**5. Conjugate electron excitations in the bulk of a solid**

The CEE phenomena occur with high probability in the near-surface layer of 2–3 ML [7–10]. Electronic affinity between surface and bulk atoms assumes similar channels of the inelastic electron scattering under the core-level excitation, no matter by primary electrons or X-ray irradiation. The photoelectron can partially lose its energy for the CEE transition and exhibits the VB peculiarities in fine XPS spectral structure. By analogy with the above findings, CEE phenomena in the bulk of a multicomponent material, under the nonresonant X-ray core-level excitation, can be characterized as follows:


electrons missing in HOPG [60].This satellite originates rather from the shake-off than from

transitions in the unit cell *hex* C24 as compared to energy losses above the core-level energy (284.6 eV) in the XPS C1s

There is expected similarity between higher energy tails of the (F)C1s (C is bound to F) and F1s XPS spectrum in **Figure 4**, which emphasizes the indifference of the VB with respect to energy source for the CEE transition. Fine XPS spectral structures above 10 eV conform well to shake-up (**Figure 4(a)**) and shake-off (**Figure 4(b)**) CEE transitions calculated by Eq. (1). The matrix elements *f(E, σ)* in Eq. (1) were accepted unity for a *W(E)* basis set, while the Y-scale factors in **Figure 4** (a, b) evaluate *f(E, σ)* and the contribution of a particular CEE transition, in total theoretical energy consumption, to fit the experimental photoelectron energy losses.

The *π* plasmon at ~5.4 eV in **Figure 4(a**, **b)** is assigned to the conjugated π bonds in a chain of

*π* plasmon is certainly not observed the XPS C1s spectrum under the lack of *π-*bonds [61]. Discovery of a similar satellite in the F1s spectrum is rather confusing because F atoms have nothing to do with the π bond between C atoms), but it is quite in line with the CEE model.

The base line shift relative to C1s = 285.1 eV in **Figure 4** (C is not bound to F) saves accordance with the DFT data and enables to make a contribution to the energy loss ~5.4 eV to the other

0.008 eV higher than that of the *hex* C24F12 cell, while pDOSs of both structures are very similar.

F fragment, respectively [30].

intercalation into C<sup>2</sup>

and the lack of new features compared to XPS spectra of the pristine C<sup>2</sup>

F1s spectra are in agreement with the same CEE transitions as before the Br<sup>2</sup>

is exhibited in the C K- and not exhibited in F K-edge XANES spectra of C<sup>2</sup>

transition and accompanies the C1s spectrum,

F matrix reveal stoichiometry C<sup>2</sup>

). The formation energy of the Bernal unit cell C24F12 is by

accepts plasmon oscillations that can give the energy loss at 9.1 or 12.9 eV

atom, not bound to F. Similar feature

F [29], and the

); (b) shake-up and shake-off CEE

Hidden Resources of Coordinated XPS and DFT Studies http://dx.doi.org/10.5772/intechopen.80002 155

FBr0.15

F (**Figure 4**); C1s and

embedding. DFT

the shake-up *p*<sup>z</sup>

spectrum of HOPG. Adapted from [31].

transition (**Figure 3(b)**).

**Figure 3.** (a) Partial DOS for the unit cells C24 with the Bernal and *hex* structure (*p*<sup>x</sup> = *p*<sup>y</sup>

C atoms [37]. This feature fits the shake-up Cpz

shake-off transitions ((F)Cpy

for 1 or 2 free electrons per a C<sup>2</sup>

XPS spectra obtained after the Br<sup>2</sup>

**F**

**-embedded C<sup>2</sup>**

A sizable DOS at *E*<sup>F</sup>

**5.2. Br2**

since the π bond is localized exclusively at the Csp<sup>2</sup>

, Fpy , Fpz

This chapter focuses on the DAPS omitting the allied threshold excitation techniques of the Auger electron and soft X-ray appearance potential spectroscopies, which follow the core hole decay and are complicated, at least, by the electron-core hole interaction [32]. In contrast, the DAPS fixes origination of the core hole, when the electron–hole interaction is not yet occurred or minimal. The same is true for energy losses in the XPS spectra because the photon absorption and the CEE energy dissipation can proceed at once or shortly, thus eliminating or minimizing the electron-core hole interaction, respectively. It is worth noting that CEE events are enabled by belonging of former core and valence band electrons to the same configuration and can be hardly detectable by electron energy loss spectroscopy, while the AES spectra are usually complicated by the background.

