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Deeper Insight into Depth-Profiling of Aqueous Solutions Using Photoelectron Spectroscopy Olle Björneholm,† Josephina Werner,†,‡ Niklas Ottosson,†,§ Gunnar Ö hrwall,∥ Victor Ekholm,† Bernd Winter,⊥ Isaak Unger,⊥ and Johan Söderström*,† †

Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden Department of Chemistry and Biotechnology, Swedish University of Agricultural Sciences, P.O. Box 7015, SE-750 07 Uppsala, Sweden ∥ MAX IV Laboratory, Lund University, Box 118, SE-221 00 Lund, Sweden ⊥ Joint Laboratory for Ultrafast Dynamics in Solutions and at Interfaces, Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany ‡

ABSTRACT: X-ray photoelectron spectroscopy (XPS) is widely used to probe properties such as molecular stoichiometry, microscopic distributions relative to the surface by so-called “depth-profiling”, and molecular orientation. Such studies usually rely on the core-level photoionization cross sections being independent of molecular composition. The validity of this assumption has recently been questioned, as a number of gas-phase molecules have been shown to exhibit photon-energy-dependent nonstochiometric intensity oscillations arising from EXAFS-like modulations of the photoionization cross section. We have studied this phenomenon in trichloroethanol in both gas phase and dissolved in water. The gas-phase species exhibits pronounced intensity oscillations, similar to the ones observed for other gas-phase molecules. These oscillations are also observed for the dissolved species, implying that the effect has to be taken into account when performing depth-profiling experiments of solutions and other condensed matter systems. The similarity between the intensity oscillations for gas phase and dissolved species allows us to determine the photoelectron kinetic energy of maximum surface sensitivity, ≈100 eV, which lies in the range of pronounced intensity oscillations.

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INTRODUCTION X-ray photoelectron spectroscopy (XPS) is widely used in many research fields to probe the electronic and geometric structure of atoms, molecules, clusters, liquids, surfaces, and solids.1 One fundamental observable in XPS is the intensity of energetically separated peaks associated with atoms of the same element but in different chemical environments. These intensities have been used to deduce information about sample properties such as stoichiometry, distribution of inequivalent atomic sites relative to the surface by so-called “depth profiling”, and molecular orientation.1−6 A more or less implicit assumption in these studies has been that at photon energies sufficiently high above the ionization threshold, the photoionization cross sections for core-level orbitals are independent of the molecular composition. It is well-known that near threshold this is not true due to the so-called shape resonances, which typically extend up to photon energies 50 eV above the photoionization threshold.7 At higher excitation energies though, the observed intensity variations of chemically shifted photoemission (PE) lines in condensed samples have mainly been interpreted as being due to differing spatial distributions of the emitting species relative to the surface, as the intensity © XXXX American Chemical Society

variations have been regarded as mainly arising from loss of signal due to inelastic scattering. This approach has been used extensively by both the Hemminger group and us to probe the distribution of solutes in the aqueous phase.2−5,8 In the gas phase, on the other hand, the reasoning above would imply that core-level intensity ratios at the so-called magic angle (where the dependence on the photoemission asymmetry parameter β vanishes1,9) should reflect the stoichiometry of the emitting sites, at excitation energies above all shape resonances. For chlorine-substituted ethanes in the gas phase, however, this has recently been shown not to be the case. Instead, the C 1s signals exhibit oscillating ratios over a wide range of photon energies. The amplitude of these oscillations decrease with increasing energy above threshold, but can still be observed several hundred of eV above threshold.10 Furthermore, the observed asymptotic value of the ratio is also different from the expected stoichiometric value Special Issue: John C. Hemminger Festschrift Received: June 5, 2014 Revised: August 24, 2014

