S Cohen

Energy dependence of photoelectron angular distributions from two- and four-photon ionization of Mg in the vicinity of the 3p2 1S0 doubly excited state

We report on measurements concerning the variation of photoelectron angular distributions (PADs) from two- and four-photon ionization of ground state Mg, as the laser wavelength is scanned across the 3p2 1S0 resonance. The energy dependence of the asymmetry parameters determined from the PADs and corresponding to two-photon ionization is found to be in good agreement with prior theoretical predictions. For the four-photon PADs there are at present no theoretical results. Nevertheless, the resonant character is found considerably red-shifted in accordance with earlier experimental and theoretical studies of the single and double ionization yields. In either case, the PADs show a dominant s-wave on-resonance contribution, while the d-wave contribution apparently dominates under non-resonant excitation within the scanned wavelength range. The PADs offer detailed wavelength- and channel-dependent information on the dynamics of the autoionization resonances under pulsed laser excitation, corroborating and elucidating the conclusions reached from ionization yield studies.

Strong laser-induced coupling between autoionizing states: the case of the four-photon-excited 3p2 1S0 state of magnesium

We investigate the interaction of ground-state Mg atoms with tuneable laser radiation of ?5?ns duration and moderate intensity (?1012?W cm?2), capable of exciting the 3p2 1S0?autoionizing state by four photons. For the corresponding photon energy range, this level is strongly one-photon coupled both with bound (third photon) and, primarily, a multitude of other autoionizing states (fifth photon). The coupling is strong enough to induce the absorption of up to six photons before the atom is ionized, creating thus population into the first excited 3pj Mg+?levels which subsequently decay radiatively to the 3s1/2?ionic ground state. The earlier studies devoted to this excitation scheme were thorough but partial. Particularly, the issues concerning the five-photon excited levels were not addressed. In this work, the examined wavelength range is greatly extended to include these states. Moreover, the interaction is characterized in more detail by employing an array of experimental techniques, namely Mg+?ion and photoelectron spectroscopy (including photoelectron angular distributions (PADs) from four- and five-photon ionization) as well as fluorescence spectroscopy where the 3pj?3s1/2?decay is monitored. The experimental data show that most of these five-photon excited levels undergo blue ac Stark shifts which are occasionally nonlinear with respect to the laser intensity and comparable to or even larger in magnitude than the large redshift of the 3p2 1S0?state itself. Finally, for the latter state, the wavelength dependence of four-photon PAD measurements is suggestive of a situation reminiscent of an ac Stark splitting picture.

Construction of RKR-QDT atomic model potentials for the calculation of lithium polarizabilities and hyper-polarizabilities

Atomic local model potentials, reproducing experimental energy levels, are built by a combination of the Rydberg–Klein–Rees (RKR) inversion method with the quantum defect theory (QDT). An efficient iterative procedure leads to the production of spectrum-equivalent potentials containing a number of free parameters. These parameters can be adjusted in order to improve the accuracy of other physical properties, calculated via the constructed potentials. The high efficiency of the method is demonstrated by calculating the lifetimes of singly excited states as well as the dipole and quadrupole polarizability and hyper-polarizability of the ground 1s22s and first excited 1s22p state of Lithium. The obtained value γ = 3390 au for the ground state hyper-polarizability is in excellent agreement with other elaborate theoretical calculations. The scalar hyper-polarizability of 1s22p acquires a much higher value, γ0 = 1.002 × 107 au, while its tensor part is γ2 = −0.621 × 107 au.

Ionization of strontium atoms by ultrashort pulses of a Ti-Sapphire laser

We present experimental photoelectron spectra obtained by ionizing two-valence electron Sr atoms with ≈25 fs Ti- Sapphire laser pulses and we show preliminary evidence that electron-electron correlation effects dominate for laser intensities <0.4×1013 W-cm-2, while a single electron picture prevails when the laser intensity is increased further.