#### **5.1. HOPG and half-fluorinated graphite C<sup>2</sup> F**

DFT runs have revealed that density of states in **Figure 3(a)**, obtained for the C24 unit cells with the Bernal and *hex* structure, are similar and close to the DOS of graphite/graphene [58]. The optimized C-C bond length *d*C-C = 1.42 Å and the interlayer distance *d*layer = 3.34 Å in *hex* C24 also fit to those of the graphite. The Bernal C24 structure reveals the larger formation energy and smaller *d*layer by ~0.3 Å due to stronger interaction between the layers as compared to *hex* C24 structure.

Conventional satellites at higher energy sides of the XPS spectra correspond indeed to the photoelectron energy consumption. The CEE approach enables complete description of the HOPG XPS C1s spectrum in **Figure 3(b)** by the combination of shake-up and shake-off CEE transitions.

The satellite ~5.5 eV in **Figure 3(b)** is assigned to a π plasmon responsible for the π → π\* transition [59], although the standard plasmon is related to the collective oscillations of free

**Figure 3.** (a) Partial DOS for the unit cells C24 with the Bernal and *hex* structure (*p*<sup>x</sup> = *p*<sup>y</sup> ); (b) shake-up and shake-off CEE transitions in the unit cell *hex* C24 as compared to energy losses above the core-level energy (284.6 eV) in the XPS C1s spectrum of HOPG. Adapted from [31].

electrons missing in HOPG [60].This satellite originates rather from the shake-off than from the shake-up *p*<sup>z</sup> transition (**Figure 3(b)**).

There is expected similarity between higher energy tails of the (F)C1s (C is bound to F) and F1s XPS spectrum in **Figure 4**, which emphasizes the indifference of the VB with respect to energy source for the CEE transition. Fine XPS spectral structures above 10 eV conform well to shake-up (**Figure 4(a)**) and shake-off (**Figure 4(b)**) CEE transitions calculated by Eq. (1). The matrix elements *f(E, σ)* in Eq. (1) were accepted unity for a *W(E)* basis set, while the Y-scale factors in **Figure 4** (a, b) evaluate *f(E, σ)* and the contribution of a particular CEE transition, in total theoretical energy consumption, to fit the experimental photoelectron energy losses.

The *π* plasmon at ~5.4 eV in **Figure 4(a**, **b)** is assigned to the conjugated π bonds in a chain of C atoms [37]. This feature fits the shake-up Cpz transition and accompanies the C1s spectrum, since the π bond is localized exclusively at the Csp<sup>2</sup> atom, not bound to F. Similar feature is exhibited in the C K- and not exhibited in F K-edge XANES spectra of C<sup>2</sup> F [29], and the *π* plasmon is certainly not observed the XPS C1s spectrum under the lack of *π-*bonds [61]. Discovery of a similar satellite in the F1s spectrum is rather confusing because F atoms have nothing to do with the π bond between C atoms), but it is quite in line with the CEE model.

The base line shift relative to C1s = 285.1 eV in **Figure 4** (C is not bound to F) saves accordance with the DFT data and enables to make a contribution to the energy loss ~5.4 eV to the other shake-off transitions ((F)Cpy , Fpy , Fpz ). The formation energy of the Bernal unit cell C24F12 is by 0.008 eV higher than that of the *hex* C24F12 cell, while pDOSs of both structures are very similar. A sizable DOS at *E*<sup>F</sup> accepts plasmon oscillations that can give the energy loss at 9.1 or 12.9 eV for 1 or 2 free electrons per a C<sup>2</sup> F fragment, respectively [30].