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dx.doi.org/10.1021/jp505569c | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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of unity. These photon-energy-dependent nonstochiometric intensity oscillations have been explained as arising from EXAFS-like modulations of the photoionization cross section. In a simple picture, this can be seen to arise from the outgoing photoelectron wave being backscattered from neighboring atoms and then interfering with itself. Depending on the photoelectron wavelength, connected to its kinetic energy via the de Broglie relation, the distance to the scatterers and the phase shift upon scattering, the interference may be either constructive or destructive. Hence, variation of the photon energy causes oscillations in the photoionization cross section. The backscattering cross-section increases with atomic number and, in the originally investigated gaseous molecules chloroethane, dichloroethane, and trichloroethane, the backscattering is dominated by the chlorine atoms. Trichloroethane exhibited the largest intensity oscillations since it has the largest number of heavy scatterers. The absence of heavy scatterers does however not guarantee constant, stoichiometric intensity ratios between chemically equivalent atoms of the same element; 2butyne (CH 3 CCCH 3 ) in the gas phase has been demonstrated to exhibit similar variations in the ratio between the carbons close to the triple bond (C2 and C3) and the terminal methyl groups (C1 and C4).11 Even small symmetric molecules such as CH4, CF4, and BF3 in the gas phase shows similar intensity variations when comparing the vibrational components to each other; this effect has been attributed to photoelectron diffraction.12 Other examples where similar effects have been observed in the gas phase include C6013 and C7014 and small metal clusters.15 These intensity oscillations observed in free, gas phase organic molecules would, if present in the condensed phase, have implications for many applications of XPS. As organic molecules are an important class of aqueous solutes, it is of general interest to study whether the photon-energy-dependent nonstochiometric intensity oscillations are observed also for solvated species. As all the above-mentioned molecules investigated in the gas phase have low solubility in water, we have instead chosen to study the water-soluble trichloroethanol (Cl3C−CH2OH), a compound closely related to the chloroethanes studied in ref 10. As trichloroethanol is rather volatile, spectra of the gas phase and hydrated species can be recorded simultaneously and photon-energy-dependent intensity oscillations be investigated for both phases. Implications of these results for XPS studies from dense samples, as liquids or solids, will be discussed.

Details of the experimental setup have been described elsewhere.17 Briefly, the liquid sample was injected into a vacuum chamber through a glass nozzle with 20 μm inner diameter, forming a liquid microjet at 10 ± 5 °C at roughly 45 m s−1. Both the nozzle and the pump generating the necessary backing pressure were obtained from Microliquids GmbH.18 The lens axis for electron detection was positioned at an angle of 54.7° (the so-called magic angle)1 relative to the polarization plane of the synchrotron light polarization vector. In this geometry, the differential cross section is independent of the photoemission anisotropy parameter β1. This means that the influence of photoelectron elastic scattering on the relative intensities for measurements not performed at the magic angle (as the magnitude of β values are reduced in the liquid phase19) is not a concern for the present measurements. The lens axis was furthermore perpendicular to the flow of the liquid jet as well as to that propagation direction of the ionizing X-rays, that is, in the so-called dipole plane.1 The photoelectrons emitted from the liquid surface entered the differentially pumped electron analyzer through a skimmer with a diameter of 1 mm. Due to the relatively large focal size at the I411 beamline, we could simultaneously measure signals from both the liquid and the gas evaporating from the jet. The large focal size also means that the measurements are not as sensitive to minor fluctuations in the relative beam positions, as they would have been for the case where the photon beam and the liquid beam are of comparable size, allowing us to relatively reliably compare the relative intensities from the liquid and gaseous phase. Curve fitting was carried out using the SPANCF20 fitting routine for IGOR Pro. The measured C 1s photoelectron lines originating from the gas phase were fitted using post collision interaction (PCI) profiles.21 The asymmetry of the PCI profiles were calculated according to equations in ref 20, assuming an average kinetic energy for the Auger electron of 280 eV (the exact kinetic energy of the Auger electrons will only affect the results presented here to a negligible extent). The lifetime broadening for both the liquid and the gaseous phase was held constant for all photon energies, the obtained value is based on a best-fit basis, and not used to extract physical constants. The Gaussian broadening was allowed to vary for each excitation energy but was assumed to be the same for both aggregation states (liquid and gaseous) at a given excitation energy. The measured photoelectron lines originating from the liquid were fitted20 using Voigt line shapes without PCI contribution, as the PCI contribution of the lines originating from the liquid phase was negligible compared to the symmetric broadening mechanisms, and no asymmetry of the liquid phase peaks could be discerned. In the fits, the energy separations between the chemically shifted components in the gas phase were kept fixed, as were the energy separations between the chemically shifted components in the liquid phase; however, the shift between the gas phase and the liquid phase was allowed to vary. This should be regarded as an effective shift accounting for several phenomena that can influence the energy positions of the gas phase lines relative to the positions of the lines originating from the liquid. This shift can for example depend on the overlap of the liquid jet with the photon beam, which may change slightly when the photon energy is changed. To explore the exact origins of this shift is however beyond the scope of this paper. Note that if a fit is done so that the relative shift of all four peaks is held constant or allowed to freely vary, the results are within the error bars in Figure 2.

