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Detection of hydrogen adsorbed on tungsten surfaces using the 1H(15N, αγ)12C reaction
574
Section
Nuclear Instruments and Methods in Physics Research B33 (1988) 574-577 North-Holl~d, Amsterdam
VIII. ion beam analysis
DETECTION OF HYDROGEN ADSORBED USING THE ‘H(“N, ay)“C REACTION
ON TUNGSTEN
SURFACES
Yasushi IWATA, Fuminori FUJIMOTO, Eugeni VILALTA, Akio OOTUKA, Ken-ichiro Koichi KOBAYASHI I), Hiroshi YAMASHITA ‘) and Yoshitada MURATA ‘)
KOMAKI,
College of Arts and Sciences, University of Tokyo, Komaba 3-8-I. Meguro-ku, Tokyo 153, Japan ‘) Research Center for Nuclear Science & Technolo~, university of Tokyo, Yayoi t-ii-16, Bunkyo-k~ Tokyo 113, Japan 2, Department of Physics, Faculty of Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ky Tokyo 113, Japan ‘) Institute for Solid State Physics, University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106, Japan
An ultrahigh vacuum (UHV) system for the ‘H(lSN, ay)“C reaction analysis of hydrogen adsorbed on well-defined surfaces was constructed. The detection limit of hydrogen coverage on the W(OO1)surface was 4 X lOi H atoms/cm2 (l/50 of the full monolayer coverage). The ion-stimulated desorption process for hydrogen atoms due to the “N ion beam was studied by measuring the decay of hydrogen coverage on W(OO1)at low background gas pressures, and the desorption cross section was obtained as 2.4~ lo-r6 cm*. Hydrogen coverage on the W(110) surface compared with that on W(OO1) was estimated from the intensity profiles given by measurement of the resonance energy width.
1. Introduction Hydrogen adsorption is one of the most fundamental processes in gas-surface interactions. Detection of surface hydrogen, however, is so difficult that the adsorbed position and dynamics of hydrogen atoms on surfaces have scarcely been studied. In a previous letter we reported that a nuclear reaction, ‘H(i’N, ay)‘*C, was applied for the first time to the detection of adsorbed hydrogen atoms on the W(OO1) surface, and made it clear that this reaction is an excellent method for the quantitative chemical analysis of adsorbed hydrogen on the surface [l]. The reaction at the resonance energy of 6.385 MeV with a large cross section of 300 mb has a large Q-value, and high energy y-rays of 4.43 MeV are released. The resonance energy width of 1.8 keV was measured with high precision by Maurel and Amsel [2]. Using this narrow resonance, the vibrational mode of hydrogen atoms on the surface can be examined by the Doppler broadening of the resonance energy width. This prominent application of the nuclear reaction was first performed by Zinke-Allmang et al. [3], and was subs~uently examined for hydrogen atoms on the well-defined W(OO1) surface by the present authors [l]. The zero-point energy of hydrogen atoms vibrating in the direction perpendicular to the surface was obtained as (64 + 16) meV, which is consistent with 65 meV obtained from HREELS [4,5] and with 68 meV from IRAS [6]. Hydrogen on W(OO1) is the most extensively studied system for the hydrogen adsorption on a well-defined metal surface, and the absolute coverage is 2.0 x 1015 H atoms/cm2 at the full monolayer coverage. In the case of the (110) surface the full monolayer 0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holl~d Physics abusing Division)
B.V.
coverage based on ratios relative to W(OO1) was shown to have values from 0.7 to 0.9 ]7,8]. In this article we describe the ultrahigh vacuum (UHV) system constructed for the nuclear reaction analysis of hydrogen on well-defined surfaces, demonstrate the detection limit of the ‘H(t5N, ay)“C reaction method for surface hydrogen and investigate the ion-stimulated desorption process. Hydrogen coverage on W(110) compared with that on W(OO1) was also measured.