#### **5.2. Br2 -embedded C<sup>2</sup> F**

**5. Conjugate electron excitations in the bulk of a solid**

excitation, can be characterized as follows:

154 Advanced Surface Engineering Research

usually complicated by the background.

C24 structure.

transitions.

**5.1. HOPG and half-fluorinated graphite C<sup>2</sup>**

the XPS spectra of different components of a sample.

The CEE phenomena occur with high probability in the near-surface layer of 2–3 ML [7–10]. Electronic affinity between surface and bulk atoms assumes similar channels of the inelastic electron scattering under the core-level excitation, no matter by primary electrons or X-ray irradiation. The photoelectron can partially lose its energy for the CEE transition and exhibits the VB peculiarities in fine XPS spectral structure. By analogy with the above findings, CEE phenomena in the bulk of a multicomponent material, under the nonresonant X-ray core-level

• Shake-off transitions are available, where pDOSs must be considered due to probable difference in their matrix elements. The same ground state (VB), the common destination (the vacuum level), and enough energy excess of any of the photoelectrons should result in

• Shake-up transitions are available, in which the convolution should include pDOSs of the same atom. The VB of chemically bound atoms has no preference for a photoelectron to detach its energy for the CEE transition, and so similar energy losses are also expected in

This chapter focuses on the DAPS omitting the allied threshold excitation techniques of the Auger electron and soft X-ray appearance potential spectroscopies, which follow the core hole decay and are complicated, at least, by the electron-core hole interaction [32]. In contrast, the DAPS fixes origination of the core hole, when the electron–hole interaction is not yet occurred or minimal. The same is true for energy losses in the XPS spectra because the photon absorption and the CEE energy dissipation can proceed at once or shortly, thus eliminating or minimizing the electron-core hole interaction, respectively. It is worth noting that CEE events are enabled by belonging of former core and valence band electrons to the same configuration and can be hardly detectable by electron energy loss spectroscopy, while the AES spectra are

**F**

DFT runs have revealed that density of states in **Figure 3(a)**, obtained for the C24 unit cells with the Bernal and *hex* structure, are similar and close to the DOS of graphite/graphene [58]. The optimized C-C bond length *d*C-C = 1.42 Å and the interlayer distance *d*layer = 3.34 Å in *hex* C24 also fit to those of the graphite. The Bernal C24 structure reveals the larger formation energy and smaller *d*layer by ~0.3 Å due to stronger interaction between the layers as compared to *hex*

Conventional satellites at higher energy sides of the XPS spectra correspond indeed to the photoelectron energy consumption. The CEE approach enables complete description of the HOPG XPS C1s spectrum in **Figure 3(b)** by the combination of shake-up and shake-off CEE

The satellite ~5.5 eV in **Figure 3(b)** is assigned to a π plasmon responsible for the π → π\* transition [59], although the standard plasmon is related to the collective oscillations of free

analogous energy losses in the XPS spectra of different components of a sample.

XPS spectra obtained after the Br<sup>2</sup> intercalation into C<sup>2</sup> F matrix reveal stoichiometry C<sup>2</sup> FBr0.15 and the lack of new features compared to XPS spectra of the pristine C<sup>2</sup> F (**Figure 4**); C1s and F1s spectra are in agreement with the same CEE transitions as before the Br<sup>2</sup> embedding. DFT

**Figure 4.** XPS C1s and F1s spectrum of C<sup>2</sup> F (relative to *E*Core = 287.6 eV and 687.4 eV, respectively; the background of external and surface energy losses are subtracted [35]) and shake-up (a) and shake-off (b) transitions of (F)C and F atoms forming the C-F bond in the unit cell *hex* C24F12 (all F are outside). (c) Shake-up and (d) shake-off CEE transitions for different arrangements of the F atoms (**Figure 1**). Adapted from [31].

The difference F1s spectrum in **Figure 5** exhibits a distinct structure, which conforms to shakeup transitions of the pDOS responsible for C-F bonding and which is interpreted as the C-F

**Table 2.** Br-Br distance *d*BrBr, angle *α* between the Br-Br axis and C-planes, and difference *Δ*s-p between the weighted

.

