Photon Factory Activity Report 2005 #23Part B (2006)
Surface and Interface

7A/2004G273

Configuration dependence of photon stimulated ion desorption
from methyl ester compounds induced by core excitation
Hiroyuki KIZAKI*, Yoshihiro MATSUMOTO, Hidenori BAN
Kenta MORISHITA, Shin-ichi WADA and Kenichiro TANAKA
Department of Physical Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
Introduction
Recently, it was found that site-selective ion desorption
is observed at resonant core excitations of C1s,O1s(OCH3)→ σ*(O-CH3) and O1s(OCH3)→ σ*(C-OCH3)
in methyl ester terminated self-assembled monolayer(MHDA-SAM, CH3OCO(CH2)15SH) [1,2]. Selective
chemical bond scission is achieved by core-electron
excitation due to its localization and atomic selectivity. Inaddition, it was suggested that the site-selective ion
desorption is closely related to inter- and intra-molecularconfigurations of reactive site [3].
In this work, photon-stimulated ion desorption (PSID)
from methyl mercaptoacetate (MA, CH3OCOCH2SH) andMHDA-SAM on Ag surfaces has been investigated using
soft x-ray around C and O K edges. MHDA has a long
methylene chain (-(CH2)15-), whereas MA has only one CH2-. Changes in site-selective ion desorption are
expected to be observed due to positional difference of
methyl ester groups with respect to the metal surface.
Experimental
Experiments were carried out at BL7A, whose details
were described elsewhere [3]. Partial ion yield (PIY)
spectra were measured by detecting selected ions using
time-of-flight (TOF) mass spectrometer, respectively.
Thin films were prepared by immersing Ag substrate into
1.0 mM ethanol solutions of MHDA and MA.
Results and Discussion+Figure1 shows PIY spectra of CH3 and OCH+ for MAand MHDA obtained in C and O K-edge core excitation
regions. Ion yield exhibited enhancement at C1s,O1s(OCH3) → σ*(O-CH3). Especially for MA, smaller ion
yield were observed at C1s(OCH3) → σ*(O-CH3) as+compared with that of MHDA. Moreover, OCH ion
intensity is also enhanced characteristically at O1s (OCH3)→ σ*(C-OCH3) in MHDA, while the intensity in MA did
not increase. The decrease of PIY at C1s(OCH3) → σ*(OCH3) is induced by change of neutralization probability in+desorbing ions. Figure 2 shows TOF spectra of CHn (n:1+3) in the C K-edge. The different fragment pattern in CHnions at C1s(OCH3) → σ*(O-CH3) represents massdependence of neutralization process, that is, heavier ions
are neutralized easily for desorption reaction. In the O K+edge, the PIY for CH3 in MHDA resembles that in MA,+
but not for OCH . Though the peaks around 532 and+536eV are observed for OCH slightly, the site-selectiveion desorption disappear in MA. This result can beinterpreted as relaxation effect on ion desorption due tostrong interaction with the metal surface.

60Users' Report

Fig. 1 Typical comparison of PIY spectra for MHDA+
SAM and MA on Ag surfaces. Left: CH3 in C K-edge.
++
Right: CH3 and OCH in O K-edge.

+

Fig.2 TOF spectra of CHn (n:1-3) for MHDA-SAM andMA in C K-edge.
References[1] S. Wada et al., Nucl. Instr. and Meth. in Phys. Res. B
199, 361 (2003).
[2] H. Kizaki et al., J. Electron Spectrosc. Relat. Phenom.
144-147, 447 (2005).
[3] S. Wada et al., Surf. Sci. 528, 242 (2003).
*kizaki@hiroshima-u.ac.jp