STRUCTURE OF MATERIALS

152 ESRF

as well as the axial recoil approximation, are not applicable.

A well-established way to measure the emission direction of photoelectrons in the molecular frame is the measurement of all charged fragments occurring after the photoreaction in coincidence. The following reaction of single nitrogen molecules in the gas phase was examined by using the cold target recoil ion momentum spectroscopy (COLTRIMS) method:

If the aforementioned axial recoil approximation is valid, the two N+ ions repel each other, yielding a rapid back-to-back emission. In that case, their relative momentum prel = p(NL+) p(NR+) corresponds to the molecular orientation at the instant of photoionisation. For core ionisation of N2 by low-energy photons, the axial recoil approximation is known to hold. The present work, however, shows that this approximation is largely violated at high photon energies, as becomes apparent by the results shown in Figure 134. The first row shows theoretical predictions of MFPADs using Coulomb waves (CW) and accurate continuous molecular waves (MW) for different spatial orientations of the N2 molecule. The left-right asymmetry is caused by nondipole contributions due to the high photon energy (the photons travel from left to right in the figure). The up-down asymmetries in the MW calculations result from the molecular orientation with respect to the polarisation vector. By definition, the CW calculations are independent of the molecular orientation and symmetric within the polarisation plane. In the lower row of Figure 134, the measured photoelectron angular distributions are depicted for different orientations of prel. Much stronger asymmetries are observed than can be explained by the calculations of Figures 134a-c.

Fig. 134: Theoretical predictions (a-f) and experimental data (g-i) for angular distributions of the 1s photoelectrons.

Fig. 135: Angular distributions of the 1s photoelectrons (a-d)

and angular distributions of the two N+ fragments for a fixed

photoelectron emission angle and different KER gates (e-h).

hv + N2 g N2+*(1s) + ephoto g N+ + N+ + ephoto + eAuger