PERSPECTIVE pubs.acs.org/JPCL
Photoelectron Spectroscopy Meets Aqueous Solution: Studies from a Vacuum Liquid Microjet Robert Seidel, Stephan Th€urmer, and Bernd Winter* Helmholtz-Zentrum Berlin f€ur Materialien und Energie, and BESSY, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany ABSTRACT: Characterization of the structure and properties of matter would be incomplete without the detailed knowledge of electronic structure, and yet, for aqueous solutions, not even the binding energies of the valence electrons are generally known. Thus, fundamental interactions between solute electronic structure and water, essentially the key to chemical reactivity, have remained poorly understood. This work describes how, by the development of the vacuum liquid microjet technique for X-ray photoelectron spectroscopy, electronic structure measurements from aqueous solutions have advanced to date. Direct and resonant second-order electron emission processes are discussed in light of the specific electron structure information accessible from aqueous solutions. Several examples of solutes in their natural aqueous environment will be presented along with future research directions and prevailing challenges in the field.
G
iven waters’ important role for life on earth and for the environment, it is surprising that the seemingly simplest electronic structure quantity, that is, the binding energy (BE) of electrons in the highest occupied molecular orbital (HOMO), from given aqueous solutions is relatively unknown. Arguably, the most direct method to access such information is photoelectron (PE) spectroscopy, which has only recently been successfully applied to highly volatile samples. What has become common practice in ultrahigh vacuum PE studies targeting well-defined crystals has remained largely elusive and still is challenging for aqueous solutions. A great advance in liquid PE spectroscopy has emerged with the development of the vacuum liquid microjet technique, which has been described in our previous works.1-3
electron dynamics. It is useful to sort the different processes by whether the electron is emitted directly or via Auger electron decay or some other second-order de-excitation channels. Figure 1 illustrates the different electron emission processes to be addressed in this work. We will show explicitly how a given relaxation mechanism can be used to reveal structure specificities of a given atomic or molecular solute in water. Probably the most common association made with PE spectroscopy is its element specificity and sensitivity to the structural environment and chemical state (e.g., pH-induced) through electron binding energies. Indeed, in many cases, the PE spectra from solution are simple enough, and ground-state electron binding energies are readily obtained as the difference between excitation photon energy and kinetic energy, BE = hν - KE (eq 1; compare Figure 1A).4 Often, though, core-level PE spectra exhibit complex structures from multielectron and core-hole interactions, connected with electronic screening. As a result, ground-state properties may be difficult to reveal; we will explore some of these effects in aqueous solution below. Another substantial feature of direct PE spectroscopy is its ability to sample electronic structure information from different depths into solution. As eq 1 indicates, for a given BE value, an increase of the excitation photon energy results in an increase of the KE of the same amount. One can in fact vary the electron KE over an energy range large enough to exploit changes in the inelastic mean-free
What has become common practice in ultrahigh vacuum PE studies targeting well-defined crystals has remained largely elusive and still is challenging for aqueous solutions. In this Perspective, we address several important aspects of liquid jet PE spectroscopy for studies from aqueous solutions, beginning with electron BEs and their shifts, and then focusing on more complex issues, which often relate to the induced r 2011 American Chemical Society
Received: December 6, 2010 Accepted: February 22, 2011 Published: March 02, 2011 633
dx.doi.org/10.1021/jz101636y | J. Phys. Chem. Lett. 2011, 2, 633–641
The Journal of Physical Chemistry Letters
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Figure 1. Schematic energy-level diagram for the electronic excitation and de-excitation processes encountered in the present work, depicting unoccupied valence, and occupied valence and core-level states (A, C, D, E). Depiction of electron escape from the solution at both low and high kinetic energy illustrating the effect of elastic and inelastic scattering (B; left). Qualitative representation of PE emission and exponentially decaying attenuation function vs probing depths for short and large IMFP (B; right). Direct photoionization (A). Resonant Auger-electron decay (C): Spectator Auger decay (left); normal-like Auger decay with delocalized excited electron (right). Intermolecular Coulombic decay, ICD (D). Energy-transfer mediated decay, ETMD (E). In (C, D, E) we have assumed promotion of a core electron to an empty state.
