Exploring Protonation and Deprotonation Effects with Auger Electron

Letter pubs.acs.org/JPCL

Exploring Protonation and Deprotonation Effects with Auger Electron Spectroscopy Nikolai V. Kryzhevoi* and Lorenz S. Cederbaum Theoretical Chemistry, Institute of Physical Chemistry, Heidelberg University, D-69120 Heidelberg, Germany S Supporting Information *

ABSTRACT: Auger electron spectroscopy is demonstrated to be a very efficient tool to probe alterations in local chemical environment due to changes in protonation states. We show that electronic and geometric structure changes induced by protonation or deprotonation are well reflected in Auger spectra through characteristic chemical shifts and spectral shape variations. We also present evidence that Auger spectra are sensitive to relative concentrations of compounds in different protonation states. Special attention is paid to the high kinetic energy spectral regions that exhibit remarkable features resulting from core ICD-like transitions in normal species and Auger transitions in deprotonated fragments. The latter contribution was so far ignored when explaining Auger spectra of species embedded in the environment. This contribution should be reconsidered, taking into account the recently discovered possibility of ultrafast dissociation of core-ionized hydrogen-bonded systems in media. SECTION: Spectroscopy, Photochemistry, and Excited States

I

mentary information on the occupied valence orbitals primarily localized on the core ionization site. These orbitals are obviously affected when a proton is attached to or removed from the system. We are not aware of any experimental AES explorations of protonation or deprotonation (hereafter, for brevity, [de]protonation) effects. From the available theoretical study of gas-phase neutral and hydrated zwitterionic forms of glycine,15 one can, however, infer that the impact of these effects on Auger spectra should be strong, and AES can thus be efficient in distinguishing protonation states. Yet, in these calculations, the solute−solvent interactions were restricted to electrostatic ones because the water molecules were simulated by point charges, and therefore, the effect of nonlocal intermolecular transitions following core ionization was not taken into account. These transitions, known also as core ICDlike processes because of their resemblance to the intermolecular Coulombic decay (ICD),16 are able to make noticeable marks on the Auger spectra17−22 and even spectacularly dominate over the local transitions, as revealed in the case of aqueous hydroxide.23 The present work serves to show the high utility of AES in exploring protonation states. By examples of small ammonia clusters, we demonstrate that the effect of [de]protonation on Auger spectra is strong and very resolvable. This is attributed to significant alterations of the geometric and electronic structures of the [de]protonated molecule and its impact on the neutral molecules in the neighborhood. We note that Auger spectra are

t is essential in many branches of chemistry to have a detailed knowledge of the protonation state of the compounds studied. For instance, protonation states of amino acids, peptides, and proteins largely determine properties and the biochemical activity of these systems.1 Of great interest, in particular for the atmospheric chemistry community, is dissociation of acids at liquid/vapor aqueous interfaces and the ensuing chemical reactions.2 By making changes in protonation states, a control over relaxation dynamics of electronically excited systems can be achieved.3 Core-level spectroscopies are well suited for distinguishing protonation states because of their ability to study selectively chemically distinct elements and their sensitivity to local changes. Recently, pH-induced changes in the protonation states of various aqueous molecules4−7 as well as relative propensities of several acids and their conjugate bases in water/ vapor interface layers8−10 were successfully identified by analyzing chemical shifts and intensities of the spectral peaks in X-ray photoelectron spectra. Note that the chemical shifts normally result from the action of several factors whose individual contributions are rather difficult to assess unless theoretical calculations are available.6,10 In this respect, X-ray absorption spectroscopy has an advantage as it probes also unoccupied orbitals that are very sensitive, for example, to the protonation/deprotonation-induced geometry changes or to the variations in the hydrogen bonding between the molecule and its nearest surrounding. These changes are well reflected in X-ray absorption spectra, which thus provide much additional insight.11−14 A core vacancy creation is usually followed by an Auger process. Auger electron spectroscopy (AES) is, of course, also element-specific but provides additionally essential comple© 2012 American Chemical Society