2, Experimental The experiment was carried out at a 5 MV vertical tandem type Van de Graaff accelerator in the Research Center for Nuclear Science & Technology, University of Tokyo. CN- ions produced from a CH, and ‘sNH, (95% enriched) gas mixture in a duo-plasmatron ion source were injected into the accelerator with the energy of 60 keV. 15N *+ ions accelerated at the resonance energy of 6.385 MeV were selected by a 90° magnetic energy analyzer. The orbital radius of the ion beam in the analyzer was 800 mm. The terminal voltage of the accelerator was stabilized by a slit feedback system. The magnetic field stability of the energy analyzer was better than l/75000. Fig. 1 shows the IJHV system which was constructed following the accelerator. In the main chamber for the specimen two samples were set up: one was W(OO1) for reference and the other was W(110) which was of primary interest. These samples could be moved up and
575
Y. Iwota et al. / Hydrogen adsorption on tungsten
AES
(
0
2i011sec) Ion
were detected by a 50 mm x 50 mm BGO (Bi,Ge,O,,) scintillation detector. Since BGO has a large absorption coefficient because of its large effective atomic number, a small BGO, which detects lower background yields, has a large efficiency for high energy y-rays. Output signals of a photomultipfer tube connected to the BGO were amplified and collected via a single channel analyzer over the energy range from 3.0 to 4.8 MeV. The in-beam background level in this region was as low as 0.05 cps, so that a low detection limit for the hydrogen was achieved.
‘RhW;;C= pump
\,
Faraday
cup
3. Results and discussion
20 cm
Fig. 1. Ultrahigh vacuum (UHV) system constructed for the nuclear reaction analysis of hydrogen adsorbed on well-defined surfaces. i5NZ+ ion beam was introduced into the system via two apertures set for differential pumping, and deflected to avoid surface contamination.
down along the center axis of the cylindrical main chamber. A large port of 178 mm in diameter was behind the target for the y-ray detector which could be placed close to the target at a distance of 15 mm while remaining outside the UHV chamber. A 4-fold LEED-AES optics system was mounted in the chamber, and the inner space of the chamber was shielded from magnetic field by p-metal. The main chamber was evacuated by a sputter ion pump. The vacuum of the beam duct was 5 x lo-’ Torr and that of the main chamber was 2 X lo- lo Torr. The UHV system was connected to the beam duct of lower vacuum via a differential pumping system with a turbomolecular pump. Two apertures of 3.2 and 3.4 mm in diameter and of 8 mm in thickness were set at the entrance and the exit of the differential pumping system, respectively. The 15N2+ ion beam introduced into the UHV system was deflected 2.5” from the incident direction by an electrostatic deflector placed at the middle chamber, in order to avoid surface contamination due to neutral contaminant molecules, such as hydrocarbons, coming from the lower vacuum region in the beam duct. The electrostatic potential was turned off when the beam current was measured by Faraday cup. The beam current was 15-20 nA (particle ampere). The beam current was monitored at the two apertures of the differential pumping system in order to prevent hitting these plates. This monitoring is very important, because the incident beam at resonant energy hitting other parts than the target gives rise to background nuclear reaction yields from the hydrogen on them. The pressure of introduced H, gas was controlled by a variable leak valve. The W(OO1) and W(110) clean surfaces were prepared in the same way as mentioned before [l]. y-rays
The y-ray yield per unit dose, Y, is written as Y= P,%(~o>S,
(1)
where pu is the hydrogen density on the surface, uN( E,) the nuclear reaction cross section at the energy E, of incident ions and r~ the efficiency of the detection. The ratio of Y to Yr, the yield at full monolayer coverage, indicates the relative hydrogen coverage, 0 = Y/Y,,
(2)
Fig. 2 shows the hydrogen adsorption process on W(OO1) at several hydrogen pressures, p(H,). The relative hydrogen coverage gradually increases with time until it reaches a saturation value, which increases with hydrogen pressure. The pressure dependence of the saturation value shows that the desorption of hydrogen is caused by the incident ion beam. At hydrogen
.
