Ankur Saha - Academia.edu (original) (raw)
Papers by Ankur Saha
Indian Journal of Physics
ABSTRACT: The photodissociation dynamics of fumaryl chloride (ClCO—CHdCH—COCl) has been studied i... more ABSTRACT: The photodissociation dynamics of fumaryl chloride (ClCO—CHdCH—COCl) has been studied in a supersonic molecular beam around 235 nm using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique by detecting the nascent state of the primary chlorine atom. A single laser has been used for excitation of fumaryl chloride and the REMPI detection of chlorine atoms in their spin-orbit states, Cl (2P3/2) and Cl * ( 2P1/2). We have determined the transla-tional energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio for chlorine atom elimination channels. To obtain these, measured polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment recoil anisotropies, βi. The TOF profiles for both Cl and Cl * are found to be independent of laser polarization; i.e., β is well characterized by a value of 0.0, within the...
In recent time, studying the dynamics and structure of pollutants in gas phase and as well as at ... more In recent time, studying the dynamics and structure of pollutants in gas phase and as well as at airliquid interface has got considerable interest in view of both environmental and fundamental research. In this context, in our division we have developed and installed a Molecular beam-Resonance Enhanced Multiphoton Ionization Time of Flight (MB-REMPI-TOF) set up to understand the UV-photodissociation of halogenated volatile organic compounds (VOCs) in gas phase. We have also installed Vibrational Sum-frequency Generation (VSFG) set up to study the structure, dynamics and interaction of molecules having environmental and biological importance, at surfaces and interfaces. Applications of these two techniques with specific examples are presented in this article.
We use second harmonic generation spectroscopy, molecular dynamics simulation, and theoretical mo... more We use second harmonic generation spectroscopy, molecular dynamics simulation, and theoretical modeling to study the response of the neat liquid water-air interface to changes in the potential of an external electrode positioned above the liquid. We observe a parabolic dependence of second harmonic intensity on applied potential, indicating that water’s net interfacial dipole responds linearly. We also observe a minimum intensity when the potential is tuned to a specific positive value. Interpreting this minimum based on the macroscopic electrostatic potential profile yields misleading physical conclusions because it neglects the internal bias exerted on molecular orientations by the excess surface concentrations of OH- or H3O+. We thus find that water’s net interfacial dipole orientation is primarily responsive to the effects of these ionic species rather than the external electric field.
The composition and lifetime of sea spray aerosols are driven by the molecular and biological com... more The composition and lifetime of sea spray aerosols are driven by the molecular and biological complexity of the air-seawater interface. We explore in situ the surface properties of marine algal blooms of diatom monocultures by utilizing surface techniques of Brewster angle microscopy (BAM) imaging, vibrational sum frequency generation spectroscopy (SFG), and infrared reflection absorption spectroscopy (IRRAS). Over the course of the bloom, the marine algae produce surface-active biogenic molecules that temporally partition to the topmost interfacial layers and are selectively probed through surface imaging and spectroscopic measurements. BAM images show morphological structural changes and heterogeneity in the interfacial films with increasing density of surface-active biogenic molecules. Film thickness calculations quantified the average surface thickness over time. The image results reveal an ~5 nm thick surface region in the late stages of the bloom which correlates to typical sea surface nanolayer thicknesses. These measurements serve to extract trends in air-seawater gas exchange dynamics, thus potentially 2 aiding in climate modeling. Our surface-specific SFG spectroscopy results show significant diminishing in the intensity of the dangling OH bond of surface water molecules consistent with organic molecules partitioning and replacing water at the air-seawater interface as the algal bloom progresses. Interestingly, we observe a new broad peak appear between 3500 cm-1 to 3600 cm-1 in the late stages of the bloom that is attributed to weak hydrogen bonding interactions of water to the surface-active biogenic matter. IRRAS confirms the presence of organic molecules at the surface as we observe increasing intensity of vibrational alkyl modes and the appearance of a proteinaceous amide band. Our work shows the often overlooked but vast potential of tracking changes in the interfacial regime of small-scale laboratory marine algal blooms. By coupling surface imaging and vibrational spectroscopies to complex, time-evolving, marine-relevant systems, we provide additional insight into unraveling the temporal complexity of sea spray aerosol compositions.
