Fragmentation and charge-neutralization pathways depending on molecular orientation at surfaces

In order to elucidate how the fragmentation dynamics differs depending on chemical-bond orientation on the surface, we have measured the polarization-angle dependence of the photon-stimulated ion-desorption (PSID) yield spectra from condensed formic acid around the C 1s region. The measurements were done using linearly polarized synchrotron radiation combined with a newly constructed time-of-flight mass spectrometer. Photoabsorption (or X-ray absorption near edge structures) and PSID yield spectra showed considerably different polarization dependencies. Fragment-ion yields for C 1s→σ*C–H resonance were greatly increased when the direction of the electric field vector (E) of the light became close to perpendicular to the surface. The results imply that the fragments formed from the perpendicularly protruding molecular plane or bond axis preferentially survive the charge-transfer de-excitation by surrounding molecules or underlayers and lead to desorption.