Low-energy constraints on photoelectron spectra measured from liquid water and aqueous solutions

Malerz, Sebastian; Trinter, Florian; Hergenhahn, U.; Ghrist, Aaron; Ali, Hebatallah; Nicolas, Christophe; Saak, Clara‐Magdalena; Richter, Clemens; Hartweg, Sebastian; Nahon, Laurent; Lee, Chin; Goy, Claudia; Neumark, Daniel M.; Meijer, Gerard; Wilkinson, Iain; Winter, Bernd; Thürmer, Stephan

doi:10.1039/d1cp00430a

Cited by 41 publications

(88 citation statements)

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“…For the corresponding eKEs, below ~18 eV, the direct photoelectrons experience such severe scattering that the nascent photoelectron peak position cannot be reliably extracted. 30 However, we deliberately include these misleading values in Figure 4 to highlight to the reader that utmost care must be taken when trying to determine any meaningful energy in this regime. Solely applying an energy referencing scheme, be it Method 1 or 2, without consideration of possible energy shifts due to electron scattering, will inevitably lead to erroneous results.…”

Section: Resultsmentioning

confidence: 99%

“…LJ-PES experiments commonly use rather high photon energies, typically some tens or more electron volts above the relevant ionization thresholds. Such photon energies sufficiently separate directly-produced photoelectron peaks from the low-energy background of inelastically scattered electrons 23 and minimize scattering-induced distortions of the PE peaks themselves 30 (owing to the fact that electron scattering is almost exclusively governed by electronic excitations at such photon and kinetic energies 39 ). The vast majority of LJ-PES studies have adopted such photon energies to establish solute core -level energies, with the measured chemical shifts serving as a reporter of changes in the chemical environment.…”

Section: Lj-pes From Water and Aqueous Solutionmentioning

confidence: 99%

“…Thus, the term SEED cannot be used for aqueous solution spectra recorded at ∼20 eV photon energies and below. 30 The term LET is adopted to avoid misleading connotations about the origin of this low energy signal. The LET comprises electrons which have lost most of their energy due to various inelastic scattering processes, and have just enough energy to overcome the surface barrier of the sample.…”

Section: Lj-pes From Water and Aqueous Solutionmentioning

confidence: 99%

Section: Lj-pes From Water and Aqueous Solutionmentioning

confidence: 99%

“…We address these deficiencies here and identify the need for additional spectroscopic information. For this particular task, the missing quantity is the (yet-to-be-discussed, although previously alluded to 4,30,31 ) low-energy electron cutoff in the liquid-water PES spectrum, a commonly measured parameter in solid-state PES. 23,[32][33][34][35] Motivated by a possible depth dependence of VIE vac,1b1(l) (i.e., of neat water), we utilize the cutoff spectral feature to report the first systematic study of this quantity over a large range of photon energies, spanning the (vacuum) ionization threshold region up to more than 900 eV above it.…”

Section: Introductionmentioning

confidence: 99%

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Accurate vertical ionization energy and work function determinations of liquid water and aqueous solutions

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Section: Resultsmentioning

confidence: 99%

Section: Lj-pes From Water and Aqueous Solutionmentioning

confidence: 99%

Section: Lj-pes From Water and Aqueous Solutionmentioning

confidence: 99%

Section: Lj-pes From Water and Aqueous Solutionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Accurate vertical ionization energy and work function determinations of liquid water and aqueous solutions

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Absolute Electronic Energetics and Quantitative Work Functions of Liquids from Photoelectron Spectroscopy

