Fragmentation of the $^4$He$_2$ dimer by relativistic highly charged projectiles in collisions with small kinetic energy release

TL;DR

This paper theoretically investigates the fragmentation of the helium dimer into ions during collisions with relativistic highly-charged projectiles, highlighting the dominant direct ionization mechanism and its sensitivity to the dimer's binding energy.

Contribution

It provides a detailed analysis of the fragmentation process, including energy and angular distributions, and demonstrates the impact of relativistic effects and the potential for precise binding energy measurement.

Findings

Direct ionization dominates the fragmentation process below 4-5 eV.

Relativistic effects significantly influence ion distribution shapes.

The energy spectrum shape is sensitive to the helium dimer's binding energy.

Abstract

We study theoretically the fragmentation of the helium dimer, $^4$He$_2$, into singly charged ions in collisions with relativistic highly-charged projectiles. We discuss the main mechanisms driving this process with the focus on the fragmentation caused by direct ionization of both atomic sites of the dimer in a single collision with the projectile. This direct mechanism dominates the He$_2$ $\to $ He$^+$ + He$^+$ breakup events with the kinetic energies of the emerging ionic fragments below $4$--$5$ eV. We explore the energy and angular distributions of the He$^+$ ions produced in collisions with $1$ and $7$ GeV/u U$^{92+}$ projectiles and show that their shape is significantly affected by relativistic and higher-order effects in the interaction between the projectile and the dimer. We also show that the shape of the energy spectrum is quite sensitive to the binding energy of the He$_2$ dimer which can be exploited for its precise determination. The contribution of the direct mechanism to the total cross section for the He$_2$ fragmentation by $1$ and $7$ GeV/u U$^{92+}$ was calculated to be $3.65$ Mb and $2.4$ Mb, respectively, representing roughly half of this cross section.