The Journal of Physical Chemistry A, 2020
There is a critical need for receptors that are designed to enhance anion binding selectivity at ... more There is a critical need for receptors that are designed to enhance anion binding selectivity at aqueous interfaces in light of the growing importance of separation technologies for environmental sustainability. Here, we conducted the first study of anion binding selectivity across a series of prevalent inorganic oxoanions and halides that bind to a positively charged guanidinium receptor anchored to an aqueous interface. Vibrational sum frequency generation spectroscopy and infrared reflection absorption spectroscopy studies at the water−air interface reveal that the guanidinium receptor binds to an oxoanion series in the order SO 4 2− > H 2 PO 4 − > NO 3 − > NO 2 − while harboring very weak interactions with the halides in the order I − > Cl − ≈ Br −. In spite of large dehydration penalties for sulfate and phosphate, the more weakly hydrated guanidinium receptor was selective for these oxoanions in contradiction to predictions made from ion partitioning alone, like the Hofmeister series and Collins's rules. Instead, sulfate binding is likely favored by the suppression of dielectric screening at the interface that consequently boosts Coulombic attractions, and thus helps offset the costs of anion dehydration. Geometric factors also favor the oxoanions. Furthermore, the unique placement of iodide in our halide series ahead of the stronger hydrogen-bond acceptors (Cl − , Br −) suggests that the binding interaction also depends upon single-ion surface partitioning from bulk water to the interface. Knowledge of the anion binding preferences displayed by a guanidinium receptor sheds light on the receptor architectures needed within designer interfaces to control selectivity.
Atmospheric Environment, 2020
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ChemistrySelect, 2018
Cl atoms in both ground [Cl(2 P 3/2)] and excited spin orbit states [Cl*(2 P 1/2)] were detected ... more Cl atoms in both ground [Cl(2 P 3/2)] and excited spin orbit states [Cl*(2 P 1/2)] were detected as photodissociation products of tetrachlorocyclopropene (TCCP) at 235 nm by Resonance Enhanced Multiphoton Ionisation-Time of Flight (REMPI-TOF) technique. From the experimental REMPI-TOF profile, the anisotropy parameter (b) for both the channels was found to be nearly zero. TOF profiles of both the Cl and Cl* channels could be fitted into single component translational energy distributions, with average energy of 7.7 and 8.6 kcal mol À1 respectively. Time Dependent-Density Functional Theory (TD-DFT) calculations using mpw1pw91/aug-cc-pVTZ level of theory suggested that, near 235 nm, the initially excited n Cl-p* state of TCCP may cross over to the nearby dissociative states, from where Cl/Cl* can be formed directly. Otherwise, the excited molecules may relax rapidly to the ground state, where the lowest energy dissociation channel is the CC bond cleavage (DH = 30.4 kcal mol À1), as C-Cl bond breaking channels need high energy (DH = 73.7 kcal mol À1 and 85.5 kcal mol À1). The biradical product of CC bond breaking may rearrange to tetrachloroallene molecule with four equivalent C-Cl bonds, which can subsequently undergo C-Cl bond dissociation to produce Cl/Cl* atoms.
Journal of Photochemistry and Photobiology A: Chemistry, 2018
Trifluoroacetic acid (TFA) is released in the atmosphere through its use in the chemical industry... more Trifluoroacetic acid (TFA) is released in the atmosphere through its use in the chemical industry and as degradation product of chlorofluorocarbon (CFC) alternatives like hydrofluorocarbons and hydrochlorofluorocarbons. In the present study, we have investigated the OH formation dynamics in the photodissociation of TFA at 193 nm by Laser Photolysis-Laser Induced Fluorescence (LP-LIF) method, as well as stable product formation by GCeMS and FTIR. It was found that, ∼26% of the available energy is partitioned into the relative translation of the photoproducts (f(T) = 0.26), which could be explained by presence of an exit barrier of ∼13 kcal/mol in OH formation channel. This result is very similar to OH formation from acetic acid (AA) and difluoroacetic acid (DFA), indicating fluorination at the side chain of aliphatic carboxylic acids does not significantly change the mechanism of CeOH bond scission. Our experimental results tallied with the theoretical studies, which suggested that the major OH formation channel in acetic acid and fluoroacetic acid is direct dissociation from the optically excited S 1 state through an exit barrier, with some competition from the T 1 state. However, quantum yield of OH formation from TFA (0.4) was found to be much smaller than AA (0.8), which is probably caused by higher reaction barrier in T 1 state of TFA, compared to AA. CHF 3 , C 2 F 4 , C 2 F 6 , CO 2 , CO, CF 3 CFO, CF 2 O and hexafluoropropylene oxide (HFPO) were detected as the stable products of the photolysis of TFA. The theoretically optimized ground state dissociation channels showed significant difference between TFA and AA.
