Recoil Effects in Valence Band Photoemission of Organic Solids

Article pubs.acs.org/ac

Recoil Effects in Valence Band Photoemission of Organic Solids Ming-Hui Shang, Takashi Fujikawa,* and Nobuo Ueno Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage, Chiba 263-8522, Japan S Supporting Information *

ABSTRACT: Recoil effects in valence band X-ray photoelectron spectroscopy (XPS) are studied for both abb-trifluorostyrene and styrene molecular crystal systems. The gradual changes of XPS spectra excited by several photon energies are theoretically investigated within the tight-binding approximation and harmonic approximation of lattice vibrations and have been explained in terms of not only atomic mass but also atomic orbital (AO) population. The recoil effect of valence band photoemission strongly depends on the population and partial photoionization cross section (PICS) of AOs as well as the masses of composite atoms. In abb-trifluorostyrene F 2p dominant bands show the recoil shift close to free F atom recoil shift, and C 2s dominant bands show that to free C atom recoil shift, whereas the mixed bands of C and F give rise to the peak asymmetries due to their different recoil shifts. For these systems, hydrogen contribution is negligibly small which is in contrast to our previous results for the crystals composed of small organic molecules. We also discuss some potential uses of the recoil shifts for these systems.

P

copy (SXPS) is quite useful for studying the surface properties.24−26,29,30 However, some problems are raised such as the breakdown of dipole approximation and recoil effects by experimentalists31−33 and theoreticians.28,34−41 In the hard X-ray photoemission process, photoelectrons leave the mother atom with quite large momentum and kinetic energy. The photoelectron emitter must be recoiled due to momentum conservation. Additionally, photoelectrons suffer scatterings from surrounding atoms during the propagation in solids, which also have some contributions to the recoil effects.41 If the photoemission process occurs in small molecular systems, center-of-mass translational motion, vibrational, and rotational excitations should be considered.28,34−38 However, in the case of solids, the rotational excitation and center-of mass translational motions are negligible because of the huge mass of the system. Hence, the vibrational excitation becomes the most important, which is well-known as phonon excitation for solid systems.39,40,42,43 High-energy-resolution C 1s XPS spectra excited by soft and hard X-rays have been measured for graphite. The C 1s peak shifts to higher binding energy with increasing hν,42 which can be well explained by use of the Mössbauer formula. Such a kind of shift was also observed for valence band XPS from aluminum.43 We however notice that the electron scatterings from surrounding atoms are completely neglected. A theory on recoil effects of photoelectrons excited by highenergy X-rays beyond the single-site approximation has also

hotoelectron spectroscopy (PES), including ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS), has become a powerful tool to explore the electronic, geometrical, and magnetic structures of various materials.1−4 Up until now, PES has been applied to many directions such as physics, chemistry, biology as well as the crossing domain among these.1,3,5−9 More explicitly, XPS is now widely applied in the analysis of various surfaces and chemical composition.10−15 The application of XPS to organic samples are also rapidly increasing.16−21 For example, the presence of both organic and inorganic forms of fluorine was clarified by analyzing the XPS from chlorofluoromethanes (CFxCly).16 The molecular nature of surface attachment and changes in electronic and wetting properties of indium−tin oxide electrodes modified by fluorinated alkyl and aryl phosphoric acids were characterized by XPS and UPS.19 A theophylline complex with 5-sulfosalicylic acid dihydrate was analyzed as a salt by XPS which quite sensitively determines the protonation state of nitrogen functional groups in the solid state.20 XPS analyses showed that n-propyltriethoxysilane was adsorbed on the clean surface Si(001), 2 × 1 at room temperature via the scission of at least two Si−O bonds.21 For these studies, chemical shifts of core levels were extensively used. On the other hand, the combination of core- and valenceXPS is generally necessary for picking up a clearer picture of the electronic structures. For instance, metal−insulator phase transitions22 and valence fluctuation and Kondo resonance23 were studied by applying core- and valence-XPS jointly. Measuring hard X-ray photoelectron spectroscopy (HAXPS) is inevitable22,24−28 for investigating the bulk electronic structures, even though the soft X-ray photoelectron spectros© 2013 American Chemical Society