Ok
0
p(H2)
= 9.1 x 10’B(T~rr)
*
4.5.x
x
9.1 x 10-g
0
4.5x
I
10-8
10-g I
300
600 Time
( set
900
1200
)
Fig. 2. Hydrogen adsorption on W(OO1) at several hydrogen pressures, p(H,). The relative coverage gradually increases with time until it reaches a saturation value. The pressure dependence of the value shows that the ion-stimulated desorption of hydrogen atoms occurs due to the incident ion beam. At hydrogen pressures above 4.5X10-* Torr the saturation values reach the full monolayer coverage. VIII. ION BEAM ANALYSIS
Y. Iwaia et al. / Hydrogen adsorption on tungsten
576
pressures above 4.5 X lo-* Torr the saturation values reach a constant value. This fact indicates that the hydrogen pressure was so high that the ion-stimulated desorption process is negligible. Thus the constant saturation value at hydrogen pressures above 4.5 X lo-* Torr was taken as the full monolayer coverage. Yr was 50 times as large as twice the number of the background counts, which was considered as the lower limit for the counting rate of y-ray yields for the present 15N2+ ion beam current of 15 nA. The detection limit of adsorbed hydrogen was estimated from these data as 4 x lOI3 H atoms/cm2 which was l/50 of the full monolayer coverage on W(OO1). The ion-stimulated desorption, appearing as the hydrogen-pressure dependence of the saturation values in fig. 2, can more clearly be seen in the decay process of the hydrogen coverage which occurred after the variable leak valve was closed. The time variation of the hydrogen coverage is given by
d@ ‘de)
-=
df
_zo
PW
B
d >
(4)
where 8, is the initial coverage at the time of closing the valve and et, the saturation coverage in the background pressure. At the present decay process the beam current was 20.1 nA, tit, = 0.53 and 6, = 0.15. The background pressure was 5 X lo- to Torr after the valve was closed. Fig. 3 shows the desorption process on a semi-logarithmic scale. The slope shown as a solid line was nsa/p, + la, = (2.1 k 0.1) x 10K3/s. Since at the present background pressure this value is 20 times as large as the value of ns,/p w , ns,/p w + lad can be replaced by lo,. To determine the desorption cross section we must consider the intensity distribution of the ion beam. In the analysis mentioned above we assumed that the beam intensity in the irradiated area was uniform. The ion-stimulated desorption rate, however, is locally different in accordance with the intensity distribution of the ion beam. Then the rate equation for the local density of hydrogen, P&T), at the distance r from the beam center is written as
dt
z(2pW
-
----__
Process
------_---
2 -0.5. F? I
aii
-1.0
Ii3 -
c
-
-1.5. -2.0. -24
SO0
!OOO
1500 Tim
2000
2500
lsscl
Fig. 3. Desorption process plotted on a semi-logarithmic scale. The solid line shows lo, and the dotted line indicates the best fit curve given by Ok= 2.6 X lo-l6 cm*.
where i(r) is the intensity distribution of ion beam normalized as 2 r j?r i(r) d r = 1. Solving eq. (5), the y-ray yield is obtained as m
s(r)=e~-(8,-8,)e\p(-(~+lo,)i),
dPH(r) =
ion StimulatedDcsorpti on
o.o!
Y(t)
where n is the impinging rate of hydrogen molecules, s( 6) the sticking probability, pw the number density of the top-most W atoms on W(OOl), Z the current density of the incident beam and Us the ion-stimulated desorption cross section. Since n is small in the desorption process, the B-dependence of s(B) has little influence on de/d t. Then it is convenient to adopt the simplest form of s,(l - 8) for s( 6) [9]. Solving the rate equation (3), we obtain the solution of B(t) as
~
0.5
pH(r)) - i(r)uddPH(Y)%
c5)
10 ri(r)pH(r,
= 2n-7jUN
t) dr,
Pr_r(U,t> = Pb(r) + (PO- P*(r))
at z = 0, PH( r) is considered to be pa and at = pb(r). Fitting the decay process by the relative coverage 0 = Y( t)/Y, obtained from eq. (6), the desorption cross section was determined to be cd = 2.4 X lo-l6 cm2. The fitting curve is shown as a dotted line in fig. 3. The relative coverage derived from eq. (6) differs from the one obtained by eq. (4) after long irradiation times. The narrow resonance energy width of 1.8 keV broadened to (9.2 i 1.0) keV in the case of the normal incidence of the 15N ion beam to the W(OO1) surface, due to the Doppler effect of hydrogen atoms vibrating in the direction perpendicular to the surface [l]. This effect was evident from the intensity distribution of y-ray yields for the relative energy of “N to ‘H in the vicinity of the resonance energy. The distribution represents the state of hydrogen atoms in the momentum space on the surface. The y-ray yields measured for several adsorption systems at constant ion beam energy, e.g. the resonance energy, take different values depending on the states, even if the same atomic densities of hydrogen are adsorbed on the surfaces. Thus, in order to compare the hydrogen coverages of the different adsorption systems, the areas of their intensity profiles of y-ray yields must be examined. Fig. 4 shows the observed intensity profiles, using an ion beam with a sufficiently good energy stability, for H on W(OO1) and H on W(110). The full widths at half-maximum of the yield curve for H on W(110) are a little larger than where
t = 00 as pH(r)
571
Y. Iwata et al. / Hydrogen adsorption on tungsten
0.0
H an
WI0011
r.xH on WC1101
was determined W(OO1) system.
