[Paper Review] Generation of ultrashort ion pulses from ultrafast electron-stimulated desorption

Marius Constantin Chirita Mihaila, Gabriel L. Szabo+4 more|arXiv (Cornell University)|Oct 4, 2023

Ion-surface interactions and analysisEngineering3 citations

TL;DR

This paper presents a novel method for generating picosecond ion pulses using ultrafast electron-stimulated desorption (UESD) from a metallic surface, where short electron pulses induce ion desorption with controlled timing. The technique achieves 220 ps full width at half maximum (FWHM) proton pulses at 8.5 keV energy, setting a new benchmark for keV-range ion pulses and enabling time-resolved studies of ultrafast ion-surface interactions.

ABSTRACT

We present an efficient method to produce laser-triggered proton pulses well below 500 ps pulse width at keV energies. We use femtosecond photoelectron pulses emitted from a cathode to enable ultrafast electron-stimulated desorption of adsorbates on a stainless steel plate under ultrahigh vacuum conditions. While direct photoionization of atoms to form well-timed ion pulses can suffer from a laser-focus-limited large starting volume, in our method the two-dimensional starting plane of the ions is defined with nanometer precision at a solid surface. We clearly outline how the method could be used in the future to efficiently produce ion beam pulses in the (sub)picosecond range for pump-probe experiments with ions.

Motivation & Objective

• To overcome the lack of sub-picosecond ion pulses in the keV energy range, which limits time-resolved studies of ultrafast ion-surface interactions.
• To develop a method for generating precisely synchronized, ultrashort ion pulses using low laser intensity (10^9–10^10 W/cm²) and ultrafast electron emission.
• To break the current pulse width barrier in ion sources by engineering the initial momentum distribution of desorbed ions through directional electron-stimulated desorption.
• To demonstrate experimentally that ion pulses with sub-250 ps duration can be achieved at keV energies using a tailored UESD setup.
• To characterize the ion source design and identify pathways toward even shorter ion pulses through optimization of electron beam geometry and polarization.

Proposed method

• Utilizes ultrafast electron-stimulated desorption (UESD) by irradiating a metallic surface with femtosecond laser pulses to generate short electron pulses.
• Laser pulses with 10^9–10^10 W/cm² intensity trigger electron emission from a photocathode, which then induces desorption of adsorbed H₂O, H₂, or CH₃ species.
• The emitted electrons cause Franck-Condon transitions, promoting adsorbed molecules into repulsive excited states that desorb as ions.
• Ion emission is initiated by the electron pulse, with timing synchronized to the laser, enabling precise control over ion pulse start time.
• Ion trajectories are simulated using SIMION software to model ion extraction and filtering by the collector electrode (DT3), accounting for electric fields and geometric constraints.
• System jitter is measured via UV reflection timing between MCP and PIN diode signals, yielding a 140 ps electronic jitter limit.

Experimental results

Research questions

• RQ1 Can ultrafast electron-stimulated desorption generate ion pulses with sub-250 ps duration at keV energies?
• RQ2 How does laser polarization influence the timing and width of ion pulses in the UESD setup?
• RQ3 What role does the geometry and voltage of the extraction electrodes (DT2, DT3) play in shaping the time-of-flight distribution of ions?
• RQ4 To what extent can the initial momentum distribution of desorbed ions be engineered to reduce pulse broadening?
• RQ5 Can a low-power laser system (10^9–10^10 W/cm²) achieve ion pulses comparable to those from high-intensity (>10^15 W/cm²) laser-driven sources?

Key findings

• The experiment demonstrates 220 ps full width at half maximum (FWHM) proton pulses at 8.5 keV kinetic energy, representing a record for keV-range ion pulses.
• Laser polarization tuning via a half-wave plate modulates the ion pulse amplitude, timing shift, and width, with optimal performance at 0° polarization.
• Two distinct ion peaks are observed in time-of-flight spectra, separated by 33 ns in experiment and 41 ns in simulations, indicating multiple ion emission sites on the DT3 surface.
• The ion source design acts as a geometric filter: only ions emitted from specific starting positions on the DT3 surface are extracted, reducing timing spread.
• System jitter is measured at ~140 ps, indicating that electronic and detector timing limitations are not the primary source of pulse broadening.
• Heavier ions such as C⁺ and N⁺ are also detected, confirming the method’s applicability beyond protons.