- Email: [email protected]
Scattering of very-low-energy D2+ ions from an Al(111) surface: neutralization and dissociation
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Surface Science 283 (1993) 41-45 North-Holland
surface sciencb .:,>:. ..... ..,,,,,i( .,,,, ..:::::::: ...> ..A.. ........:::.:.. i:.s-.,........... “” ...\ :-‘::.:“i.-.,, “.‘h’.‘. “““.A’.‘.. ...i. ...i.. ..:.:.: .........(. ../.......... ,,,,,_ :’ ‘.‘.‘.‘.““.‘i.‘.: ........:.: ..>...:.:,:.:.:.:~:::~:j:: ..(._ :,y,: ,,:,.;, ,..,(,ziiB i.
“h’
‘...‘Vi. ..... ,...“,:;.,.\;; ;, ““.‘..... _...,.,..~..,,,,_(......
Scattering of very-low-energy D2+ ions from an Al( 111) surface: neutralization and dissociation Michio Okada
and Yoshitada
Murata
The Institute for Solid State Physics, The University of Tokyo, 7-22-1, Roppongi, Minato-ky
Tokyo 106, Japan
Received 20 April 1992; accepted for publication 12 August 1992
The incident-energy dependence and the angular distribution of the scattered Dl-ion yield on Al(111) were measured and the molecular-orientation effect in the collision is discussed, comparing it with the scattering yield of D+ and He+ ions. The incident-energy dependence of the scattered ion yield for the dissociated D + ion on Al(111) was also measured in the Dl incidence and a resonance-like feature was found, which is considered to be caused by dissociative neutralization of the primary D+2 ions
duction, taking the molecular-orientation on neutralization into account.
1. Introduction The interaction between the very-low-energy molecular ions (I 100 eV> and the surface is interesting as a new pathway of chemical reaction on and with the surface [l-4]. In this energy region, energy exchange of molecular ions on the surface is considered to depend on orientation of their molecular axis against the surface [5-111. Although neutralization of the very-low-energy molecular ions near the surface has been discussed in recent years [12,131, the molecular orientation effect on neutralization has not been discussed in the very-low-energy region. We observed this effect on neutralization using the Di-Al(111) collision, based on the result calculated by Imke et al. [8]. Dissociative scattering of the molecular ion in the Dl -Al(lll) system was also observed. Two mechanisms of the translational-to-internal energy transfer process in dissociative scattering have been discussed in this energy region. One is the rotational excitation mechanism [14-161 and the other is the vibrational excitation mechanism [5,6]. Besides these mechanisms, it has been discussed that dissociation proceeds via electronic excitation [1,17,18]. In the present paper, we discuss the mechanism of dissociated D+ ion pro0039-6028/93/$06.00
effect
2. Experiment The measurements were performed with almost the same apparatus described previously [13]. The ion beam line consists of a Menzingertype ion source, a mass-selecting magnet and a decelerating-lens system. Incident ion current was l-20 nA even at very low energies (E I 100 eV). The scattered ions were detected with a quadrupole mass filter which is rotatable in two axes around the crystal surface. The trajectories of ions after passing through the mass filter were bent by 90” with a deflecting plate mounted in front of an electron multiplier. The voltage applied to the deflecting plate was adjusted at the maximum intensity of detected ions. The ion yield was obtained by integrating the mass peak and normalizing it with the incident beam current. An Al(111) surface was mechanically polished and cleaned by the repetition of Ar+ ion bombardment and annealing at 470-670 K. Low-energy electron diffraction showed a sharp 1 x 1 pattern and Auger electron spectra showed no detectable
0 1993 - Elsevier Science Publishers B.V. All rights reserved
42
M. Okada, Y Murata
contamination. The surface cleanliness was also verified by secondary ion emission measurements.
/ 0:
scattering from Al(lI1)
~1011) EV
D2
D
He n=l
3. Results and discussion Fig. 1 shows the incident-energy dependence of the specularly-scattered-ion yield in the Dl and the D+ ion incidence. The angle of incidence is 60”. The scattered survival ion yield in the Dl ion incidence is nearly the same as that in the D+ ion incidence. However, an enhancement appears around N 80 eV in the former, in contrast to the monotonous decrease of the latter with increasing incident energy. This difference can be interpreted by the molecular-orientation effect on neutralization.
n=,
-
‘so
Fig. 2. Energy diagram to discuss the charge exchange tween the Al surface and D$, D+, and He+.