P o+oo .O 0. .
xx .
x
-10
-5
0 E - Eres
*
5
:x 54 I..
IO
IkeVl
Fig. 4. Normalized y-ray yield curves for W(OO1)and W(110). The area of the profiles, representing the hydrogen coverage on the surface, is (0.7?0.1) times smaller for H on W(110) than that for H on W(OO1).
those for H on W(OO1). The area of the profiles for H on W(110) showing the hydrogen coverage on the surface is (0.7 f 0.1) times smaller than that for H on W(OO1).
4. Conclusion The ‘H(15N, cxy)i2C reaction has several excellent properties for the applications to hydrogen analysis on well-defined surfaces. The detection limit of hydrogen coverage, examined for the adsorption system of H on W(OOl), was 4 X 1013H atoms/cm2 which was l/50 of the full monolayer coverage. Ion-stimulated desorption of hydrogen due to the energetic “N ion beam takes place during the analysis. The desorption cross section
to be 2.4 x lo-l6
cm2 for the H on
The authors are grateful to Dr T. Nozaki, Institute of Physical and Chemical Research, and Professor K. Ono, Institute for Solid State Physics, for supplying us a BGO scintillator. They are also indebted to members of the Machine Shops of the College of Arts and Sciences and the Institute for Solid State Physics for the construction of their apparatus. This work is supported by a Grant-in-Aid for the Special Research Project on Ion Beam Interactions with Solids from Ministry of Education. Science and Culture.
References [l] Y. Iwata, F. Fujimoto, E. Vilalta, A. Ootuka, K. Komaki, K. Kobayashi, H. Yamashita and Y. Murata, Jpn. J. Appl. Phys. 26 (1987) L1026. [2] B. Maurel and G. Amsel, Nucl. Instr. and Meth. 218 (1983) 159. See also H. Damjantschitsch, M. Weiser, G. Heusser, S. Kalbitzer and H. Mannsperger, Nucl. Instr. and Meth. 218 (1983) 129. [3] M. Zinke-Allmang, S. Kalbitzer and M. Weiser, Z. Phys. A323 (1986) 183. [4] W. Ho, R.F. Wills and E.W. Plummer, Phys. Rev. Lett. 40 (1978) 1463. [5] M.R. Barnes and R.F. Wills, Phys. Rev. Lett. 41 (1978) 1729. [6] Y.J. Chabal, Phys. Rev. Lett. 55 (1985) 845. [7] B.D. Barford and R.R. Rye, J. Chem. Phys. 60 (1974) 1046. [S] P.W. Tamm and L.D. Schmidt, J. Chem. Phys. 54 (1971) 4115. [9] T.E. Madey, Surf. Sci. 36 (1973) 281.
VIII. ION BEAM ANALYSIS
Section
Nuclear Instruments and Methods in Physics Research B33 (1988) 574-577 North-Holl~d, Amsterdam
VIII. ion beam analysis
DETECTION OF HYDROGEN ADSORBED USING THE ‘H(“N, ay)“C REACTION
ON TUNGSTEN
SURFACES
Yasushi IWATA, Fuminori FUJIMOTO, Eugeni VILALTA, Akio OOTUKA, Ken-ichiro Koichi KOBAYASHI I), Hiroshi YAMASHITA ‘) and Yoshitada MURATA ‘)
KOMAKI,
College of Arts and Sciences, University of Tokyo, Komaba 3-8-I. Meguro-ku, Tokyo 153, Japan ‘) Research Center for Nuclear Science & Technolo~, university of Tokyo, Yayoi t-ii-16, Bunkyo-k~ Tokyo 113, Japan 2, Department of Physics, Faculty of Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ky Tokyo 113, Japan ‘) Institute for Solid State Physics, University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106, Japan
An ultrahigh vacuum (UHV) system for the ‘H(lSN, ay)“C reaction analysis of hydrogen adsorbed on well-defined surfaces was constructed. The detection limit of hydrogen coverage on the W(OO1)surface was 4 X lOi H atoms/cm2 (l/50 of the full monolayer coverage). The ion-stimulated desorption process for hydrogen atoms due to the “N ion beam was studied by measuring the decay of hydrogen coverage on W(OO1)at low background gas pressures, and the desorption cross section was obtained as 2.4~ lo-r6 cm*. Hydrogen coverage on the W(110) surface compared with that on W(OO1) was estimated from the intensity profiles given by measurement of the resonance energy width.