**Species/unit cell** *d***BrBr (Å)** *α* **(optimized)** *Δ***s-p (eV) Br state**

<sup>0</sup> 2.29 — 12.806 Molecular

<sup>−</sup><sup>1</sup> 2.89 — 12.061 Molecular

<sup>0</sup> — — 12.285 Atomic

<sup>−</sup><sup>1</sup> — — 11.730 Atomic

#1 *hex* (*α*<sup>0</sup> = 0°) 3.24 0.0° 11.66 Atomic #2 *hex* (*α*<sup>0</sup> = 90°) 2.45 51.5° 12.06 Molecular #3 Bernal (*α*<sup>0</sup> = 0°) 2.375 6.2° 12.32 Molecular #4 Bernal (*α*<sup>0</sup> = 90°) 3.18 0.1° 12.68 Atomic #5 Bernal (*α*<sup>0</sup> = 52°) 3.34 0.8° 12.61 Atomic

2.73 (next nearest)

4.96 (next nearest)

2.73 (next nearest)

4.96 (next nearest)

between carbon layers, which should be accompanied with the enrichment of the occupied DOS of C and F. Arrangement of the F atoms in a cell makes no matter for the conclusion.

Each of nine local structures is appropriate; no preference can be given to a particular cell from the conventional DFT study.Each of these unit cells is characterized by the specific Br pDOS structure [30]. DFT examinations of separate bromine species revealed a strong difference *Δ*s-p = 0.2–1.3 eV between the weighted average energy *<E*s,p *>* of the Br *s*- and *p*- DOS, far beyond the accuracy of DFT runs ~0.01 eV (**Table 2**). The parameter *<E*s,p *>* was determined

*s*- and *p*- DOS, respectively. In the same way, as the binding energy determines the oxidation rate in extensive XPS practice; the parameter *Δ*s-p was taken as a descriptor of the Br state [30].

local geometry and the state (atomic, molecular, and chain type) of the embedded Br<sup>2</sup>

embedding weakens the interactions

17.4° 12.09 Chain type

Hidden Resources of Coordinated XPS and DFT Studies http://dx.doi.org/10.5772/intechopen.80002 157

12.9° 12.82 Molecular

17.2° 12.15 Chain type

18.7° 12.97 Molecular

(**Table 2**).

with the optimized parameters and different

for the Br atom with single, several localized, and diffused

bond strengthening [30]. It can be the case since the Br<sup>2</sup>

DFT studies have found a set of cells C24F12Br<sup>2</sup>

and middle of ∫*<sup>σ</sup> <sup>i</sup>*

*d Ei*

as *E*, ∑*<sup>σ</sup> <sup>i</sup>*

Free *Br*<sup>2</sup>

Free *Br*<sup>2</sup>

Free *Br*<sup>1</sup>

Free *Br*<sup>1</sup>

All F out a cell (**Figure 1**)

Half F in & half F out a cell (**Figure 1**)

#6 *hex* (*α*<sup>0</sup> = 90°) 2.44 (nearest)

#7 *hex* (*α*<sup>0</sup> = 0°) 2.30 (nearest)

#8 Bernal (*α*<sup>0</sup> = 90°) 2.44 (nearest)

#9 Bernal (*α*<sup>0</sup> = 0°) 2.29 (nearest)

average Br *s*- and *p*- DOS in free species and unit cells C24F12Br<sup>2</sup>

*Ei* /∑*σ<sup>i</sup>*

studies were performed for the Bernal and *hex* C24F12Br<sup>2</sup> unit cells #1–9 in **Table 2** at the entry angles *α*<sup>0</sup> = 0 and 90° between the Br-Br axis and C planes and different arrangements of the F atoms (**Figure 1**).