The Journal of Physical Chemistry C, 2018
We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular level un... more We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular level understanding of interaction of L-phenylalanine (Phe) with lipid monolayers of zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air−water interface. The measured VSFG spectra in the CH stretch region due to the lipid and Phe, and the OH stretch region due to interfacial water molecules were analyzed. These results in combination with surface pressure studies reveal that the Phe molecules at acidic pH of 5.6 intercalate into DPPC monolayers, and replace some interfacial water molecules. Consequently, there is a decrease in the VSFG intensity in the OH stretch region in the Phe subphase, and a concomitant increase in the surface pressure of the DPPC monolayer. The exclusion of the water molecules is controlled by both the bulk concentration of Phe, and the surface concentration of DPPC. In contrast, at the neutral pH of 7.3 there is an increase in the VSFG intensity due to the interfacial water molecules in the Phe subphase, and a decrease in the surface pressure of the DPPC monolayer. A decrease in the surface pressure, implying an increase in the surface tension toward values of pure water, suggests condensation of lipids to some extent and exposing water regions in the surface. At the neutral pH, the Phe molecules interact mostly with the headgroup of the lipid monolayer without affecting directly the hydrophobic interaction with the tail region. At both the pHs, the microscopic order of the hydrocarbon chains of the DPPC molecules is observed to increase with increased VSFG intensity due to the terminal CH 3 group, and decreased intensity due to the CH 2 group. The interaction of DPPC monolayer with Phe is compared to that with sodium dodecyl sulfate (SDS), and the hydrophobic interactions between the side chains in the latter are found to be relatively much stronger.
Atmospheric Environment, 2018
The reactions of 1-chlorocyclopentene with tropospheric oxidants, Cl, OH and O 3 are studied to m... more The reactions of 1-chlorocyclopentene with tropospheric oxidants, Cl, OH and O 3 are studied to measure the rate coefficients at 298 K and 800 Torr of N 2 using the relative rate method with butene, cyclopentene and butane as references. The concentrations of the organics are measured by Gas Chromatograpy technique. The measured average rate coefficient values of k Cl , k OH and k O3 (in cm 3 molecule −1 s −1) are (3.51 ± 1.26) x 10 −10 , (5.97 ± 1.08) x 10 −11 and (1.50 ± 0.19) x 10 −17 , respectively. In addition, the rate coefficient for reaction of OH with 1-chlorocyclopentene at 298 K and 30 Torr was also measured using an absolute method of laser photolysis-laser induced fluorescence technique, and the k OH value is found to be (6.01 ± 0.70) x 10 −11 cm 3 molecule −1 s −1. The stable products formed during the reaction of Cl, OH and O 3 initiated oxidation of the 1chlorocyclopentene in presence of air are characterized by GCMS. The abstraction and addition products are identified among them, the latter are found to be major products. The experimental results are supported by molecular orbital calculations, and abstraction of the allylic hydrogen atoms is predicted to be the major abstraction channel. Calculations predict the preferential addition of chlorine atom to the carbon atom of the unsaturation center not having Cl attached to it. The rate coefficients of 1-chlorocyclopentene are compared with that of cyclopentene and substituted cyclopentene to understand the effect of substituent Cl on the reactivity. The measured rate coefficients have been used to calculate tropospheric lifetime of the compound to be 316, 2 and 26 h for Cl, OH and O 3 , respectively. Based on these values, the major degradation pathway of 1-chlorocyclopentene is suggested to be its reaction with OH. Atmospheric impact of these molecules is local as tropospheric lifetime (τ) < 2 h. Using the tropospheric lifetime of 1-chlorocyclopentene, and its IR absorption cross-section in tropospheric IR window and under the assumption of a well mixed atmosphere, the radiative forcing of 1-chlorocyclopentene was estimated to be 22.41 mWm −2 ppb −1 , and the global warming potentials for 20 and 100 years time horizon were calculated to be 0.01 and 0.003, respectively.
The Journal of Physical Chemistry C, 2017
We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular level un... more We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular level understanding of interaction of anionic surfactant sodium dodecylsulfate (SDS), in low bulk concentration at the µM level, with lipid monolayer zwitterionic 1,2-dipalmitoyl-snglycero-3-phosphocholine (DPPC) at the air-water interface. These results are different from that reported at higher bulk concentration of SDS at mM level. At very low concentration neither DPPC nor SDS produces any VSFG signal in the CH stretch region in the water subphase. But, with the same concentration DPPC produces typical VSFG spectra at the SDS subphase due to interaction between these two molecules. The interaction leads to polar ordering of DPPC molecules with enhancement of VSFG intensity in the CH vibrational region of the hydrophobic tails. The interaction between the lipid and SDS molecules is influenced by concentrations of both lipid and SDS. Hydrophobic interactions between long alkyl chains of SDS and DPPC are responsible for an increase in the conformational order of the alkyl chain of DPPC with a decrease in the gauche defect and increase in trans conformer. Similarly, the orientation and concentration of interfacial water molecules of DPPC monolayer at SDS subphase are controlled by concentration of both SDS and DPPC. The VSFG results are complemented by the surface pressure measurements.
