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

8A/2004G017

Isotope effects in H+ (D+) desorption induced by4a1 ← O 1s resonant transition of condensed H2O (D2O)
Akira NAMBU1*, Eiichi KOBAYASHI1,2, Kazuhiko MASE1,3
1

KEK-PF, Tsukuba, Ibaraki 305-0801, Japan
Inoue Foundation for Science, 15-15 Nanpeicho, Shibuya-ku, Tokyo 150-0036, Japan
3PRESTO, JST, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan(* Present Address: Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000)

2ExperimentalAll the experiments were performed with a homemade
EICO analyzer which consists of a coaxially symmetricmirror analyzer for electron and a miniature time-of-flight(TOF) mass spectrometer for ions [1]. A soft x-ray
beamline of 8A was employed for the experiments.
Water (H2O/D2O) was purified by the cycles of freezepump-thaw and the liquid-nitrogen-cooled (~80 K) goldplated surface was exposed to 50 Langmuir (1 Langmuir-6= 1 × 10 Torr·s) of H2O/D2O molecules to form anamorphous condensed layer.

3000

D2O
H2O1400

2500

12002000

1000

15001000
500
440

800

α'γ'460600

β'480400500200520

Auger Kinetic Energy (eV)Fig. 1 AEPICO Spectra of H2O and D2O.

60Users' Report

Coincidence Intensity (H 2O) / 600 sec

Coincidence Intensity (D 2O) / 2400 sec

Results and Discussion
The result of AEPICO is shown in Fig. 1. In AEPICOspectrum, at first, the amount of photoion desorbed at thesame time (within ~1 ns) of certain energy Auger electron
is measured as a TOF spectrum. Next, those amounts ofdesorbed ion are plotted as a function of (Auger) electronkinetic energy to reproduce the AEPICO spectra. Thus,the AEPICO spectra show the relationship of the Auger-

final-state and ion desorption probability. Also, theanalysis of the TOF data allows us to reveal the kinetic
energy distribution of desorbed ion (Fig. 2).
As shown in Fig. 1 and Fig. 2, several kinds of isotope
effects were observed in AEPICO spectrum and kinetic
energy distribution of desorbed ion kinetic energy; (1)peak shifts in AEPICO (Fig.1) and resonant Auger
electron spectra (not shown), (2) the relative intensity
differences in AEPICO spectra (Fig. 1), (3) the
differences in ion kinetic energy for the selected Augerfinal-states (Fig. 2) and (4) the intensity difference inprominent peak in ion yield spectra (not shown).
Those effects can be explained by the four-stepdesorption mechanism which employs three potential
energy surfaces and ultrafast O-H (O-D) bond extension
within the lifetime of the O 1s core-hole [1]. With thismechanism, when the H is replaced by the D whose mass
is as twice as H, the length of ultrafast extension should
be shorter than that of H, thus that the desorption isaffected by this mass difference [2].
References[1] K. Mase, M. Nagasono, S. Tanaka, T. Sekitani and S.
Nagaoka, Fizika Nizkikh Temperatur 29 (2003) 321 (LowTemperature Physics 29 (2003) 243).
[2] A. Nambu et al., Surf. Sci., in press.
* akira.nambu@bnl.gov

Ion Kinetic Energy (eV)Introduction
In order to study the interactions between watermolecules and photon at 4a1 ← O 1s resonant transitionenergy, Auger electron – photoion coincidence (AEPICO)
experiments have been performed for condensed water(H2O and D2O). Several kinds of isotope effect were++observed in H (D ) desorption. Those isotope effects are
successfully explained by a four-step ion desorption
mechanism involving O-H (O-D) bond extension within
the lifetime of the O 1s core hole.

5.0

D2O
H2O

4.5
4.0
3.5
3.0
2.5
460

470

480
490
500
510
Auger Kinetic Energy (eV)520

Fig. 2 Kinetic Energy Distribution ofDesorbed Photoion.