DFT calculations have revealed that the Br<sup>2</sup> embedding enlarges the interlayer distance, but insignificantly affects the pDOS of the C, (F)C and F [30]. The latter conforms to chemical inertness of the pristine C<sup>2</sup> F cell and to low Br content in the product C<sup>2</sup> FBr0.15 [62]. Invariant pDOS of the C and F atoms and slight change in the C1s and F1s XPS spectra, after the Br<sup>2</sup> embedding into C24F12 framework, restrain the correlation between XPS and DFT outputs. The novelties of XPS and DFT data, which are resulted from the Br<sup>2</sup> embedding, concern the bromine only and are considered in more detail.


**Table 2.** Br-Br distance *d*BrBr, angle *α* between the Br-Br axis and C-planes, and difference *Δ*s-p between the weighted average Br *s*- and *p*- DOS in free species and unit cells C24F12Br<sup>2</sup> .

The difference F1s spectrum in **Figure 5** exhibits a distinct structure, which conforms to shakeup transitions of the pDOS responsible for C-F bonding and which is interpreted as the C-F bond strengthening [30]. It can be the case since the Br<sup>2</sup> embedding weakens the interactions between carbon layers, which should be accompanied with the enrichment of the occupied DOS of C and F. Arrangement of the F atoms in a cell makes no matter for the conclusion.

studies were performed for the Bernal and *hex* C24F12Br<sup>2</sup>

different arrangements of the F atoms (**Figure 1**). Adapted from [31].

ties of XPS and DFT data, which are resulted from the Br<sup>2</sup>

DFT calculations have revealed that the Br<sup>2</sup>

**Figure 4.** XPS C1s and F1s spectrum of C<sup>2</sup>

156 Advanced Surface Engineering Research

only and are considered in more detail.

F atoms (**Figure 1**).

ness of the pristine C<sup>2</sup>

angles *α*<sup>0</sup> = 0 and 90° between the Br-Br axis and C planes and different arrangements of the

external and surface energy losses are subtracted [35]) and shake-up (a) and shake-off (b) transitions of (F)C and F atoms forming the C-F bond in the unit cell *hex* C24F12 (all F are outside). (c) Shake-up and (d) shake-off CEE transitions for

insignificantly affects the pDOS of the C, (F)C and F [30]. The latter conforms to chemical inert-

ding into C24F12 framework, restrain the correlation between XPS and DFT outputs. The novel-

F cell and to low Br content in the product C<sup>2</sup>

of the C and F atoms and slight change in the C1s and F1s XPS spectra, after the Br<sup>2</sup>

unit cells #1–9 in **Table 2** at the entry

FBr0.15 [62]. Invariant pDOS

embedding, concern the bromine

embed-

embedding enlarges the interlayer distance, but

F (relative to *E*Core = 287.6 eV and 687.4 eV, respectively; the background of

DFT studies have found a set of cells C24F12Br<sup>2</sup> with the optimized parameters and different local geometry and the state (atomic, molecular, and chain type) of the embedded Br<sup>2</sup> (**Table 2**). Each of nine local structures is appropriate; no preference can be given to a particular cell from the conventional DFT study.Each of these unit cells is characterized by the specific Br pDOS structure [30]. DFT examinations of separate bromine species revealed a strong difference *Δ*s-p = 0.2–1.3 eV between the weighted average energy *<E*s,p *>* of the Br *s*- and *p*- DOS, far beyond the accuracy of DFT runs ~0.01 eV (**Table 2**). The parameter *<E*s,p *>* was determined as *E*, ∑*<sup>σ</sup> <sup>i</sup> Ei* /∑*σ<sup>i</sup>* and middle of ∫*<sup>σ</sup> <sup>i</sup> d Ei* for the Br atom with single, several localized, and diffused *s*- and *p*- DOS, respectively. In the same way, as the binding energy determines the oxidation rate in extensive XPS practice; the parameter *Δ*s-p was taken as a descriptor of the Br state [30].

**Figure 5.** Difference XPS F1s spectrum (F1s C<sup>2</sup> F was subtracted from F1s C<sup>2</sup> FBr0.15) as compared to shake-up VB transitions for the unit cells C24F12Br<sup>2</sup> (a) #2 and (b) #6 in **Table 2** with different layout of the F atoms (**Figure 1**).