Indian Journal of Physics, 2012
The photodissociation dynamics of trichloroethylene was investigated near 235 nm, (p,p*) transiti... more The photodissociation dynamics of trichloroethylene was investigated near 235 nm, (p,p*) transition, by detecting the nascent products, Cl (2 P 3/2) and Cl* (2 P 1/2), via 2 ? 1 resonance enhanced multiphoton ionization. The photofragment speed distribution, the recoil anisotropy parameter b and the spin-orbit branching ratio for chlorine atom elimination channels were determined from time of flight profiles in polarization experiments. Based on the dynamical information, the features of potential energy surface along the reaction coordinate were discussed.
Chemical Physics Letters, 2016
Abstract The dynamics of the C⿿Br bond dissociation on UV excitation of methyl 2-bromopropionate ... more Abstract The dynamics of the C⿿Br bond dissociation on UV excitation of methyl 2-bromopropionate mainly to the 1 (nϿ * ) state, repulsive in the C⿿Br bond, has been investigated, employing resonance-enhanced multiphoton ionization. Both the ground state and spin-orbits excited bromine atoms were detected, with the former being the major channel. Bromine fragments show bimodal translational energy distributions, with slow and fast (major) bromine atoms arising mainly from the ground and excited electronic states, respectively. The measured recoil anisotropy suggests isotropic angular distributions of bromine atoms. Molecular orbital calculations reveal an important role of avoided curve crossing on C⿿Br bond dissociation dynamics.
Chemical Physics, 2014
ABSTRACT Photodissociation of 2-chloro-6-nitrotoluene (ClNT) at 193, 248 and 266 nm and nitrocycl... more ABSTRACT Photodissociation of 2-chloro-6-nitrotoluene (ClNT) at 193, 248 and 266 nm and nitrocyclopentane (NCP) at 193 nm leads to the formation of OH, as detected by laser-induced fluorescence (LIF). The nascent OH produced from the photolysis of ClNT at all the wavelengths is vibrationally cold, with the Boltzmann type rotational state distributions. However, the nascent OH product from NCP is in the ground and vibrationally excited states with the measured average relative population in nu '' = 1 to that in nu '' = 0 of 0.12 +/- 0.03, and these levels are characterized by rotational temperatures of 650 +/- 180 K and 1570 +/- 90 K, respectively. The translational energy partitioned in the OH fragment has been measured for photodissociation of both ClNT and NCP. On the basis of both the experimental results and the ground state molecular orbital (MO) calculations, a plausible mechanism for the OH formation has been proposed.
The Journal of Physical Chemistry C, 2014
The effects of various compositions in mixed surfactant solutions of sodium dodecyl sulfate (SDS)... more The effects of various compositions in mixed surfactant solutions of sodium dodecyl sulfate (SDS), an anionic surfactant, and cetyltrimethylammonium bromide (CTAB), a cationic surfactant, at an air−water interface have been investigated using vibrational sum-frequency generation (VSFG) nonlinear optical spectroscopy. The work is focused on understanding the temporal evolution of aggregation behavior of the catanionic system of CTAB and SDS at the charged air−water interface. For the mixed surfactants, the VSFG intensity of the OH stretching bands decreases, whereas that of the CH stretching of alkyl chains increases with time. For the 1:1 ratio of surfactants, the VSFG intensity of the OH stretch vanishes much earlier than the complete growth of the CH stretching modes. Thus, the polar ordering of interfacial water molecules is faster than the time-evolution of alkyl chains of the catanionic system. The temporal growth of the complex between surfactants CTAB and SDS exhibited an induction time (up to ∼2000 s), followed by a rapid growth (∼30 s) and then a slow growth for hours. The effects of different compositions of the surfactants on the induction time and the adsorption kinetics have been investigated. Our results on VSFG are supported by measurements, employing the surface pressure-time (π−t) kinetics and Brewster angle microscopy (BAM). The nature of π−t curves and its dependence on composition of surfactants is qualitatively similar to VSFG temporal profiles, except for the absence of the rapid adsorption growth in the former. This difference in the adsorption kinetics is explained based on formation of surfactant domains, as detected by BAM, at the air−water interface.