Received: August 6, 2012 Accepted: September 10, 2012 Published: September 10, 2012 2733

dx.doi.org/10.1021/jz301130t | J. Phys. Chem. Lett. 2012, 3, 2733−2737

The Journal of Physical Chemistry Letters

Letter

superimposed spectra of neutral molecules and of ions and thus provide insight into the potential usefulness of AES in studying acid−base balance in solutions. To the benefits of the present study belongs also the explicit treatment of solvent molecules, which are, in our case, the neutral ammonia molecules. This allows for core ICD-like transitions, which remarkably modify the computed Auger spectra, predominantly on their high kinetic energy ends. It turns out that these transitions overlap energetically with the intramolecular transitions occurring in core-ionized deprotonated species. This significant finding makes the interpretation of the high kinetic energy shoulders occurring in the experimental Auger spectra of some hydrogenbonded systems more complex. So far, these spectral features were assigned to the core ICD-like processes. It might, however, happen that parts of their intensities originate from ultrafast dissociation of core-ionized molecules loosing a proton on the time scale of a few femtoseconds. For simulating Auger spectra, core ionization potentials (IPs) are required. These have been computed together with the whole core ionization spectra. Figure 1 shows the theoretical

Table 1. Computed IPs (eV) of the N1s Core Electrons in the Normal, Protonated, and Deprotonated Ammonia Dimers and Trimersa system normal protonated deprotonated

dimer 405.72 413.97 395.23

404.65 411.12 399.16

trimer 404.77 412.41 395.73

404.77 410.17 399.45

404.77 410.17 399.45

a

The ionization potentials of the ammonium cations and amide anions are underlined.

Satellites in core ionization spectra contain a wealth of information on the geometric and electronic structures of coreionized systems. Each molecule possesses a characteristic satellite appearance whose complexity is determined by many factors. In molecules of low symmetry, the satellite structures are much richer because more transitions are allowed than in highly symmetric systems (see the theoretical core ionization spectra of isolated NH+4 , NH3, and NH−2 in the Supporting Information). The spectra of compounds containing nonequivalent atoms of one kind acquire additional complexity due to overlapping individual spectral contributions (like in Figure 1). In weakly bound clusters, intermolecular charge- and energy-transfer processes accompany core ionization.24 These are very sensitive to the local environment and may considerably modify spectral shapes.24−27 In the spectra of the ammonia clusters studied here, charge- and energy-transfer satellites start to appear already 3 eV above the main lines. Intensities of these states are moderate to weak, as seen from Figure 1. In general, [de]protonation influences noticeably the appearance of satellites in core ionization spectra. If not for their weak intensities, these secondary states would be very useful in exploring protonation states. Weak satellites imply that their role in subsequent electronic decay processes is minor and can be discarded. The Auger spectra discussed below describe decay processes of the main core-ionized states only. Figure 2 shows the computed Auger spectra of ammonia dimers in different protonation states. Each panel represents individual spectral contributions from distinct cluster subunits and their superposition. The spectrum of the normal dimer has been already discussed in detail before (see ref 21). As one can clearly see, alterations in the protonation state cause significant changes in the Auger spectral shapes. Importantly, the effect of [de]protonation is not restricted to the resulting ions but extends to their neighbors whose Auger spectra experience strong variations as well. Like in X-ray photoelectron spectra, electronic transitions in Auger spectra acquire characteristic chemical shifts upon [de]protonation. Auger electrons emitted from a protonated system have lower kinetic energies than those emitted from a normal system. In contrast, detaching a proton from a system shifts the Auger spectrum toward higher kinetic energies. Chemical shifts have important implications on uncovering changes in the local chemical environment. Explorations of Auger spectral shapes are able to provide finer details on these changes. By attaching a proton, an isolated ammonia molecule transforms to a highly symmetric tetrahedrally shaped ammonium cation, and its outer valence 3a1 and doubly degenerate 1e orbitals transform to the three-fold degenerate 1t2 orbitals. Because the energy spread of the multiplets resulting from removing two electrons from these 1t2 orbitals is rather small, the Auger spectrum of NH+4 in the outer-valence

Figure 1. Theoretical X-ray photoelectron spectra (XPS) of the protonated, normal, and deprotonated ammonia dimers. The contributions from distinct cluster subunits are distinguished by different colors: ammonium cation (blue), amide anion (red), hydrogen-donor (dark green), and hydrogen-acceptor (light green) ammonia molecules. Cluster geometries are shown in the upper left corner.