Received: January 10, 2013 Accepted: February 26, 2013 Published: February 26, 2013 3739

dx.doi.org/10.1021/ac4000865 | Anal. Chem. 2013, 85, 3739−3745

Analytical Chemistry

Article

been proposed,41 which can naturally provide us with Debye− Waller factors, recoil factors, and their interference terms. A multiple scattering photoemission formula from extended levels has been obtained by use of the tight-binding approach.44−46 The interference effects between photoelectron waves emanating from different atomic sites can be negligible compared with the one-center term because the two-center terms should be suppressed due to Debye−Waller factors in the high energy region. Then the Gelius formula is safely applied for calculating the XPS intensities from organic47 and inorganic48 valence bands. On the basis of the Gelius formula, we have discussed valence photoemission accompanying the recoil shifts.47,48 In the previous paper,47 the valence band photoemission bands excited by intermediate- and high-energy photons are calculated for acetylene and diacetylene molecular solids. In those molecules some MOs have a large contribution from hydrogen, which show prominent recoil shifts even in the soft X-ray region. In contrast, molecules considered here, styrene (C8H8) and abb-trifluorostyrene (C8H5F3), have a small hydrogen contribution to each MO and the latter has influence from F atoms. It is thus interesting to study some new aspects of recoil effects in those molecular crystals.

I0(k)i = I0(k)1i + I0(k)i2

where I0(k)1i and I0(k)2i are the one- and two-center terms neglecting elastic scatterings from surrounding atoms. Now we introduce phonon effects on the photoemission intensity, where uα in eq 2.2 is treated as an operator and we take a thermal average over the ensemble. Thus these two terms can be written by39,40 I0(k)1i =

′

αα ′

=

I0(k)i2 =

α

L

+

(2.5)

−∞

dt exp(iεt ) 2π

exp(iAα )⟩ × exp[iQ· (R α0 − R0β)] * ∑ Y L*(k̂)YL ′(k̂)MLL

β

LL ′

ML

′

′ Lα′

(2.6)

with Âα (t ) = exp(iHvt )Aα exp( −iHvt ) ε = ωq + E0 − E0* − εk

(2.7)

where Aα = Q · uα, ωq stands for the photon energy, εk the kinetic energy of the photoelectron, and E0 and E0* are the energy of the target before and after the photoemission process, respectively. If we assume that the linear approximation works well even for the valence band photoemission, then the phonon Hamiltonian H*v in the excited state can be written as

′

Hv* = Hv +

∑ (λsbs + λs*bs†) s

(2.2)

(2.8)

where Hv is the harmonic ground-state vibrational Hamiltonian,

where Δ is the electron-photon interaction operator, uα denotes the deviation from the equilibrium position R0α of the αth atomic site, and the atomic excitation amplitude MLLα′ describes the photoexcitation from the AO χα′ with angular momentum Lα′ = (lα′,mα′) to the photoelectron state with L = (l,m). Q = k − q is the difference between the photoelectron momentum (k) and the incident photon momentum (q). The photoelectron wave function ψ−k under the influence of the optical potential is expanded by applying the site T-matrix, ϕ0k is the plane wave with momentum k, and the site T-matrix tβ satisfies

tβ = vβ + vβg0tβ

′ Lα′

× ⟨exp( −iAβ̂ (t )) exp(iHvt ) exp(−iHv*t )

∑

⟨ϕk0|tβgα Δ|χα ⟩ + ...) ′ β≠α ,α′

∞

∑ ∑ Ci*α ′Ciβ ′ ∫

β(≠ α)α α ′ β ′

′

αα ′

ML

α ′′

and

(2.1)

