**4. Inelastic electron scattering in near-surface layer**

in the Perdew-Burke-Ernzerhof parameterization [40]. The interactions between ionic cores and electrons were described by the projected augmented wave (PAW) method [41] with the kinetic energy cutoff *E*cut = 40 Ry (320 Ry for the charge-density cutoff) for a plane-wave basis set. The Gaussian spreading for the Brillouin-zone integration was 0.02 Ry; the Marzari-Vanderbilt cold

The Pt DOS was calculated using the Perdew-Burke-Ernzerhof functional [44] and the PAW with the optimized lattice constant 3.99 Å. The kinetic energy cutoff *E*cut = 40 Ry and a

HOPG was modeled with a bilayer C24 unit cell of the Bernal and Hexagonal (*hex*) structure

*a* = 2.49 Å × 3, *b* = 2.48 Å × 2 for the *hex* structure and *a* = 2.50 Å × 3, *b* = 2.48 Å × 2 for the Bernal structure. The F was attached to C atoms all outside and half inside and half outside a cell (**Figure 1**) with 40 Bohr space between slabs to prevent the interactions. In latter case, the entry

ation of the system), in line with experimental measurements [46]. All atoms were allowed to move free under an optimization of the unit cells. The Brillouin-zone integration was performed on a 20 × 20 × 1 grid of Monkhorst-Pack k-points [47]. The accuracy was verified by testing the energy convergence. The default numbers of bands were used for free bromine particles.

**Figure 1.** Unit cells C24 with the Bernal and *hex* structure (top panel) and unit cell C24F12 Bernal with arrangement of the

F atoms half inside and half outside and all outside a cell. Adapted from [30].

unit cell, respectively, with the optimized lattice parameters

F formation or F ↔ Br<sup>2</sup>

unit cell was taken larger by 2 Å than optimized *d*layer in a

molecule, was modeled with

replacement under relax-

smearing was used [42], and the van der Waals (vdW) corrections were included [43].

(**Figure 1**) and optimized lattice parameters (*a* = 2.46 Å × 3, *b* = 2.46 Å × 2) [45].

12 × 12 × 12 grid of Monkhorst-Pack k-points were applied.

Half-fluorinated graphite, pristine and imbedded with the Br<sup>2</sup>

a bilayer C24F12 and C24F12Br<sup>2</sup>

150 Advanced Surface Engineering Research

interlayer distance *d*layer in a C24F12Br<sup>2</sup>

C24F12 unit cell (in order to avoid unrealistic Br<sup>2</sup>

DAPS theory directs core and primary electrons to nearest vacant states above *E*<sup>F</sup> [32]. The larger is the vacant DOS, the larger is the spectral dip, and the lack of free DOS gives no signal. The DAPS technique discovers all channels of the elastic electron consumption, which are specifically related to CEE phenomena and consisted of shake-up and shake-off VB transitions coupled with the threshold core-level excitation of an atom. These channels are electron transitions from the ground *σ*VB to vacant DOS *σ*Vac and the vacuum level, whose probability *W(E)* is in proportion to the corresponding convolution and *σ*VB, respectively, on the absolute energy relative to *E*<sup>F</sup> with a matrix element *f(E, σ)*:

$$\begin{aligned} \mathcal{W}\_{\text{up}}(\mathcal{E}) &= \int\_0^{\mathcal{E}} f(\mathcal{E}, \sigma) \, \sigma\_{\text{VB}}(-\mathcal{E}) \, \sigma\_{\text{low}}(\mathcal{E} - \varepsilon) d\varepsilon \\ \mathcal{W}\_{\text{eff}}(\mathcal{E}) &= f(\mathcal{E}, \sigma) \, \sigma\_{\text{VB}}(\mathfrak{p} - \mathcal{E}) \end{aligned} \tag{1}$$

The shake-off CEE moves *σ*VB to free DOS at the vacuum level. According to the Rutherford relation *ds*/*d* ~sin<sup>4</sup> (*Θ*/2), the cross-section for the nonrelativistic scattering is efficient for small angles *Θ* [48, 49]. The probability *W(E)*off in Eq. (1) therefore includes one-dimensional (1D) free DOS. According to Van Hove singularities, the 1D DOS is equal to 0, infinity, and 1/ √ \_\_\_\_\_\_\_\_\_ *<sup>E</sup>* <sup>−</sup> *EF* <sup>−</sup> <sup>ϕ</sup>*Pt* at the energies below, at, and above the vacuum level, respectively [50], as shown in **Figure 2(a)** (where ϕ*Pt* = 5.6 eV is the work function of Pt(100) [51]). This provides the resonant CEE behavior and multiple tracing over the adsorbed species (including hydrogen atoms and reaction intermediates) around different thresholds [8–11]. The shake-off satellite of adsorbed particle is an intense peak of the 1–2 eV base-width and coverage-proportional intensity, which is located at its ionization potential above the Pt threshold. The DAPS spectra in **Figure 2** particularly exhibit the σ state of the *H*ad atom and 1π, 5σ, and 4σ states of the *CO*ad molecule, which fit published UPS data in **Table 1** [52–55]. Similar accordance between the DAPS and UPS measurements has been found for the adsorbed O, N, NO, and NH species [7].

