Broadband 2DES detection of vibrational coherence in the Sx state of canthaxanthin.

The nonadiabatic mechanism that mediates nonradiative decay of the bright S2 state to the dark S1 state of carotenoids involves population of a bridging intermediate state, Sx, in several examples. The nature of Sx remains to be determined definitively, but it has been recently suggested that Sx corresponds to conformationally distorted molecules evolving along out-of-plane coordinates of the isoprenoid backbone near a low barrier between planar and distorted conformations on the S2 potential surface. In this study, the electronic and vibrational dynamics accompanying the formation of Sx in toluene solutions of the ketocarotenoid canthaxanthin (CAN) are characterized with broadband two-dimensional electronic spectroscopy (2DES) with 7.8 fs excitation pulses and detection of the linear polarization components of the third-order nonlinear optical signal. A stimulated-emission cross peak in the 2DES spectrum accompanies the formation of Sx in <20 fs following excitation of the main absorption band. Sx is prepared instantaneously, however, with excitation of hot-band transitions associated with distorted conformations of CAN's isoprenoid backbone in the low frequency onset of the main absorption band. Vibrational coherence oscillation maps and modulated anisotropy transients show that Sx undergoes displacements from the Franck-Condon S2 state along out-of-plane coordinates as it passes to the S1 state. The results are consistent with the conclusion that CAN's carbonyl-substituted ß-ionone rings impart an intramolecular charge-transfer character that frictionally slows the passage from Sx to S1 compared to carotenoids lacking carbonyl substitution. Despite the longer lifetime, the S1 state of CAN is formed with retention of vibrational coherence after passing through a conical intersection seam with the Sx state.

The role of viscosity in various dynamical processes of different fluorophores in ionic liquid-cosolvent mixtures: a femtosecond fluorescence upconversion study.

Literature reports provide ample evidence of the dynamical studies of various fluorophores in different room-temperature ionic liquid (RTIL)-cosolvent mixtures. However, most of the experimental and simulation studies reveal that ∼50% of the spectral relaxation dynamics is fast and cannot be resolved using traditional time correlated single photon counting (TCSPC) measurements. Our group has also investigated the dynamics of a solvatochromic probe coumarin 153 (C153) in a RTIL-cosolvent mixture using a TCSPC setup (S. Sarkar, R. Pramanik, C. Ghatak, P. Setua and N. Sarkar, J. Phys. Chem. B, 2010, 114, 2779-2789). Consequently, a major portion of the solvation dynamics remained undetected and moreover we could not monitor the dynamics beyond 0.4 mole fraction of the cosolvents. Thus in this study, we have rekindled our interest to sufficiently capture the rotational anisotropy and solvation dynamics of C153 beyond 0.4 mole fraction of the cosolvents in the RTIL-cosolvent mixture employing femtosecond fluorescence upconversion measurements. Additionally, we have utilized another RTIL with a higher alkyl chain length and viscosity to obtain a comprehensive and quantitative picture of the role of viscosity on the dynamics of the probe molecule. The most interesting observation of the present work is that the viscosities of different RTIL-cosolvent mixtures can efficiently control the cis-trans isomerization kinetics of the anionic fluorophore merocyanine 540 (MC 540) and the translational diffusion of a hydrophobic probe. The optimization of geometrical structures of [EmimOs]- and [EmimOs]-cosolvent mixtures followed by frequency analyses in both gas and solution phases have been carried out using quantum chemical calculations with the aid of density functional theory (DFT) methods. The computation based on the bond distances, electron densities and non-covalent interactions (NCI) has also been used to investigate the existence of the hydrogen-bond (H-bond). Again to comprehend van der Waals interactions and the conventional hydrogen-bond, the evolution of NCI plots are simulated. Therefore, the detailed experimental and theoretical studies presented in this manuscript lead to the inference that addition of the conventional solvents finely tunes the physicochemical properties of RTILs and broadens their scope of applications in the fields of chemistry and biology.

Classification of eco-zones from the factors and processes controlling phytoplankton biomass.

Phytoplankton is of utmost importance to the marine ecosystem and, subsequently, to the Blue Economy. This study aims to explain the reasons for variability of phytoplankton by estimating the dependency of Chlorophyll-a (Chl-a) on various limiting factors using statistics. The global oceans are classified into coherent units that display similar sensitivity to changing parameters and processes using the k-means algorithm. The resulting six clusters are based on the limiting factors (PAR, iron, or nitrate) that modulate Chl-a yield divisions of the oceans, similar to regions of different trophic statuses. The clusters range from the polar and equatorial regions with high nutrient values limited by light, to open oceanic regions in downwelling gyres limited by nutrients. Some clusters also show a high dependency on marine dissolved iron. Further, oceans are also divided into eight clusters based on the processes (stratification, upwelling, topography, and solar insolation) that impact ocean productivity. The study shows that considering temporal variations is crucial for segregating oceans into ecological zones by utilizing correlation of time-series data into classification. Our results provide valuable insights into the regulation of phytoplankton abundance and its variability, which can help in understanding the implications of climate change and other anthropogenic effects on marine biology.

On the evidence of helico-spiralling recirculation within coherent cores of eddies using Lagrangian approach.