∑ Ciα ′(exp[iQ ·(R 0α + uα)] ∑ YL(k̂)MLL

* ∑ Y L*(k̂)YL ′(k̂)MLL LL ′

∑ Ciα ′⟨ψk−|Δ|χα ⟩ αα ′

∞

∫ dt exp(iεt ) ′ −∞ 2π

exp(iAα )⟩ ×

where χα′, χα′′,... are atomic orbitals (AO) on the site Rα and Ciα′s are the expansion coefficients. As discussed before, the photoemission amplitude is now given by49 ⟨ψk−|Δ|ϕi⟩ =

Ci*α ′′Ciα

× ⟨exp( −iÂ α (t )) exp(iHvt ) exp(−iHv*t )

■

∑ Ciα ′χα (r − R α)

∑ αα ′ α ′′

GENERAL FORMULATIONS Here we discuss the recoil effects in high-energy photoemission from delocalized levels where the initial one-electron state φi(r) is assumed to be written by the tight-binding approximation (approximately, linear combination of atomic orbitals, LCAO) ϕi(r) =

(2.4)

Hv =

⎛

∑ ωs⎝bs†bs + ⎜

s

1 ⎞⎟ 2⎠

(2.9)

bs (b†s )

designates the phonon annihilation (creation) operator for the mode s = (p, j) with crystal momentum p and its branch j. We thus write the one-center term as I0(k)1i =

∑ αα ′′ α ′

Ci*α ′′Ciα

∞

∫ dt exp[iεt + γ(t ) − Fα(t ) ′ −∞ 2π

* ML L − Gα(t )]∑ Y L*(k̂)YL (k̂)MLL α ′′ ′ ′ α′ LL ′

(2.3)

for the site potential vβ. The first term in eq 2.2 describes the direct photoemission amplitude without suffering elastic scattering from neighboring atoms, and the second term describes the amplitude of single scattering and so forth. In the high energy region we keep only the first term in eq 2.2 and obtain the photoemission intensity formula for the excitation from ϕi

(2.10)

where γ(t) is closely related to the Franck−Condon effect, while Fα(t) describes the recoil effect, and Gα(t) describes the interference between the two terms. All explicit forms of the three terms are given in the previous papers.40,41 In the highenergy region, the interference between the recoil and Franck− Condon effects plays an important role for strongly distorted systems. 3740



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imation.50−52 Trzhaskovskaya et al. also calculated PICS from 100.0 eV to 5.0 keV within the central Dirac−Fock−Slater potential under the dipole + quadrupole approximation.53 Published PICS for several AOs employing different approaches are shown in Table 2, where the data is taken with respect to the PICS of F 2p. The PICS of AO rapidly decrease with increasing photon energy ω. Molecular crystal of styrene and abb-trifluorostyrene are selected as examples since all composite atoms are quite light. The molecular structures and MO energies of these two systems are calculated using Gaussian09 code54 based on density functional theory (DFT)55 with Becke 3-parameter Lee−Yang−Parr semiemipirical hybrid functional (B3LYP) describing the exchange-correlation potential.56 Spin unrestricted calculations are employed throughout for this paper. Figure 1 shows the geometric structures of abb-trifluorostyrene (Figure 1a) and styrene (Figure 1b) molecules, where 3 hydrogen atoms are substituted by fluorine in the former. Photoemission bands are calculated for a single molecule fixed in space which can be regarded as the photoemission from molecular crystals, where the rotational fine structures can be neglected and intermolecular interaction is supposed to be small enough. Partial density of states (PDOS) can be obtained by use of Gaussian broadening of each MO to properly describe solid state effects. The parameter Fα is set as constant 0.1 a.u. throughout the calculations, which implies that the corresponding full width at half-maximum (fwhm) is about 0.22 eV. Figure 2 shows the PDOS of the valence bands of abb-trifluorostyrene (a) and styrene (b). The binding energy of the highest occupied molecular orbital (HOMO) is shifted to the Fermi level. The relative weight of AOs in the molecular crystal abbtrifluorostyrene system is displayed in Figure 2a. Fluorine 2p considerably contributes to PDOS in this frontier valence band region (12.0 ≥ Eb ≥ 0.0 eV: Eb is binding energy) in addition to C 2p and H 1s. C 2s contributes the major to the MOs with much larger binding energy than 13.0 eV. The top 3 MOs of abb-trifluorostyrene have π-type symmetry. PDOS of molecular crystal of styrene is displayed in Figure 2b, and the top 3 MOs of styrene are primarily dominated by C 2p with π-type symmetry.

