The strongest reported pattern in this laboratory preprint was tied to the charge state helium ions carried on exit from a solid. Photon emission coincident with He2+ detection was very low, while the measured photon yield for He+ was about twice the yield for neutral helium. The He2+ measurements had low statistics.
By comparison, photon-yield differences among carbon, silicon and silicon carbide were small despite about a 1.5-fold difference in electronic stopping power and silicon carbide's high atomic density. Silicon measured under (100) axial channeling also showed no reported change in photon yield, even though electronic energy deposition was demonstrably different.
Following the ions through the target
The study examined how specific electronic excitations in keV helium projectiles varied with kinetic energy, target system and the charge state measured when the ions exited. Its primary readout was the fraction of transmitted helium projectiles that emitted a photon, separated by exit charge state and energy across target materials.
The measurements used incident He+ on carbon foils and on 50 to 200 nm single-crystalline silicon and silicon carbide membranes. Silicon was tested in the (100) axial-channeling orientation and in a pseudo-random crystal orientation; silicon carbide was recorded only in the pseudo-random condition.
To associate a photon with the ion that crossed the target, the time-of-flight acquisition used pulses at 236 kHz lasting 0.5 to 3 ns. The beam current was set so that the vast majority of cycles contained at most one ion, enabling photon-ion association.
The photon detector had an approximately 10 eV threshold and approached constant efficiency above 13 eV. Within its open area, the raw photon detection probability above 13 eV was 10%. After accounting for the detector's 54% open-area ratio, the photon detection probability was 5.4%; below 10 eV, the stated probability was zero.
Noise coincidences were estimated by applying the coincidence condition in a 200 ns-wide region before the photon peak. The postprocessing correction was recalculated four more times before charge-state-resolved yields were extracted.
Speed mattered, but the material did not change much
As projectile energy changed, photon yields broadly followed a linear-velocity or square-root scaling. At lower exit velocities below the Bohr velocity, however, the yields declined more sharply.
The authors also compared the results with an electron-loss and capture model. They reported that it held for EUV photons from silicon, including ion channeling, and for silicon carbide in transitions ending in the ground state. In the model-based analysis, the electron-loss probability beta increased with projectile energy, while the capture probability alpha seemed to decrease overall; the capture probability showed no sharp drop above the Bohr velocity.
What the photons can and cannot show
The authors caution that the reported photon yields are only a lower limit on overall projectile excitation. The detected photon count should therefore not be read as a complete measure of all excitation in the projectile.
The paper adds another caveat about exit charge. The authors expect measurable Auger-electron yields from doubly excited helium, written He**, and state that the charge-state distribution measured after transmission can differ from the distribution inside the solid.
The authors acknowledge Swedish Research Council VR-RFI support for accelerator operation under Contract No. 2023-00155.
Paper data and sources
Original title: Charge-State Dependence of Electronic Excitations in keV Ions Transmitted Through Solids Probed by Ion-Photon Coincidence Measurements
Authors: Kevin Vomschee, Radek Holeňák, Svenja Lohmann et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-28
DOI: Not available
Original paper · Full text