0 . OO X.0 .
. l.
0
0 0
. 0.
00 I
I
100 INCIDENT
I
.
200 ENERGY (eV)
II@ ,.
I
0
300
Fig. 1. Incident-energy dependence of the scattered Dt(closed circle) and D+- (open circle) ion yield for 60” specular scattering geometry along the [llz] azimuth on Al(111) in the D$- and the D+-ion incidence, respectively. The inset shows the incident-energy dependence of the dissociated D+-ion yield in the D:-ion incidence. The yield is given in arbitrary units, which is used in all figures of this paper.
be-
Fig. 2 shows an energy diagram relevant to the charge exchange of Dl, D+ and He+ with the Al(111) surface. In the D: case, the resonant electron transition is energetically allowed from the metal-occupied valence state to the b3Zl and X12,’ states of D,, as well as Auger electron transition to the X’Z: state over a wide range of internuclear separations. The b3Zz state of D, appearing due to resonance neutralization is an unstable dissociative state. Both resonance and Auger neutralizations influence on the ion survival probability, as discussed by Snowdon et al. [19] and O’Connor et al. [20]. Nearly the same ion yield of Dz as that of D+ shown in fig. 1 can be interpreted on the basis of the contribution of both Auger and resonance neutralization to the ion survival probability. Moreover, resonance neutralization to the b3C: state is a dominant process and takes a large molecular-orientation effect, based on the result calculated by Imke et al. [8]. Since the transition rate of this process takes the maximum value for the D: ion with its molecular axis perpendicular to the surface as understood from the consideration of symmetry in the overlapping of the wavefunctions between the neutralized D, molecule and the target, it is concluded that a dominant part of the survival
M. Okada, Y Murata / 0:
INCIDENT
ENERGY
(eV)
Fig. 3. Incident-energy dependence of the scattered He+-ion yield for the 60” specular scattering geometry along the [1?2] azimuth on AlUll). Inset shows the angular distribution of He+ ions with incident energy Ei = 50 (closed triangle) and 100 (closed square) eV.
ions takes a molecular axis nearly parallel to the surface at the turning point in the scattering process. On the other hand, in the D+ case, resonant and Auger electron transitions from the metal-occupied valence state to the 1s state of D are energetically allowed. The enhancement of the scattered-D: ion yield was observed at 70-80 eV, as shown in fig. 1. The same trend was observed in the case of scattering of the He+ ion, which has the same mass and the same apparent atomic number as the Dz ion, as shown in fig. 3. The enhancement was not observed in the D+ ion scattering. The V-shaped energy dependence in the scattering yield was also observed in the rare-gas-ion scattering from the Pt(001) surface 1211. Since the prominent V-shaped dependence can be interpreted on the basis of scattering by the repulsive potential, the present result indicates that D: ions are scattered by a rare-gas-like repulsive potential. The rare-gas-like repulsive potential in the Dl scattering may be different from that in the He+ scattering. However, Di and its neutralized D, have only valence electron(s) and can be described as the united-atom picture. The orientation effect can be ignored in this case. How-
scattering from Al(lIl)
43
ever, in the energy region where the enhancement of the D: ion yield was observed, the symmetry of the wavefunction is considered to regulate more strongly the transition rate of neutralization in the Dg scattering. So, the enhancement of the D:-ion yield which was not observed in the case of D+ is considered to be due to the molecular-orientation effect on the ion survival probability. The dominant part of survival ions takes the molecular axis nearly parallel to the surface, as mentioned above. The monotonous decrease in the scattered yield of Dl ions was observed above 80 eV like that of reactive D+ ions, which is clearly different from that in He+ scattering shown in fig. 3. The monotonous decrease is caused by collisional neutralization where charge exchange happens at the diabatic crossing of the energy levels. In the D: scattering, the non-adiabatic charge exchange may dominantly take place within the residencetime-dominated region [21]. So, the enhancement of the Dl ion yield at 80 eV can be observed. In the D+ scattering, the non-adiabatic may take place more easily than in D: scattering, compared at the same energy, because the velocity of D+ is larger than that of Dl. So, the non-adiabatic process may occur outside the residencetime-dominated region in the D+ scattering. This is another possible interpretation of the enhancement of the Dz ion yield relative to the D+ ion yield. The angular distribution of the scattered ions in the Dl and the D+ ion incidence is shown in fig. 4. The maximum lobe position of the scattered D,+ ions locates at N 10” from the surface parallel. The lobe position and the shape of the scattered reactive D+ ions are nearly the same as those of the scattered D: ions. On the other hand, the lobe position of scattered He+ is nearly in the specular direction, as shown in the inset of fig. 3. The difference in the lobe position depends on whether the ion is reactive (D+, Dl> or nonreactive (He+). In the reactive ion scattering, a substantial amount of the normal component of the kinetic energy will be dissipated via phonon and plasmon excitations, or electron-hole pair creation. So, the ion will enter the potential well and be trapped moving parallel to the surface.
