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

2C, 7A/2004G320

Development of time-resolved XPS system and its application to the observationof transient adsorption states of NO on Pt(111) surfaces
Ikuyo NAKAI, Hiroshi KONDOH*, Kenta AMEMIYA,Toru SHIMADA, Masanari NAGASAKA, and Toshiaki OHTAThe University of Tokyo, Hongo, Bunkyo-ku, 113-0033, Japan
at the surface (∆t=0) and decreases in coincidence withthe decrease of the flux of the molecular beam. Theseresults indicate that transient adsorption of NO is inducedby the NO molecular beam.
On Pt(111) at 290 K, NO occupies the fcc-hollow site
(0.25 ML) [Ref. 2]. Unfortunately, no chemical shift isobserved for the molecular-beam induced species and it isdifficult to determine its adsorption sites from the XPspectrum. It was previously revealed that NO occupies
also the atop site at 240 K, which exhibits no N 1s
chemical shift [2]. We tentatively attribute the molecularbeam induced species to the NO molecules transiently
occupying the empty atop site. It decreases with almost
the same time profile as seen for the molecular beam (Fig.
2), which indicates that the lifetime of the transientlyadsorbed NO is shorter than the time resolution of thebeam width, 3.5 ms.
References[1] I. Nakai et al., PF Activity Report 21, 46 (2003).
[2] P. Zhu et al., Surf. Sci. 565, 232 (2004).
* kondo@chem.s.u-tokyo.ac.jp
Intensity [arb. units]

Without MB
With MB (∆t=0)
Difference

404403402

401

400399398

Binding Energy [eV]Fig.1 N 1s TRXP spectra with (∆t=0) andwithout NO molecular-beam irradiation.
Coverage ofnew species
TOF signal

0.8Coverage / θsat.

Introduction
Direct observations of adsorbates in transient states
during adsorption or reaction processes are important toelucidate their mechanisms. X-ray photoelectron
spectroscopy (XPS) measurements from monolayer
adsorbates at a time scale of several seconds have been
made possible. Further improvement of the time
resolution is necessary for the direct observation ofreaction intermediates or transient adsorption states. We
developed a time-resolved XPS (TRXPS) method byapplying the pump-probe technique to surface repeatablephenomena. To obtain photoemission signals in a shortperiod, we have used chopped x-ray generated by amechanical chopper, and have achieved a time resolutionof several milliseconds [1]. In order to improve the time
resolution further and to stabilize the incident x-rays, newdetection method was applied where the photoemission
signal is detected with a gated CCD camera undercontinuous x-ray irradiation. We applied the new method
to the observation of the transient adsorption states of NO
on Pt(111) surfaces, which is important as a fundamental
model for the NOx reduction catalyst.
ExperimentalThe experiments were carried out at BL-2C with anUHV system. The photoelectron spectra are obtained witha gated CCD camera for a short period after the arrival ofthe molecular beam at the surface with a delay time of ∆t.
A series of spectra are obtained with systematic variationof the delay time. The time resolution of the wholesystem is limited by the width of the molecular beam: thehighest time resolution was 500 µs. The time distributionof the molecular beam was measured with a time-of-flightmass spectrometer (TOF-MS) placed on the line.
A saturated overlayer of NO was prepared at 290 K.
TRXPS measurements were done under NO molecularbeam irradiation. The photon energy for the N 1s XPS
measurement was 530 eV. The time resolution was 2 ms,
and the repetition frequency was 0.2 Hz.
Results and discussion
Figure 1 shows N 1s TRXP spectra with (∆t=0ms)
and without the irradiation of the NO molecular beam on
the NO saturated surface at 290 K. It is evident that thecoverage of NO is increased by the molecular-beamirradiation. The increase in coverage is plotted in Fig. 2 asa function of the delay time ∆t with the profile of themolecular beam measured with the TOF-MS. The
coverage of the molecular-beam induced NO adsorbates
jumps up at the same time as the molecular beam arrives

0.6
0.4
0.2
0.0-10-5051015Time [ms]Fig.2 coverage of molecular-beam induced NO
adsorption as a function of the delay time and the
profile of the molecular beam measured with TOF.

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