24P130-R
Development of a pulsed beam method for slow metastable helium atoms
低速準安定ヘリウム原子のパルスビーム技術開発
Yasushi Yamauchi, Mitsunori Kurahashi and Taku Suzuki
山内泰，倉橋光紀，鈴木拓National Institute for Materials Science1-2-1, Sengen, Tsukuba 305-0047, Japan
物質･材料研究機構, 〒305-0047 つくば市千現1-2-1
The use of desorption measurements to identify the adsorbed species on surfaces is important forcharacterizing plasma facing and tritium breeder materials. Among them the knowledge of molecules
adsorbed on outermost surfaces is indispensable to elucidate the rate-determining surface reaction. A slow
metastable helium atom beam can exclusively stimulate the adsorbate to desorb. To efficiently detect
desorption signals and to attain a higher mass resolution, a mechanical chopper is considered here as afundamental technique of pulsing a slow metastable helium beam.

atoms are reflected above the surface so thatinteractions are definitely restricted within atoms at
the outermost surface. This is the metastable–atomstimulated desorption (MSD) [1]. Furthermore, theelectron spin of metastable atom beams that can bepolarized has the potential to offer an insight into
DIET [2].
Electrons
0hν & Fast He(arb.
units)

1. Introduction
To develop plasma facing materials with highplasma resistance, surface reactions, such as
chemical sputtering or tritium inventory, have to beclarified. In the case of tritium release from breeder
materials, the rate-determining surface reaction canbe explained as the adsorption / desorption
characteristics of surfaces. The dissociation of thesurface constituents followed by desorption fromthe surface during dissipation of the electronic
excitation energy is known as desorption inducedby electron transition (DIET), which provides thebasis for surface analysis techniques.

400He*

Positive ions200

Pass energy 7eV0400
200

6 eV

0400

Intensity (Counts)

2. Desorption Measurements
The major approaches for DIET have been
electron- and photon-stimulated desorption (ESD,PSD). It is usually assumed in ESD and PSD thatthe desorbed ions stem from the outermost layer
although beams of energetic electron or photons areused and their transmissivities are large enough toexcite the atoms in deeper layers. The possibilityof ion transport from deeper layers to the vacuum
has arisen against the conventional assumption.
Shorter transmissivity can be provided by using
low-energy ions as the primary beam. Since ionscarry potential energies (ionization energy) in theirinternal electron systems besides kinetic energies,
ion-stimulated desorption (ISD) (or ion sputtering)consists of the kinetic energy-dependent component
and the potential component (or potentialsputtering). The potential component is due to
DIET at the outermost surface where the potential
energy transferred from the primary ion. ISD can be
purified into genuine DIET right on the outermost
surface by using thermal atoms in their metastablestates instead of primary ions. Thermal metastable200

5 eV0400

M/e
1200

1 15M/e15

4 eV0400200

3 eV0400
2 eV200

1 150400

M/e M/e
1152000400x7200

1 eV

0.5 eV

00100

200300400Time of flight (μsec)

500

Fig.1. TOF spectra for dodecanethiolate-SAM
formed on an Au substrate obtained by a pulsed
beam consisting of metastable He, fast He and UV
photon. Top panel displays emitted electron
intensity. Emitted positive ion intensities are plottedin the lower eight panels for various pass energies[3].

4. Double Chopper
The rather poor mass resolution in this MSD
measurement is a result of the wide pulse width ofthe primary beam at the sample position. Though
the pulse width is narrow at the metastable helium
beam source which is operated in the pulsed
discharge mode, it becomes wider at the sample
point due to the flight distance and wide energy
distribution of an effusive beam. In order to attain afine mass resolution we have to introduce amechanical chopper close to the sample to get a

Intensity (arb. units)

3. Metastable-Atom-Stimulated Desorption
As an example of MSD, the results oftime-of-flight (TOF) measurement for a SAM
surface are shown in Fig. 1. Larger peaks
distributed from 0 to 200 μ sec in the TOF spectraof positive ions (the second to the seventh panel
from the top) correspond to secondary ions of H+and CHx + sputtered by fast helium neutral atoms
contained in the primary beam. Smaller and broader
peaks after 200 μ sec in the fifth to the eighth panelof Fig. 1 correspond to MSD ions of H+ and CHx +induced by slow metastable helium atoms. From thecomparison of these peaks and the arrows, each ofwhich indicates the calculated arrival time for themass charge ratios (M/e) of 1 (H) or 15 (CH3), it
can be seen that the species of M/e 15 or so isdominant at a pass energy of 1 eV.

FWHM 5μsec

05101520253035404550

Time (μsec)

Fig.3. Chopping performance of the double tuning
fork chopper system. A laser beam was used tosimulate pulsing the metastable He beam.

narrow pulse width at the sample position.
Aside from conventional chopper wheels, weadopted tuning fork choppers instead because of
their ultra high vacuum compatibility (ElectroOptical Products Corporation CH10). By vibrating
two choppers in an almost opposite phase, anarrow pulse can be formed as illustrated in Fig. 2.
Figure 3 shows its pulsing performance examinedby the laser beam experiment. With the aid of aphase- locked loop that precisely controls the phasedifference between two choppers, the chopping
system successfully pulsed the laser beam down toan FWHM of 5 μ sec at a repetition rate of 300 Hz.
We infer that the chopping performance isfeasible for use in pulsing the metastable-atombeam. The pulsed beam method employing thedouble chopper will be expected to improve themass resolution by an order of magnitude or so.
Acknowledgment
This work was partly supported by a Grant-inAid for Scientific Research 23656580.
References[1] M. Kurahashi and Y. Yamauchi: Phys. Rev. Lett. 84(2000) 4725.
[2] T. Suzuki, M. Kurahashi,; Y. Yamauchi, T. Ishikawa
and T. Noro: Phys. Rev. Lett. 86 (2001) 3654.
[3] Y. Yamauchi, T. Suzuki, M. Kurahashi and X. Ju:
Phys. Chem. B 107 (2003) 4107.

Fig.2. Operation schematics of the double tuning
fork chopper system. The first and the second
chopper are placed as close as possible.