Photon Factory Activity Report 2004 #22 Part B (2005)
Surface and Interface

11A/2002G032

Mechanism of state-specific enhancement in photon-stimulated desorption
as studied using a polarization-dependent technique1
Tetsuhiro SEKIGUCHI*,1, Yuji BABA1, Iwao SHIMOYAMA, Guohua Wu2, and Yoshinori
3
KITAJIMA
1

Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki, 319-1195, Japan
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Anhui, 230026, China
3
Institute of Materials Structure Science, Tsukuba, 305-0801, Japan2

We measured and compared similar angle dependent data for++D - and Cl -desorption at C 1s and Cl 2s edges and found
the following intriguing tendencies: (i) marked angulardependences are observed only when resonant
enhancement in desorption is observed; (ii) the excitationof molecules with standing upward C-Cl (or C-D) bondleads to more efficient desorption (Orientation effect).
To explain the orientation effect, we assume first thatultra-fast dissociation (UFD) is involved in the desorption
mechanism; and second that the charge-neutralization rateof desorbing ions decays exponentially with a normal
distance from the surface. Figure 2 displays thedesorption sequence proposed for our system. According
to UFD, C-Cl (or C-D) bond is presumably elongated andCl atoms (or D atoms) migrate a substantial distancewithin the lifetime of the core-hole states owing to antibonding nature of unoccupied orbitals. Therefore, the ion
survival rate depends on the surface normal projection of
the elongation. This view fits with the observed angulardependences.
[1] T. Sekiguchi et al., J.Elect.Spect.144/147, 437 (2005).
[2] T. Sekiguchi et al., Surf.Sci.(2005) in press.
ABDC

θ = 20°A BCDθ = 20°

030°

030°

040°

050°

060°

070°

080°

090°

+(a)

40°
0(b)

45°
055°
060°
0Total Electron Yields (Arbitrary Units)

0

Cl Yields (Arbitrary Units)

Introduction
We have incorporated photon-stimulated iondesorption (PSID) technique withpolarization
spectroscopy around NEXAFS (near-edge x-ray
absorption fine structure) region. We believe it will giveinsights into orientation nature at top-most layers if
desorption mechanism is fully understood[1]. However,there are still only few data of polarization-angle
dependence of PSID spectra. Here we studied the angledependent
PSIDfrom
condensed
deuterated
chlorobenzene (C6D5Cl)[2]. This system is interesting
because it extends to 2p-edge excitation from 1s-edges
studied so far, also because we can compare the angle
dependences between core excitations of different
elements, i.e. carbon and chlorine.
Experimental
The experiments were performed at BL11A station.
The dependence of fragment-ion yields on photon
incidence angles were measured using a rotatable time-offlight mass-spectrometer (R-TOF-MS) that was developedin our laboratory. TOF measurements were done using
pulsed soft X-rays during the single bunched operation.
Polarization-dependence of NEXAFS was measured byrecording sample drain current as total electron yields(TEY). The sample was prepared by depositing thickdeuterated chlorobenzene onto a sputter-cleaned Si(100)wafer at 80K.
Results and discussion+Figure 1 compares the chlorine 2p-edge (a) Cl -PSIDand (b) TEY-NEXAFS spectra of condensed C6D5Cl forvarious polarization angles θ. According to the gaseous
ISEELS study, resonances A and B are respectively dueto Cl 2p3/2→π*(b1) and Cl 2p1/2→π*(b1).
+It is noteworthy that Cl desorption yields inresonances A and B depend on polarization angles, while
TEY show no significant angle dependence. Assuming
2p-d(π*) transitions, we speculate the following twotransitions are plausible for the A, B resonances taking
into account photo-selection rule for C2v symmetry.
Cl 2px → dxz π* with Mz .....(1),Cl 2pz → dxz π* with Mx .....(2),where Mz and Mx refer to transition moments with C-Cl
axis and perpendicular to benzene plane, respectively.
Here we rule out the possibility (1). For the transition(1), all C-Cl bonds have to be parallel to the surface whennormal incidence (E//surface, E: electric vector of thelight). But, this is not the case considering that strong
enhancement of ion desorption is observed at normalincidence. The transition (2) is reasonable because all CCl bonds should be close to parallel to the surface when
grazing incidence (E⊥ surface), while parts of C-Cl bonds
are upwards when normal incidence and have apossibility to end in enhanced desorption.

75°
090°
00200

220
Photon Energy (eV)200220
Photon Energy (eV)+Fig. 1 Cl 2p-edge (a)Cl - and (b)TEY- NEXAFS spectra of solid C6D5Cl
for various polarization angles (θ). "θ" shows the incidence angles of Xray beam. The E vector of polarized light and the surface normal has anangle of θ.

Fig.2 Desorption mechanism proposed for observed angular dependenceof fragment-ion desorption.

*sekiguch@popsvr.tokai.jaeri.go.jp

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