2023

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Conspectus Liquid-jet photoelectron spectroscopy (LJ-PES) enabled a breakthrough in the experimental study of the electronic structure of liquid water, aqueous solutions, and volatile liquids more generally. The novelty of this technique, dating back over 25 years, lies in stabilizing a continuous, micron-diameter LJ in a vacuum environment to enable PES studies. A key quantity in PES is the most probable energy associated with vertical promotion of an electron into vacuum: the vertical ionization energy, VIE, for neutrals and cations, or vertical detachment energy, VDE, for anions. These quantities can be used to identify species, their chemical states and bonding environments, and their structural properties in solution. The ability to accurately measure VIEs and VDEs is correspondingly crucial. An associated principal challenge is the determination of these quantities with respect to well-defined energy references. Only with recently developed methods are such measurements routinely and generally viable for liquids. Practically, these methods involve the application of condensed-matter concepts to the acquisition of photoelectron (PE) spectra from liquid samples, rather than solely relying on molecular-physics treatments that have been commonly implemented since the first LJ-PES experiments. This includes explicit consideration of the traversal of electrons to and through the liquid’s surface, prior to free-electron detection. Our approach to measuring VIEs and VDEs with respect to the liquid vacuum level specifically involves detecting the lowest-energy electrons emitted from the sample, which have barely enough energy to surmount the surface potential and accumulate in the low-energy tail of the liquid-phase spectrum. By applying a sufficient bias potential to the liquid sample, this low-energy spectral tail can generally be exposed, with its sharp, low-energy cutoff revealing the genuine kinetic-energy-zero in a measured spectrum, independent of any perturbing intrinsic or extrinsic potentials in the experiment. Together with a precisely known ionizing photon energy, this feature enables the straightforward determination of VIEs or VDEs, with respect to the liquid-phase vacuum level, from any PE feature of interest. Furthermore, by additionally determining solution-phase VIEs and VDEs with respect to the common equilibrated energy level in condensed matter, the Fermi level—the generally implemented reference energy in solid-state PES—solution work functions, eΦ, and liquid-vacuum surface dipole effects can be quantified. With LJs, the Fermi level can only be properly accessed by controlling unwanted surface charging and all other extrinsic potentials, which lead to energy shifts of all PE features and preclude access to accurate electronic energetics. More specifically, conditions must be engineered to minimize all undesirable potentials, while maintaining the equilibrated, intrinsic (contact) potential difference between the sample and apparatus. The establishment of these liquid-phase,...

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Core-Level Photoelectron Angular Distributions at the Liquid–Vapor Interface

et al. 2023

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Metrics & MoreArticle Recommendations * sı Supporting Information CONSPECTUS: Photoelectron spectroscopy (PES) is a powerful tool for the investigation of liquid−vapor interfaces, with applications in many fields from environmental chemistry to fundamental physics. Among the aspects that have been addressed with PES is the question of how molecules and ions arrange and distribute themselves within the interface, that is, the first few nanometers into solution. This information is of crucial importance, for instance, for atmospheric chemistry, to determine which species are exposed in what concentration to the gas-phase environment. Other topics of interest include the surface propensity of surfactants, their tendency for orientation and self-assembly, as well as ion double layers beneath the liquid−vapor interface. The chemical specificity and surface sensitivity of PES make it in principle well suited for this endeavor. Ideally, one would want to access complete atomic-density distributions along the surface normal, which, however, is difficult to achieve experimentally for reasons to be outlined in this Account. A major complication is the lack of accurate information on electron transport and scattering properties, especially in the kinetic-energy regime below 100 eV, a pre-requisite to retrieving the depth information contained in photoelectron signals.In this Account, we discuss the measurement of the photoelectron angular distributions (PADs) as a way to obtain depth information. Photoelectrons scatter with a certain probability when moving through the bulk liquid before being expelled into a vacuum. Elastic scattering changes the electron direction without a change in the electron kinetic energy, in contrast to inelastic scattering. Random elastic-scattering events usually lead to a reduction of the measured anisotropy as compared to the initial, that is, nascent PAD. This effect that would be considered parasitic when attempting to retrieve information on photoionization dynamics from nascent liquid-phase PADs can be turned into a powerful tool to access information on elastic scattering, and hence probing depth, by measuring core-level PADs. Core-level PADs are relatively unaffected by effects other than elastic scattering, such as orbital character changes due to solvation. By comparing a molecule's gas-phase angular anisotropy, assumed to represent the nascent PAD, with its liquid-phase anisotropy, one can estimate the magnitude of elastic versus inelastic scattering experienced by photoelectrons on their way to the surface from the site at which they were generated. Scattering events increase with increasing depth into solution, and thus it is possible to correlate the observed reduction in angular anisotropy with the depth below the surface along the surface normal.We will showcase this approach for a few examples. In particular, our recent works on surfactant molecules demonstrated that one can indeed probe atomic distances within these molecules with a high sensitivity of ∼1 Å resolution along the ...

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Low-energy constraints on photoelectron spectra measured from liquid water and aqueous solutions

Abstract: We report on the effects of electron collision and indirect ionization processes, occurring at photoexcitation and electron kinetic energies well below 30 eV on the photoemission spectra of liquid water....

Cited by 41 publications

References 81 publications

Accurate vertical ionization energy and work function determinations of liquid water and aqueous solutions

Accurate vertical ionization energy and work function determinations of liquid water and aqueous solutions

Absolute Electronic Energetics and Quantitative Work Functions of Liquids from Photoelectron Spectroscopy

Core-Level Photoelectron Angular Distributions at the Liquid–Vapor Interface

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