The Journal of Physical Chemistry A, 2014
The photodissociation dynamics of benzoyl chloride at 235 nm has been investigated and compared w... more The photodissociation dynamics of benzoyl chloride at 235 nm has been investigated and compared with that of 2-furoyl chloride. Atomic Cl and molecular HCl channels have been detected in benzoyl chloride by employing resonance-enhanced multiphoton ionization technique and time-of-flight mass spectrometry. Both the Cl fragments, Cl((2)PJ=3/2, relative quantum yield 0.70 ± 0.15) and Cl*((2)PJ=1/2), show isotropic angular distribution and bimodal translational energy distributions. The predominant high kinetic energy channel contributes 72% to the C-Cl bond scission and arises from the S1 state having nπ* character of benzoyl chloride. However, the low-energy Cl and HCl channels originate from the ground electronic state. The most plausible mechanism of HCl formation is proposed based on molecular orbital calculations. In contrast to benzoyl chloride, the HCl channel is not observed in 2-furoyl chloride on excitation at 235 nm, and this is attributed to an energy constraint.
The Journal of Physical Chemistry A, 2013
The Journal of Physical Chemistry A, 2012
The photodissociation dynamics of halogen-substituted thiophenes, namely, 2-chlorothiophene and 2... more The photodissociation dynamics of halogen-substituted thiophenes, namely, 2-chlorothiophene and 2-bromo-5-chlorothiophene, has been studied in a supersonic molecular beam around 235 nm, using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique, by detecting the nascent state of the primary halogen atoms. A single laser has been used for excitation of halothiophenes, as well as for the REMPI detection of photoproducts, namely, chlorine and bromine atoms, in their spin-orbit states X((2)P(3/2)) and X*((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio, for chlorine and bromine atom elimination channels. State-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies, β(ι). The TOF profiles for Cl, Cl*, Br, and Br* are found to be independent of laser polarization; i.e., the β is well characterized by a value of ~0.0, within the experimental uncertainties. For 2-chlorothiophene, we have observed two components for the Cl and only one component for the Cl* atom elimination channel in the translational energy distributions. The average translational energies for the fast and the slow components of the Cl channel are 3.0 ± 1.0 and 1.0 ± 0.5 kcal/mol, respectively. For Cl*, the average translational energy is 3.5 ± 1.0 kcal/mol. For 2-bromo-5-chlorothiophene, we have observed only one component for Cl, Cl*, Br, and Br* in the translational energy distributions. The average translational energies for the Cl and Cl* channels are 3.5 ± 1.0 and 5.0 ± 1.0 kcal/mol, respectively, whereas the average translational energies for the Br and Br* channels are 2.0 ± 1.0 and 3.5 ± 1.0 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. The ΔH(f)(298) value for 2-chlorothiophene has been estimated theoretically to be 23.5 kcal/mol.
The Journal of Physical Chemistry A, 2010
The photodissociation dynamics of phosphorus trichloride (PCl(3)) has been studied in a supersoni... more The photodissociation dynamics of phosphorus trichloride (PCl(3)) has been studied in a supersonic beam by resonance enhanced multiphoton ionization (REMPI), using time-of-flight (TOF) mass spectrometry. The study is focused on the nascent state of the primary chlorine atom, formed on excitation of the (n, sigma*) transition of the molecule around 235 nm. Dissociation of PCl(3) and the REMPI detection of chlorine atoms are performed, using the same laser around 235 nm. The photofragments, namely, Cl((2)P(3/2)) and Cl*((2)P(1/2)), are probed, using the 2+1 REMPI scheme in the 234-236 nm region. We have determined the photofragment speed distribution, the recoil anisotropy parameter beta, and the spin-orbit branching ratio for chlorine atom elimination channels. Polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies. The anisotropy parameters for Cl and Cl* are characterized by values of 0.0 +/- 0.05 and 0.20 +/- 0.05, respectively. Two components, namely, the fast and the slow, are observed in the speed distribution (P(v)) of Cl and Cl* atoms, formed from different potential energy surfaces. The average translational energies for the Cl and Cl* channels for the fast component are 29.7 and 30.6 kcal/mol, respectively. Similarly, for the slow component, the average translational energies for the Cl and Cl* channels are 9.5 and 9.1 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of an impulsive model, for the fast component, and a statistical model, for the slow component. Apart from the chlorine atom elimination channel, molecular chlorine (Cl(2)) elimination is also observed in the photodissociation of PCl(3). The observation of the molecular chlorine in the dissociation process and the bimodal translational energy distribution of the chlorine atom clearly indicate the existence of a crossover mechanism from the initially prepared state to the ground state.
