S V Fomichev

Laser-induced nonlinear excitation of collective electron motion in a cluster

We consider the nonlinear collective electron dynamics inside a large cluster irradiated by a strong linearly polarized short (0.1 ?ps) laser pulse. The equation of the centre-of-mass motion of the electron cloud driven by the strong laser field is derived using the approximation of an incompressible medium. The analysis of this equation demonstrates the presence of odd harmonics of the fundamental frequency in the cluster dipole moment, and in both the internal and the scattered electric field. Both neutral and ionized clusters are considered. For clusters with radii R 100?? irradiated by a femtosecond titanium?sapphire laser with a peak intensity of I 1016?W?cm?2, the internal electric field strength near the tripled fundamental frequency is shown to be of the same order as the field of the fundamental. The reason is that both for metallic and for laser-ionized van der Waals clusters the Mie surface-plasmon energy ?M is around 5 eV, which is close to three times the energy of a titanium?sapphire laser-field quantum. On the other hand, the condition for first-order resonance with the Mie frequency is not met during the presence of the main laser pulse, but only temporarily, either at the first onset of inner ionization on the leading edge of the pulse (for van der Waals clusters) or during the subsequent Coulomb explosion. In both cases, the ion density is reduced. The presence of a strong third harmonic leads, in particular, to the enhanced production of multiply charged ions in clusters irradiated by a strong laser field, as compared with isolated atoms. This point is discussed in the light of recent experimental results on the production of multiply charged ions in laser?cluster experiments. Third-harmonic generation by a cluster in a strong laser field, as a function of both the cluster and the laser-field parameters, is also considered.

Nonlinear excitation of the Mie resonance in a laser-irradiated cluster

The nonlinear collective electron dynamics inside a large heated cluster irradiated by a strong linearly polarized short laser pulse are considered in the approximation of an incompressible medium. When the incident radiation frequency is near three-photon resonance with the Mie frequency, the field inside the cluster exhibits a third harmonic with an amplitude comparable with that of the fundamental. In the same parameter range, due to shielding, the field inside the cluster at the fundamental frequency is strongly reduced with respect to the incident field. The presence of the third harmonic can lead to a strong enhancement of the production of multiply charged ions. Third-harmonic generation by a cluster under the same conditions is analyzed, too.

Classical modelling of the nonlinear properties of clusters in strong low-frequency laser fields

The nonlinear collective electron dynamics of a cluster irradiated by a strong near-infrared linearly polarized short laser pulse are studied by classical molecular-dynamics simulations for a small model cluster with ~103 particles. The model naturally yields all the features of a cluster exposed to a strong laser pulse, such as inner and outer ionization, expansion of the ion core, etc. When the frequency of the incident radiation is near three-photon resonance with the Mie frequency in the laser-ionized cluster, both the electron acceleration and the local electric field acting on the ions inside the cluster exhibit a third harmonic of the fundamental frequency. The time evolution during the laser pulse of the third harmonic of the total electron acceleration as well as of the electric field inside the cluster is discussed for different parameters of the laser?cluster interaction. The effect of ion motion is investigated. The presence of a strong non-resonant second harmonic in the local electric field at ion positions off the cluster centre is reported.

Vlasov theory of Mie resonance broadening in metal clusters

Dynamical electromagnetic properties of metal clusters with radii R<10 nm have been theoretically considered in the dipole approximation in a jellium model taking into exact account the spatial dispersion of free electron gas. The Vlasov equation for the plasma electron distribution function inside a spherical cluster exposed to a low-intensity external electromagnetic field is solved analytically in the linear response approximation for the cases of both diffuse and mirror reflection of electrons from the cluster boundary. The solution found is used to obtain the non-local relation between the current density and the electric field inside the cluster, which is substituted into the Maxwell equations and boundary conditions for the electromagnetic field. In the diffuse reflection case the complete set of field equations is reduced to a single integral equation which has been solved numerically to obtain the distributions of both the electric field and the electron density inside the cluster, as well as the cluster dynamical electric polarizability and the photoabsorption and photoscattering cross sections. It has been found, from the cluster photoabsorption spectra calculated, that in the considered model the width of the dipole Mie resonance is mainly due to the electron reflection from the cluster boundary, it slightly depends on the permittivity 0 of the environment, and in the case of clusters in vacuum it is described with a good accuracy by the conventional expression (R) = +bvF/R with b1.0, vF being the electron Fermi velocity.

Features of laser spectroscopy and diagnostics of plasma ions in high magnetic fields

Laser induced fluorescence and laser absorption spectroscopies of plasma ions in high magnetic fields have been investigated. Both the high degree of Zeeman splitting of the resonant transitions and the ion rotational movement drastically change the properties of the resonance interaction of the continuous wave laser radiation with ions in highly magnetized plasma. Numerical solution of the density matrix equation for a dissipative two-level system with time-dependent detuning from resonance was used to analyse this interaction. A theoretical simulation was performed and compared with the experimental results obtained from the laser spectroscopy diagnostics of barium plasma ions in high magnetic fields in the several tesla range.

Non-selective photoionization of uranium atoms by the third harmonic of a Nd-YAG laser: experiment and simulation

Time of flight mass spectrometry in combination with laser ablation (LA) can be successfully applied to low-isotope-ratio measurements of metallic samples. In this connection, the non-selective quasi-resonant photoionization by Nd-YAG pulse laser third-harmonic radiation of uranium atoms created by either LA on a metallic target or by radiative heating in an oven was studied in a wide range of laser fluence both experimentally and with computer simulation. In the latter case both the level structure of the uranium atom and the experimentally obtained profiles of spectral and spatial distributions of the third-harmonic radiation of the Nd-YAG laser in the photoionization zone were taken into account. Comparison of experimental and simulated results was made. Experimental and simulated results for the LA case were found to be in agreement only with the assumption that the ionization yield from the metastable level of the uranium atom is strongly suppressed. This could be explained by low population of the metastable level in the photoionization volume due to the expansion and subsequent population cooling of uranium atom vapour created by LA. The unknown parameters of uranium transitions (lifetimes and ionization cross sections) involved in the quasi-resonant photoionization process under study are estimated.