Broadband Deep UV Spectra of Interfacial Aqueous Iodide

Abstract

The behavior of ions at aqueous interfaces influences vital processes in many fields but has long remained a subject of controversy. Over the past decade, counterintuitive surface concentration enhancement of several ions in aqueous solution has been demonstrated via nonlinear laser spectroscopy and mass spectrometry. While the evidence for significant ion enhancement at the air-water interface is convincing, the mechanism remains incompletely understood. Toward this end, we present the full broadband DUV-SFG spectrum of the charge-transfer-to-solvent (CTTS) band of interfacial aqueous iodide measured in a single laser shot with a newly developed broadband deep UV-SFG technique, clearly revealing a ∼8 nm redshift and a significant linewidth narrowing relative to bulk solution spectra. KI and NaI solutions yield indistinguishable results. Additionally, we observe a dramatic change in the relative intensities of the J = 3/2 and 1/2 CTTS transitions.

Reversed interfacial fractionation of carbonate and bicarbonate evidenced by X-ray photoemission spectroscopy

“…This repulsion was originally explained by classical electrostatic theory 47 and supported by surface tension measurements which showed that the surface tension increased as a function of salt concentration. 48,49 More recently, molecular dynamics simulations, 50,51 surface specific second-order nonlinear optical experiments, [52][53][54] and XPS measurements 33,39 have predicted and observed the enhancement of certain simple ions at the air/water interface. These models have shown that weakly hydrated, charge-diffuse ions are generally enhanced at the air/water interface.…”

Soft X-Ray Second Harmonic Generation as an Interfacial Probe

“…One fundamental nonlinear probe is second harmonic generation (SHG), a second-order process which combines two photons of the same energy to generate a single photon with twice the energy [8]. At infrared, visible, and ultraviolet wavelengths, second-order nonlinear spectroscopies have become important tools in surface science, as symmetry considerations, within the dipole approximation, constrain signal generation to regions lacking centrosymmetry, such as surfaces and interfaces [9][10][11][12]. In contrast, at hard X-ray wavelengths, second harmonic and sum frequency generation (SFG) have been observed in centrosymmetric materials with a non-uniform electron density and are essentially bulk probes [1,2].…”

“…Additionally, as the pulses remains fully coherent, it should be possible to combine this technique with lens-less coherent imaging techniques with a resolution not limited by aberration [31]. Such a technique would require either an energy-sensitive nondispersive detector or the generation of a signal that is spatially separated from the input pulses, as has been achieved previously with non-collinear input laser pulses, such as in optical SFG [9][10][11][12]. If radiation damage concerns can be mitigated, the spectral sensitivity demonstrated here would enable experiments which can both spectroscopically and spatially resolve nanoparticles.…”

X-Ray Probe Targets Interfaces

“…One fundamental nonlinear probe is second harmonic generation (SHG), a secondorder process which combines two photons of the same energy to generate a single photon with twice the energy [8]. At infrared, visible, and ultraviolet wavelengths, secondorder nonlinear spectroscopies have become important tools in surface science, as symmetry considerations within the dipole approximation constrain signal generation to regions lacking centrosymmetry, such as surfaces and interfaces [9][10][11][12]. In contrast, at hard x-ray wavelengths, second harmonic and sum frequency generation (SFG) have been observed in centrosymmetric materials with a nonuniform electron density and are essentially bulk probes [1,2].…”

“…Additionally, as the pulses remain fully coherent, it should be possible to combine this technique with lensless coherent imaging techniques with a resolution not limited by aberration [41]. Such a technique would require either an energy-sensitive nondispersive detector or the generation of a signal that is spatially separated from the input pulses, as has been achieved previously with noncollinear input laser pulses, such as in optical SFG [9][10][11][12]. If radiation damage concerns can be mitigated, the spectral sensitivity demonstrated here would enable experiments which can both spectroscopically and spatially resolve nanoparticles.…”