Indian Journal of Physics
ABSTRACT: The photodissociation dynamics of fumaryl chloride (ClCO—CHdCH—COCl) has been studied i... more ABSTRACT: The photodissociation dynamics of fumaryl chloride (ClCO—CHdCH—COCl) has been studied in a supersonic molecular beam around 235 nm using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique by detecting the nascent state of the primary chlorine atom. A single laser has been used for excitation of fumaryl chloride and the REMPI detection of chlorine atoms in their spin-orbit states, Cl (2P3/2) and Cl * ( 2P1/2). We have determined the transla-tional energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio for chlorine atom elimination channels. To obtain these, measured polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment recoil anisotropies, βi. The TOF profiles for both Cl and Cl * are found to be independent of laser polarization; i.e., β is well characterized by a value of 0.0, within the...
In recent time, studying the dynamics and structure of pollutants in gas phase and as well as at ... more In recent time, studying the dynamics and structure of pollutants in gas phase and as well as at airliquid interface has got considerable interest in view of both environmental and fundamental research. In this context, in our division we have developed and installed a Molecular beam-Resonance Enhanced Multiphoton Ionization Time of Flight (MB-REMPI-TOF) set up to understand the UV-photodissociation of halogenated volatile organic compounds (VOCs) in gas phase. We have also installed Vibrational Sum-frequency Generation (VSFG) set up to study the structure, dynamics and interaction of molecules having environmental and biological importance, at surfaces and interfaces. Applications of these two techniques with specific examples are presented in this article.
We use second harmonic generation spectroscopy, molecular dynamics simulation, and theoretical mo... more We use second harmonic generation spectroscopy, molecular dynamics simulation, and theoretical modeling to study the response of the neat liquid water-air interface to changes in the potential of an external electrode positioned above the liquid. We observe a parabolic dependence of second harmonic intensity on applied potential, indicating that water’s net interfacial dipole responds linearly. We also observe a minimum intensity when the potential is tuned to a specific positive value. Interpreting this minimum based on the macroscopic electrostatic potential profile yields misleading physical conclusions because it neglects the internal bias exerted on molecular orientations by the excess surface concentrations of OH- or H3O+. We thus find that water’s net interfacial dipole orientation is primarily responsive to the effects of these ionic species rather than the external electric field.
The composition and lifetime of sea spray aerosols are driven by the molecular and biological com... more The composition and lifetime of sea spray aerosols are driven by the molecular and biological complexity of the air-seawater interface. We explore in situ the surface properties of marine algal blooms of diatom monocultures by utilizing surface techniques of Brewster angle microscopy (BAM) imaging, vibrational sum frequency generation spectroscopy (SFG), and infrared reflection absorption spectroscopy (IRRAS). Over the course of the bloom, the marine algae produce surface-active biogenic molecules that temporally partition to the topmost interfacial layers and are selectively probed through surface imaging and spectroscopic measurements. BAM images show morphological structural changes and heterogeneity in the interfacial films with increasing density of surface-active biogenic molecules. Film thickness calculations quantified the average surface thickness over time. The image results reveal an ~5 nm thick surface region in the late stages of the bloom which correlates to typical sea surface nanolayer thicknesses. These measurements serve to extract trends in air-seawater gas exchange dynamics, thus potentially 2 aiding in climate modeling. Our surface-specific SFG spectroscopy results show significant diminishing in the intensity of the dangling OH bond of surface water molecules consistent with organic molecules partitioning and replacing water at the air-seawater interface as the algal bloom progresses. Interestingly, we observe a new broad peak appear between 3500 cm-1 to 3600 cm-1 in the late stages of the bloom that is attributed to weak hydrogen bonding interactions of water to the surface-active biogenic matter. IRRAS confirms the presence of organic molecules at the surface as we observe increasing intensity of vibrational alkyl modes and the appearance of a proteinaceous amide band. Our work shows the often overlooked but vast potential of tracking changes in the interfacial regime of small-scale laboratory marine algal blooms. By coupling surface imaging and vibrational spectroscopies to complex, time-evolving, marine-relevant systems, we provide additional insight into unraveling the temporal complexity of sea spray aerosol compositions.
The Journal of Physical Chemistry A, 2020
There is a critical need for receptors that are designed to enhance anion binding selectivity at ... more There is a critical need for receptors that are designed to enhance anion binding selectivity at aqueous interfaces in light of the growing importance of separation technologies for environmental sustainability. Here, we conducted the first study of anion binding selectivity across a series of prevalent inorganic oxoanions and halides that bind to a positively charged guanidinium receptor anchored to an aqueous interface. Vibrational sum frequency generation spectroscopy and infrared reflection absorption spectroscopy studies at the water−air interface reveal that the guanidinium receptor binds to an oxoanion series in the order SO 4 2− > H 2 PO 4 − > NO 3 − > NO 2 − while harboring very weak interactions with the halides in the order I − > Cl − ≈ Br −. In spite of large dehydration penalties for sulfate and phosphate, the more weakly hydrated guanidinium receptor was selective for these oxoanions in contradiction to predictions made from ion partitioning alone, like the Hofmeister series and Collins's rules. Instead, sulfate binding is likely favored by the suppression of dielectric screening at the interface that consequently boosts Coulombic attractions, and thus helps offset the costs of anion dehydration. Geometric factors also favor the oxoanions. Furthermore, the unique placement of iodide in our halide series ahead of the stronger hydrogen-bond acceptors (Cl − , Br −) suggests that the binding interaction also depends upon single-ion surface partitioning from bulk water to the interface. Knowledge of the anion binding preferences displayed by a guanidinium receptor sheds light on the receptor architectures needed within designer interfaces to control selectivity.