The two-center term I0(k)2i can be neglected because of the strong damping due to the Debye−Waller factors.41 The sum over the off-diagonal (α′ ≠ α′′) terms are negligible in eq 2.10,44 which yields a simple formula for the one-center intensity, I0(ω)1i =

∑ |Ciα ′|2 σα ′δ(ω − εi − εk − Δεα) αα ′

Δε α = −

Q2 2Mα

(2.11)

where εi is the binding energy of the ith molecular orbital (MO) and σα′ is the partial photoionization cross section (PICS) for the α′th AO χα′ on the site α. For discussing the peak broadening, it is inevitable to take the second power of t in the exponent of eq 2.10. The integrals over t can be analytically carried out which yields Gaussian broadening40,41 I0(ω)1i =

∑ [Jαi (ω)gαi (ω)]

(2.12)

α

Jαi (ω) =

∑ |Ciα ′|2 σα ′

(2.13)

α′

gαi (ω) =

⎛ (ω − ε − ε − Δε α)2 ⎞ 2π i k ⎟ exp⎜ − Fα 2 F ⎝ ⎠ α

(2.14)

where the weighted atomic cross section of the αth atom in the ith MO photoemission Jiα(ω) is given by the sum over composite AOs χα′ on the αth atom. For example, in the case α is carbon, α′ runs over C 1s, 2s, and 2p AOs. The width parameter Fα can be calculated by40 Fα = k 2

∫0

∞

dω ω 2aα(ω)[2n(ω) + 1]

(2.15)

The projected phonon spectral function aα in eq 2.15 on site α is given in terms of eigenvectors of the phonon dynamical matrix.40,41 The width parameters are temperature dependent; it is about ∼0.1 eV at room temperature 300 K for graphite.40

■

■

CALCULATION DETAILS According to eq 2.11, the recoil effect is remarkable for light atoms such as hydrogen, carbon, and fluorine. The estimated Δεα′s for free atoms of H, C, and F for several photoelectron energies are shown in Table 1. On the other hand, as clearly shown in eq 2.13, PICS, σα′, are also crucial parameters for calculating the photoelectron intensity I0(ω)i besides the population on AO χα′, |Ciα′|2. The PICS for photoelectron energy below 1600.0 eV has been calculated by Yeh et al. within the dipole length approx-

RESULTS AND DISCUSSION Calculated XPS Valence Band of abb-Trifluorostyrene. Calculated XPS spectra from abb-trifluorostyrene crystal for photon energy ω = 1.0 keV are presented in Figure 3 where the spectra are normalized to the maximum height. In the calculations, we employ PICS by Yeh et al.,50−52 and the results are shown in Figure 3a. Results obtained by use of PICS by Trzhaskovskaya et al.53 are shown in Figure 3b. They are quite similar. Then we use the PICS values by Trzhaskovskaya et al.53 in the subsequent calculations. XPS spectra for the valence bands are calculated for abbtrifluorostyrene based on the electronic structure predicted by the Gaussian09 package.54 Figure 4 shows the calculated XPS spectra with (red solid line) and without (black dashed line) the recoil effects for soft and hard X-rays, which are normalized to the maximum height. The relative intensity from shallow MOs decreases with ω, while the intensity from the inner MOs increases. Similar results are also obtained when we use PICS by Yeh et al.50−52 (see S-Fig. 1 in the Supporting Information). Referring to Table 2, one can say that the ratio σF2p/σC2s, σC2p/ σC2s, and σH1s/σC2s decrease with ω. σF2p is larger than σC2s

