled equations with timedependent coefficients (in the thousands) are solved to all orders by a Taylor series expansion technique. Convergence is checked as a function of the number of the numerically obtained U(ε) that span the continuous spectrum of the free electron. The robustness of the method was verified by using a model interaction in analytic form and comparing the results from two different methods for integrating the TDSE (appendix B). For the physically relevant application, the chosen state was the He − 1s2p 2 4 P shape resonance, about which very accurate theoretical and experimental relevant information exists. Calculations using accurate wavefunctions and an energy grid of 20.000 points in the range 0.0-21.77 eV show that the effective interaction depends on energy in a state-specific manner, thereby leading to state-specific characteristics of non-exponential decay (NED). For the established energy position of 0.01 eV, the results show an exponential decay over about 6 × 10 4 au of time, from which a width of = 5.2 meV and a lifetime of 1.26 × 10 −13 s is deduced. The experimentally obtained width is 7.16 meV (Walter, Seifert and Peterson 1994 Phys. Rev. A 50 664). After 12 lifetimes (about 1400 fs), at which time the survival probability is 10 −6 , NED sets in. On the other hand, due to the shape of the interaction, the NED appears at earlier times if the energy position happened to be slightly larger. For example, if E were at 0.019 eV, NED would start after nine exponential lifetimes. These facts suggest that either in this state or in other autoionizing states close to threshold, NED may have sufficient presence to make the violation of the law of exponential decay observable.

The problem of integrating the time-dependent Schrodinger equation (TDSE) describing the interaction of a polyelectronic atom with a laser pulse is treated by expanding the time-dependent wave function %(r, t) in terms of wave functions 4"E computed for discrete, autoionizing, and scattering states separately. The TDSE is transformed into a system of coupled first-order differential equations with time-dependent coeScients, whose number (in the thousands), necessary for convergence to be reliable, depends mainly on the degree of the contribution of the continuous spectrum, as a function of the frequency and strength of the field. This approach allows the systematic incorporation of the significant electronic structure, electron correlation, and spectral characteristics of each N-electron system under investigation. Furthermore, since the free-electron function is computed numerically in the polarized core potential of the remaining (N -1)-electron atom, properties such as the angular distribution and partial above-threshold ionization (ATI) of the photoelectron are directly computable. We present results from the application of our methods to H and Li . For the applications to H, which served as testing grounds for the method, the state-specific wave functions for discrete and continuum states were obtained numerically, for n and I up to 12 and 5, respectively, and for positive energies up to a= 34 eV with I up to 6. When comparisons with other time-independent and time-dependent results are possible, very good agreement is observed. On the other hand, our calculations do not confirm recent experimental results on absolute ionization rates for laser pulses of 248 nm. For Li, our results on ATI for photon energy tie=1.36 eV demonstrate the effects of initial-state electron correlation and of final-state fieldinduced coupling of open channels (the Li 1s 2s Sand 1s 2p P'), as a function of field intensity.

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We have solved the time-dependent Schrödinger equation describing the simultaneous interaction of the He 1s2s 1S state with two laser-generated pulses of trapezoidal or Gaussian shape, of duration 86 fs and of frequencies ω1 = 1.453 au and ω2 = 1.781 au. The system is excited to the energy region of two strongly correlated doubly excited states, chosen for this study according to specific criteria. It is demonstrated quantitatively that, provided one focuses on the dynamics occurring within the attosecond timescale, the corresponding orbital configurations, 2s2p and 2p3d 1Po, exist as nonstationary states, with occupation probabilities that are oscillating as the states decay exponentially into the 1sεp continuum, during and after the laser-atom interaction. It follows that it is feasible to probe by attosecond pulses the motion of configurations of electrons as they correlate via the total Hamiltonian. For the particular system studied here, the probe pulses could register the oscillating doubly excited configurations by de-exciting to the He 1s3d 1D state, which emits at 6680 Å.