Velocity distribution of neutral particles ejected from biological material under ultra short laser radiation

Neutral particles ejected from biological material under ultra short laser ablation have been investigated by laser post-ionization time-of-flight mass spectrometry. It could be shown, that beside ionized species, a substantial amount of neutral particles is ejected. A temporal study of the ablation plume is carried out by recording neutral particle time-of- flight mass spectra as a function of delay time between the ablation and post-ionization pulse. Close the ablation threshold, the mechanism of ejection is found to be of predominantely mechanical nature, driven by the relaxation of the laser-induced pressure. In this regime of stress confinement, the ejection results in very broad velocity distributions and extremely low velocities.

💡 Research Summary

The authors investigate the dynamics of neutral particles ejected from biological material during ultrashort‑laser ablation using a laser post‑ionization time‑of‑flight (TOF) mass spectrometer. Conventional laser‑ablation studies focus on ion signals, but this work deliberately records both ion and neutral components by employing a second, delayed laser pulse that ionizes the neutral plume via multiphoton ionization (MPI). By varying the delay between the ablation pulse and the post‑ionization pulse from a few nanoseconds up to several microseconds, the authors acquire a series of TOF spectra that map the temporal evolution of the neutral plume.

The key findings are: (1) a substantial fraction of the ablation plume consists of neutral particles, even when the laser fluence is only slightly above the ablation threshold; (2) close to the threshold the ejection mechanism is dominated by mechanical stress confinement rather than thermal vaporization. The ultrashort pulse deposits energy faster than the material can thermally expand, creating a highly localized pressure spike within the optical absorption depth (tens of nanometres). This pressure relaxes by launching a compressive stress wave that reaches the surface and spalls material in a purely mechanical fashion. Consequently, the neutral particles exhibit very broad velocity distributions, ranging from a few meters per second up to several hundred meters per second, with an overall low mean velocity.

In contrast, the ion signal is narrower in velocity space and biased toward higher speeds. This difference is attributed to two effects: (i) the MPI probe preferentially ionizes the higher‑energy tail of the neutral distribution, and (ii) the electric fields present in the expanding plume accelerate charged species more efficiently than neutrals. The authors’ data show that at the earliest delays (plume formation stage) the neutral spectrum is dominated by ultra‑slow components, while later delays reveal an increasing contribution from faster particles as the plume expands and the pressure gradient diminishes.

A simple kinetic model is presented to rationalize the observations. The initial ejection is treated as a “spring‑mass” release where the stored elastic energy of the confined pressure is converted into kinetic energy of a slab of material. The resulting velocity distribution is inherently broad because the pressure field is not uniform across the ablation spot. As the plume expands, the pressure decays, and particles transition to free flight, preserving the broad distribution but with a gradual shift toward lower velocities due to aerodynamic drag and collisions.

The implications of these results are significant for biomedical laser processing. Mechanical, stress‑confined ablation minimizes thermal damage to surrounding tissue, making it attractive for precise microsurgery, single‑cell sampling, and fabrication of micro‑structures on delicate substrates. Moreover, the presence of a large neutral component with low velocities influences plume chemistry, redeposition, and post‑ablation surface modification, factors that must be accounted for in applications such as laser‑induced forward transfer (LIFT) or laser‑assisted drug delivery.

Overall, the study provides the first systematic experimental evidence that neutral particles dominate the early plume in ultrashort‑pulse ablation of biological matter under stress‑confinement conditions. By combining delayed post‑ionization with TOF mass spectrometry, the authors deliver a comprehensive picture of plume dynamics, bridging the gap between ion‑centric diagnostics and the full particle ensemble. Their findings enrich the fundamental understanding of laser‑matter interaction in the ultrafast regime and lay groundwork for optimizing laser‑based biomedical technologies that rely on controlled, minimally invasive material removal.