When the second strike is harder than the first: efficient indirect ionization of He nanodroplets

Ionization of matter by energetic radiation generally causes complex secondary reactions which are hard to decipher. When a primary energetic photon hits a target, one or a few electrons are emitted which then collide multiple times inside the material. These collisions can trigger a cascade of secondary processes, such as impact-excitation and -ionization of atoms and molecules nearby which creates more slow electrons, breaking of chemical bonds and formation of radicals, and the exchange of energy and charge among the excited particles. In biological tissue, radicals and slow electrons are particularly damaging due to their ability to break the strands of the DNA molecule.

In a series of experiments carried out at the GasPhase beamline of Elettra, an international team of researchers from Denmark, India, Germany, and Italy have recently succeeded in deciphering the full chain of processes ensuing primary photoionization of a condensed-phase model system -- superfluid helium nanodroplets. The researchers used extreme ultraviolet radiation to irradiate helium nanodroplets which were about 100 nm in size and travelled in a beam through vacuum. They used two different methods for detecting the emitted electrons, velocity-map imaging and high-resolution electron spectroscopy, see Fig. 1. In this way, they were able to resolve all primary and secondary ionization processes up to the level of the excited quantum states of the droplets involved in the processes and the charge states of the products.

Figure 1: Sketch of the experimental setups used in this work. a) He nanodroplet beam source and velocity-map imaging (VMI)-time of flight (TOF) spectrometer. b) Hemispherical electron analyzer.

Researchers found that both elastic and inelastic electron collisions at the helium atoms in the nanodroplets efficiently induce further ionizations by a new process, called Interatomic Coulombic Decay (ICD), mediated by impact excitation and electron-ion recombination. In helium droplets with radius >40 nm, slow electrons are efficiently trapped in the material and recombine with their parent ions; the highly-excited species created in this way interact with one another through ICD causing their mutual ionization. This indirect ICD process even becomes the dominant channel of electron and ion emission when the size of the helium droplets is increased so as to approach the limit of bulk superfluid helium. Fig. 2 shows ICD electron spectra measured with low resolution (a) and with high resolution (b); they can mostly be related to transitions between pair-states of excited He atoms to the ionized state, He* + He* ⟶ He + He⁺, see Fig. 2 c).

Figure 2: a) Electron spectra from helium nanodroplets of radii R=5, 20, 35 and 75 nm (red to black) measured at various photon energies hν and inferred from electron images as that shown in the inset (hν=46.5 eV, R=50 nm). The cyan line is a reference spectrum at hν=21.6 eV and R=50 nm. The vertical dashed lines indicate expected electron energies based on helium atomic and molecular levels. b) High-resolution analyzer spectra measured around the main ICD peak for R=50 nm and in the regime of impact excitation (black and grey lines) and resonant excitation (cyan line). The colored stick spectrum indicates characteristic electron energies for ICD of a He*-He* pair formed in ³S and ¹S atomic states (dashed lines) and a He*-He* pair forming Σ nd Π molecular states (solid lines). The line colors match the potential energy curves involved in the ICD process, shown in c).

While helium nanodroplets are a particularly favorable system for studying these kind of light-matter interaction processes, as the researchers have demonstrated with numerous studies before, ICD-like secondary ionization processes are expected to play an important role in other condensed-phase systems exposed to ionizing radiation as well; only that their signatures are harder to detect experimentally. These results will hopefully help to better understand how ionizing radiation causes damage in matter, which is the basis for devising better radiation protection and radiotherapies schemes.

This research was conducted by the following research team:

L. Ben Ltaief¹, K. Sishodia², S. Mandal³, S. De², S. R. Krishnan², C. Medina⁴, N. Pal⁵, R. Richter⁵, T. Fennel⁶, and M. Mudrich^1
1 Department of Physics and Astronomy, Aarhus University, Aarhus C, Denmark
² Quantum Center of Excellence for Diamond and Emergent Materials and Department of Physics, Indian Institute of Technology Madras, Chennai, India
³ Indian Institute of Science Education and Research, Pune, India
⁴ Institute of Physics, University of Freiburg, Freiburg, Germany
⁵ Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy
⁶ Institute for Physics, University of Rostock, Rostock, Germany

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