A concept to generate ultrashort ion pulses for pump-probe experiments in the keV energy range

Abstract

The impact of an energetic particle onto a solid surface generates a strongly perturbed and extremely localized non-equilibrium state, which relaxes on extremely fast time scales. In order to facilitate a time-resolved observation of the relaxation dynamics using established ultrafast pump-probe techniques, it is necessary to pinpoint the projectile impact in time with sufficient accuracy. In this paper, we propose a concept to generate ultrashort ion pulses via femtosecond photoionization of rare gas atoms entrained in a supersonic jet, combined with ion optical bunching of the resulting ion package. We calculate the photoion cloud generated by an intense focused laser pulse and show that Ar q+ ions with q=1-5 can be generated with a standard table-top laser system, which are then accelerated to energies in the keV range over a very short distance and bunched to impinge onto the target surface in a time-focused manner. Detailed ion trajectory simulations show that single ion pulses of sub-picosecond duration can be generated this way. The influence of space charge broadening is included in the simulations, which reveal that flight time broadening is insignificant for pulses containing up to 10-20 ions and starts to increase the pulse width above ∼50 ions/pulse. an ultrashort laser pulse for the excitation (pump), which is then combined with a time-delayed laser-based analysis (probe). In order to transfer this methodology to the ion-surface interaction problem, it is necessary to pinpoint the impact of the projectile ion(s) in time with sufficient accuracy, i.e. with a time resolution of a picosecond or below. The generation of such ultrashort ion pulses, however, poses a significant problem. While electron pulses with sub-ps duration can fairly easily be produced via photoemission from a suitable photocathode using a femtosecond laser pulse, the same technique generally fails for low-energy heavy ions due to their larger mass. In order to achieve good time resolution, the generated charged particles must be quickly accelerated to rather high kinetic energy, leading to short flight times towards the investigated sample with small temporal dispersion accompanied by weak space charge broadening. One possibility to generate a sufficiently short ion pulse is via sheath acceleration in a laser-induced plasma, which is generated by an ultrashort highintensity laser pulse and may accelerate ions to MeV/u energies with a rather broad energy distribution [17,18]. In combination with time resolved x-ray diffraction using an ultrashort x-ray source driven by the same laser pulse, this technique has, for instance, been applied to investigate the ultrafast melting of graphite induced by proton irradiation [19]. The inherently broad energy spectrum of the generated ion bunches can be narrowed by letting the ions pass through thin foils of different thickness [20]. Ion bunches generated this way have been analyzed by an optical streaking scheme that utilizes the ultrafast excitation and relaxation of a high-energy irradiated...

Ultrashort Ne+ ion pulses for use in pump–probe experiments: numerical simulations

“…In order to verify these predictions experimentally, it is necessary to improve the time resolution of our ion detection system from currently 135 ps [13]. For that purpose, we are working to replace the currently used microchannel plate ion detector by a streak technique delivering sub-picosecond resolution.…”

“…The computational methods mimic an experimental setup designed to create ultra-short ion pulses and perform pump-probe experiments based on ion irradiation of sample surfaces and were already described in great detail in [13,15] and will only be repeated in a general way here. The experimental setup consists of 3 electrodes which are arranged plan parallel as shown in figure 1 (left panel).…”

“…While bunching techniques generally allow to produce short ion pulses down to several ten picoseconds in that regime, these pulses yield broad energy spectra due to the bunching process. Therefore, we have proposed an approach using femtosecond strong field-ionization of ultracold atoms from a supersonic neutral gas jet [13]. Earlier experiments utilizing argon at room temperature as a process gas have demonstrated pulse widths on the order 180 ps for keV Ar q+ ions.…”

Nonequilibrium Dynamics of Electron Emission from Cold and Hot Graphene under Proton Irradiation

“…In addition to two-photon photoelectron emission pump–probe experiments, − recent developments toward ion pump optical probe experiments promise sufficient temporal resolution. These allow experiments with few-picosecond ion pulses, − thereby making electronic relaxation time scales in 2D materials, such as graphene, experimentally accessible, which motivates the present work. In such modern ion beam experiments, − the time between successive ion impacts on a sample surface reaches the few-picosecond regime.…”

“…These allow experiments with few-picosecond ion pulses, − thereby making electronic relaxation time scales in 2D materials, such as graphene, experimentally accessible, which motivates the present work. In such modern ion beam experiments, − the time between successive ion impacts on a sample surface reaches the few-picosecond regime. On that time scale, graphene has not yet relaxed back to its ground state and is pre-excited, that is, thermalized to a finite, nonzero temperature.…”