Ponderomotive Acceleration of Photoelectrons in Surface-Plasmon-Assisted Multiphoton Photoelectric Emission

International audiencePhotoelectrons emitted from a gold target via a surface-plasmon-assisted multiphoton photoelectric process under a femtosecond laser pulse of moderate intensity are much more energetic than in an ordinary photoeffect without electron collective excitation. The phenomenon is interpreted in terms of time-dependent ponderomotive acceleration of the particles by the resonant field localized at the metal surface. The amplitude of the plasmon resonance may be directly estimated by means of the electron energy spectra. The development of powerful lasers more than three decades ago has allowed the investigation of the generalization of the classical photoelectric emission from metals to processes involving the absorption of several photons [1]. In recent years, the advent of laser pulses of ultra-short duration has favored studies in the femtosecond time regime [2]. These investigations can lead to the creation of new high-current ultrafast electron sources. Experimental studies have revealed that the electron emission rate can be greatly enhanced by the excitation of collective electron modes of the metal, the so-called surface plasmons [3,4]. The increase of the photoelectric signal can be qualitatively explained in terms of an assisted photoelectric effect where the energy of femtosecond light pulses is stored by the surface plasmon, creating a hot-electron population that does not have enough time to transfer its energy to the crystal lattice. While the presence of a surface-plasmon excitation is efficient in increasing the production of photoelectrons, an important open question is how the energy of the emitted electrons in such a " surface-plasmon-assisted " photoelectric process may differ from the energy predicted by the familiar photoelectric equation generalized to multiphoton processes. In this Letter, we show that the photoelectron energy is strongly affected by the surface-plasmon field, the modification from the classical values depending on the characteristics of the plasmon resonance. This fact may be easily understood by considering a simple analysis of the photo-electron behavior in the inhomogeneous high-frequency electric field surrounding the metal surface. The analysis involves simple classical concepts such as the notion of time-dependent ponderomotive effects, which have been successfully used in the context of multiphoton ionization of atoms in high-intensity lasers [5]. Consider an electron released from the metal surface after having absorbed a required number n of photons from the laser beam to overcome the work function W of the metal. While traveling in the vacuum dressed by the high-frequency field E sp of the surface plasmon, the total energy of the electron consists of the sum of its kinetic energy § n (given by the Einstein multiphoton photoelectric equation § n ෇ n ¯ hv 2 W) and its quiver energy U sp ෇ e 2

Attosecond chirp-encoded dynamics of light nuclei Attosecond chirp-encoded dynamics of light nuclei

International audienceWe study the spectral phase of high-order harmonic emission as an observable for probing ultrafast nuclear dynamics after the ionization of a molecule. Using a strong-field approximation theory that includes nuclear dynamics, we relate the harmonic phase to the phase of the overlap integral of the nuclear wavefunctions of the initial neutral molecule and the molecular ion after an attosecond probe delay. We determine experimentally the group delay of the high harmonic emission from D 2 and H 2 molecules, which allows us to verify the relation between harmonic frequency and the attosecond delay. The small difference in the harmonic phase between H 2 and D 2 calculated theoretically is consistent with our experimental results

Status of the SPARC-X Project

SPARC-X is a two branch project consisting in the SPARC test facility dedicated to the development and test of critical subsystems such as high brightness photoinjector and a modular expandable undulator for SASE-FEL experiments at 500 nm with seeding, and the SPARX facility aiming at generation of high brilliance coherent radiation in the 1.5-13 nm range, based on the achieved expertise. The projects are supported by MIUR (Research Department of Italian Government) and Regione Lazio. SPARC has completed the commissioning phase of the photoinjector in November 2006. The achieved experimental results are here summarized together with the status of the second phase commissioning plans. The SPARX project is based on the generation of ultra high peak brightness electron beams at the energy of 1 and 2 GeV generating radiation in the 1.5-13 nm range. The construction is at the moment planned in two steps starting with a 1 GeV Linac. The project layout including both RF-compression and magnetic chicane techniques has been studied