M. Okada, Y Murata / 0:
SO’ /I
1 0”
1
1OOeV
1OOeV
0” so”
SO”
Fig. 4. Angular distribution of the scattered Dz- (closed circle) and the D&- (open circle) ion yield in the Dl incidence with incident energy Ei = 50 and 100 eV for the left part of the figure. Angular distribution of the scattered D+-ion yield in the D + incidence with incident energy Ei = 50 and 100 eV for the right part of the figure. The angle of incidence measured from the surface normal is 60”. The scale of the Dl-ion yield is different from that of the D&-ion yield.
The reactive ions scattered parallel to the surface are considered to suffering a considerable energy loss. Moreover, the lobe width for D: scattering observed at 100 eV is slightly narrower than that at 50 eV. This result shows that the collisional neutralization and the dissociation of the ions occur effectively above 100 eV. Thus, the D: ions scattered parallel to the surface are considered to be scattered via soft collisions, which is consistent with the above-mentioned result that the orientation of the scattered ion is nearly parallel to the surface. The incident-energy dependence of the scattered yield for dissociated D+ ions, DA,, in the DC incidence is shown in the inset of the fig. 1. In the incident energy dependence of the scattered D& ion yield, a resonance-like peak was found at an incident energy 100 eV. The angular distribution of the scattered D& ions is shown in fig. 4. The lobe of D& ions is broad and locates at the supra-position of the lobe of the scattered Dl ions. So, the dissociation via translational-tointernal energy transfer through an impulsive col-
scattering from Al(lI1)
lision proposed by Akazawa and Murata (vibrational excitation mechanism) [5,6] and a FOM group (rotational excitation mechanism) [7,15,16] is a dominant dissociation process in the threshold-energy region. That is, collision-induced dissociation occurs. On the other hand, the threshold energy for the detection of D& ions is > 20 eV, which is independent of the incidence angle. The estimated value of the dissociation threshold on the basis of the vibrational excitation model proposed by Akazawa and Murata [5,6] is N 4 eV. This value is much smaller than the observed threshold energy. It is difficult to consider the reionization process of D in this energy region from the energy diagram shown in fig. 2. However, since the velocity of the particle is large and the interaction potential between the molecule and the metal atom is of short range, a non-adiabatic process is considered to be possible even at the observed threshold energy region - 20 eV. So, the Df, ion is considered to originate in the reionization of the D atom which is dissociated from once neutralized D, via impulsive collision and the observed threshold energy - 20 eV corresponds to the reionization threshold. The impulsive nature of the scattering is considered to be prominent in the scattering of molecules with the molecular axis perpendicular to the surface. Neutralization to the dissociative state b3Zz occurs dominantly at higher energy in the case of D$ ions with the molecular axis nearly perpendicular to the surface [8]. Then, the D, molecules, which are collisionally dissociated and reionized to D& ions, decrease due to neutralization to the dissociative state. So, a resonance-like peak can be observed in the incidentenergy dependence of the D&-ion yield. The recurring increase and nearly constant value of D&-ion yield at higher energy may be caused by the increase of the survival probability of the reionized D,& ions on the outgoing trajectory.