path (IMFP) lengths of electrons in the solution. Then, at shorter IMFPs, PE spectroscopy becomes appreciably surface-sensitive, whereas at larger IMFPs, one primarily samples electron signal from bulk solution.5 This is illustrated in Figure 1B. Electrons with suitably low KE (corresponding to the minimum in the IMFP curve4,5) can only escape into vacuum if they were generated close enough to the solution/vacuum interface, whereas electrons at higher KE can be detected from deeper layers (Figure 1B, left). The actual signal measured in the experiment is the integrated intensity over the exponential attenuation length into the solution (Figure 1B, right). Hence, any bulk PE measurement will detect some signal from the surface, and the weight will decrease with increasing IMFP. This feature of PE spectroscopy is particularly useful when investigating the density distribution of solute species across the solution interface,5,6 for which we will provide some examples. Direct PE emission upon X-ray excitation is usually accompanied by the emission of Auger electrons, with the exception for inner electron shells in high-Z atoms, where the competitive fluorescence decay would be more favorable.4 However, the second-order electron emission processes occurring at resonant excitation (Figure 1, bottom) are arguably more interesting and more challenging. Resonant PE (RPE) processes, typically referring to near-core-level excitation, can lead to substantial intensity enhancement in the valence spectrum. Here, Auger decay leads to the same final state reached in direct ionization, and interference of the identical final states gives rise to the resonant enhancement.7 Relaxation details and pathways, and hence spectral characteristics, depend on the specific
configuration of the hydration complex. For instance, structure can determine whether electronic de-excitation remains localized on the initial molecular center or rather delocalizes into a neighboring water molecule. RPE processes occur within the very short lifetime of the core hole (approximately 4 fs for oxygen 1s7). Electronic relaxation dynamics, including charge and energy exchange between solute and solvent, can thus be studied at very early times after the initial excitation. Here, we discuss present and future PE measurements from aqueous solutions of alkali halide salts, of small inorganic and biologically relevant molecules, of water’s ionic products, and of transition-metal (TM) cations. The RPE spectra shown here have not been reported before; these are, in fact, the very first RPE measurements from any aqueous solution.
Electronic relaxation dynamics, including charge and energy exchange between solute and solvent, can be studied at very early times after the initial excitation. Liquid Vacuum Microjet for PE Spectroscopy: Brief History and Applications. What has taken PE spectroscopy so long to become 634
dx.doi.org/10.1021/jz101636y |J. Phys. Chem. Lett. 2011, 2, 633–641
The Journal of Physical Chemistry Letters
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emitted from the liquid microjet upon X-ray excitation. In practice, one selects an energy window of interest; see, for instance, the valence PE spectrum from liquid water of Figure 2B. While energy measurements are more or less straightforward, the quantitative analysis of PE signals, for example, from two solute species as a function of KE (hence depth), is rather intricate. One complication is connected with the inevitable change of photon energy when varying the KE of the PEs (compare Figure 1B); reliable photoionization cross sections for solutes are barely available, but they are needed when comparing intensities of different species (often at different orbital energies) at a given photon energy. The dependence of the electron signal on detection angle is also not known; this is probably most crucial for emission from the surface.5 Full understanding would require exact knowledge of the photoemission process at the solution interface and knowledge of both elastic and inelastic electron scattering. Electron probing depths in water are approximately known, that is, attenuation lengths illustrated in Figure 1B (right) are qualitative. Experimental and computational work, aiming at fully establishing angle-resolved PE spectroscopy from aqueous solutions, ultimately enabling the determination of molecular orientation at the aqueous interface, is in progress. A rather serious experimental hurdle along these lines is that current measurements are made from a curved sample, that is, from cylindrical liquid jets, or droplets, yielding a distribution of probing depths and emission angles. Concepts for generating a planar micrometer-sized free water surface in vacuum are of considerable interest and challenge. Our current best estimate of electron IMFP in water is