Br<sup>2</sup>

Br<sup>2</sup>

while there is a few difference between the Br *p*<sup>x</sup>

different F layout (a) #2 (*hex*), and (b) #8 and #9 (Bernal).

−1

discordance with the XPS data due to their specific *p*<sup>x</sup>

and #9; the state close to *Br*<sup>2</sup>

**Figure 6.** Br3d XPS spectrum of C<sup>2</sup>

respect to the shake-off *p*<sup>y</sup>

the molecular Br<sup>2</sup>

cell #8 (**Table 2**).

, while nearest distance between the Br atoms of the adjacent cells 2.73 Å is smaller than the nearest intermolecular distance in a solid Br ~3.37 Å [63] and still enough for the vdW interaction [45]. There is no visible difference in the Br pDOS of cell #6 and #8 (chain type Br),

). Finally, the weighted average *Δ*s-p values are line with the molecular Br state for cells #7

The ~20 eV shoulder in the Br3d spectrum in **Figure 6(b)** conforms well to shake-off transition of the *s*-state in unit cells #8 and #9 with different arrangement of the Br atoms, and there is no solid reason to give preference to a particular cell. The shape and location of the 5–12 eV spectral fragment in **Figure 6(b)**, with due regard to the baseline of this energy region, are consistent with a comparable mixture of CEE transitions calculated for the unit cells #8 and #9. According to DFT data, there is a little difference between cells #6 and #8 (chain type Br) with

#6, with the chain type Br layout, is preferable among others, whereas the energy losses in the

Experimental data have reported the angle *α* ~30° between the Br-Br axis and C-planes and

XPS and DFT outputs suggests possibility of the chain like bromine arrangement, which can

On the contrary, using the cell #7 *hex* instead of #9 Bernal (molecular Br<sup>2</sup>

Br3d XPS spectrum suggest a mixture ~1:1 of unit cells #6 and #9.

state for similar to C<sup>2</sup>

for cell #6; and the intermediate, between *Br*<sup>1</sup>

and *p*<sup>y</sup>

FBr0.15 as compared to relevant shake-off VB transitions of the Br atom in unit cells with

transition, while the cell #6 wins #8 by ~5% in the formation energy.

states for cell #7 and #9 (molecular

, Br state for

) results in larger

0 and *Br*<sup>2</sup> −1

Hidden Resources of Coordinated XPS and DFT Studies http://dx.doi.org/10.5772/intechopen.80002 159

structure. Finally, the unit cell C24F12Br<sup>2</sup>

FBr0.15 systems [38, 46]. The current combination of

For the unit cells with all F atoms outside, the minimal deviations *Δ*s-p of 0.02, 0, and 0.04 eV make preferable the cells #1, #2, and #3, respectively (**Table 2**). Moreover, in case of the large *d*BrBr (the Br-Br bond is lost), the difference *Δ*s-p in a cell should be close to that of free *Br*<sup>1</sup> 0 . Otherwise (the Br-Br bond retains), the difference *Δ*s-p should be close to that of the *Br*<sup>2</sup> 0 species. In this case, *d*BrBr for cell #1 (3.24 Å) and cell #2 (2.45 Å) is adequate to the lack of the Br-Br bond in *Br*<sup>1</sup> −1 and to the eigenvalue of *Br*<sup>2</sup> 0 (2.29 Å), respectively. On the contrary, the cell #3 should be ruled out, because its *d*BrBr = 2.37 Å indicates retaining the Br-Br bond while the *Br*<sup>1</sup> 0 specimen, with nearest *Δ*s-p, has no bond. Finally, the cell #2 wins the cell #1 in the formation energy (**Table 1**). Besides, the reaction C24F<sup>12</sup> + Br<sup>2</sup> → C24F12Br<sup>2</sup> is endothermic for cell #1, in contrast to other cells [30].