Atmospheric Environment, 2020
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ChemistrySelect, 2018
Cl atoms in both ground [Cl(2 P 3/2)] and excited spin orbit states [Cl*(2 P 1/2)] were detected ... more Cl atoms in both ground [Cl(2 P 3/2)] and excited spin orbit states [Cl*(2 P 1/2)] were detected as photodissociation products of tetrachlorocyclopropene (TCCP) at 235 nm by Resonance Enhanced Multiphoton Ionisation-Time of Flight (REMPI-TOF) technique. From the experimental REMPI-TOF profile, the anisotropy parameter (b) for both the channels was found to be nearly zero. TOF profiles of both the Cl and Cl* channels could be fitted into single component translational energy distributions, with average energy of 7.7 and 8.6 kcal mol À1 respectively. Time Dependent-Density Functional Theory (TD-DFT) calculations using mpw1pw91/aug-cc-pVTZ level of theory suggested that, near 235 nm, the initially excited n Cl-p* state of TCCP may cross over to the nearby dissociative states, from where Cl/Cl* can be formed directly. Otherwise, the excited molecules may relax rapidly to the ground state, where the lowest energy dissociation channel is the CC bond cleavage (DH = 30.4 kcal mol À1), as C-Cl bond breaking channels need high energy (DH = 73.7 kcal mol À1 and 85.5 kcal mol À1). The biradical product of CC bond breaking may rearrange to tetrachloroallene molecule with four equivalent C-Cl bonds, which can subsequently undergo C-Cl bond dissociation to produce Cl/Cl* atoms.
Journal of Photochemistry and Photobiology A: Chemistry, 2018
Trifluoroacetic acid (TFA) is released in the atmosphere through its use in the chemical industry... more Trifluoroacetic acid (TFA) is released in the atmosphere through its use in the chemical industry and as degradation product of chlorofluorocarbon (CFC) alternatives like hydrofluorocarbons and hydrochlorofluorocarbons. In the present study, we have investigated the OH formation dynamics in the photodissociation of TFA at 193 nm by Laser Photolysis-Laser Induced Fluorescence (LP-LIF) method, as well as stable product formation by GCeMS and FTIR. It was found that, ∼26% of the available energy is partitioned into the relative translation of the photoproducts (f(T) = 0.26), which could be explained by presence of an exit barrier of ∼13 kcal/mol in OH formation channel. This result is very similar to OH formation from acetic acid (AA) and difluoroacetic acid (DFA), indicating fluorination at the side chain of aliphatic carboxylic acids does not significantly change the mechanism of CeOH bond scission. Our experimental results tallied with the theoretical studies, which suggested that the major OH formation channel in acetic acid and fluoroacetic acid is direct dissociation from the optically excited S 1 state through an exit barrier, with some competition from the T 1 state. However, quantum yield of OH formation from TFA (0.4) was found to be much smaller than AA (0.8), which is probably caused by higher reaction barrier in T 1 state of TFA, compared to AA. CHF 3 , C 2 F 4 , C 2 F 6 , CO 2 , CO, CF 3 CFO, CF 2 O and hexafluoropropylene oxide (HFPO) were detected as the stable products of the photolysis of TFA. The theoretically optimized ground state dissociation channels showed significant difference between TFA and AA.