Table 1. Estimated Recoil Energy Δεα in eV for Free H, C, and F Atoms with Respect to That at εk = 100.0 eV εk (keV) 0.2 0.5 1.0 2.0 4.0 8.0

H1 1.03 2.67 5.39 1.08 2.17 4.35

× × × × × ×

C12 −1

10 10−1 10−1 100 100 100

4.57 1.83 4.11 8.69 1.78 3.61

× × × × × ×

F19 −3

10 10−2 10−2 10−2 10−1 10−1

2.89 1.16 2.60 5.49 1.13 2.28

× × × × × ×

10−3 10−2 10−2 10−2 10−1 10−1 3741



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Table 2. Published PICS for H 1s, C 2s, C 2p, and F 2s Subshells Corresponding to Several Photon Energiesa Yb

Tc

H 1s C 2s C 2p F 2s F 2p H 1s C 2s C 2p F 2s F 2p

0.1 keV

0.2 keV

0.5 keV

1 keV

2 keV

4 keV

0.0089 0.1723 0.0794 0.2600 1.0000 0.0189 0.1814 0.0712 0.2731 1.0000

0.0058 0.2537 0.0550 0.5859 1.0000 0.0123 0.2485 0.0517 0.2594 1.0000

0.0045 0.4628 0.0428 1.5024 1.0000 0.0088 0.4197 0.0389 1.2889 1.0000

0.0045 0.7672 0.0361 2.9766 1.0000 0.0082 0.6643 0.0323 2.5230 1.0000

no data

no data

0.0087 1.0707 0.0258 4.8810 1.0000

0.0107 1.7900 0.0218 9.5640 1.0000

a

The PICS data is relative value with respect to that of F 2p at each photon energy. bCalculated by Yeh et al.50−52 cCalculated by Trzhaskovskaya et al.53

Figure 1. Geometric structures of abb-trifluorostyrene (a) and styrene (b) molecules.

Figure 3. Calculated XPS spectra from the valence band of abbtrifluorostyrene crystal excited by X-ray with ω = 1.0 keV. Results using PICS by Yeh et al.50−52 and Trzhaskovskaya et al.53 are displayed in parts a and b.

Figure 2. Valence band partial density of states (PDOS) obtained by Gaussian broadening of each level in abb-trifluorostyrene (a) and styrene (b).

when ω ≤ 1.0 keV, but σF2p is smaller than σC2s in the harder Xray region (≥1.0 keV). Therefore, MOs with a large population of F 2p give strong intensity for soft X-ray excitation while weaker for hard X-ray excitation. For the 4 keV X-ray photons, the peak observed at ∼3.5 eV shows the recoil shift of about 0.12 eV which is close to that for

Figure 4. Calculated XPS spectra from the valence band of the abbtrifluorostyrene system using the PICS by Trzhaskovskaya et al.53 XPS spectra by six excitation photons from 100.0 eV to 4.0 keV are displayed from parts a to f.

3742



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F, but the one at ∼14.3 eV shows the shift about 0.17 eV close to that for C as shown in Table 1. This observation implies that the PDOS near 3.5 eV has a dominant contribution from F and that near 14.3 eV has a dominant contribution from C. As demonstrated by eq 2.12, the XPS bands from valence levels can be separated out into each atomic contribution in the high-energy region. Figure 5 shows the individual contribution

are expected for soft X-ray excitations. In abb-trifluorostyrene, however, the contribution by H is quite weak and no more observable because the PICS of H decays rapidly with increasing excitation energy and these peaks are easily suppressed by the main peaks as shown in Figure 6.

