The Binary Encounter Peak in Radiative Ionization

It is shown that the radiative electron capture to continuum in energetic collisions with fast heavy projectiles also leads to a binary-encounter type peak in the high-energy electron spectrum. In contrast to the nonradiative target ionization this peak results from a higher-order process, involving near-elastic scattering, and its shape is different from the target Compton profile. Numerical results within the impulse approximation are presented for 30 MeV/amu Kr + H collisions, and the dependence of the peak on collision velocity, target binding and electron emission angle is discussed. With the feasibility of the coincident detection of forward electrons and simultaneously emitted photons during relativistic ion-atom collisions the interest in radiative processes has been revived. The photon is able to carry away the excess energy without notably changing the momentum balance. Therefore radiative processes are, in contrast to the corresponding nonradiative processes, predominantly induced by distant collisions. This leads to a much weaker decrease of the cross section with increasing collision velocity v, thereby compensating for the smallness of the radiative coupling constant e provided v is sufficiently high. In a recent pilot experiment the cusp from radiative capture to the projectile continuum (also termed radiative ionization, RI) could be identified in both the electron and photon spectra from 90 MeV/amu U + N2 collisions (v = 56.24 a.u.) [1]; although even for these collision parameters the RI cusp intensity is still an