4. Summary Both resonance and Auger neutralizations contribute to the ion survival probability. The
M. Okada, Y Murata / 0:
orientation effect of the survival D: ions could be observed in the incident-energy dependence of the scattered ion yield. The once neutralized D, molecule with the molecular axis nearly perpendicular to the surface contributes to the D&-ion production. In the threshold energy region, the dissociation mechanism is mainly internal-state excitation, which is proposed by Akazawa and Murata and the FOM group. The Ddis atom may be reionized via non-adiabatic level crossing. Neutralization to the dissociative state b3Ci of the DC ion with the molecular axis nearly perpendicular to the surface may lead to the resonance-like peak in the incident-energy dependence of the D&-ion yield.
References
111S.R. Kasi, H. Kang, C.S. Sass and J.W. Rabalais, Surf. Sci. Rep. 10 (1989) 1.
121P. Haochang, T.C.M. Horn and A.W. Kleyn, Phys. Rev. Lett. 57 (1986) 3035. [31 P.H.F. Reijnen, U. van Slooten and A.W. KIeyn, J. Chem. Phys. 94 (1991) 695. [41 H. Akazawa and Y. Murata, J. Chem. Phys. 88 (1988) 3317. PI H. Akazawa and Y. Murata, Surf. Sci. 207 (1989) L971.
scattering from Al(lII)
4.5
[6] H. Akazawa and Y. Murata, J. Chem. Phys. 92 (1990) 5560. [7] P.J. van den Hoek, T.C.M. Horn and A.W. Kleyn, Surf. Sci. 198 (1988) L335. [8] U. Imke, K.J. Snowdon and W. Heiland, Phys. Rev. B 34 (1986) 41, 48. [9] J.W. Gadzuk, Surf. Sci. 180 (19871 225. [lo] S. Ron, Y. Shima and M. Baer, Chem. Phys. 101 (1986) 45. [ll] J.H. Rechtien, R. Harder, G. Herrmann, C. Rothig and K.J. Snowdon, Surf. Sci. 272 (1992) 240. [12] H. Akazawa and Y. Murata, Phys. Rev. Lett. 61 (1988) 1218. [13] H. Akazawa and Y. Murata, J. Chem. Phys. 92 (1990) 5551. [14] R.B. Geber and R. Elber, Chem. Phys. Lett. 107 (1984) 141; E. Kolodney and A. Amirav, J. Chem. Phys. 79 (1983) 4648. [15] P.J. van den Hoek and A.W. KIeyn, J. Chem. Phys. 91 (1989) 4318. [16] U. van Slooten, P. Andersson, A.W. Kleyn and E.A. Gialason, Chem. Phys. Lett. 185 (1991) 440. [17] B. WilIerding, W. Heiland, K.J. Snowdon, Phys. Rev. Lett. 53 (1984) 2031. [18] C.S. Sass and J.W. Rabalais, J. Chem. Phys. 89 (19881 3870. [19] K.J. Snowdon, R. Hentschke, A. Nannann and W. Heiland, Surf. Sci. 173 (1986) 581. [20] D.J. O’Connor, Y.G. Shen, J.M. Wilson and R.J. MacDonald, Surf. Sci. 197 (19881 277. [21] H. Akazawa and Y. Murata, Phys. Rev. B 39 (1989) 3449.
.A..(.‘.‘.:.: . .. . . . . .. . . . . :f . . . . . .. . :.:.:.: .,.......,.. . ..>..:.:.:.:.:.:.::::::::.+:.:.::y::::?~.::::> .:.:. :(.:.::~;:j>>
.,,
,), “i:~:~,~~ ::::
::
“‘.........,.‘...... . . . . . . . . . . . . . . . . . . .. . . . . . . . .. . . . . .. . . . .. .:::::::::.:.:.:,::::::::;:::.:+:.:.:.:.l ...i.... I’.‘. . . . .../. :.:.+:.: :.:.:::: ):A’.
Surface Science 283 (1993) 41-45 North-Holland
surface sciencb .:,>:. ..... ..,,,,,i( .,,,, ..:::::::: ...> ..A.. ........:::.:.. i:.s-.,........... “” ...\ :-‘::.:“i.-.,, “.‘h’.‘. “““.A’.‘.. ...i. ...i.. ..:.:.: .........(. ../.......... ,,,,,_ :’ ‘.‘.‘.‘.““.‘i.‘.: ........:.: ..>...:.:,:.:.:.:~:::~:j:: ..(._ :,y,: ,,:,.;, ,..,(,ziiB i.
