Abstract
A mixed beam of hyperthermal N atoms and N_{2} molecules was scattered from the N-covered Ag(111) surface held at 300 K. The angular distribution of scattered N atoms is very broad. In contrast, N_{2} molecules exhibit a sharp angular distribution. Taking into account the relative mass ratio, N loses more energy at the surface than N_{2}. In terms of energy loss, the atoms approximately follow the binary collision model while the molecules do not. Instead, the energy curves of scattered N_{2} are more comparable to the parallel momentum conservation model for near specular outgoing angles (40°–65°). For both atoms and molecules the angle-resolved intensity and final energy curves are very similar to those from the bare surface. However, the N-covered surface yields non-negligible N_{2} intensity for a broad range of outgoing angles, including along the surface normal. This was not the case from the clean surface, where the measured intensity distribution was confined to the narrower angular range indicated above. Backscattering and direct abstraction reactions are evaluated as possible origins of this additional N_{2} signal. Of these, an abstraction mechanism appears to be the most consistent with the measured data.
This work is a part of the research programme of the “Stichting voor Fundamenteel Onderzoek der Materie (FOM)” and is supported financially by the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO).” It is supported by the European Communities under the contract of Association between EURATOM and FOM and carried out within the framework of the European Fusion Programme. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
I. INTRODUCTION
II. EXPERIMENTAL
III. RESULTS
A. N Scattering
B. N_{2}scattering
IV. DISCUSSION
A. N scattering
B. N_{2}scattering
V. CONCLUSIONS
Key Topics
- Surface scattering
- 40.0
- Atom surface collisions
- 39.0
- Surface cleaning
- 30.0
- Surface states
- 30.0
- Atom scattering
- 29.0
Figures
(a) Angular-resolved density distributions of N atoms (〈E _{i} 〉 ∼ 4.3 eV; θ _{i} = 60°) scattered from bare and N-covered Ag(111) at T _{S} = 500 K and 300 K, respectively. The scattered intensities have been normalized to the intensity of the corresponding direct beam. (b) Angle-resolved ratios of final-to-initial energy (〈E _{f}〉/〈E _{i}〉) for N atoms scattered from the bare and N-covered surfaces. The solid line represents the model of single-collision hard-sphere scattering of the incident atoms from an isolated “silver” atom (mass ratio of m _{N}/M _{Ag} = 14/108).
(a) Angular-resolved density distributions of N atoms (〈E _{i} 〉 ∼ 4.3 eV; θ _{i} = 60°) scattered from bare and N-covered Ag(111) at T _{S} = 500 K and 300 K, respectively. The scattered intensities have been normalized to the intensity of the corresponding direct beam. (b) Angle-resolved ratios of final-to-initial energy (〈E _{f}〉/〈E _{i}〉) for N atoms scattered from the bare and N-covered surfaces. The solid line represents the model of single-collision hard-sphere scattering of the incident atoms from an isolated “silver” atom (mass ratio of m _{N}/M _{Ag} = 14/108).
N_{2} (〈E _{i} 〉 ∼ 5.6 eV; θ _{i} = 60°) angular intensity distribution from the bare and N-covered surfaces. These distributions were obtained by integrating the TOF distribution recorded at each outgoing angle. The data have been normalized to the intensity of the corresponding direct beam distribution.
N_{2} (〈E _{i} 〉 ∼ 5.6 eV; θ _{i} = 60°) angular intensity distribution from the bare and N-covered surfaces. These distributions were obtained by integrating the TOF distribution recorded at each outgoing angle. The data have been normalized to the intensity of the corresponding direct beam distribution.
A series of N_{2} TOF spectra (thin black lines) collected from the N-covered surface at T _{S} = 300 K at various θ _{f} (θ _{i} = 60°). The fitted curves (thick black lines) are based on the assumption of a single shifted MB distribution arising from scattering of N_{2}. Corresponding spectra collected from the bare surface at T _{S} = 500 K are also shown as red lines in the panels for θ _{f} = (a) 0°, (b) 15°, (c) 30°, and (f) 60°. The chopper frequency was 400 Hz.
A series of N_{2} TOF spectra (thin black lines) collected from the N-covered surface at T _{S} = 300 K at various θ _{f} (θ _{i} = 60°). The fitted curves (thick black lines) are based on the assumption of a single shifted MB distribution arising from scattering of N_{2}. Corresponding spectra collected from the bare surface at T _{S} = 500 K are also shown as red lines in the panels for θ _{f} = (a) 0°, (b) 15°, (c) 30°, and (f) 60°. The chopper frequency was 400 Hz.
