Saptarsi Mondal Ultrafast & multidimensional IR spectroscopy
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Research

My research examines how local hydrogen-bonding and solvation environments govern molecular dynamics, transport and reactivity.

Schematic of surface-enhanced 2D IR at a gold surface. Infrared pump and probe pulses reach a gold disc bearing tethered nitrile probe molecules; beyond it, water and DMSO molecules are shown hydrogen bonded to one another. Labels mark two-dimensional confinement, the electrostatic environment, isolated water dynamics, and the plasmonically enhanced 2D IR signal that emerges.
Surface-enhanced 2D IR isolates hydrogen-bond rearrangement within a few nanometres of the electrode, where confinement and the interfacial electric field both act on the probe.

■ THEME I · CURRENT · 4 PAPERS

Interfacial and electrochemical dynamics

My research explores the nanoscale behaviour of water at electrified interfaces. I investigate how the hydrogen-bond network reorganises near an electrode in response to applied potential and added cosolvents. Operando SEIRAS and surface-enhanced 2D IR spectroscopy provide complementary views of interfacial structure and ultrafast dynamics, and constant-potential molecular dynamics simulations help connect the spectra to microscopic solvation structure.

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Two-panel schematic. On the left, a Fabry–Perot cavity formed by two distributed Bragg reflector mirrors separated by half a wavelength, with the vacuum cavity field drawn between them and molecular vibrations represented as arrows; the strong-coupling condition is written beneath. On the right, a PMMA film above an array of indium tin oxide nanocrystal blocks, with an infrared wave entering from below and an arrow marking vibrational energy transport through the polymer along the carbonyl mode.
Two routes to coupling a molecular vibration to a confined optical mode: a closed high-Q Fabry–Perot cavity, and an open-faced, continuously tunable plasmonic metasurface.

■ THEME II · CURRENT · 1 PAPERS

Polaritonic and plasmonic control of molecular behaviour

A central focus of this work is whether hybridising a molecular vibration with a confined optical mode can measurably alter molecular dynamics or chemical reactivity. I study both Fabry–Pérot microcavities and tunable plasmonic metasurfaces, using broadband, polarisation-controlled 2D IR spectroscopy to probe the resulting molecular dynamics.

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Three-panel comparison titled "Battery separators are not passive". On the left, bulk electrolyte with free, mobile lithium ions and hexafluorophosphate anions shown separately and labelled high conductivity. In the centre, a polypropylene separator with roughly 43 nanometre nanopores. On the right, a magnified view inside a pore where the same ions have associated into contact ion pairs and clusters, with an arrow marking an approximately tenfold collapse in conductivity.
The same electrolyte behaves differently inside the separator: nanoconfinement promotes contact ion pairing and clustering, and conductivity falls sharply.

■ THEME III · ONGOING · 3 PAPERS

Electrolytes, confinement and ion transport

I study how ions are solvated and transported in battery electrolytes, and how that changes when the electrolyte is confined inside a separator. Using the electrolyte's own carbonyl and anion stretches as reporters, I resolve ion speciation with FTIR, two-dimensional correlation analysis and polarisation- resolved pump–probe spectroscopy, and relate it to macroscopic transport through impedance measurements.

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Two-panel schematic of the surface-sensitive techniques. On the left, a SEIRAS spectro-electrochemical cell in which an infrared beam enters a zinc selenide prism and produces an evanescent wave enhanced at a gold film working electrode, with reference and counter electrodes and an arrow marking DMSO enrichment under negative potential. On the right, the surface-enhanced 2D IR geometry in which pump and probe pulses interact with gold-tethered nitrile probe molecules and a plasmonically enhanced signal emerges.
The two surface-specific methods that underpin the interfacial work: SEIRAS for potential-resolved structure, surface-enhanced 2D IR for picosecond dynamics.

■ THEME IV · ONGOING · 6 PAPERS

Vibrational probes, methods and analysis

Extracting reliable dynamical information from congested vibrational spectra requires careful experimental design, modelling and signal processing. I use isotope substitution and 2D IR spectroscopy to separate overlapping spectral contributions, together with analytical models, molecular dynamics simulations, wavelet-based denoising and machine-learned interatomic potentials where appropriate.

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Six weak noncovalent interactions arranged around a central node labelled cooperative weak interactions: an O–H···O hydrogen bond between two water molecules, an O–H···F–C contact to organic fluorine, C–F···F–C dispersion between perfluorinated groups, charge-shifted S···O and C···O contacts in a carbonyl sulfide–water complex, an O–H···π hydrogen bond to acetylene, and a nitrogen lone-pair contact to CO₂ on a triazine ring.
Interactions weaker than a conventional hydrogen bond, from organic fluorine and fluorous dispersion in liquids to charge-shifted contacts in atmospheric clusters and CO₂ binding in porous polymers.

■ THEME V · COMPLETED · 20 PAPERS

Weak noncovalent interactions and functional materials

My doctoral work examined whether interactions weaker than conventional hydrogen bonds can nevertheless organise liquids in experimentally measurable ways. Infrared spectroscopy and molecular dynamics connected the observed spectra to liquid structure, while quantum-chemical calculations and atoms-in-molecules analysis characterised the underlying interactions. Fluorescence upconversion and time-correlated single-photon counting provided complementary dynamical information.

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Projects

Individual projects

Interfaces

Cosolvent and potential control of interfacial water

I am establishing cosolvent composition as a second control variable, alongside applied potential, for the structure and dynamics of water at an electrode surface, and measuring both with surface-specific vibrational spectroscopy.

Current work Collaborative

Polaritons

Tunable plasmonic metasurfaces for vibrational coupling

I use self-assembled doped-oxide nanocrystal metasurfaces, tunable across the mid-infrared, as an open alternative to high-Q microcavities for coupling light to molecular vibrations, and broadband 2D IR to measure what that coupling does to molecular dynamics.

Current work Collaborative

Probes & methods

Water structure and free volume in biocompatible polymers

I used ultrafast infrared spectroscopy to address two connected questions about amorphous polymers: how water behaves at their surface, and how large the free- volume elements inside them are.

Completed Collaborative