Figure 6. XPS spectra with and without recoil effects using the PICS by Trzhaskovskaya et al.53 for ω = 100.0 eV, 1.0 keV, and 4.0 keV. The hydrogen component for ω = 100.0 eV and 1.0 keV is 100 times larger, while it is 200 times larger for excitation ω = 4.0 keV.

Figure 5. Calculated XPS spectra from the valence bands of the abbtrifluorostyrene system excited by X-rays with ω = 4.0 keV. The spectra without recoil effects are presented in part a, while part b displays the results with recoil effects obtained using the PICS by Trzhaskovskaya et al.53

The expanded XPS spectra in the low binding energy region is shown in Figure 6 including the spectra with and without recoil effects for three excitations ω = 100.0 eV, 1.0 keV, and 4.0 keV. Total PES and hydrogen components are displayed. Figure 6a−c shows the photoemission bands from the outer valence region, while parts d−f show the photoemission bands from the inner valence region. The majority contribution of total PES is that from F 2p whereas the hydrogen contribution is about 1/100−1/200 of F 2p. The photoelectron intensity by hydrogen is rather weak compared with that from fluorine due to the small PICS of H 1s. As indicated by arrows, shifts of the H-component gets larger with increasing excitation energy. By comparing the component PES from H in Figure 6f, the most obvious recoil effect is obtained for H atoms with ∼2.17 eV which is quite close to the value shown in Table 1 for photoelectron kinetic energy εk = 4.0 keV. However, its influence on total recoil shift is negligible due to quite weak intensity. Near the Fermi level, the peak mainly composed of C 2p and F 2p is quite broad, so that no prominent recoil shift is observed even in this expanded scale. Calculated XPS Valence Band of Styrene. On the basis of the PDOS shown in Figure 2b, the recoil effects for styrene are also investigated by use of PICS by Trzhaskovskaya et al.53 Figure 7 shows the calculated XPS spectra from the valence band of styrene molecular crystal for six excitation energies in the region 100.0 eV to 4.0 keV with and without recoil effects. One can easily find that XPS bands shift to higher binding energy as a result of the recoil effects caused by X-ray excitation, and the shifts depend on the excitation energy ω. Photoelectron intensity in the frontier region above 7.0 eV decays with ω where the spectra are normalized to the maximum at around 14.3 eV. Similar results are also obtained when we use PICS by Yeh et al.50−52 (see S-Fig. 2 in the Supporting Information). If one notice the PDOS of styrene,

without (a) and with (b) recoil shifts. In Figure 5a all Δεα′s in eq 2.14 are set to be 0, whereas finite Δεα′s are used in Figure 5b. Because of negligibly small PICS of H 1s, the PES from H is not shown. Figure 5 clearly shows that F dominantly contributes to the photoelectron intensity in the low binding energy region. The populations of C 2p and F 2p are comparable in this region (see Figure 2a). However, we should note that the PICS also plays an important role in addition to the PDOS. Actually, σC2p is much smaller than σF2p (see Table 2). Hence the combined effect of PDOS and PICS enhance the F 2p contribution to the photoelectron intensity in this low binding energy region. Then the recoil shift (0.12 eV) at ∼3.5 eV is quite close to the recoil energy of free fluorine ΔεF at photoelectron kinetic energy εk = 4.0 keV (shown in Table 1). In the deeper region, the carbon component plays a dominant role in the photoelectron intensity. Figure 2a shows PDOS of C 2s is the dominant in this region, and σC2s is much larger than σF2p. Then large population combining with large PICS of C 2s certainly yields stronger intensity than F 2p. As a result, the recoil shift (0.17 eV) is quite close to the recoil energy of free carbon ΔεC. In Figure 5b, asymmetric bell-shapes can be found in the peaks around ∼14.2 eV and ∼16.0 eV. This asymmetry is caused by different recoil shifts of carbon and fluorine. Therefore study of recoil effects can be one promising approach for picking up the PDOS in condensed systems. Generally, larger recoil energy shift can be expected on lighter elements than heavy elements under the same X-ray excitation level (as explicitly shown in Table 1). The recoil energy of free hydrogen ΔεH is estimated as nearly 12 times of carbon ΔεC at each excitation level. In the previous work,47 the hydrogen component is not so small for small organic molecules and detectable recoil shifts due to hydrogen recoil 3743



