In order to elucidate how the fragmentation dynamics differs depending on c
hemical-bond orientation on the surface, we have measured the polarization-
angle dependence of the photon-stimulated ion-desorption (PSID) yield spect
ra from condensed formic acid around the C Is region. The measurements were
done using linearly polarized synchrotron radiation combined with a newly
constructed time-of-flight mass spectrometer. Photoabsorption (or X-ray abs
orption near edge structures) and PSID yield spectra showed considerably di
fferent polarization dependencies. Fragment-ion yields for C 1s --> sigma (
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 protr
uding molecular plane or bond axis preferentially survive the charge-transf
er de-excitation by surrounding molecules or underlayers and lead to desorp
tion. (C) 2001 Elsevier Science B.V. All rights reserved.