# Forward-angle electron spectroscopy

In collisions of heavy highly-charged projectile ions with atomic targets, the energy distribution of the emitted electrons is a characteristic observable for the underlying elementary charge-transfer processes. At the ESR, a dedicated magnetic electron spectrometer was installed downstream from the gas-jet target, which enables the measurement of high-energetic electrons emitted in ion-atom collisions with velocities similar to the projectile velocity within a small cone in the forward direction. This provides the ability to extend the well known study of cusp electrons towards heavy-ion atom collisions at near-relativistic projectile energies. Through the electron-loss-to-continuum cusp, double-differential cross sections of projectile ionization can be studied even for the heaviest few-electron projectiles. But also a new channel opens up, the radiative electron capture to continuum, which can be directly compared to its non-radiative counterpart. Using the electron spectrometer in combination with detectors for emitted x rays and charge-exchanged projectiles, the study of the collision system **U ^{88+} + N_{2} @ 90** MeV/u revealed three processes, each characterized by a unique shape of the electron cusp [1]:

- The process of
**electron loss to continuum**(ELC) corresponds to the ionization of an electron from the projectile into the projectile continuum during the collision with the target,**U**. For the ELC, the measured spectrum has been compared to first-order perturbation theory using fully-relativistic Dirac wavefunctions [2].^{88+}+ N_{2}→ U^{89+}+ [N_{2}]^{*}+ e^{-} - The process of
**electron capture to continuum**(ECC) corresponds to the capture of a target electron into the projectile continuum, while the excess energy is carried away by the recoil of the generated target ion:**U**. For the ECC, the measured spectrum has been compared to calculations in the impulse approximation using semi-relativistic Sommerfeld-Maue wavefunctions, and to calculations in the continuum-distorted-wave (CDW) approach [3].^{88+}+ N_{2}→ U^{88+}+ [N_{2}^{+}]^{*}+ e^{-} - The process of
**radiative electron capture to continuum**(RECC) corresponds to the capture of a target electron into the projectile continuum, while the excess energy is carried away by a photon:**U**. This process can be seen as the high-energy endpoint of bremsstrahlung studied in inverse kinematics. For the RECC, the measured spectra have been compared to calculations using fully-relativistic Dirac wavefunctions, and to calculations in the impulse approximation using semi-relativistic Sommerfeld-Maue wavefunctions [4].^{88+}+ N_{2}→ U^{88+}+ [N_{2}^{+}]^{*}+ e^{-}+ γ

Furthermore, the process of ELC was investigated for **multi-electron projectiles** in the collision systems

**U ^{28+} +H_{2} → U^{29+} +[H_{2}]^{*} +e^{−}, U^{28+} +N_{2} → U^{29+} +[N_{2}]^{*} +e^{−} and U^{28+} + Xe → U^{29+} + Xe^{*} +e^{−}**.

The experimental data revealed a significant electron cusp asymmetry, which increases towards heavier targets. This behavior is not yet consistent with presently available theories based on first-order perturbation using fully-relativistic wavefunctions [5].

In a more recent study, the RECC was measured for the collision system **U ^{89+} + N_{2} @ 76 MeV/u**, and an im- proved agreement of the experimental data and theory was achieved [6]. Within the same experimental campaign, the ELC for

**U**colliding with

^{89+}**N**and Xe was studied, showing a deviation of the electron energy distribution from first-oder perturbation for the Xe target due to the effect that the electron emitted by the projectile is attracted by the target nucleus [7].

_{2}Future concepts of applying the same technique to positron spectroscopy in relativistic heavy-ion atom collisions at the HESR of FAIR are currently under consideration [8].

# References / Selected Publications

# | Title | Author | Reference |
---|---|---|---|

1 | Forward-angle electron spectroscopy in heavy-ion atom collisions studied at the ESR | P-M. Hillenbrand et. al. | |

2 | Electron-loss-to-continuum cusp in U | P-M. Hillenbrand et. al. | |

3 | Electron-capture-to-continuum cusp in U | P-M. Hillenbrand et. al. | |

4 | Radiative-electron-capture-to-continuum cusp in U | P-M. Hillenbrand et. al. | |

5 | Strong asymmetry of the electron-loss-to-continuum cusp of multielectron U | P-M. Hillenbrand et. al. | |

6 | Radiative electron capture to the continuum in U | P-M. Hillenbrand et. al. | |

7 | Electron-loss-to-continuum cusp in collisions of U | P-M. Hillenbrand et. al. | |

8 | Experimental concepts of positron spectroscopy at HESR | P-M. Hillenbrand et. al. |