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EXPERIMENTAL SECTION Soft XPS experiments on a liquid microjet were conducted at the I411 undulator beamline at the MAX IV Laboratory, the Swedish national synchrotron facility, Lund University.16 The sample consisted of a solution of 0.5 mol/dm 3 (M) trichloroethanol in highly demineralized water (18.2 MΩ×cm, Millipore Direct-Q). In addition, 0.2 M of NaCl was added to make the aqueous sample sufficiently conducting to avoid sample charging. Commercially available chemicals (Trichloroethanol: 99+%; NaCl ≥ 99.5%) were obtained from Sigma-Aldrich and used without further purification. Furthermore, prior to the XPS experiments, each sample was filtered (Whatman Puradisc FP30 syringe filters, 1.2 μm) to remove solid particles and sonicated to remove air bubbles, both which may disturb the flow in the liquid microjet or cause the injection system to fail. B



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We do not attempt to fit various vibrational components in the PE lines, which are instead assumed to result in a broadening of the profiles used. The fitting employed here follows the general ideas used in ref 10 where the fit was performed on a best-fit basis, which is a valid procedure as long as the discussion is related to the relative intensities of various peaks.

when comparing XPS spectra of molecules in the gas phase and in the aqueous phase, which has been recently discussed in detail; see, for example, ref 22. The lower binding energy of the peaks from the aqueous phase is primarily caused by final-state screening of the charged core-hole species created in the PE process. The widths of the gas-phase peaks are in this case mainly determined by the experimental resolution (combined with broadenings caused by vibrational, rotational, thermal movement, PCI, etc.), while the larger width of the aqueous phase peaks is caused by inhomogeneous broadening arising due to the distribution of nonequivalent local surroundings in the liquid, resulting in a distribution of final-state screening energies. The chemical shifts between the carbonyl and chlorinated methyl carbons and the binding energy shift between gas and aqueous phase peaks are such that four separate peaks are easily identified and well fitted, as seen in Figure 1. Having identified the spectral features in terms of binding energies, we will now proceed to discuss their relative intensities. PE signals from the condensed phase can generally be expressed as the integral over the density function, ρ(z), of the emitting species, exponentially attenuated with the effective electron mean free path λ1. If the liquid interface is considered sharp, that is, the density changes abruptly from the bulk value to that of the gas-phase, the intensity from site n (COH or CCl) in the liquid phase can be written as

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RESULTS AND DISCUSSION In Figure 1, C 1s XPS spectra of trichloroethanol recorded at four different photon energies are shown. The recorded spectra

Iliq, n = kliq, nσliq, n

∫ ρliq,n (z)·exp(−z/λ)dz

(1)

where kliq,n depends on a number of parameters, such as photon dose on the sample, spectrometer transmission, detector efficiency, and so on, that we for the present discussion can consider constant for each excitation energy during the experiment. Furthermore, σliq,n is the photon-energy-dependent photoionization cross-section. For a given excitation energy λ is constant and hence we can consider the integral in eq 1 to yield an efficient probability Pliq,n to generate a photoelectron from sites n in the liquid that escapes into the gas phase. We can thus write

Figure 1. C 1s XPS spectra of gas phase and aqueous trichloroethanol recorded at photon energies of 320, 360, 400, and 500 eV. The data points shown in Figure 2 are obtained from several spectra recorded at each photon energy; here we show four representative spectra, one from each photon energy. The fitted peaks are assigned to photoemission from CCl at high binding energies and COH at lower binding energies in both aggregation states. These spectra are arbitrarily calibrated so that the Cliq,OH peak is centered at 292 eV. The noise in the spectra at higher photon energies are both due to a lower cross section for the photoemission process as well as a significant lower photon flux at the I411 beamline. These data points were measured for a longer time, however the error bars in the observed ratio (see Figure 2) are still larger for high photon energies.

