1,721,479 research outputs found
High-order harmonic generation in laser-aligned molecules.
No full textStrong-field interactions with molecules are not as well understood as for atoms. In atoms, both fully quantum-mechanical calculations [1] and semiclassical models [2] reproduce single-electron ionization and related phenomena—high-harmonic generation (HHG) and above-threshold ionization (ATI )—-with good accuracy. In contrast, for many molecules there is now consistent experimental evidence indicating that ionization rates are significantly lower than those in atoms with the same ionization potential (IP) [3], evidencing the critical role played by the ground state wavefunction in such processes, thus suggesting the possibility to use HHG to probe this ground state wave-function. In order to make progresses in the understanding the molecular electron dynamics in strong laser field the knowledge of the angular position of the molecules is required; to this end the molecular alignment by laser field has been shown to be effective in producing a macroscopic ensemble of ordered molecules [4]. We analyze the electron dynamics and high-order harmonic generation in molecules exposed to high intensity (1013-1015 W cm-2) ultrashort pulse laser fields. The use of adiabatic laser alignment techniques, using relatively long duration (300 ps) laser pulses of moderately high intensity (1011-1012 W cm2) enabled us to perform the first experiments investigating the role of the angle between the laser polarization vector and the molecular axis [5-7]. We have shown that alignment of a sample can enhance the harmonic yield by a small but significant factor. The unique influence of the dipole phase in molecular HHG is clearly demonstrated and is shown to offer a consistent explanation for our experimental observations. The electron wavepacket dynamics exhibit strong interference effects that result in the reduction or enhancement of harmonic yield at certain alignment angles. Experiments to further investigate these effects and to increase the modulation of the harmonics by molecular alignment are discussed.[1] M. Lewenstein, Ph. Balcou, M. Yu. Ivanov, A. L'Huillier, and P. Corkum, Phys. Rev. A 49, 2117 (1994).[2] P. B. Corkum, Phys. Rev. Lett. 71, 1994 (1993).[3] E. Wells, E. Merrick, J. DeWitt, and R. R. Jones, Phys. Rev. A 66, 013409 (2002).[4] J. Larsen, I. Wendt-Larsen, and Henrik Stapelfeldt, Phys. Rev. Lett. 83, 1123 (1999). [5] R. Velotta, N. Hay, M. Mason, M. Castillejo, and J. P. Marangos, Phys. Rev. Lett. 87, 183901 (2001).[6] N. Hay, R. Velotta, M. Lein, R. de Nalda, E. Heesel, M. Castillejo, and J. P. Marangos, Phys. Rev A, 65 053805 (2001).[7] R. de Nalda, E. Heesel, M. Lein, N. Hay, R. Velotta, E. Springate, M. Castillejo, and J. P. Marangos, Phys. Rev.A 69, 031804(R) (2004)
continuous electron beam focusing induced by collective plasma interactions in D2 and H2
No full textAn experimental analysis of the focusing of a continuous electron beam, induced by the mutual interaction with charges produced by electron impact, is presented. The phenomenon has been observed by measuring the beam current on two different detector active areas as a function of the gas pressure in H-2 and D2. The stationary solutions of a three-fluid model, already developed, allow us to measure the maximum beam density. Moreover, we carry out information about the main processes responsible for ionization and their cross sections in the case of a three-component quasiequilibrium plasma. RI Berardi, Vincenzo/H-4550-2011; Velotta, Raffaele/I-5298-201
Experimental analysis of H_{3}- and D_{3}-molecule autoionization
No full textExcited neutral triatomic hydrogen and deuterium molecules formed by electron-impact excitation have been investigated. We report on the autoionization of H-3* and D3* molecules produced by the reactions H-2+ + H-2 --> H-3* + H+ and D2+ + D2 --> D3* + D+. We measured the lifetime of H-3* and D3* and the branching ratio between H-3*, D3*, and H-3+, D3+ production channels. RI Berardi, Vincenzo/H-4550-2011; Velotta, Raffaele/I-5298-201
Chargel species analysis as a diagnostic tool for laser produced plasma characterization
No full textIn this work we report on a study of the plasma produced by UV (lambda = 351 nm) laser ablation of an aluminum target. In particular, we have directly measured the flux velocity of ablated ions along the normal to the metal target, the corresponding ion temperature, and the average kinetic energy by using an electrostatic energy analyzer. Moreover, by interpreting the measured ion energy distributions within the framework of the hydrodynamical model, we have also inferred an estimate of plasma electron density and temperature, which provide significant information on the laser plasma characterization in a number of applications. RI amoruso, salvatore/E-3941-2012; Berardi, Vincenzo/H-4550-2011; Velotta, Raffaele/I-5298-201
Ellipticity effects in HHG from aligned molecules.
No full textTwo-centre interference in the recombination step of molecular high harmonic generation (HHG) has been probed in CO2 and O2. We report the order dependence of characteristic enhancements or suppressions of high harmonic production in aligned samples of both molecules. The ellipticity dependent studies in CO2 clearly show how the destructive interference can be “switched off” by increasing the degree of ellipticity and thus shifting the effective resonance condition. The presence of an elliptically polarised field leads to a tilting in the electron momentum which in turn breaks the cylindrical symmetry introducing a dependence on the azimuthal angle . The observed shift can be explained by assuming that molecules for which the tilting leads to a reduction of (angle between the molecular axis and the recolliding electron momentum), give larger contribution to HHG. In a simple classical approach we have considered uniform contribution from half the molecules obtaining a result in agreement with the experimental findings. References.1) C. Vozzi et al., Phys. Rev. Lett. 95, 153902 (2005).2) C. Altucci et al., in preparation
Consiglio di Amministrazione del Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia
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