Détermination temporelle et spatiale de la fonction de distribution en énergie des électrons dans une post-décharge

Afin d'obtenir directement, de façon précise, dans une post-décharge, la fonction de distribution en énergie des électrons avec une bonne résolution spatiale, temporelle et énergétique, un dispositif de mesure de haute sensibilité, par sonde de Langmuir, a été construit et est ici présenté. Il s'applique à n'importe quel type de décharge répétitive et permet de mesurer, à des pressions relativement élevées, la partie haute énergie des fonctions de distribution

Collisional ionization under strong laser irradiation

The main laser-induced collisional ionization processes are examined according to the shape and the relative position of the input and output channel potential curves. The different cases are illustrated by a few systems under experimental investigation, with emphasis on beam experiments

EFFET DE LA DIFFUSION DES ÉLECTRONS SUR LA FONCTION DE DISTRIBUTION ET LE BILAN D'ÉNERGIE ÉLECTRONIQUES DANS UNE POST-DÉCHARGE D'HÉLIUM

Au cours de la post-luminescence d'un gaz d'hélium faiblement ionisé, la désexcitation des métastables atomiques et moléculaires crée des électrons "chauds" à 15 et 20 eV. La relaxation en énergie de ces électrons est étudiée dans des conditions où la diffusion n'est pas négligeable. L'expression du coefficient de diffusion, compte tenu du champ de charge d'espace crée par la distribution électronique totale, permet de calculer le nombre d'électrons disparaissant au cours de leur relaxation et de déterminer la perturbation de la fonction de distribution maxwelliene qui en résulte. L'importance de cet effet est présentée dans quelques cas expérimentaux.During the helium afterglow, the deactivation of atomic and molecular metastables produces "hot" electrons at 15 and 20 eV. The energy relaxation of these electrons is studied when diffusion is not negligible. With the knowledge of the diffusion coefficient, taking into account the space charge field due to the total distribution, one can calculate the number of electrons lost during their relaxation, and determine the deviation to the maxwell distribution. Some examples show the importance of this effect

Resonant electron emission of silver spheroids induced by laser surface plasmon excitation

International audienceLaser excitation of surface plasmons on a silver granular surface has been studied as a function of light frequency. Both light absorption and electron emission have been measured by varying continuously the wavelength between 275 and 700 nm. Resonance effects at the Fröhlich frequencies of the silver spheroids are displayed and a correlation between absorption and emission is clearly exhibited. The lux-ampere characteristics performed at selected increasing wavelengths display slopes with integer orders, indicating that the emission process remains a photoeffect even in the presence of a surface-plasmon resonance. The photoemission process shows net switches of these slopes that turn from linear into quadratic and then cubic integer values. The position of the first switch yields W s = 4.15 ± 0.05 eV for the work function of the granular surface. The position of the second switch defines the threshold energy for the two-photon emission process, which is found to differ significantly from the expected photon minimum energy ¯ hω = 1 2 W s

Interaction laser - métal en présence de plasmons de surface : vers une source de rayonnement UV et X

On présente une analyse expérimentale des électrons émis lors de l'interaction d'un laser ultra-bref avec des surfaces bi-couches Au/Al, où la resonance plasmon de surface du métal exterieur, excitée par le laser, excite elle-même une resonance à l''interface entre les deux métaux. On montre que l'émission, de durée quasi identique à celle de l'impulsion laser, augmente considérablement en présence de cette resonance. Un effet optimal est obtenu pour une épaisseur d'or de 40 nm. Les électrons émis sont issus principalement de l'interface, et ont des énergies cinétiques pouvant atteindre plusieurs dizaines d'eV. En considérant que la traversée de la couche d'or, de l'interface à la surface, s'accompagne d'une émission de rayonnement de type bremsstrahlung, on dispose ainsi, grâce à cette résonance d'interface, d'une source ultra-brève et intense de rayonnement UV