Anthony F. Starace

Effect of elliptically polarized light on the angular distribution of photoelectrons

The angular distribution of photoelectrons predicted for elliptically polarized light is shown to be the same as that predicted for partially polarized light having incoherent perpendicular electric field components equal to the electric field components along the major and minor axes of the ellipse.

Attosecond pulse carrier-envelope phase effects on ionized electron momentum and energy distributions: roles of frequency, intensity and an additional IR pulse

The effects of the carrier-envelope phase (CEP) of a few-cycle attosecond pulse on ionized electron momentum and energy spectra are analyzed, both with and without an additional few-cycle IR pulse. In the absence of an IR pulse, the CEP-induced asymmetries in the ionized electron momentum distributions are shown to vary as the 3/2 power of the attosecond pulse intensity. These asymmetries are also found to satisfy an approximate scaling law involving the frequency and intensity of the attosecond pulse. In the presence of even a very weak IR pulse (having an intensity of the order of 10 11 -10 12 W cm 2 ), the attosecond pulse CEP-induced asymmetries in the ionized electron momentum distributions are found to be significantly augmented. In addition, for higher IR laser intensities, we observe for low electron energies peaks separated by the IR photon energy in one electron momentum direction along the laser polarization axis; in the opposite direction, we find structured peaks that are spaced by twice the IR photon energy. Possible physical mechanisms for such asymmetric, low-energy structures in the ionized electron momentum distribution are proposed. Our results are based on single- active-electron solutions of the three-dimensional, time-dependent Schrodinger equation including atomic potentials appropriate for the H and He atoms.

Entanglement evolution in the presence of decoherence

The entanglement of two qubits, each defined as an effective two-level, spin 1/2 system, is investigated for the case that the qubits interact via a Heisenberg XY interaction and are subject to decoherence due to population relaxation and thermal effects. For zero temperature, the time-dependent concurrence is studied analytically and numerically for some typical initial states, including a separable (unentangled) initial state. An analytical formula for non-zero steady state concurrence is found for any initial state, and optimal parameter values for maximizing steady state concurrence are given. The steady state concurrence is found analytically to remain non-zero for low, finite temperatures. We also identify the contributions of global and local coherence to the steady state entanglement.

Analytic formulae for high harmonic generation

Analytic formulae describing harmonic generation by a weakly bound electron are derived quantum mechanically in the tunnelling limit. The formulae confirm the classical three-step model and provide an analytic explanation for oscillatory structures on the harmonic generation plateau.

Interaction of laser radiation with a negative ion in the presence of a strong static electric field

This paper provides a general theoretical description of a weakly bound atomic system (a negative ion) interacting simultaneously with two (generally strong) fields, a static electric field and a monochromatic laser field having an arbitrary elliptical polarization. The zero-range δ-potential is used to model the interaction of a bound electron in a negative ion as well as the interaction of a detached electron with the residual atom. Our treatment combines the quasistationary (complex energy) and quasienergy (Floquet) approaches. This quasistationary, quasienergy state (QQES) formalism is the most appropriate one for analysing a decaying quantum system under the influence of a periodic external perturbation. Existing QQES theory is reviewed and some new results are discussed: the Hellmann-Feynman theorem and the normalization procedure for QQES, and the definition of the dipole moment and the dynamic polarizability for a decaying atomic system (in strong static electric and/or laser fields). These results are illustrated using analytical formulae obtained from an exact solution of the QQES problem for a δ-model potential in two strong fields. Finally, from the imaginary part of the dynamic polarizability we obtain analytic results (to first order in the laser-field intensity) for the photodetachment cross section. Our results are then compared with those of previous theoretical studies.

Multiphoton detachment of a negative ion by an elliptically polarized, monochromatic laser field

The quasistationary quasienergy state approach (QQES) is applied to the analysis of partial (n-photon) decay rates and angular distributions (ADs) of photoelectrons produced by an elliptically polarized laser field. The problem is formulated for a weakly bound electron with an energy E0 in the threedimensional δ-model potential (which approximates the short-range potential of a negative ion) interacting with a strong monochromatic laser field having an electric vector F (ωt) .T he resu lts presented cover weak (perturbative), strong (nonperturbative), and superstrong field regimes as well as a wide interval of frequencies ω extending from the tunnelling (¯ hω �| E0|) and multiphoton (¯ ω |E0|). For a weak laser field, exact equations for the normalization factor and for the Fourier coefficients of the QQES wavefunction at the origin (|r |→ 0) (that are key elements of the QQES approach for a δ-model potential) as well as for the detachment amplitudes are analysed analytically using both standard Rayleigh– Schr¨ odinger perturbation theory (PT) in the intensity, I ,o f the lase rfi eld and Brillouin–Wigner PT expansions involving the exact (complex) quasienergy � . The lowest-order perturbative results for the n-photon ADs are presented in analytic form, and the parametrization of ADs in terms of polarization- and angular-independent atomic parameters is discussed for the general case of elliptical polarization. The major emphasis is on the analysis of an ellipticity induced distortion of three-dimensional ADs and, especially, on the elliptic dichroism (ED) effect, i.e. the dependence of the photoelectron yield in a fixed direction n on the sign of the ellipticity (or on the helicity) of a laser field. The dominant role of binding potential effects for a correct description of ED and threshold effects is demonstrated, and the intimate relationship between atomic ED factors and scattering phases o ft hedetached electron is established for multiphoton detachment, including the above-threshold case.