Iliq, n = kliq, nσliq, nPliq, n

(2)

Furthermore, the recorded intensity from the gas-phase molecules can similarly be written as Igas, n = kgas, nσgas, nPgas, n

(3)

We will now for a given excitation energy assume the two inequivalent COH and CCl sites in the trichloroethanol molecule in a given aggregation state x to have equal spatial distributions and be attenuated by the same mean free path, yielding Px,OH = Px,Cl. Furthermore, as the measurements of the two signals are done simultaneously, it also holds that kx,OH = kx,Cl. Note, however, that Px,n and kx,n will differ between the two aggregation states, that is, Pliq,n ≠ Pgas,n and kliq,n ≠ kgas,n, which we will return to below. The observed intensity ratio between the signals from COH and CCl in a certain aggregation state then becomes

contains contributions from both the hydrated and the gas phase molecules, the latter due to evaporation of trichloroethanol from the jet into vacuum, observed since the focus of the X-ray beam at the I411 beamline is considerably larger than the diameter of the liquid jet. The C 1s XPS spectra of both gaseous and aqueous trichloroethanol consist each of two C 1s PE lines associated with the chemically inequivalent carbon atoms, that is, of the carbonyl (COH) and of the chlorinated methyl (CCl) group, respectively. These two PE lines are separated with 1.9(1) eV, where the lower binding energy component is associated with COH, whereas the higher binding energy component is associated with CCl. Relative to the two gas phase peaks, the two peaks associated with the hydrated molecules are broader and are located at approximately 1.2(1) eV lower binding energies. This is typical

Ix ,Cl /Ix ,OH = σx ,Cl /σx ,OH

(4)

As mentioned above, the intensity ratio Ix,Cl/Ix,OH between the two inequivalent carbons naively expected from stoichiometry is unity. Upon variation of the photon energy, we notice that this is generally not the case though, as clearly seen in C



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Figure 2. Observed intensity ratio IxCl/IxOH for the C 1s XPS signals from the two inequivalent carbons COH and CCl in trichloroethanol in the gas phase and dissolved in liquid water. Most data points were measured several times. By fitting each individual data set using various constraints in the fit a spread in the calculated ratio was observed; this spread was then used to estimate the error bars. It is possible that the overlap between the liquid beam and the ionizing radiation changes slightly when changing photon energy (it can be assumed to be constant for one each individual measurement), the observed ratio will however not be sensitive to such changes.

any significant differences within the experimental uncertainty. The largest differences between Ix,Cl/Ix,OH for gaseous and aqueous molecules, respectively, are observed for photon energies above 420 eV. We note that the statistical quality of the spectra decrease with increasing photon energies due to the decreased photon flux, the decreased spectrometer efficiency, and the gradual decrease of the C 1s ionization cross section with increasing electron kinetic energy,23 meaning that the differences at higher photon energies may be due to an increased experimental uncertainty and not due to any physical effect. This means that, at least for the present case of trichloroethanol, the EXAFS-like oscillations observed for the gas-phase species are also observed for the aqueous species. This also suggests that the observed oscillations indeed originate from intramolecular scattering and that the geometry of trichloroethanol is not, or at least not significantly, changed upon hydration. Several studies of the distribution of species relative to the surface, so-called depth profiling, are based on very similar measurements, that is, changes in relative peak intensities from chemically inequivalent species of the same element with photon energy, see, for example, refs 4 and 5. The basic assumptions in such studies are that the cross-section of such chemically inequivalent species exhibits the same photonenergy dependence and that any changes in relative intensity between such chemically inequivalent species is due to their distribution relative to the surface since the surface sensitivity of XPS, reflected by the so-called universal curve of the electron inelastic mean free path (IMFP),1 which has a minimum at electron kinetic energies around 50−100 eV, above and below which it monotonously increases. The present study shows that the PE of such chemically inequivalent species of the same element but in different functional groups, in this case COH and CCl in trichloroethanol, do not necessarily exhibit the same photon-energy dependence. This implies that one of the assumptions underlying depth profiling using PE intensities is not generally valid, and that depth-profiling measurements consequently could be affected