The bromine *p*<sup>z</sup> - and *s*- shake-off transition conforms to the nonresolved 5–12 eV fragment and ~20 eV shoulder of the Br3d spectrum in **Figure 6(a)**, respectively. The higher energy parts of the XPS Br3d, F1s and C1s spectra are similar [30], but they do not correlate with any of the Br CEE transitions. This indicates such a bonding between the Br<sup>2</sup> molecule and the C<sup>2</sup> F frame that provides the Br3d photoelectron energy losses via CEE transitions of pDOS of the C and F atoms.

DFT calculations for the unit cells C24F12Br<sup>2</sup> , with the F atoms half inside and half outside a cell, have revealed two stable Br<sup>2</sup> states. The first state, in cells #7 and #9, corresponds to Br<sup>2</sup> pairs (**Table 2**), which are separated from each other in the adjacent cells and exhibit the same Br-Br distance *d*BrBr ~2.29 Å as in a free Br<sup>2</sup> molecule. The second state (cells #6 and #8) corresponds to Br arrangement as the chain, in which *d*BrBr ~2.44 Å within a unit cell is larger than in a free

**Figure 6.** Br3d XPS spectrum of C<sup>2</sup> FBr0.15 as compared to relevant shake-off VB transitions of the Br atom in unit cells with different F layout (a) #2 (*hex*), and (b) #8 and #9 (Bernal).

Br<sup>2</sup> , while nearest distance between the Br atoms of the adjacent cells 2.73 Å is smaller than the nearest intermolecular distance in a solid Br ~3.37 Å [63] and still enough for the vdW interaction [45]. There is no visible difference in the Br pDOS of cell #6 and #8 (chain type Br), while there is a few difference between the Br *p*<sup>x</sup> and *p*<sup>y</sup> states for cell #7 and #9 (molecular Br<sup>2</sup> ). Finally, the weighted average *Δ*s-p values are line with the molecular Br state for cells #7 and #9; the state close to *Br*<sup>2</sup> −1 for cell #6; and the intermediate, between *Br*<sup>1</sup> 0 and *Br*<sup>2</sup> −1 , Br state for cell #8 (**Table 2**).

For the unit cells with all F atoms outside, the minimal deviations *Δ*s-p of 0.02, 0, and 0.04 eV make preferable the cells #1, #2, and #3, respectively (**Table 2**). Moreover, in case of the large *d*BrBr

F was subtracted from F1s C<sup>2</sup>

(a) #2 and (b) #6 in **Table 2** with different layout of the F atoms (**Figure 1**).

*Δ*s-p, has no bond. Finally, the cell #2 wins the cell #1 in the formation energy (**Table 1**). Besides,

~20 eV shoulder of the Br3d spectrum in **Figure 6(a)**, respectively. The higher energy parts of the XPS Br3d, F1s and C1s spectra are similar [30], but they do not correlate with any of the

that provides the Br3d photoelectron energy losses via CEE transitions of pDOS of the C and

(**Table 2**), which are separated from each other in the adjacent cells and exhibit the same Br-Br

to Br arrangement as the chain, in which *d*BrBr ~2.44 Å within a unit cell is larger than in a free

(2.29 Å), respectively. On the contrary, the cell #3 should be ruled out,

states. The first state, in cells #7 and #9, corresponds to Br<sup>2</sup>


is endothermic for cell #1, in contrast to other cells [30].

0

specimen, with nearest

species. In this case,

0

FBr0.15) as compared to shake-up VB transitions

molecule and the C<sup>2</sup>

0

, with the F atoms half inside and half outside a cell,

molecule. The second state (cells #6 and #8) corresponds

. Otherwise

−1 and

F frame

pairs

(the Br-Br bond is lost), the difference *Δ*s-p in a cell should be close to that of free *Br*<sup>1</sup>

*d*BrBr for cell #1 (3.24 Å) and cell #2 (2.45 Å) is adequate to the lack of the Br-Br bond in *Br*<sup>1</sup>

(the Br-Br bond retains), the difference *Δ*s-p should be close to that of the *Br*<sup>2</sup>

because its *d*BrBr = 2.37 Å indicates retaining the Br-Br bond while the *Br*<sup>1</sup>

Br CEE transitions. This indicates such a bonding between the Br<sup>2</sup>

to the eigenvalue of *Br*<sup>2</sup>

for the unit cells C24F12Br<sup>2</sup>

158 Advanced Surface Engineering Research

The bromine *p*<sup>z</sup>

F atoms.