Angle-resolved 〈E _{f}〉/〈E _{i}〉 ratios as a function of θ _{f} for N_{2} scattering from the N-covered surface (filled circles). The values were derived from the single shifted MB distribution fittings that are illustrated in Fig. 3. The solid line represents the model of single-collision hard-sphere scattering of the incident atoms from an isolated “silver” atom (mass ratio of m _{N2}/M _{Ag} = 28/108). The broken line corresponds to parallel momentum conservation. The open triangles are the corresponding 〈E _{f}〉/〈E _{i}〉 values derived from TOF spectra acquired from the clean surface.
Angle-resolved 〈E _{f}〉/〈E _{i}〉 ratios as a function of θ _{f} for N_{2} scattering from the N-covered surface (filled circles). The values were derived from the single shifted MB distribution fittings that are illustrated in Fig. 3. The solid line represents the model of single-collision hard-sphere scattering of the incident atoms from an isolated “silver” atom (mass ratio of m _{N2}/M _{Ag} = 28/108). The broken line corresponds to parallel momentum conservation. The open triangles are the corresponding 〈E _{f}〉/〈E _{i}〉 values derived from TOF spectra acquired from the clean surface.
N_{2} TOF spectra collected from the N-covered surface at T _{S} = 300 K (same spectra as shown in Fig. 3). These have been fitted with two shifted MB distributions under the assumption that both originate from scattering of incident N_{2}.
N_{2} TOF spectra collected from the N-covered surface at T _{S} = 300 K (same spectra as shown in Fig. 3). These have been fitted with two shifted MB distributions under the assumption that both originate from scattering of incident N_{2}.
Angle-resolved 〈E _{f}〉/〈E _{i}〉 ratios as a function of θ _{f} for N_{2} scattering from the N-covered surface at T _{S} = 300 K. These values were derived from the fitting with two shifted MB distributions illustrated in Fig. 5. The solid line represents the model of single-collision hard-sphere scattering of the incident N_{2} from an isolated “silver” atom (mass ratio of m _{N2}/M _{Ag} = 28/108). The broken line corresponds to parallel momentum conservation.
Angle-resolved 〈E _{f}〉/〈E _{i}〉 ratios as a function of θ _{f} for N_{2} scattering from the N-covered surface at T _{S} = 300 K. These values were derived from the fitting with two shifted MB distributions illustrated in Fig. 5. The solid line represents the model of single-collision hard-sphere scattering of the incident N_{2} from an isolated “silver” atom (mass ratio of m _{N2}/M _{Ag} = 28/108). The broken line corresponds to parallel momentum conservation.
N_{2} TOF spectrum measured for θ _{f} = 0° (T _{S} = 300 K; θ _{i} = 60°) compared with a simulated 300 K MB distribution (dashed line), which assumes incident N_{2} trapping on the surface followed by thermal desorption. In addition, the results of fitting the spectrum with two shifted MB distributions under the assumption that the measured N_{2} is the result of a direct recombination reaction involving incident N atoms are shown. The chopper frequency was 200 Hz.
N_{2} TOF spectrum measured for θ _{f} = 0° (T _{S} = 300 K; θ _{i} = 60°) compared with a simulated 300 K MB distribution (dashed line), which assumes incident N_{2} trapping on the surface followed by thermal desorption. In addition, the results of fitting the spectrum with two shifted MB distributions under the assumption that the measured N_{2} is the result of a direct recombination reaction involving incident N atoms are shown. The chopper frequency was 200 Hz.
Intensity and energy distributions determined from two-component fittings of the N_{2} TOF distributions measured for θ _{f} = 0–45°. (a) and (b) show angle-resolved N_{2} intensity and energy distributions, respectively, that were derived based on the assumption of two N_{2} components at these outgoing angles, both originating from recombination reactions involving incident N atoms (〈E _{i}〉 ∼ 4.3 eV). (c) and (d) show the corresponding distributions derived on the assumption of two N_{2} components, both originating from scattering of incident N_{2} molecules (〈E _{i}〉 ∼ 5.6 eV).
Intensity and energy distributions determined from two-component fittings of the N_{2} TOF distributions measured for θ _{f} = 0–45°. (a) and (b) show angle-resolved N_{2} intensity and energy distributions, respectively, that were derived based on the assumption of two N_{2} components at these outgoing angles, both originating from recombination reactions involving incident N atoms (〈E _{i}〉 ∼ 4.3 eV). (c) and (d) show the corresponding distributions derived on the assumption of two N_{2} components, both originating from scattering of incident N_{2} molecules (〈E _{i}〉 ∼ 5.6 eV).
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