IR spectroscopy in BARC: from incandescent lamp to synchrotron

The excitement in spectroscopy using infrared (IR) rays, discovered 2-centuries ago, gets renewed with the evolution of sources and techniques. In BARC, scientists right from the 1st batch of training school adopted it for analytical characterization, D₂O monitoring, ro-vibrational studies of heavy molecules etc. The advent of Fourier Transform spectrometer, computational tools and intense sources from incandescent quasi-blackbody lamps, diode lasers to SiC blackbody thermal radiators were the milestones, leading to notable contributions in NH₃ inversion studies, UF₆ energy levels, complex rotational levels of symmetric/ asymmetric top molecules etc. While the incorporation of synchrotron source (SRS), having limited utility at low energies, boosted the parallel programs in X-ray spectroscopy, the high brightness of SRS in IR opened yet another domain: IR micro-spectroscopy. The experimental signatures of all the three possibilities resulting in novel high-pressure phases have been obtained in complexes of oxalic acid and glycine. We found proton migration in curcumin, oxalic acid dihydrate forming hydronium ion, which showed peculiar features of umbrella inversion in condensed phase. Phase transition to a non-centrosymmetric structure in bis(glycinium)oxalate resulted in a potential ferroelectric phase. The symmetric state has been reported previously in ice, formic acid and goethite, which all lack unambiguous evidences. We provided experimental signatures of disordered hydrogen bond symmetrization in glycinium oxalate. Our most recent results using SR-IR show that in formic acid, the simplest carboxylic acid, the hydrogen bond symmetrization gets impeded due to phase transition to a polymer phase forming unprecedented infinite ether-chains