“h’
‘...‘Vi. ..... ,...“,:;.,.\;; ;, ““.‘..... _...,.,..~..,,,,_(......
Scattering of very-low-energy D2+ ions from an Al( 111) surface: neutralization and dissociation Michio Okada
and Yoshitada
Murata
The Institute for Solid State Physics, The University of Tokyo, 7-22-1, Roppongi, Minato-ky
Tokyo 106, Japan
Received 20 April 1992; accepted for publication 12 August 1992
The incident-energy dependence and the angular distribution of the scattered Dl-ion yield on Al(111) were measured and the molecular-orientation effect in the collision is discussed, comparing it with the scattering yield of D+ and He+ ions. The incident-energy dependence of the scattered ion yield for the dissociated D + ion on Al(111) was also measured in the Dl incidence and a resonance-like feature was found, which is considered to be caused by dissociative neutralization of the primary D+2 ions
duction, taking the molecular-orientation on neutralization into account.
1. Introduction The interaction between the very-low-energy molecular ions (I 100 eV> and the surface is interesting as a new pathway of chemical reaction on and with the surface [l-4]. In this energy region, energy exchange of molecular ions on the surface is considered to depend on orientation of their molecular axis against the surface [5-111. Although neutralization of the very-low-energy molecular ions near the surface has been discussed in recent years [12,131, the molecular orientation effect on neutralization has not been discussed in the very-low-energy region. We observed this effect on neutralization using the Di-Al(111) collision, based on the result calculated by Imke et al. [8]. Dissociative scattering of the molecular ion in the Dl -Al(lll) system was also observed. Two mechanisms of the translational-to-internal energy transfer process in dissociative scattering have been discussed in this energy region. One is the rotational excitation mechanism [14-161 and the other is the vibrational excitation mechanism [5,6]. Besides these mechanisms, it has been discussed that dissociation proceeds via electronic excitation [1,17,18]. In the present paper, we discuss the mechanism of dissociated D+ ion pro0039-6028/93/$06.00
effect
2. Experiment The measurements were performed with almost the same apparatus described previously [13]. The ion beam line consists of a Menzingertype ion source, a mass-selecting magnet and a decelerating-lens system. Incident ion current was l-20 nA even at very low energies (E I 100 eV). The scattered ions were detected with a quadrupole mass filter which is rotatable in two axes around the crystal surface. The trajectories of ions after passing through the mass filter were bent by 90” with a deflecting plate mounted in front of an electron multiplier. The voltage applied to the deflecting plate was adjusted at the maximum intensity of detected ions. The ion yield was obtained by integrating the mass peak and normalizing it with the incident beam current. An Al(111) surface was mechanically polished and cleaned by the repetition of Ar+ ion bombardment and annealing at 470-670 K. Low-energy electron diffraction showed a sharp 1 x 1 pattern and Auger electron spectra showed no detectable
0 1993 - Elsevier Science Publishers B.V. All rights reserved
42
M. Okada, Y Murata
contamination. The surface cleanliness was also verified by secondary ion emission measurements.
/ 0:
scattering from Al(lI1)
~1011) EV
D2
D
He n=l
3. Results and discussion Fig. 1 shows the incident-energy dependence of the specularly-scattered-ion yield in the Dl and the D+ ion incidence. The angle of incidence is 60”. The scattered survival ion yield in the Dl ion incidence is nearly the same as that in the D+ ion incidence. However, an enhancement appears around N 80 eV in the former, in contrast to the monotonous decrease of the latter with increasing incident energy. This difference can be interpreted by the molecular-orientation effect on neutralization.
n=,
-
‘so
Fig. 2. Energy diagram to discuss the charge exchange tween the Al surface and D$, D+, and He+.
0 . OO X.0 .
. l.
0
0 0
. 0.
00 I
I
100 INCIDENT
I
.
200 ENERGY (eV)
II@ ,.