Figure 2. For both the gas and the liquid phases, the spectra recorded at 320 and 400 eV exhibit ratios close to 1, while the spectrum recorded at 360 eV exhibits a ratio Ix,Cl/Ix,OH = 1.6 ± 0.05. This shows that gas-phase trichloroethanol does exhibit photon-energy-dependent nonstochiometric intensity variations, implying different photon-energy dependences of the two cross sections σxCl and σxOH, a behavior similar to what has been previously reported for chloroethane, dichloroethane, trichloroethane,10 and 2-butyne.11 Moreover, our results show that such variations are also present for hydrated molecules. We will now proceed to study the photon-energy dependence of these variations in more detail. In Figure 2 the C 1s XPS intensity ratio IxCl/IxOH of the gas and liquid phases between CCl and COH is shown as a function of photon energy, where the photon energy was varied between 300 and 500 eV. Both of these ratios are very close to 1.05 ± 0.05 for hν ≈ 320 eV, above which comes a local maximum of 1.25 ± 0.15/1.15 ± 0.05 around hν ≈ 330 eV, followed by a local minimum of 1.15 ± 0.15/1.1 ± 0.05 around hν ≈ 340 eV. This in turn is followed by a maximum of 1.50 ± 0.05/1.65 ± 0.2 around hν ≈ 360 eV, which is followed by a minimum of slightly above 1 around hν ≈ 400 eV. After this minimum we see the onset of what could be a broader maximum, which then extends toward higher photon energies after our last data point. Returning to eq 4, the observed variations in Ix,Cl/Ix,OH imply variations in σx,Cl/σx,OH, the ratio between the ionization cross sections of CCl and COH. We note that these observed photonenergy-dependent variations of the cross-section ratio are strongly reminiscent of the behavior of the related molecules chloroethane, dichloroethane, and trichloroethane in the gas phase.10 We therefore attribute the here-observed photonenergy-dependent variations in the cross-section ratio of gaseous trichloroethanol to the same physical phenomenon, that is, intramolecular EXAFS-like oscillations, as for the earlier reported gaseous cases chloroethane, dichloroethane, and trichloroethane10 and 2-butyne.11 As can be seen in Figure 2, the oscillations for the gaseous and hydrated species are very similar, and we cannot identify D



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maximum at 360 eV, and in the ratio I(Cliq,Cl)/I(Cgas,OH), a local minima near 400 eV can be discerned. The differences indicate that we cannot fully treat σliq/σgas as a constant. This could partly be explained by variations in the absolute cross sections for the two aggregation states; however, we have already shown (see Figure 2) that the relative cross sections within an aggregation state change similarly in the two aggregation states. Another possibility is that the overlap between the liquid microjet and the ionizing radiation changes slightly when changing photon energy. When comparing spectra obtained at different times with the same photon energy it is possible to see variations in the relative intensity between the two aggregation states (not shown in figure). This indicates that the observed changes likely are not due to variations in the absolute cross sections for the two aggregation states, but rather to instabilities in the overlap of the two beams. As discussed earlier, these changes do not significantly affect the ratio within the same aggregation state presented in Figure 2. However, comparisons between the two aggregation states, as done here, will be affected by such changes. Even though not strictly correct, for simplicity we will continue to treat σliq/σgas as a constant. Alternatively, it would be possible to introduce an energy-dependent function that attempts to “correct” for the differences between Figure 2 and Figure 3; however, we believe that such an approach will introduce even larger systematic errors into the following discussion. Thus, we treat σliq/σgas as a constant, and we can proceed to analyze the significance of the intensity ratio Iliq/Igas shown in Figure 4. Taken these considerations together with eq 5 we find that

by similar scattering mechanisms. In the present case, a (erroneous) conclusion could be that at the surface the C−C bond of the molecule is either parallel to the water surface or randomly oriented (since the CCl/COH ratio is close to the stoichiometric value for the lowest photon energies corresponding to the highest surface sensitivity) but that subsurface molecules would be aligned with CCl toward the water surface (since the CCl/COH ratio is >1 for somewhat less surfacesensitive conditions around 400 eV photon energy). The microscopic mechanism responsible for such a hypothetical orientational behavior would be difficult to understand. However, since the photon-energy-dependent variations of the CCl/COH intensity ratio is an intrinsic property of the molecule, observed also for the gaseous species, there is no basis for drawing such conclusions about any molecular orientation. We now consider the ratio Iliq,n/Igas,n of the total PE intensity originating from the liquid phase divided by the total intensity originating from the gas phase, as a function of photon energy, which we will show contains some information about the kinetic energy dependence of the mean free path in the liquid phase. Combining eqs 1−3 and omitting the indices n for the two inequivalent sites CCl/COH we obtain Iliq /Igas = kliq /kgasσliq /σgasPliq /Pgas