0

DFT calculations for the unit cells C24F12Br<sup>2</sup>

have revealed two stable Br<sup>2</sup>

distance *d*BrBr ~2.29 Å as in a free Br<sup>2</sup>

the reaction C24F<sup>12</sup> + Br<sup>2</sup> → C24F12Br<sup>2</sup>

**Figure 5.** Difference XPS F1s spectrum (F1s C<sup>2</sup>

The ~20 eV shoulder in the Br3d spectrum in **Figure 6(b)** conforms well to shake-off transition of the *s*-state in unit cells #8 and #9 with different arrangement of the Br atoms, and there is no solid reason to give preference to a particular cell. The shape and location of the 5–12 eV spectral fragment in **Figure 6(b)**, with due regard to the baseline of this energy region, are consistent with a comparable mixture of CEE transitions calculated for the unit cells #8 and #9. According to DFT data, there is a little difference between cells #6 and #8 (chain type Br) with respect to the shake-off *p*<sup>y</sup> transition, while the cell #6 wins #8 by ~5% in the formation energy. On the contrary, using the cell #7 *hex* instead of #9 Bernal (molecular Br<sup>2</sup> ) results in larger discordance with the XPS data due to their specific *p*<sup>x</sup> structure. Finally, the unit cell C24F12Br<sup>2</sup> #6, with the chain type Br layout, is preferable among others, whereas the energy losses in the Br3d XPS spectrum suggest a mixture ~1:1 of unit cells #6 and #9.

Experimental data have reported the angle *α* ~30° between the Br-Br axis and C-planes and the molecular Br<sup>2</sup> state for similar to C<sup>2</sup> FBr0.15 systems [38, 46]. The current combination of XPS and DFT outputs suggests possibility of the chain like bromine arrangement, which can be realized in appropriate experimental conditions. Besides, the experimental angle (~30°) is close to the weighted average (~29°) of *α* = 51.5, 17.4, and 18.7° in **Table 2**, which have been found for the most probable unit cells #2, #6, and #9, respectively.

**7. Conclusion**

graphite-based materials.

the atoms in a sample.

**Acknowledgements**

**Acronyms and abbreviations**

AES auger electron spectroscopy CEE conjugate electron excitation

DFT density functional theory

DOS density of states

pDOS partial DOS

DAPS disappearance potential spectroscopy

Catalysis.

the development of advanced composites.

Primary collecting of the extra data by a routine technique is always desirable. This chapter highlights a rational model that gives a chance to realize this desire using the conventional

Hidden Resources of Coordinated XPS and DFT Studies http://dx.doi.org/10.5772/intechopen.80002 161

• Electronic configuration of the atoms in a solid holds the traps for the energy absorption, such as valence band electron transitions; and the core-level excitation of any origin fills

• These channels can be traced by the XPS, as the photoelectron energy losses, and by the DFT, as the valence band electron transitions. This pattern does not conflict with general concepts of electron-solid interaction and has been well verified in model studies of Pt and

• Intersection of the XPS and DFT outputs carries out two duties. First, it rejects those DFT results which do not conform to the fine XPS spectral structures. Second, it justifies the assignment of the refined DFT data, related to an appropriate unit cell, to a given XPS sample. As a result, the correlated XPS and DFT study discloses hidden potentialities of both techniques and provides the extra data on chemical behavior and local geometry of

• The procedure of a coordinated XPS and DFT study being highlighted can provide a deeper insight into the mechanism of wear performance of the material, thus facilitating

This work has been supported by the Russian Foundation for Basic Research (Grant 17-03- 00049) and conducted within the framework of the budget project for Boreskov Institute of

XPS and DFT outputs. The model is based on following statements.

those traps forming the multiple channels for energy dissipation.