1. Introduction Hydrogen adsorption is one of the most fundamental processes in gas-surface interactions. Detection of surface hydrogen, however, is so difficult that the adsorbed position and dynamics of hydrogen atoms on surfaces have scarcely been studied. In a previous letter we reported that a nuclear reaction, ‘H(i’N, ay)‘*C, was applied for the first time to the detection of adsorbed hydrogen atoms on the W(OO1) surface, and made it clear that this reaction is an excellent method for the quantitative chemical analysis of adsorbed hydrogen on the surface [l]. The reaction at the resonance energy of 6.385 MeV with a large cross section of 300 mb has a large Q-value, and high energy y-rays of 4.43 MeV are released. The resonance energy width of 1.8 keV was measured with high precision by Maurel and Amsel [2]. Using this narrow resonance, the vibrational mode of hydrogen atoms on the surface can be examined by the Doppler broadening of the resonance energy width. This prominent application of the nuclear reaction was first performed by Zinke-Allmang et al. [3], and was subs~uently examined for hydrogen atoms on the well-defined W(OO1) surface by the present authors [l]. The zero-point energy of hydrogen atoms vibrating in the direction perpendicular to the surface was obtained as (64 + 16) meV, which is consistent with 65 meV obtained from HREELS [4,5] and with 68 meV from IRAS [6]. Hydrogen on W(OO1) is the most extensively studied system for the hydrogen adsorption on a well-defined metal surface, and the absolute coverage is 2.0 x 1015 H atoms/cm2 at the full monolayer coverage. In the case of the (110) surface the full monolayer 0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holl~d Physics abusing Division)
B.V.
coverage based on ratios relative to W(OO1) was shown to have values from 0.7 to 0.9 ]7,8]. In this article we describe the ultrahigh vacuum (UHV) system constructed for the nuclear reaction analysis of hydrogen on well-defined surfaces, demonstrate the detection limit of the ‘H(t5N, ay)“C reaction method for surface hydrogen and investigate the ion-stimulated desorption process. Hydrogen coverage on W(110) compared with that on W(OO1) was also measured.
2, Experimental The experiment was carried out at a 5 MV vertical tandem type Van de Graaff accelerator in the Research Center for Nuclear Science & Technology, University of Tokyo. CN- ions produced from a CH, and ‘sNH, (95% enriched) gas mixture in a duo-plasmatron ion source were injected into the accelerator with the energy of 60 keV. 15N *+ ions accelerated at the resonance energy of 6.385 MeV were selected by a 90° magnetic energy analyzer. The orbital radius of the ion beam in the analyzer was 800 mm. The terminal voltage of the accelerator was stabilized by a slit feedback system. The magnetic field stability of the energy analyzer was better than l/75000. Fig. 1 shows the IJHV system which was constructed following the accelerator. In the main chamber for the specimen two samples were set up: one was W(OO1) for reference and the other was W(110) which was of primary interest. These samples could be moved up and
575
Y. Iwota et al. / Hydrogen adsorption on tungsten
AES
(
0
2i011sec) Ion
were detected by a 50 mm x 50 mm BGO (Bi,Ge,O,,) scintillation detector. Since BGO has a large absorption coefficient because of its large effective atomic number, a small BGO, which detects lower background yields, has a large efficiency for high energy y-rays. Output signals of a photomultipfer tube connected to the BGO were amplified and collected via a single channel analyzer over the energy range from 3.0 to 4.8 MeV. The in-beam background level in this region was as low as 0.05 cps, so that a low detection limit for the hydrogen was achieved.
‘RhW;;C= pump
\,
Faraday
cup
3. Results and discussion
20 cm
Fig. 1. Ultrahigh vacuum (UHV) system constructed for the nuclear reaction analysis of hydrogen adsorbed on well-defined surfaces. i5NZ+ ion beam was introduced into the system via two apertures set for differential pumping, and deflected to avoid surface contamination.