Application of Coulomb wave function discrete variable representation to atomic systems in strong laser fields.

We present an efficient and accurate grid method for solving the time-dependent Schrodinger equation for an atomic system interacting with an intense laser pulse. Instead of the usual finite difference (FD) method, the radial coordinate is discretized using the discrete variable representation (DVR) constructed from Coulomb wave functions. For an accurate description of the ionization dynamics of atomic systems, the Coulomb wave function discrete variable representation (CWDVR) method needs three to ten times fewer grid points than the FD method. The resultant grid points of the CWDVR are distributed unevenly so that one has a finer grid near the origin and a coarser one at larger distances. The other important advantage of the CWDVR method is that it treats the Coulomb singularity accurately and gives a good representation of continuum wave functions. The time propagation of the wave function is implemented using the well-known Arnoldi method. As examples, the present method is applied to multiphoton ionization of both the H atom and the H(-) ion in intense laser fields. The short-time excitation and ionization dynamics of H by an abruptly introduced static electric field is also investigated. For a wide range of field parameters, ionization rates calculated using the present method are in excellent agreement with those from other accurate theoretical calculations.

Perturbation theory in a strong-interaction regime with application to 4d-subshell spectra of Ba and La

The authors investigate the behaviour of a bound electron-hole excitation in an energy region where the response of the system is highly resonant and shows strong collective behaviour. Formulae for calculating discrete energy levels and wavefunctions are presented for the case where the perturbation is strong enough to change the nodal structure of the perturbed wavefunctions relative to that of the unperturbed wavefunctions. This change in nodal structure requires a renaming of the associated atomic states. Neglect of such renaming is shown to have been at the root of a recent controversy between differing interpretations, based on alternative calculational procedures, of the 4d-subshell photoionisation spectra of Ba and La. It is demonstrated explicitly that these alternative calculational procedures are equivalent and that their interpretations are consistent.

Quasi-Landau spectrum of a hydrogen like atom in a high magnetic field

A two dimensional semiclassical approximation is used to examine the sigma polarization spectrum of a hydrogen like atom in a strong magnetic field. Resonances above the zero field ionization limit are found and their energy separations are computed over the region from the ionization limit to 40 h(cross) omega c above, where omega c is the cyclotron frequency, for magnetic fields of 10, 17, 25, 32, 40 and 47 kG. At the ionization limit the separation between resonances is found to be 1.500 h(cross) omega c, independent of magnetic field strength, which is in agreement with a separation of 1.5 h(cross) omega c observed experimentally by Garton and Tomkins (1969) in the sigma polarization spectrum of barium in a magnetic field of 24 kG. The energy separations are found to decrease above the ionization limit toward the value h(cross) omega c, which is characteristic of the levels of a free electron in a homogeneous magnetic field.

Enhanced asymmetry in few-cycle attosecond pulse ionization of He in the vicinity of autoionizing resonances

By solving the two-active-electron, time-dependent Schr¨ odinger equation in its full dimensionality, we investigate the carrier-envelope phase (CEP) dependence of single ionization of He to the He + (1s) state triggered by an intense few-cycle attosecond pulse with carrier frequency ! corresponding to the energy ¯! = 36eV. Effects of electron correlations are probed by comparing projections of the final state of the two-electron wave packet onto field-free highly correlated Jacobi matrix wave functions with projections onto uncorrelated Coulomb wave functions. Significant differences are found in the vicinity of autoionizing resonances. Owing to the broad bandwidths of our 115 and 230 as pulses and their high intensities (1-2PWcm 2 ), asymmetries are found in the differential probability for ionization of electrons parallel and antiparallel to the linear polarization axis of the laser pulse. These asymmetries stem from interference of the one- and two-photon ionization amplitudes for producing electrons with the same momentum along the linear polarization axis. Whereas these asymmetries generally decrease with increasing ionized electron