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or 15 times. Relative intensity of H-component decays with ω. As indicated by arrows, the shift of the H-component gets larger with excitation energy. However, its influence on total XPS bands is still negligible due to quite weak intensity. The recoil shifts due to hydrogen could not be observed even for soft X-ray excitations.

■

CONCLUDING REMARKS The recoil effects in XPS bands for molecular crystals, abbtrifluorostyrene and styrene, are discussed on the basis of harmonic approximation of atomic vibrations in solids where the rotational excitations and translational motions are neglected. The interference between photoelectron waves emanating from different atomic sites is suppressed by the Debye−Waller factor in the high energy region, and the Gelius formula is safely applied to the XPS calculation, which provides us with simple tools for the analyses of XPS bands. The recoil effects are found to be strongly atomic mass-dependent and also AO-dependent. We can obtain useful information on electronic structures of molecular crystals from recoil shift analyses of XPS bands. The Gelius formula provides us with PDOS, and the recoil shift analyses furthermore gives us element specific PDOS in particular of light atoms. The calculated XPS bands by use of the Gelius formula can be compared with the observed XPS bands, which gives us useful information on |Ciα|2. The recoil shift analyses shown by eq 2.12 of high-energy XPS bands additionally provide us with the information only on |Ciα|2 on light atoms. High-resolution high energy XPS could offer isotope dependent PDOS with aid of the recoil shifts. For example, contribution by 1s of H and D to each MO can be separated out with aid of the recoil effects in the soft X-ray region XPS: This is a very unique feature. We thus believe that the recoil effects on XPS will open a new field in XPS analyses.

Figure 7. Calculated XPS spectra from the styrene crystal valence band by use of PICS by Trzhaskovskaya et al.53.

such behavior can be acceptable. As shown in Figure 2b, PDOS of C 2p is dominant in the low binding energy region (12.0 eV). The ratio σC2p/σC2s rapidly decreases with photon energy ω as shown in Table 2. As a result, we only observe C 2s predominant bands for the high-energy excitations. Comparing the results from abb-trifluorostyrene, we observe uniformly shifted spectra from nonrecoiled spectra. The shift at ω = 4.0 keV is about 0.18 eV, which is quite close to the free carbon recoil shift as shown in Table 1. Figure 8 shows the expanded spectra from the frontier valence band (≤6.0 eV) with and without recoil effects by three

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: (+81) 043-290-3699. E-mail: [email protected]. jp. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to Dr. K. Niki for putting the manuscript into final version. Dr. T. Kaneko and Mr. Y. Ohori are acknowledged for pertinent comments on the manuscript. Ming-Hui Shang is grateful to Dr. Jing Zhang for productive discussion and he thanks the Global COE program (Advanced School for Organic Electronics, Chiba University) for financial support.

Figure 8. XPS spectra with and without recoil effects using the PICS by Trzhaskovskaya et al.53 The carbon and hydrogen components are displayed in addition to the total spectra for photon energies ω = 100.0 eV, 1.0 keV, and 4.0 keV. The hydrogen component for ω = 100.0 eV is 10 times larger, while it is 15 times larger for excitation ω = 1.0 and 4.0 keV .

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REFERENCES

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excitations, ω = 100.0 eV, 1.0 keV, and 4.0 keV. In this figure it is clearly shown that the majority of total PES is by carbon. The photoelectron intensity by hydrogen is rather weak compared with the carbon component due to the small PICS of hydrogen, even though hydrogen components are shown in the scale of 10 3744



Article

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Recoil Effects in Valence Band Photoemission of Organic Solids

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