down along the center axis of the cylindrical main chamber. A large port of 178 mm in diameter was behind the target for the y-ray detector which could be placed close to the target at a distance of 15 mm while remaining outside the UHV chamber. A 4-fold LEED-AES optics system was mounted in the chamber, and the inner space of the chamber was shielded from magnetic field by p-metal. The main chamber was evacuated by a sputter ion pump. The vacuum of the beam duct was 5 x lo-’ Torr and that of the main chamber was 2 X lo- lo Torr. The UHV system was connected to the beam duct of lower vacuum via a differential pumping system with a turbomolecular pump. Two apertures of 3.2 and 3.4 mm in diameter and of 8 mm in thickness were set at the entrance and the exit of the differential pumping system, respectively. The 15N2+ ion beam introduced into the UHV system was deflected 2.5” from the incident direction by an electrostatic deflector placed at the middle chamber, in order to avoid surface contamination due to neutral contaminant molecules, such as hydrocarbons, coming from the lower vacuum region in the beam duct. The electrostatic potential was turned off when the beam current was measured by Faraday cup. The beam current was 15-20 nA (particle ampere). The beam current was monitored at the two apertures of the differential pumping system in order to prevent hitting these plates. This monitoring is very important, because the incident beam at resonant energy hitting other parts than the target gives rise to background nuclear reaction yields from the hydrogen on them. The pressure of introduced H, gas was controlled by a variable leak valve. The W(OO1) and W(110) clean surfaces were prepared in the same way as mentioned before [l]. y-rays
The y-ray yield per unit dose, Y, is written as Y= P,%(~o>S,
(1)
where pu is the hydrogen density on the surface, uN( E,) the nuclear reaction cross section at the energy E, of incident ions and r~ the efficiency of the detection. The ratio of Y to Yr, the yield at full monolayer coverage, indicates the relative hydrogen coverage, 0 = Y/Y,,
(2)
Fig. 2 shows the hydrogen adsorption process on W(OO1) at several hydrogen pressures, p(H,). The relative hydrogen coverage gradually increases with time until it reaches a saturation value, which increases with hydrogen pressure. The pressure dependence of the saturation value shows that the desorption of hydrogen is caused by the incident ion beam. At hydrogen
.
Ok
0
p(H2)
= 9.1 x 10’B(T~rr)
*
4.5.x
x
9.1 x 10-g
0
4.5x
I
10-8
10-g I
300
600 Time
( set
900
1200
)
Fig. 2. Hydrogen adsorption on W(OO1) at several hydrogen pressures, p(H,). The relative coverage gradually increases with time until it reaches a saturation value. The pressure dependence of the value shows that the ion-stimulated desorption of hydrogen atoms occurs due to the incident ion beam. At hydrogen pressures above 4.5X10-* Torr the saturation values reach the full monolayer coverage. VIII. ION BEAM ANALYSIS
Y. Iwaia et al. / Hydrogen adsorption on tungsten
576
pressures above 4.5 X lo-* Torr the saturation values reach a constant value. This fact indicates that the hydrogen pressure was so high that the ion-stimulated desorption process is negligible. Thus the constant saturation value at hydrogen pressures above 4.5 X lo-* Torr was taken as the full monolayer coverage. Yr was 50 times as large as twice the number of the background counts, which was considered as the lower limit for the counting rate of y-ray yields for the present 15N2+ ion beam current of 15 nA. The detection limit of adsorbed hydrogen was estimated from these data as 4 x lOI3 H atoms/cm2 which was l/50 of the full monolayer coverage on W(OO1). The ion-stimulated desorption, appearing as the hydrogen-pressure dependence of the saturation values in fig. 2, can more clearly be seen in the decay process of the hydrogen coverage which occurred after the variable leak valve was closed. The time variation of the hydrogen coverage is given by
d@ ‘de)
-=
df
_zo
PW
B
d >
(4)
where 8, is the initial coverage at the time of closing the valve and et, the saturation coverage in the background pressure. At the present decay process the beam current was 20.1 nA, tit, = 0.53 and 6, = 0.15. The background pressure was 5 X lo- to Torr after the valve was closed. Fig. 3 shows the desorption process on a semi-logarithmic scale. The slope shown as a solid line was nsa/p, + la, = (2.1 k 0.1) x 10K3/s. Since at the present background pressure this value is 20 times as large as the value of ns,/p w , ns,/p w + lad can be replaced by lo,. To determine the desorption cross section we must consider the intensity distribution of the ion beam. In the analysis mentioned above we assumed that the beam intensity in the irradiated area was uniform. The ion-stimulated desorption rate, however, is locally different in accordance with the intensity distribution of the ion beam. Then the rate equation for the local density of hydrogen, P&T), at the distance r from the beam center is written as
dt
z(2pW
-
----__
Process
------_---
2 -0.5. F? I
aii
-1.0
Ii3 -
c
-
-1.5. -2.0. -24
SO0
!OOO
1500 Tim
2000
2500
lsscl
Fig. 3. Desorption process plotted on a semi-logarithmic scale. The solid line shows lo, and the dotted line indicates the best fit curve given by Ok= 2.6 X lo-l6 cm*.