(5)

The exact value of kliq/kgas strongly depends on the overlap between the photon beam and the liquid beam, but under stable experimental conditions (which is aided by the large focus size at the I411 beamline), kliq/kgas can be considered constant. Even though we do not know the absolute values of the C 1s cross sections σxOH and σxCl for the two inequivalent carbons CCl and COH in trichloroethanol, we have shown above that they exhibit the same photon energy dependence for both gaseous as well as dissolved molecules (note that σxOH and σxCl do not have to be the same for the ratios to be the same). It is possible to check the assumption that σliq/σgas can be treated as a constant by studying the ratios I(Cgas,Cl)/I(Cliq,OH) or I(Cliq,Cl)/I(Cgas,OH) and comparing this to the ratios in Figure 2. If σliq/σgas can be treated as a constant, then these ratios should closely resemble those in Figure 2, and strong deviations indicate that σliq/σgas cannot be treated as a constant. These ratios are shown in Figure 3, normalized to 1 at the photon energy 320 eV. These ratios are not identical to the ratios in Figure 2, however, they do clearly show the same trend with a low value at low photon energies rising toward a

Iliq /Igas ∼ Pliq /Pgas

(6)

Since the molecular densities in both the liquid and gas phase are independent of excitation energy, a variation in Pliq/Pgas must hence be due to a variation in the mean free path λ in the liquid phase. The photon-energy dependence in the measured Iliq/Igas will thus reflect the probability for inelastic scattering of the outgoing photoelectron on molecules in the liquid. The functional form of Iliq/Igas is related, but not directly proportional, to the electron mean free path curve, λ(KE).25 Iliq/Igas will, however, mimic the behavior of λ(KE) in the sense that an increase in λ(KE) always results in an increase of Iliq/Igas and vice versa. The experimentally obtained ratios Il/Ig for both the COH and CCl signals are shown in Figure 3. As can be seen in the figure, the minimum of Iliq/Igas, corresponding to the minimum of λ(KE), is at an electron kinetic energy of approximately 100 eV. We note that around this kinetic energy, corresponding to photon energies around 400 eV, the cross section oscillations are quite strong. Maximum surface sensitivity is thus, at least in this case, not compatible with stoichiometric intensity ratios, a fact that has to be taken into consideration in depth-profiling studies, as well as studies where the photon energy is not kept constant. To the best of our knowledge there is only one previous experimental study of λ(KE) of liquid water in the literature.8 These were based on O 1s PE intensity measurements from pure water and NaI electrolytes, where comparisons were made to ion distribution profiles obtained from classical MD simulations. In that study the KE of the minimum λ(KE) value was found to be somewhat higher than here, close to 175 eV. This could, at least in part, be due to the fact that the study was not performed in the magic angle. As a consequence, elastic scattering might have partly affected the results. However, the overall shape of the

Figure 3. Comparison of the ratios I(CCl)/I(COH) between the two aggregation states, see text for further discussion. E



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Figure 4. Observed intensity ratio Iliq,n/Igas,n for the C 1s XPS signals from each of the two carbon atoms in trichloroethanol (n = COH or CCl) as a function of photon energy and electron kinetic energy. The error bars were estimated according to the description given in the footnote.24

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two λ(KE) curves are similar; the most striking feature being the weak increase in λ(KE) upon going to low KEs below the minimum, being much less pronounced than commonly found in estimates of the universal mean free path curve.1 This is consistent with the conclusion in ref 19, namely, that at nearthreshold energies the electron probing depth in liquid water is much shorter than previously thought.