(not superimposed) core-level spectra of the monomers constituting normal, deprotonated, and protonated ammonia dimers. The general trend is clearly seen; IPs increase upon protonation, and the effect of deprotonation is the reverse. Compared to the ammonia molecules in the normal dimer, the IP of NH+4 is higher by 8.3−9.3 eV, while that of NH−2 is lower by 9.4−10.5 eV. Similar energy differences were found recently between IPs in the inner-valence spectral region, where they play a decisive role in regulating conventional ICD processes.3 In larger clusters, the above IP differences become less pronounced (see Table 1) because of different dependences of IPs of neutral molecules and ions on cluster size. Notice that distinct monomers in each cluster have different chemical shifts and can, in principle, be distinguished spectroscopically. 2734



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Figure 2. Computed Auger spectra of protonated (left panel), normal (middle), and deprotonated (right) ammonia dimers. The upper part of each panel shows the total Auger spectrum, which is a superposition of the Auger spectra of distinct cluster subunits displayed in the lower parts. A and D are used in the middle panel to distinguish between the hydrogen-acceptor and hydrogen-donor molecules in the normal ammonia dimer. Thin solid and thick dashed vertical lines describe individual Auger and core ICD-like transitions, respectively. The high kinetic energy structure appearing due to core ICD-like processes in the NH+4 monomer of the protonated dimer is enhanced.

eV), which experiences a 10-fold intensity enhancement. Such a profound increase of the ICD efficiency compared to the normal dimer can hardly be explained by the enhanced overlap of orbitals28 of the two monomers induced by shortening of the intermolecular distance (3.25 Å in the neutral dimer versus 2.70 Å in the protonated one). Indeed, the same ICD states are populated by Auger decay of NH+4 in the protonated dimer where they acquire rather weak intensities (see the spectral enhancement in Figure 2). The main reason why core ICD-like transitions in NH3 in protonated ammonia clusters are highly efficient lies in the near degeneracy of Auger and ICD states and in the strong coupling between them (for the energies and interactions of the doubly and singly ionized states of the ammonia clusters studied here, see ref 3). This finding lends support to the recently suggested explanation29 of the dominant ICD processes in the core excited aqueous hydroxide,23 highlighting the pivotal role of the energy overlap and mediated hybridization of the valence orbitals of an excited solute and its neighboring solvent molecules. We note that the demonstrated dependence of the ICD efficiency on the place where the initial core vacancy is created ([de]protonated or normal species) has implication for X-ray absorption spectroscopy as well. By varying the photon energy across the absorption edge and thus resonantly exciting molecules in different protonation states, one can enhance or suppress core ICD-like processes. When going from [de]protonated dimers to trimers and larger clusters, the relative concentration of the normal and [de]protonated cluster subunits changes. The comparison of the total Auger spectra of the trimers (shown in Figure 3) with those of the dimers (Figure 2) indicates that AES is sensitive to these concentration changes. In the Auger spectra of the protonated clusters, major variations occur in the interior spectral region where the most intense peak belonging to the ammonium cation gradually looses intensity with decreasing concentration of NH+4 . The Auger spectra of the deprotonated clusters experience changes predominantly on the high kinetic

energy region is dominated by a single peak that is remarkably at variance with the structured shape of the Auger spectrum of NH3 (see computed Auger spectra of isolated NH+4 , NH3, and NH−2 in the Supporting Information). The shape of the Auger spectrum of the ammonium cation in the protonated dimer is similar to that of the isolated NH+4 . The main peak is somewhat broader in response to a slight degeneracy lifting of the ammonium 1t2 orbitals, and the spectrum includes contributions from states that are not available in the isolated NH+4 and refer to core ICD-like transitions. Core ICD-like transitions give rise also to the faint spectral structure at higher kinetic energies that is shown to be enhanced in the middle part of the left panel in Figure 2. The amide anion obtained by deprotonating an ammonia molecule has the lowest symmetry among the three species NH+4 , NH3, and NH−2 . All of its outer-valence orbitals are nondegenerate, so that creating two vacancies in these orbitals leads to a manifold of states broadly distributed in energy. As shown in the Supporting Information, the Auger spectrum of NH−2 exhibits a series of overlapping peaks and is rather complex. The peculiarity of this spectrum is largely retained in the deprotonated dimer. Note that the core-ionized NH−2 anion in the dimer can relax electronically via the Auger and core ICD-like mechanisms. Several intense core ICD-like transitions manifest themselves in the spectral interior but not on the high kinetic energy side. This contrasts dramatically the Auger spectrum of OH− in a deprotonated water dimer (not shown), which reveals a prominent high kinetic energy shoulder resulting from core ICD-like processes. [De]protonation also affects the Auger spectra of neighboring molecules. The comparison of the spectra of NH3 in all three dimers in Figure 2 provides clear evidence that this effect can be substantial. Particularly strong spectral shape alterations are found in the protonated dimer. Here, the two major spectral peaks on the high kinetic energy side associated with the local Auger transitions in the ammonia molecule remarkably redistribute their intensities in favor of the ICD peak (∼367 2735



