Picosecond Ion Pulses from Adsorbate-Covered Tungsten Nanotips

Abstract

Ultrashort ion pulses enable time-resolved investigation of particle-surface interactions, where some processes following ion impact evolve on picosecond timescales. Accessing these dynamics experimentally requires ion sources capable of delivering precisely timed ion pulses in the same temporal regime. Laser-stimulated desorption (LSD) from metallic nanotips provides such a pathway by exploiting the strong local electric field enhancement as well as nanometric confinement of the emission region [1]. Here, we investigate the temporal characteristics of the ions emitted from a molecular-loaded tungsten nanotip when driven by femtosecond laser pulses. The ion source consists of an electrochemically etched tungsten nanotip with an apex radius of ~150 nm, operated in ultrahigh vacuum and biased at voltages of up to +6.5 kV. Due to the strong geometric field enhancement at the tip apex, local static fields on the order of ~35–65 MV/cm are achieved in the emission region. Introducing background gases at partial pressures of 10-8–10-6 mbar enables adsorption of molecular species on the tip surface. Upon irradiation with ultraviolet laser pulses of <300 fs and 5–12 nJ/pulse, surface-bound species are ionized and desorbed from the nanotip apex. While the ionization surface is defined by the laser focal spot (~3.6 μm diameter), the effective emission region contributing to the detected signal is further confined by the extraction geometry, in particular by apertures formed by the extraction and focusing drift tubes. This geometrically imposed confinement restricts the range of possible ion trajectories and is essential for limiting the temporal dispersion of the ion pulses [2]. Despite this geometric filtering, the source maintains a high ion yield exceeding one ion per laser pulse at repetition rates of up to 1 MHz, corresponding to a cw-equivalent ion current of >100 fA. The resulting ions are extracted by the electrostatic field, accelerated to kinetic energies in the sub-10 keV range, and detected using time-of-flight (TOF) spectroscopy. The measured TOF spectra directly reflect the pulse-to-pulse jitter of the ions. For hydrogen, this results in ion peaks with full-width at half-maximum (FWHM) of less than 100 ps (cf. Fig. 1a). As shown in Fig. 1b, several other adsorbate species were investigated, including deuterium and nitrogen. Under cryogenic cooling of the nanotip, ion emission from weakly bound species such as noble gases including krypton and xenon become observable, indicating the crucial role of surface-adsorption and surface-catalyzed ionization in the emission process. In all cases, the ion arrival time distribution exhibit characteristic asymmetries, suggesting that the temporal structure of the ion pulses is not governed by the femtosecond laser excitation itself, but rather by the emission geometry and subsequent propagation through the strong electrostatic field near the nanotip apex, as expected for heavy ions in contrast to ultrashort electron pulses. To investigate the cause of the observed picosecond pulse width, particle trajectory simulations using SIMION [3] are employed to model ion propagation from the nanotip surface to the detector. By initializing ions at different positions and emission angles on the tip apex, the simulations reproduce the experimentally observed temporal broadening of TOF spectra as seen in Fig. 1c. These results demonstrate that even small variations in initial conditions, which are inherent to emission from a curved nanoscale structure, lead to significant dispersion in flight

122 Symposium on Surface Science times. Surface geometry and field-driven trajectory effects are therefore identified as dominant contributors to the measured ion arrival-time jitter. The intrinsic synchronization between the ion pulse and the driving laser, combined with the inherently high brilliance of a nanotip source, makes LSD an attractive approach for pump-probe experiments with picosecond resolution. More generally, this system provides a versatile platform for exploring ultrafast surface processes and ion-surface interactions, bridging surface science and time-resolved ion physics.