(1) Hüfner, S. Photoelectron Spectroscopy; Springer-Verlag: Berlin, Heidelberg, 1995. (2) Ghosal, S.; Hemminger, J. C.; Bluhm, H.; Mun, B. S.; Hebenstreit, E. L. D.; Ketteler, G.; Ogletree, D. F.; Requejo, F. G.; Salmeron, M. Electron Spectroscopy of Aqueous Solution Interfaces Reveals Surface Enhancement of Halides. Science 2005, 307, 563. (3) Brown, M. A.; D’Auria, R.; Kuo, I. F. W.; Krish, M. J.; Starr, D. E.; Bluhm, H.; Tobias, D. J.; Hemminger, J. C. Ion Spatial Distributions at the Liquid-Vapor Interface of Aqueous Potassium Fluoride Solutions. Phys. Chem. Chem. Phys. 2008, 10, 4778. (4) Ottosson, N.; Vácha, R.; Aziz, E. F.; Pokapanich, W.; Eberhardt, W.; Svensson, S.; Ö hrwall, G.; Jungwirth, P.; Björneholm, O.; Winter, B. Large Variations in the Propensity of Aqueous Oxychlorine Anions for the Solution/Vapor Interface. J. Chem. Phys. 2009, 131, 124706. (5) Lewis, T.; Winter, B.; Stern, A. C.; Baer, M. D.; Mundy, C. J.; Tobias, D. J.; Hemminger, J. C. Does Nitric Acid Dissociate at the Aqueous Solution Surface? J. Phys. Chem. C 2011, 115, 21183. (6) Ottosson, N.; Romanova, A. O.; Söderström, J.; Björneholm, O.; Ö hrwall, G.; Fedorov, M. V. Molecular Sinkers: X-ray Photoemission and Atomistic Simulations of Benzoic Acid and Benzoate at the Aqueous Solution/Vapor Interface. J. Phys. Chem. B 2012, 116, 13017. (7) Piancastelli, M. N. The Neverending Story of Shape Resonances. J. Electron Spectrosc. Relat. Phenom. 1999, 100, 167. (8) Ottosson, N.; Faubel, M.; Bradforth, S. E.; Jungwirth, P.; Winter, B. Photoelectron Spectroscopy of Liquid Water and Aqueous Solution: Electron Effective Attenuation Lengths and EmissionAngle Anisotropy. J. Electron Spectrosc. Relat. Phenom. 2010, 177, 60. (9) Cooper, J.; Zare, R. N. Angular Distribution of Photoelectrons. J. Chem. Phys. 1968, 48, 942. (10) Söderström, J.; Mårtensson, N.; Travnikova, O.; Patanen, M.; Miron, C.; Sæthre, L. J.; Børve, K. J.; Rehr, J. J.; Kas, J. J.; Vila, F. D.; et al. Nonstoichiometric Intensities in Core Photoelectron Spectroscopy. Phys. Rev. Lett. 2012, 108, 193005. (11) Carroll, T. X.; Zahl, M. G.; Børve, K. J.; Sæthre, L. J.; Decleva, P.; Ponzi, A.; Kas, J. J.; Vila, F. D.; Rehr, J. J.; Thomas, T. D. Intensity Oscillations in the Carbon 1s Ionization Cross Sections of 2-Butyne. J. Chem. Phys. 2013, 138, 234310. (12) Ueda, K.; Miron, C.; Plésiat, E.; Argenti, L.; Patanen, M.; Kooser, K.; Ayuso, D.; Mondal, S.; Kimura, M.; Sakai, K.; et al. Intramolecular Photoelectron Diffraction in the Gas Phase. J. Chem. Phys. 2013, 139, 124306.

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CONCLUSIONS We have shown that the type of EXAFS-like oscillations observed in ref 10 persists for a solution of 0.5 M trichloroethanol in water and that the oscillations of the dissolved molecules are similar to those of the free (gaseous) molecules. This result has potential implications for essentially all studies where the photon energy is changed to probe various depths in the liquid, as already suggested in refs 10 and 26. Furthermore, by comparing the PE signal from the gaseous phase with that of the liquid phase we are able to show a curve where we attribute the overall shape to the inelastic mean free path of PEs in the liquid phase, displaying a minimum at roughly 100 eV KE.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +46 (0)18 471 36 05. Present Address §

FOM Institute AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands. Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

The authors would like to acknowledge financial support by the Swedish Research Council (VR) and the Knut and Alice Wallenberg Foundation, as well as the help of the MAX-lab staff. F