I
0
300
Fig. 1. Incident-energy dependence of the scattered Dt(closed circle) and D+- (open circle) ion yield for 60” specular scattering geometry along the [llz] azimuth on Al(111) in the D$- and the D+-ion incidence, respectively. The inset shows the incident-energy dependence of the dissociated D+-ion yield in the D:-ion incidence. The yield is given in arbitrary units, which is used in all figures of this paper.
be-
Fig. 2 shows an energy diagram relevant to the charge exchange of Dl, D+ and He+ with the Al(111) surface. In the D: case, the resonant electron transition is energetically allowed from the metal-occupied valence state to the b3Zl and X12,’ states of D,, as well as Auger electron transition to the X’Z: state over a wide range of internuclear separations. The b3Zz state of D, appearing due to resonance neutralization is an unstable dissociative state. Both resonance and Auger neutralizations influence on the ion survival probability, as discussed by Snowdon et al. [19] and O’Connor et al. [20]. Nearly the same ion yield of Dz as that of D+ shown in fig. 1 can be interpreted on the basis of the contribution of both Auger and resonance neutralization to the ion survival probability. Moreover, resonance neutralization to the b3C: state is a dominant process and takes a large molecular-orientation effect, based on the result calculated by Imke et al. [8]. Since the transition rate of this process takes the maximum value for the D: ion with its molecular axis perpendicular to the surface as understood from the consideration of symmetry in the overlapping of the wavefunctions between the neutralized D, molecule and the target, it is concluded that a dominant part of the survival
M. Okada, Y Murata / 0:
INCIDENT
ENERGY
(eV)
Fig. 3. Incident-energy dependence of the scattered He+-ion yield for the 60” specular scattering geometry along the [1?2] azimuth on AlUll). Inset shows the angular distribution of He+ ions with incident energy Ei = 50 (closed triangle) and 100 (closed square) eV.
ions takes a molecular axis nearly parallel to the surface at the turning point in the scattering process. On the other hand, in the D+ case, resonant and Auger electron transitions from the metal-occupied valence state to the 1s state of D are energetically allowed. The enhancement of the scattered-D: ion yield was observed at 70-80 eV, as shown in fig. 1. The same trend was observed in the case of scattering of the He+ ion, which has the same mass and the same apparent atomic number as the Dz ion, as shown in fig. 3. The enhancement was not observed in the D+ ion scattering. The V-shaped energy dependence in the scattering yield was also observed in the rare-gas-ion scattering from the Pt(001) surface 1211. Since the prominent V-shaped dependence can be interpreted on the basis of scattering by the repulsive potential, the present result indicates that D: ions are scattered by a rare-gas-like repulsive potential. The rare-gas-like repulsive potential in the Dl scattering may be different from that in the He+ scattering. However, Di and its neutralized D, have only valence electron(s) and can be described as the united-atom picture. The orientation effect can be ignored in this case. How-
scattering from Al(lIl)
43
ever, in the energy region where the enhancement of the D: ion yield was observed, the symmetry of the wavefunction is considered to regulate more strongly the transition rate of neutralization in the Dg scattering. So, the enhancement of the D:-ion yield which was not observed in the case of D+ is considered to be due to the molecular-orientation effect on the ion survival probability. The dominant part of survival ions takes the molecular axis nearly parallel to the surface, as mentioned above. The monotonous decrease in the scattered yield of Dl ions was observed above 80 eV like that of reactive D+ ions, which is clearly different from that in He+ scattering shown in fig. 3. The monotonous decrease is caused by collisional neutralization where charge exchange happens at the diabatic crossing of the energy levels. In the D: scattering, the non-adiabatic charge exchange may dominantly take place within the residencetime-dominated region [21]. So, the enhancement of the Dl ion yield at 80 eV can be observed. In the D+ scattering, the non-adiabatic may take place more easily than in D: scattering, compared at the same energy, because the velocity of D+ is larger than that of Dl. So, the non-adiabatic process may occur outside the residencetime-dominated region in the D+ scattering. This is another possible interpretation of the enhancement of the Dz ion yield relative to the D+ ion yield. The angular distribution of the scattered ions in the Dl and the D+ ion incidence is shown in fig. 4. The maximum lobe position of the scattered D,+ ions locates at N 10” from the surface parallel. The lobe position and the shape of the scattered reactive D+ ions are nearly the same as those of the scattered D: ions. On the other hand, the lobe position of scattered He+ is nearly in the specular direction, as shown in the inset of fig. 3. The difference in the lobe position depends on whether the ion is reactive (D+, Dl> or nonreactive (He+). In the reactive ion scattering, a substantial amount of the normal component of the kinetic energy will be dissipated via phonon and plasmon excitations, or electron-hole pair creation. So, the ion will enter the potential well and be trapped moving parallel to the surface.