where i(r) is the intensity distribution of ion beam normalized as 2 r j?r i(r) d r = 1. Solving eq. (5), the y-ray yield is obtained as m
s(r)=e~-(8,-8,)e\p(-(~+lo,)i),
dPH(r) =
ion StimulatedDcsorpti on
o.o!
Y(t)
where n is the impinging rate of hydrogen molecules, s( 6) the sticking probability, pw the number density of the top-most W atoms on W(OOl), Z the current density of the incident beam and Us the ion-stimulated desorption cross section. Since n is small in the desorption process, the B-dependence of s(B) has little influence on de/d t. Then it is convenient to adopt the simplest form of s,(l - 8) for s( 6) [9]. Solving the rate equation (3), we obtain the solution of B(t) as
~
0.5
pH(r)) - i(r)uddPH(Y)%
c5)
10 ri(r)pH(r,
= 2n-7jUN
t) dr,
Pr_r(U,t> = Pb(r) + (PO- P*(r))
at z = 0, PH( r) is considered to be pa and at = pb(r). Fitting the decay process by the relative coverage 0 = Y( t)/Y, obtained from eq. (6), the desorption cross section was determined to be cd = 2.4 X lo-l6 cm2. The fitting curve is shown as a dotted line in fig. 3. The relative coverage derived from eq. (6) differs from the one obtained by eq. (4) after long irradiation times. The narrow resonance energy width of 1.8 keV broadened to (9.2 i 1.0) keV in the case of the normal incidence of the 15N ion beam to the W(OO1) surface, due to the Doppler effect of hydrogen atoms vibrating in the direction perpendicular to the surface [l]. This effect was evident from the intensity distribution of y-ray yields for the relative energy of “N to ‘H in the vicinity of the resonance energy. The distribution represents the state of hydrogen atoms in the momentum space on the surface. The y-ray yields measured for several adsorption systems at constant ion beam energy, e.g. the resonance energy, take different values depending on the states, even if the same atomic densities of hydrogen are adsorbed on the surfaces. Thus, in order to compare the hydrogen coverages of the different adsorption systems, the areas of their intensity profiles of y-ray yields must be examined. Fig. 4 shows the observed intensity profiles, using an ion beam with a sufficiently good energy stability, for H on W(OO1) and H on W(110). The full widths at half-maximum of the yield curve for H on W(110) are a little larger than where
t = 00 as pH(r)
571
Y. Iwata et al. / Hydrogen adsorption on tungsten
0.0
H an
WI0011
r.xH on WC1101
was determined W(OO1) system.
P o+oo .O 0. .
xx .
x
-10
-5
0 E - Eres
*
5
:x 54 I..
IO
IkeVl
Fig. 4. Normalized y-ray yield curves for W(OO1)and W(110). The area of the profiles, representing the hydrogen coverage on the surface, is (0.7?0.1) times smaller for H on W(110) than that for H on W(OO1).
those for H on W(OO1). The area of the profiles for H on W(110) showing the hydrogen coverage on the surface is (0.7 f 0.1) times smaller than that for H on W(OO1).
4. Conclusion The ‘H(15N, cxy)i2C reaction has several excellent properties for the applications to hydrogen analysis on well-defined surfaces. The detection limit of hydrogen coverage, examined for the adsorption system of H on W(OOl), was 4 X 1013H atoms/cm2 which was l/50 of the full monolayer coverage. Ion-stimulated desorption of hydrogen due to the energetic “N ion beam takes place during the analysis. The desorption cross section
to be 2.4 x lo-l6
cm2 for the H on
The authors are grateful to Dr T. Nozaki, Institute of Physical and Chemical Research, and Professor K. Ono, Institute for Solid State Physics, for supplying us a BGO scintillator. They are also indebted to members of the Machine Shops of the College of Arts and Sciences and the Institute for Solid State Physics for the construction of their apparatus. This work is supported by a Grant-in-Aid for the Special Research Project on Ion Beam Interactions with Solids from Ministry of Education. Science and Culture.
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VIII. ION BEAM ANALYSIS