Raman at the Ångström Scale
A delayed-probe CARS workflow suppresses metal background and detects vibrational modes from molecular layers only a few ångströms thick
A time-frequency engineered coherent Raman spectroscopy method has enabled direct detection of ångström-scale molecular layers at metal interfaces without relying on plasmonic nanostructures or electronic resonance enhancement. Developed by researchers at the Institute for Molecular Science and SOKENDAI, the approach addresses a long-standing limitation in interfacial Raman spectroscopy, in which weak vibrational signals from ultrathin molecular films are often overwhelmed by substrate background.
The advance lies in how the coherent Raman signal is timed. The team used coherent anti-Stokes Raman scattering (CARS), with femtosecond pump and Stokes pulses exciting molecular vibrations before an asymmetrically shaped picosecond probe pulse was introduced after a short delay. Because the metal background is generated almost instantaneously, this delay sharply reduced the substrate signal while preserving the longer-lived molecular response. A small residual background was then used deliberately as an internal local oscillator, turning what is usually a source of interference into a way to amplify weak interfacial Raman features.
“Our approach is expected to enable versatile Raman studies of functional interfaces that have previously been difficult to access without artificial enhancement structures,” said principal investigator Toshiki Sugimoto in the team’s press release.
As a model system, the team analyzed a benzyl mercaptan self-assembled monolayer on atomically flat gold. The method detected vibrational modes from a molecular layer only a few ångströms thick, including Raman-active but infrared-inactive phenyl-ring vibrations and lower-symmetry modes with mixed Raman and infrared activity. Maximum entropy analysis was used to reconstruct Raman-like spectral line shapes from the normalized CARS response.
The researchers suggest the method could be particularly useful for systems where interfacial molecules are difficult to probe without altering the surface. “We anticipate broad applications in real-time and operando analysis of electrochemical reactions, detection of reactive intermediates on catalyst surfaces, molecular and chemical characterization of adhesion interfaces, and studies of molecular and organic electronic devices,” Sugimoto added.
A Hidden Order in the Proteome
Single-cell Raman spectra predict condition-dependent proteome changes and highlight conserved stoichiometric groups in E. coli
By reading condition-dependent Raman spectral patterns from single Escherichia coli cells grown under 15 environmental conditions, researchers at the University of Tokyo have traced an underlying proteome architecture that helps explain how cells remain stable while adapting to environmental change.
The study measured Raman spectra from single cells in the biological fingerprint region of the spectrum, where molecular signals overlap. Rather than trying to decompose spectra into individual components, they used linear discriminant analysis to reduce the data into condition-sensitive patterns, then tested whether those patterns could predict previously published quantitative proteome profiles. Leave-one-out cross-validation supported a significant correspondence between low-dimensional Raman spectra and proteome changes.
“We demonstrated that cellular proteome profiles can be nondestructively inferred by simply exposing cells to light and analyzing their so-called Raman spectra,” said co-corresponding author Yuichi Wakamoto in a press release.
That relationship pointed to a broader organizational feature of the proteome. Many proteins involved in information storage and processing, including ribosomal and transcription-related proteins, showed conserved abundance ratios across environmental conditions. The researchers also identified larger “stoichiometrically conserved groups,” including a homeostatic core whose protein abundances increased with growth rate and smaller condition-specific groups associated with particular environments.
Similar stoichiometry-conservation patterns appeared in analyses of human transcriptome datasets, suggesting that Raman spectra may capture not just broad cellular state differences, but underlying constraints in how proteomes are organized. The authors say that, with further development, the approach could help track molecular changes before they become visible through conventional phenotypic measurements.
“It’s possible that by applying our method, we may be able to predict the early changes in cellular states associated with diseases and the molecular underpinnings that drive such changes,” co-corresponding author Ken-ichiro Kamei added.
The Shape of (Surface) Water
Depth-resolved vibrational spectroscopy shows that water at the air interface follows a more structured pattern than simple “up” or “down” models suggest
Researchers at the Fritz Haber Institute and Freie Universität Berlin have used depth-resolved vibrational spectroscopy to revise the molecular picture of water at the air-water interface. Together, the spectroscopy and simulations move beyond the usual “up” or “down” description of interfacial water, showing that the first few molecular layers follow alternating patterns of tilt and twist.
