Publication summaries¶
“Polarizability tensor invariants of H2, HD, and D2” - JCP 2018¶
Article link PDFs:
PDF (article)|PDF (SI)Premise: Accurate data on polarizability is required for estimating a wide variety of physical and optical characteristics of molecules, including molecular hydrogen (the simplest and most abundant molecule in the universe.) This work reported the accurate dataset on dipole-polarizability of the hydrogen molecule and two of its isotopologues. Dipole-polarizability, a property closely related to the electronic structure of the molecule, governs a wide array of micro- and macroscopic properties, such as molecule’s response to an external electric field, refractive index and more. This property also governs the transition probability of scattering processes such as Rayleigh and Raman scattering. Computation of Raman intensities was the motivation of this particular project.
Outcome : Extensive tabulation of transition matrix elements of polarizability invariants between various ro-vibrational states of the three molecules in the ground electronic state for incident light of different wavelengths (from UV to IR). Corresponding software is publicly available.
“Toward standardization of Raman spectroscopy: Accurate wavenumber and intensity calibration using rotational Raman spectra of H2, HD and D2, and vibration–rotation spectrum of O2” - JRS 2020¶
Article link PDFs:
PDF (article)|PDF (SI)Premise: Raman spectrometers are composed of multi-optical components through which the scattered photons travel. These optical components have varying response to different wavelengths. Hence, a calibration process for the relative sensitivity over the spectral range is required for obtaining correct Raman intensities.
This work reported the accurate wavenumber and intensity calibration of Raman spectrometer spanning the wavenumber region from -1100 to 1600 cm-1. For this purpose, we used the broad-band light source from a tungsten lamp, for an initial correction, followed by utilization of Raman band areas of pure rotational Raman transitions H2, HD and D2, and ro-vibration transitions of O2.
Ratio of band areas of specific Raman transitions in these four gases were analyzed to extract out a correction in the form of a polynomial function, which in combination with the broad-band light source provided accurate relative intensity calibration within 2-3% over the studied spectral range. This was performed for both of the polarizations.
Outcome : Scheme for accurate relative intensity calibration was reported along with software code for easy applications.
“Accurate intensity calibration of multichannel spectrometers using Raman intensity ratios” - JRS 2021¶
Article link PDFs:
PDF (article)|PDF (SI)Premise: Raman spectrometers are composed of multi-optical components through which the scattered photons travel. These optical components have varying response to different wavelengths. Hence, a calibration process for the relative sensitivity over the spectral range is required for obtaining correct Raman intensities.
This work reported the accurate wavenumber and intensity calibration of Raman spectrometer in the high-wavenumber region spanning from ~2400 to 4200 cm-1. For this purpose, measured spectra of a broad-band light source was used, followed by using the partially corrected observed Raman band areas of molecular hydrogen and its isotopologues.
Ratio of band areas of specific Raman transitions in H2, HD and D2 were analyzed to extract out a correction in the form of a polynomial function, which in combination with the broad-band light source provided accurate relative intensity calibration within 4-6% over the studied spectral range, over both the polarizations.
Outcome: A generalized scheme for relative intensity calibration of Raman spectrometers was presented based on the observed relative Raman intensities. Corresponding previous works were summarized and analogous computer code was organized (made available online) to present all available modes of calibration scheme in a clear way.
“Testing the limitations of harmonic approximation in the determination of Raman intensities” - Mol. Phys. 2022¶
Article link PDFs:
PDF (article)|PDF (SI)Premise: Estimation of Raman transition probabilities which governs the observed intensities is performed at present via crude approximations involving the vibrational wavefunction and the polarizability invariants. These approximations had to be tested to understand the errors encountered while estimating Raman intensities from quantum-chemical calculations. Did you ever wonder why the observed Raman spectra did not match the computed one even for simple gas molecules ? This is a report on this topic !
This work reported the deviations in the Raman intensities of 12 diatomic molecules by a comparison between two results, i) first, computed using accurate anharmonic wavefunctions and accurate polarizability, and ii) computed using approximated harmonic wavefunctions and the first derivative of polarizability. This second approach is the one employed in almost all of the available quantum-chemistry software packages.
We found that the harmonic approximation with the first derivative of polarizability underestimated Raman intensities by 5-10%, and this deviation was found to be consistent in all molecules where polarizability can be obtained to good accuracy.
Outcome: A rigorous test of the harmonic approximation for computation of Raman intensities for a set of 12 diatomic molecules was performed revealing that computed Raman intensities using the trasitional approach are consistently underestimated.
“Water channels aiding in nanoparticles stability”¶
Article1 | Article2 PDFs:
PDF (article 1)|PDF (article 2)Premise: Typical hydrophobic nanoparticles, consisting of alkyl groups (or other hydrophobic moieties) are generally unstable in aqueous systems sue to self-aggregation and eventual precipitation. In this set of collaborative work, Prof. Wang’s group (NYCU, Hsinchu, and now at NTU, Taipei) designed hydrophobic nanoparticles which are stable across a wide varity of physical conditions. Raman spectroscopy was used to assess the state of hydrogen bonding and extent of association of water with the polar groups of the nanoparticle.
Outcome: A new class of hydrophobic nanoparticles were found which are resilient to self-aggregation in various physical conditions. Raman spectroscopy with HAMAND analysis was used to find extent of hydrogen bonding interaction.
“Water:solvent hydrogen bonded complexes observed in the condensed phase”¶
Article1 PDFs:
PDF (article)|PDF (SI)Premise: Hydrogen bonding interactions controls great aspects of biological function including molecular recognition and reaction dynamics. From the perspective of applied chemistry, solvents are always containing small amounts of water absorbed from air which affects its properties. A large change in bulk properties are observed at larger dilution with water, for example, effects like co-nonsolvency and clathrate formation in THF. In this work, binary mixture of THF and water were studied. To our surprise, we were able to record monomeric water:THF molecular complex in the condensed phase using Raman spectroscopy at room temperature.
Outcome: Strange observation of monomeric water:THF complex and larger complexes with increasing number of water molecules, all in a dynamic equilibrium in the condensed phase !