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In conclusion, [de]protonation effects are well reflected in Auger spectra. Characteristic chemical shifts and spectral shape changes appear and provide valuable insight into geometric and electronic structure variations induced by [de]protonation. Spectral structures emerging in the high kinetic energy regions of Auger spectra are usually attributed to core ICD-like processes. They may acquire prominent intensities if ICD and Auger transitions overlap in energy. Ultrafast dissociation of core-ionized molecules taking place within the core−hole lifetime adds intensity to the spectral high kinetic energy ends as well. This should be taken into account when analyzing experimental Auger spectra of hydrogen-bonded systems.

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COMPUTATIONAL METHODS To simulate Auger and XPS spectra, we used Green’s function algebraic diagrammatic construction (ADC) methods.32,33 The ADC(4) method32,34 was applied to calculate IPs and intensities of core-ionized states. Double ionization potentials were computed by means of the ADC(2) method.33,35,36 By subtracting them from IPs of core electrons, the energies of Auger transitions were obtained. The relative Auger transition rates were evaluated with the help of a two-hole population analysis as described in ref 36. We used modified Dunning’s DZP37 and aug-cc-pVTZ38 basis sets in the ADC(4) and ADC(2) calculations, respectively. The modifications made in the basis sets and further details on the computations are described in ref 21.

Figure 3. Computed total Auger spectra of the ammonia trimers in different protonation states. The partial contributions due to the ammonium cation, the amide anion, and ammonia molecules are represented by the blue, red, and green areas, respectively. Note the distinct origins of the highest kinetic energy peaks in the spectra; in the protonated and normal clusters, these peaks emerge due to core ICDlike processes, while in the deprotonated cluster, it is mostly due to local Auger transitions in the amide anion. This contrasts water clusters, where the high-energy peaks due to core ICD-like processes do appear also in the deprotonated species.

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ASSOCIATED CONTENT

S Supporting Information *

Theoretical XPS and Auger spectra of the isolated NH+4 , NH3, and NH−2 molecules. This material is available free of charge via the Internet at http://pubs.acs.org.

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energy side where NH2− largely contributes. To other consequences of growing cluster size belong systematic chemical shifts of the spectra toward higher kinetic energies and spectral broadenings. Noticeable changes happen also with the ICD features, which gain intensity due to the increasing number of molecules involved in core ICD-like processes.21 The high kinetic energy regions in the Auger spectra of hydrogen-bonded systems deserve special attention. Figures 2 and 3 indicate that the explanation of the spectral structures appearing there may not be so clear as thought before, and the high kinetic energy peaks and shoulders can originate not only due to core ICD-like transitions but also due to electronic relaxation in deprotonated species. These species can naturally be present in the medium, and their abundances can be varied by changing the environmental pH value. Deprotonated fragments can also be produced through dissociation induced by core ionization. In order to be able to influence Auger spectra noticeably, this process should proceed within the ultrashort lifetimes of core vacancies. In aqueous media, this is achievable.30,31 For systems containing ammonia molecules, the possibility of core-ionization-induced ultrafast dissociation has to be verified. Additional explorations are needed to determine the relative impact of ICD and dissociation channels on Auger spectra of hydrogen-bonded systems. It is important, in particular for theoretical simulations, that all three relaxation pathways, Auger, dissociation, and core ICD-like processes, including those occurring in the produced molecular fragments, be considered simultaneously. We remind that the core ICDlike processes become more efficient when more neighbors are present.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by the DFG. REFERENCES

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Exploring Protonation and Deprotonation Effects with Auger Electron

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