The study focused on the H–O–H bending vibration, a more localized structural probe than the more commonly studied O–H stretching region. Using phase-resolved sum-frequency generation (SFG) and difference-frequency generation (DFG) spectra, the team separated anisotropic interfacial signals from bulk-like quadrupolar contributions, a long-standing complication in second-order spectroscopy of water. Once the bulk-like component was removed, the extracted interfacial dipolar response matched molecular dynamics simulations in both line shape and amplitude.
That agreement allowed the team to analyze the interface layer by layer. Rather than fitting the spectrum to a simple picture of water molecules pointing “up” or “down,” the simulations revealed four preferred orientational motifs across a roughly 6–8 Å interfacial region. These motifs alternated with depth, with molecular tilt and twist changing together from the air-facing side toward the bulk.
The findings suggest that the hydrogen-bond network imposes more structured orientational constraints on interfacial water than previously recognized. The authors also observed depth-dependent frequency shifts that do not follow a simple transition from vapor-like to bulk-like water, pointing to unusually complex hydrogen-bonding behavior at the interface.
The authors suggest the approach could be extended to more complex aqueous interfaces, including electrochemical systems such as batteries.
The Missing Middle of Bismuth Vanadate
The study suggests precursor-to-material transformations may contain more useful chemistry than final-product analysis reveals
A closer look at precursor-to-oxide synthesis has revealed hidden amorphous and crystalline phases, including vanadium oxide intermediates and a previously unknown β form of bismuth vanadate, a material widely studied for solar fuel generation.
To capture those intermediate stages, the team combined solid-state nuclear magnetic resonance spectroscopy, pair distribution function analysis, and variable-temperature X-ray diffraction. Together, the methods tracked changes in vanadium and proton environments, local structure in poorly crystalline phases, and the emergence of crystalline products during heating.
This workflow revealed mixed-valence amorphous vanadium oxide intermediates before crystallization to vanadium pentoxide. In bismuth-containing precursors, it also captured β-BiVO₄, a previously unknown, kinetically stabilized polymorph that appeared during heating and disappeared at higher temperatures. Calculations suggested the β phase has a larger band gap than monoclinic BiVO₄, with possible implications for solar fuel materials.
“When materials are made by heating, scientists usually focus on the final product, the ‘B’ that results from ‘A,’” said Sebastian Pike in the team’s press release. “But this study shows that there are many fascinating stages in between ‘A’ and ‘B,’ and these hidden steps could be just as important.”
For the authors, the broader message is that molecule-to-material transformations may contain more useful chemistry than final-product analysis alone can reveal. “We only studied a few precursors here, but this work points to a broader opportunity in materials science,” said Pike. “By carefully controlling temperature, precursor chemistry and reaction pathways, there may be many more ‘hidden’ but extremely useful materials to be found.”
Superhydrides Through a Sharper Lens
Radio-frequency transmission spectroscopy and proton NMR offer a volume-sensitive route to probing superhydrides under extreme pressure
A microstructured radio-frequency lens system has opened a route to studying lanthanum superhydrides under extreme pressure, where tiny, multiphase samples remain difficult to probe with conventional resistance measurements. Working with samples compressed in diamond anvil cells to pressures up to 165 GPa, an international team used Lenz lenses to focus radio-frequency fields into sample volumes only tens of micrometers across.
The setup addresses a long-standing measurement problem in superhydrides, hydrogen-rich materials that can show signs of superconductivity near room temperature but only under megabar pressure. Electrical transport measurements probe current paths between electrodes, so they can miss superconducting regions elsewhere in the sample. Contactless radio-frequency transmission and proton nuclear magnetic resonance (NMR) spectroscopy offer a more volume-sensitive alternative.
At 165 GPa, radio-frequency transmission dropped sharply at around 267–279 K in zero field. Proton NMR measurements in a 7 T magnetic field showed a related suppression of signal intensity below about 260 K, alongside changes in spectral linewidth, frequency shift, and spin-lattice relaxation behavior. The authors interpret these features as consistent with the onset of superconductivity, while noting that their goal was to demonstrate contactless methods for probing superhydrides under pressure, rather than definitive proof of near-room-temperature superconductivity.
“We had to focus the high-frequency fields precisely where the sample is located between the diamond anvils, over an area of just a few tens of micrometers, which is smaller than the diameter of a human hair,” said co-author Florian Bärtl. “With the use of Lenz lenses, we were able to amplify the high-frequency signal to such an extent that, for the first time, meaningful NMR data became accessible for superhydrides.”
For superhydrides, where superconducting phases can be microscopic, metastable, and unevenly distributed, the combined approach offers a way to look beyond the limited current paths measured by electrical transport alone.
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