M. Okada, Y Murata / 0:
SO’ /I
1 0”
1
1OOeV
1OOeV
0” so”
SO”
Fig. 4. Angular distribution of the scattered Dz- (closed circle) and the D&- (open circle) ion yield in the Dl incidence with incident energy Ei = 50 and 100 eV for the left part of the figure. Angular distribution of the scattered D+-ion yield in the D + incidence with incident energy Ei = 50 and 100 eV for the right part of the figure. The angle of incidence measured from the surface normal is 60”. The scale of the Dl-ion yield is different from that of the D&-ion yield.
The reactive ions scattered parallel to the surface are considered to suffering a considerable energy loss. Moreover, the lobe width for D: scattering observed at 100 eV is slightly narrower than that at 50 eV. This result shows that the collisional neutralization and the dissociation of the ions occur effectively above 100 eV. Thus, the D: ions scattered parallel to the surface are considered to be scattered via soft collisions, which is consistent with the above-mentioned result that the orientation of the scattered ion is nearly parallel to the surface. The incident-energy dependence of the scattered yield for dissociated D+ ions, DA,, in the DC incidence is shown in the inset of the fig. 1. In the incident energy dependence of the scattered D& ion yield, a resonance-like peak was found at an incident energy 100 eV. The angular distribution of the scattered D& ions is shown in fig. 4. The lobe of D& ions is broad and locates at the supra-position of the lobe of the scattered Dl ions. So, the dissociation via translational-tointernal energy transfer through an impulsive col-
scattering from Al(lI1)
lision proposed by Akazawa and Murata (vibrational excitation mechanism) [5,6] and a FOM group (rotational excitation mechanism) [7,15,16] is a dominant dissociation process in the threshold-energy region. That is, collision-induced dissociation occurs. On the other hand, the threshold energy for the detection of D& ions is > 20 eV, which is independent of the incidence angle. The estimated value of the dissociation threshold on the basis of the vibrational excitation model proposed by Akazawa and Murata [5,6] is N 4 eV. This value is much smaller than the observed threshold energy. It is difficult to consider the reionization process of D in this energy region from the energy diagram shown in fig. 2. However, since the velocity of the particle is large and the interaction potential between the molecule and the metal atom is of short range, a non-adiabatic process is considered to be possible even at the observed threshold energy region - 20 eV. So, the Df, ion is considered to originate in the reionization of the D atom which is dissociated from once neutralized D, via impulsive collision and the observed threshold energy - 20 eV corresponds to the reionization threshold. The impulsive nature of the scattering is considered to be prominent in the scattering of molecules with the molecular axis perpendicular to the surface. Neutralization to the dissociative state b3Zz occurs dominantly at higher energy in the case of D$ ions with the molecular axis nearly perpendicular to the surface [8]. Then, the D, molecules, which are collisionally dissociated and reionized to D& ions, decrease due to neutralization to the dissociative state. So, a resonance-like peak can be observed in the incidentenergy dependence of the D&-ion yield. The recurring increase and nearly constant value of D&-ion yield at higher energy may be caused by the increase of the survival probability of the reionized D,& ions on the outgoing trajectory.
4. Summary Both resonance and Auger neutralizations contribute to the ion survival probability. The
M. Okada, Y Murata / 0:
orientation effect of the survival D: ions could be observed in the incident-energy dependence of the scattered ion yield. The once neutralized D, molecule with the molecular axis nearly perpendicular to the surface contributes to the D&-ion production. In the threshold energy region, the dissociation mechanism is mainly internal-state excitation, which is proposed by Akazawa and Murata and the FOM group. The Ddis atom may be reionized via non-adiabatic level crossing. Neutralization to the dissociative state b3Ci of the DC ion with the molecular axis nearly perpendicular to the surface may lead to the resonance-like peak in the incident-energy dependence of the D&-ion yield.
References
111S.R. Kasi, H. Kang, C.S. Sass and J.W. Rabalais, Surf. Sci. Rep. 10 (1989) 1.
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scattering from Al(lII)
4.5
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