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

7A/2002G273

Polarization-dependent ionic dissociation of methyl ester terminatedself-assembled monolayers by core-electron excitations

Experimental
Measurements of NEXAFS and polarization-dependent
PSID were carried out at the beamline BL7A. Although
the polarization dependence of NEXAFS can be measured
by rotating a sample holder, time-of-flight mass
spectrometer (TOF-MS) is generally installed in achamber at a fixed direction. So, if the surface normal
direction deviates from the TOF direction, detection yieldof desorbing ions falls down rapidly. In this study, therotatable sample holder is equipped with additionalcorrection electrodes at 2 mm away from the each side of
the sample to apply appropriate gradient of electricalpotential for the desorbing ions in the off-normalarrangements between the sample and TOF-MS. As there
is limitation of off-normal angle range measurable by thecorrection electrodes, two equivalent TOF devices areinstalled in the chamber at 30° and 70° (30° and 70°TOFs) from the incident SR to make measurements
possible in wide incident angle range (10 - 80°) [3].
Results and Discussion
Typical results obtained for deuterated methyl esterterminated SAM (CD3OCO(CH2)15S/Au) in the C K-edge++region are shown in Fig. 2. Intensities of CD3 and D ions*in the σ (O-CD3) excitation obviously depend on the+incident angle of SR. On the other hand, H desorption is
independent of the angle in whole excitation region. Ions

92

Users' Report(a) CD3+(b) Dθ / degree

1020406070802010+
MCP20TOFSRSample

θ1000286

28829029228629430(c) H288290292294

Photon Energy / eV

Photon Energy / eV
Intensity / arb. units

Introduction
Polarization dependence of near edge X-ray absorption
fine structure (NEXAFS) spectroscopy using linearlypolarized SR is one of powerful tools especially toinvestigate orientation and structure of adsorbed
molecules on surfaces conveniently [1]. As opposed toabundant spectroscopic NEXAFS studies, however,polarization dependence of photon stimulated ion
desorption (PSID) induced by core excitations has rarely
been carried out because of much experimental difficulties.
Recent our studies have found the site-selective ion
desorption reactions of methyl ester compounds, such as
polymer thin film and self-assembled monolayers (SAMs)[2]. The purpose of this study is to measure polarization
dependence of each PSID in core excitations of methyl
ester-terminated SAMs as an oriented molecular systemand to investigate whether the polarization dependence isconserved even in ion desorption phenomena and what
information about desorption dynamics can be derived
from the dependence.

Intensity / arb. units

Shin-ichi WADA*, Ryohei SUMII, Hiroyuki KIZAKI, Yoichi IIZUKA,
Yoshihiro MATSUMOTO, Tetsuji SEKITANI, Kenichiro TANAKA
Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan+

25

20

10(d) σ*(O-CD3)
+

20

H+D+
CD2

15CD3+

105028628829029229400Photon Energy / eV

10 20 30 40 50 60 70 80 90

Incident Angle, θ / degree

Fig. 1 (a)-(c) Normalized PIY spectra of CD3+ ,D+, and H+ ions measured at different incident anglesof SR. Spectra at 10-40° and 60-80° were measured
using 70°-TOF and 30°-TOF, respectively. (e)Incident angle dependence of integrated PIY
intensities of H+, D+, CD2+, and CD3+ ions for the net
σ*(O-CD3) excitation.
which desorb site-selectively indicate significant
polarization dependence, but non-selective ions rarely
show the dependence. These findings indicate that siteselective ion desorption takes place only within thefunctional group where core electron is excited and theinitial memory of the core excitation is effectively
conserved during desorption.
Trends of the incident angle dependence of the net*σ (O-CD3) intensities depicted in Fig. 2(d) differ indesorbing ions as well as resonant excitations. Variation of
the dependence can be understood by the different
superiority of dissociation/desorption mechanisms: direct
dissociation induced by core excitation and indirect
dissociation caused by rapid energy relaxation and X-rayinduced electron stimulated desorption.
References[1] J. Stöhr, NEXAFS Spectroscopy, Surface Sciences
vol.25 (Springer, Berlin, 1992).
[2] S. Wada et al., Surf. Sci. 528 (2003) 242.
[3] S. Wada et al., Surf. Sci. (2005) in press.
* swada@sci.hiroshima-u.ac.jp