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(13) Xu, Y. B.; Tan, M. Q.; Becker, U. Oscillations in the Photoionization Cross Section of C60. Phys. Rev. Lett. 1996, 76, 3538. (14) Liebsch, T.; Hentges, R.; Rüdel, A.; Viefhaus, J.; Becker, U.; Schlögl, R. Evidence for Oscillations in the C70 Valence Photoionization Cross Sections. Chem. Phys. Lett. 1997, 279, 197. (15) Jänkälä, K.; Tchaplyguine, M.; Mikkelä, M.-H.; Björneholm, O.; Huttula, M. Photon Energy Dependent Valence Band Response of Metallic Nanoparticles. Phys. Rev. Lett. 2011, 107, 183401. (16) Bässler, M.; Forsell, J.-O.; Björneholm, O.; Feifel, R.; Jurvansuu, M.; Aksela, S.; Sundin, S.; Sorensen, S.; Nyholm, R.; Ausmees, A.; et al. Soft X-ray Undulator Beam Line I411 at MAX-II for Gases, Liquids and Solid Samples. J. Electron Spectrosc. Relat. Phenom. 1999, 101, 953. (17) Bergersen, H.; Marinho, R. R. T.; Pokapanich, W.; Lindblad, A.; Bjö rneholm, O.; Sæthre, L. J.; Ö hrwall, G. A Photoelectron Spectroscopic Study of Aqueous Tetrabutylammonium Iodide. J. Phys.: Condens. Matter 2007, 19, 326101. (18) Microliquids GmbH, http://www.microliquids.com. (19) Thürmer, S.; Seidel, R.; Faubel, M.; Eberhardt, W.; Hemminger, J. C.; Bradforth, S. E.; Winter, B. Photoelectron Angular Distributions from Liquid Water: Effects of Electron Scattering. Phys. Rev. Lett. 2013, 111, 173005. (20) Kukk, E. Spectrum Analysis by Curve Fitting (SPANCF) Macro Package for Igor Pro; University of Turku: Turku, Finland, 2012. (21) van der Straten, P.; Morgenstern, R.; Niehaus, A. Angular Dependent Post-Collision Interaction in Auger Processes. Z. Phys. D: At., Mol. Clusters 1988, 8, 35. (22) Ottosson, N.; Børve, K. J.; Spångberg, D.; Sæthre, L. J.; Faubel, M.; Bergersen, H.; Pokapanich, W.; Ö hrwall, G.; Björneholm, O.; Winter, B. On the Origins of Core−Electron Chemical Shifts of Small Biomolecules in Aqueous Solution: Insights from Photoemission and Ab Initio Calculations of Glycineaq. J. Am. Chem. Soc. 2011, 133, 3120. (23) Yeh, J. J.; Lindau, I. Atomic Subshell Photoionization Cross Sections and Asymmetry Parameters: 1 ≤ Z ≤ 103. At. Data Nucl. Data Tables 1985, 32, 1. (24) The error bars were estimated as follows. First we assume that both carbon atoms contribute equally to the error bars in Figure 2. This error was then propagated using the formula that ΔZ = [(∂Z/ ∂CCl)2(ΔCCl)2 + (∂Z/∂COH)2(ΔCOH)2]1/2, where Δ signifies the error in question and Z = (Iliq,n/Igas,n). This error was then multiplied by a factor of 3; this is to attempt to correct for possible fluctuations of the overlap of the liquid microjet and ionizing radiation, the exact magnitude of this potential uncertainty is unknown, it is likely different for various data points and the factor of 3 is to be considered as an attempt to include this. As discussed in the text, we have assumed that σliq/σgas is constant, which then implies that the overlap of the two beams does not change with photon energy. The polynomial fits are weighted with respect to the error bars. (25) The escape probability from a certain depth d is given by e−d/λ, and I is described by an integral from 0 to infinite depth. If the electron mean free path λ(KE) is such that the total inelastic scattering in the dilute gas is negligible compared to the total inelastic scattering in the liquid, as the present experimental conditions are relatively close to and the solute is evenly distributed in the solvent, Iliq/Igas will in fact be directly proportional to the electron mean free path λ(KE). (26) Mårtensson, N.; Söderstrom, J.; Svensson, S.; Travnikova, O.; Patanen, M.; Miron, C.; Sæthre, L. J.; Børve, K. J.; Thomas, T. D.; Kas, J. J.; et al. On the Relation between X-ray Photoelectron Spectroscopy and XAFS. J. Phys.: Conf. Ser. 2013, 430, 012131.

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