Oceanic eddies exhibit remarkable coherence and longevity compared to other transient features in the surrounding flow. They possess the ability to transport properties over extensive distances while maintaining their material identity intact. The Lagrangian Coherent Structure (LCS) framework has proven effective in capturing these coherent eddies, where they display a solid-body-like rotation. Although various LCS approaches have been employed to investigate different facets of coherent eddies, a comprehensive understanding of their three-dimensional structures and internal dynamics remains elusive. This study aims to advance our comprehension of coherent eddies' structural characteristics and delve into the precise nature of their internal dynamics by utilizing the Lagrangian Averaged Vorticity Deviation approach. Two eddies, one cyclonic and the other anti-cyclonic, were chosen from a high-resolution simulation carried out in the Bay of Bengal using the Regional Ocean Modeling System (ROMS). The findings unveil that these eddies have three-dimensional coherent cores resembling gently tapered cones that are broader at the surface and gradually narrow towards the bottom. Intriguingly, the dynamically coherent core of these eddies exhibits simultaneous upwelling and downwelling while maintaining their volumes during advection due to persistent material coherence. The three-dimensional trajectories followed by the fluid parcels inside the coherent core are helical. Their two-dimensional horizontal projections show alternating spiral bands of upwelling and downwelling which are the manifestations of Vortex Rossby Waves. These observations lead to a conceptual framework of a three-dimensional helico-spiralling recirculation pattern within the coherent cores of eddies.

Recent changes in the upper oceanic water masses over the Indian Ocean using Argo data.

Utilizing Argo data from 2003 to 2019, we examine thermohaline changes in the Indian Ocean within the upper 700 m. Widespread warming is observed except in the Southern Indian Ocean. Increasing salinity is obtained over all regions except the Bay of Bengal and Southern Indian Ocean. Thermohaline trends in regional water masses at various depths are first decomposed into spice and heave components, and then linked to processes like pure heave, pure freshening and pure warming. Three consistent patterns across all seven regions are: (1) Below 300 m spice dominates heave; (2) The freshening process within the spice component is the primary driver below 300 m; (3) Spice primarily influences salinity changes along isobars. The warming of Arabian Sea's Subsurface Minima and the Indian Equatorial Water are primarily dictated by spice and heave, respectively. Freshening of the Bay of Bengal Water is linked to heave changes under pure freshening and pure heave processes. In the upper 250 m of the western equatorial, southern Indian Ocean, and Seychelles-Chagos Thermocline Ridge, salinity rises due to spice under pure freshening. The southern Indian Ocean's advected mode water shows freshening and cooling trends due to pure freshening.

Excitation energy transfer and vibronic coherence in intact phycobilisomes.

The phycobilisome is an oligomeric chromoprotein complex that serves as the principal mid-visible light-harvesting system in cyanobacteria. Here we report the observation of excitation-energy-transfer pathways involving delocalized optical excitations of the bilin (linear tetrapyrrole) chromophores in intact phycobilisomes isolated from Fremyella diplosiphon. By using broadband multidimensional electronic spectroscopy with 6.7-fs laser pulses, we are able to follow the progress of excitation energy from the phycoerythrin disks at the ends of the phycobilisome's rods to the C-phycocyanin disks along their length in <600 fs. Oscillation maps show that coherent wavepacket motions prominently involving the hydrogen out-of-plane vibrations of the bilins mediate non-adiabatic relaxation of a manifold of vibronic exciton states. However, the charge-transfer character of the bilins in the allophycocyanin-containing segments localizes the excitations in the core of the phycobilisome, yielding a kinetic bottleneck that enables photoregulatory mechanisms to operate efficiently on the >10-ps timescale.

Shifting seasonality of cyclones and western boundary current interactions in Bay of Bengal as observed during Amphan and Fani.

In recent years, the seasonal patterns of Tropical Cyclones (TC) in the Bay of Bengal have been shifting. While tropical depressions have been common in March-May (spring), they typically have been relatively weaker than the TCs during October-December. Here we show that the spatial pattern of recent warming trends during the last two decades in the southwestern Bay has allowed for stronger springtime pre-monsoon cyclones such as Amphan (May 2020, Super Cyclone) and Fani (April-May 2019, Extremely Severe Cyclone). The tracks of the pre-monsoon cyclones shifted westward, concurrent with an increasing rate of warming. This shift allowed both Fani and Amphan tracks to cross the northeastward warm Western Boundary Current (WBC) and associated warm anti-cyclonic eddies, while the weaker Viyaru (April 2013, Cyclonic Storm) did not interact with the WBC. A quantitative model linking the available along-track heat potential to cyclone's intensity is developed to understand the impact of the WBC on cyclone intensification. The influence of the warming WBC and associated anti-cyclonic eddies will likely result in much stronger springtime TCs becoming relatively common in the future.

Vibronic Excitons and Conical Intersections in Semiconductor Quantum Dots.

Surface defects and organic surface-capping ligands affect the photoluminescence properties of semiconductor quantum dots (QDs) by altering the rates of competing nonradiative relaxation processes. In this study, broadband two-dimensional electronic spectroscopy reveals that absorption of light by QDs prepares vibronic excitons, excited states derived from quantum coherent mixing of the core electronic and ligand vibrational states. Rapidly damped coherent wavepacket motions of the ligands are observed during hot-carrier cooling, with vibronic coherence transferred to the photoluminescent state. These findings suggest a many-electron, molecular theory for the electronic structure of QDs, which is supported by calculations of the structures of conical intersections between the exciton potential surfaces of a small ammonia-passivated model CdSe nanoparticle.