Phase sensitive coherent control of atomic excitation in the presence of static electric fields: a frame transformation Stark theory approach

Phase sensitive coherent control on atomic Rydberg states and in the presence of a static electric field is theoretically studied using Harmins frame transformation theory. It is shown that under the presence of the static field, the total excitation rate of an atomic target by a bichromatic laser field (consisting of a fundamental radiation frequency and of its second harmonic) depends on all the elements of the density-of-states matrix dictating the Stark effect. Some of these off-diagonal matrix elements are inaccessible by any other photoexcitation scheme and affect non-trivially the optimum achievable rate modulation contrast. This fact leads to important qualitative and quantitative differences with respect to static-field-free coherent control. The obtained formal expressions for the phase-controlled excitation rate are applied to the lithium target atom. It is demonstrated that the electric field strength can be used as an additional, externally adjustable, control and manipulation parameter. Finally, it is shown that above the saddle point energy, where quasi-discrete and continuum states coexist, the lineshapes of individual Fano-like resonances can be quite efficiently modified by applying this scheme.

Two-photon ionization spectra of calcium above the 4s1/2 threshold

Two-photon ionization of ground state calcium is investigated both experimentally and theoretically in the 374–323 nm spectral range. The spectra are dominated by the presence of the 4p2 1S0 perturber and by members of even parity 3dnd J = 0, 2 (4 ≤ n ≤ 10) and 3dns J = 2 (6 ≤ n ≤ 12) autoionizing Rydberg series, the latter appearing as window resonances. No single-photon resonances are observed. The majority of recorded lines are identified using older stepwise spectroscopic data while there is a number of newly discovered low-lying members. The two previously unobserved 3dns, n ≤ 12, series members are treated using a semi-empirical multichannel quantum defect theory (MQDT) model. Moreover, the low-lying part of the observed predominantly singlet spectrum is fairly well reproduced by theoretical calculations, combining a configuration interaction approach with B-splines basis functions and neglecting relativistic effects. The agreement between theory and experiment becomes less satisfactory as we move towards higher lying resonances dominated by singlet–triplet mixing effects, which are evident in the experimental data while neglected in the calculations.

Phase conjugation through autoionizing states: a density matrix approach

We present a theoretical study of phase conjugation via degenerate four wave mixing through an autoionizing state (AIS), formulated in terms of a density matrix approach. Within this quite general and flexible formulation the probe field is handled as a perturbation, while the coupling of the pump fields to the atomic system is treated exactly. The AIS is described as an isolated Fano resonance decaying into a single continuum. In the weak-field, steady-state limit it is possible to analytically recover earlier perturbative results for the corresponding nonlinear susceptibility. However, the advantage of this approach is best revealed when applied to the detailed study of realistic examples drawn from such diverse atomic systems as Xe and Mg. Both spectral and temporal features of the produced phase-conjugated (PC) signal are studied quantitatively through the numerical solution of the density matrix equations. Moreover, time-integrated quantities, such as the reflectivity, that are usually employed to measure the PC response, are also calculated. We find that the PC response is determined by the asymmetry of the autoionizing lineshape, the peak intensity of the pump pulses and the interplay between the pump pulse duration and the autoionization lifetime. Moreover, the role of the pump–probe delay in restoring reflectivity to high values under otherwise unfavourable conditions is highlighted. Finally, the impact of pump-induced ionization on the PC response is also studied and conditions to counter its deleterious effect on the reflectivity are determined.

Phase conjugation by degenerate four-wave mixing via autoionizing states

The degenerate four-wave-mixing process in the vicinity of an autoionizing state is studied theoretically in the context of perturbation theory. The autoionizing state is represented either by an isolated Fano profile or by the appropriate expression of the multichannel quantum-defect theory for the corresponding Rydberg series of autoionizing states. Analytic expressions for the third-order susceptibility are given together with realistic numerical estimates based on atomic parameters extracted from accurate spectroscopic data. It is demonstrated that the observation of the effect is feasible in extensively studied atomic systems (e.g., alkaline earths) with existing coherent light sources. Constraints on the atomic parameters that will facilitate the experimental observation of the effect are identified and shown to be frequently met in real atomic systems.