The Pt shake-off spectrum in **Figure 2(b**, **c)** was constructed on the basis of DFT data as follows. The VB was inverted (because the larger *σ*VB the larger *W*off*(E)* in Eq. (1), and the larger the spectral dip); differentiated, and shifted to the higher energy by ϕ*Pt*. Adsorbed layer makes significant contribution into the DAPS spectrum due to superior surface sensitivity of this technique, whose probing depth 2–3 ML is determined by half the electron mean free path in a solid. The Pt shake-up spectrum in **Figure 2(b)** corresponds to convolution of the occupied and vacant *d* states by Eq. (1). The calculated Pt shake-up and shake-off spectra in **Figure 2** are close to each other due to strongly localized vacant states at *E*<sup>F</sup> and the 1D DOS at the vacuum level, respectively, and because of domination of the equal *d*zx, *d*zy, and *d*xy states in total DOS [30]. An agreement between experimental and simulated spectral fragments in **Figure 2** implies regular involvement of the Pt DOS into CEE events, as well as similar matrix elements *f(E, σ)* for different partial densities of states (pDOS) in Eq. (1) and no symmetry ban for CEE transitions.

**Figure 2(b)** demonstrates tracing over the adsorbed hydrogen atoms, which is beyond AES and XPS facilities. It is important to note that none of the satellites in the DAPS spectra was assigned to the interatomic CEE transition (from the VB of the adsorbed species to free state of

the metal atom) [8, 10]. Furthermore, the surface plasmon disappears while the bulk plasmon decreases on coverage in **Figure 2(b)** because of screening by the adsorbed layer, in contrast to behavior of the VB CEE satellites [9]; multiple plasmons were also detected at relevant energy points [7]. Conformity between the calculated and experimental data in **Figure 2** indicates

**Sample O H CO Source**

Gas 13.6 13.6 14.0 16.5 19.7 [34] H/Pt(100)-(1 × 1) 13.7 **Figure 2a** H/Pt(111) 12.8 [57] CO/Pt(100)-(1 × 1) 12.7 13.7 18.3 **Figure 2c** CO/Pt(111) 14.2 15.1 17.5 [53–55] CO/Pt(100)-*hex* 13.4–15.8 16.4–18.0 [53] O/Pt(100)-(1 × 1) 14.0 [10] O/Pt(100) 12.7 [53] O/Pt(111) 14.2 [12] O/Pt(111) 12.1 [54, 57]

*Satellite O2p H1s 1π 5σ 4σ*

**Table 1.** Peak minima in the DAPS spectra as compared with the reference UPS data (eV).

of the electrons under their CEE transitions. Present consideration confirms the generality of CEE phenomena under the inelastic electron scattering in the adsorbed system. The CEE control is an additional tool of electron analyzer for fingerprinting the adsorbed layer and an alternative to the RPES, which requires a tunable synchrotron irradiation and special instrumentation [6]. Besides that, the DAPS provides the vacant state structure and geometrical

CEE satellites of the adsorbed species accompany the threshold core-level excitation of that neighboring atom, which is bound to the above species, while core-level energies are easily distinguishable. Therefore, the CEE control empowers localization of the adsorbed species over multicomponent surface. CEE regularities in the near-surface layer can be summarized

• Shake-up transitions correspond to convolution of the occupied and unoccupied pDOS of

• Shake-off transitions are available for the substrate atom and adsorbed species as well, where the former is the energy source; the 1D DOS at the vacuum level is a common VB destination. There is no symmetry prohibition, and the satellite structure is a VB mirror-

• Plasmon oscillations give evidence for the collective CEE phenomenon.

, shifted to higher energy by the work function.

parameters similar to XANES and EXAFS, respectively [56].

the same atom and likely have no symmetry ban.

and at the vacuum level as well, are appropriate spots of destination

Hidden Resources of Coordinated XPS and DFT Studies http://dx.doi.org/10.5772/intechopen.80002 153

that the vacancies, at *E*<sup>F</sup>

as follows:

image with respect to *E*<sup>F</sup>

**Figure 2.** (a) General scheme of the CEE transitions by an example of H/Pt(100) system exposed to primary electron beam with energy above the Pt4d5/2 threshold. (b) Difference DAPS spectra obtained after exposure of Pt(100) surface to H2 ; colored curves indicate the Pt shake-up and shake-off satellites expected from (a). (c) DAPS spectra obtained after exposure of Pt(100) surface to CO; red curve shows the Pt shake-off satellites expected from (a). Adapted from [8].


**Table 1.** Peak minima in the DAPS spectra as compared with the reference UPS data (eV).

the metal atom) [8, 10]. Furthermore, the surface plasmon disappears while the bulk plasmon decreases on coverage in **Figure 2(b)** because of screening by the adsorbed layer, in contrast to behavior of the VB CEE satellites [9]; multiple plasmons were also detected at relevant energy points [7]. Conformity between the calculated and experimental data in **Figure 2** indicates that the vacancies, at *E*<sup>F</sup> and at the vacuum level as well, are appropriate spots of destination of the electrons under their CEE transitions. Present consideration confirms the generality of CEE phenomena under the inelastic electron scattering in the adsorbed system. The CEE control is an additional tool of electron analyzer for fingerprinting the adsorbed layer and an alternative to the RPES, which requires a tunable synchrotron irradiation and special instrumentation [6]. Besides that, the DAPS provides the vacant state structure and geometrical parameters similar to XANES and EXAFS, respectively [56].

CEE satellites of the adsorbed species accompany the threshold core-level excitation of that neighboring atom, which is bound to the above species, while core-level energies are easily distinguishable. Therefore, the CEE control empowers localization of the adsorbed species over multicomponent surface. CEE regularities in the near-surface layer can be summarized as follows:


**Figure 2.** (a) General scheme of the CEE transitions by an example of H/Pt(100) system exposed to primary electron beam with energy above the Pt4d5/2 threshold. (b) Difference DAPS spectra obtained after exposure of Pt(100) surface to

; colored curves indicate the Pt shake-up and shake-off satellites expected from (a). (c) DAPS spectra obtained after exposure of Pt(100) surface to CO; red curve shows the Pt shake-off satellites expected from (a). Adapted from [8].

H2

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