The Journal of Physical Chemistry C, 2018
We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular level un... more We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular level understanding of interaction of L-phenylalanine (Phe) with lipid monolayers of zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air−water interface. The measured VSFG spectra in the CH stretch region due to the lipid and Phe, and the OH stretch region due to interfacial water molecules were analyzed. These results in combination with surface pressure studies reveal that the Phe molecules at acidic pH of 5.6 intercalate into DPPC monolayers, and replace some interfacial water molecules. Consequently, there is a decrease in the VSFG intensity in the OH stretch region in the Phe subphase, and a concomitant increase in the surface pressure of the DPPC monolayer. The exclusion of the water molecules is controlled by both the bulk concentration of Phe, and the surface concentration of DPPC. In contrast, at the neutral pH of 7.3 there is an increase in the VSFG intensity due to the interfacial water molecules in the Phe subphase, and a decrease in the surface pressure of the DPPC monolayer. A decrease in the surface pressure, implying an increase in the surface tension toward values of pure water, suggests condensation of lipids to some extent and exposing water regions in the surface. At the neutral pH, the Phe molecules interact mostly with the headgroup of the lipid monolayer without affecting directly the hydrophobic interaction with the tail region. At both the pHs, the microscopic order of the hydrocarbon chains of the DPPC molecules is observed to increase with increased VSFG intensity due to the terminal CH 3 group, and decreased intensity due to the CH 2 group. The interaction of DPPC monolayer with Phe is compared to that with sodium dodecyl sulfate (SDS), and the hydrophobic interactions between the side chains in the latter are found to be relatively much stronger.
Atmospheric Environment, 2018
The reactions of 1-chlorocyclopentene with tropospheric oxidants, Cl, OH and O 3 are studied to m... more The reactions of 1-chlorocyclopentene with tropospheric oxidants, Cl, OH and O 3 are studied to measure the rate coefficients at 298 K and 800 Torr of N 2 using the relative rate method with butene, cyclopentene and butane as references. The concentrations of the organics are measured by Gas Chromatograpy technique. The measured average rate coefficient values of k Cl , k OH and k O3 (in cm 3 molecule −1 s −1) are (3.51 ± 1.26) x 10 −10 , (5.97 ± 1.08) x 10 −11 and (1.50 ± 0.19) x 10 −17 , respectively. In addition, the rate coefficient for reaction of OH with 1-chlorocyclopentene at 298 K and 30 Torr was also measured using an absolute method of laser photolysis-laser induced fluorescence technique, and the k OH value is found to be (6.01 ± 0.70) x 10 −11 cm 3 molecule −1 s −1. The stable products formed during the reaction of Cl, OH and O 3 initiated oxidation of the 1chlorocyclopentene in presence of air are characterized by GCMS. The abstraction and addition products are identified among them, the latter are found to be major products. The experimental results are supported by molecular orbital calculations, and abstraction of the allylic hydrogen atoms is predicted to be the major abstraction channel. Calculations predict the preferential addition of chlorine atom to the carbon atom of the unsaturation center not having Cl attached to it. The rate coefficients of 1-chlorocyclopentene are compared with that of cyclopentene and substituted cyclopentene to understand the effect of substituent Cl on the reactivity. The measured rate coefficients have been used to calculate tropospheric lifetime of the compound to be 316, 2 and 26 h for Cl, OH and O 3 , respectively. Based on these values, the major degradation pathway of 1-chlorocyclopentene is suggested to be its reaction with OH. Atmospheric impact of these molecules is local as tropospheric lifetime (τ) < 2 h. Using the tropospheric lifetime of 1-chlorocyclopentene, and its IR absorption cross-section in tropospheric IR window and under the assumption of a well mixed atmosphere, the radiative forcing of 1-chlorocyclopentene was estimated to be 22.41 mWm −2 ppb −1 , and the global warming potentials for 20 and 100 years time horizon were calculated to be 0.01 and 0.003, respectively.
The Journal of Physical Chemistry C, 2017
We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular level un... more We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular level understanding of interaction of anionic surfactant sodium dodecylsulfate (SDS), in low bulk concentration at the µM level, with lipid monolayer zwitterionic 1,2-dipalmitoyl-snglycero-3-phosphocholine (DPPC) at the air-water interface. These results are different from that reported at higher bulk concentration of SDS at mM level. At very low concentration neither DPPC nor SDS produces any VSFG signal in the CH stretch region in the water subphase. But, with the same concentration DPPC produces typical VSFG spectra at the SDS subphase due to interaction between these two molecules. The interaction leads to polar ordering of DPPC molecules with enhancement of VSFG intensity in the CH vibrational region of the hydrophobic tails. The interaction between the lipid and SDS molecules is influenced by concentrations of both lipid and SDS. Hydrophobic interactions between long alkyl chains of SDS and DPPC are responsible for an increase in the conformational order of the alkyl chain of DPPC with a decrease in the gauche defect and increase in trans conformer. Similarly, the orientation and concentration of interfacial water molecules of DPPC monolayer at SDS subphase are controlled by concentration of both SDS and DPPC. The VSFG results are complemented by the surface pressure measurements.
Indian Journal of Physics, 2012
The photodissociation dynamics of trichloroethylene was investigated near 235 nm, (p,p*) transiti... more The photodissociation dynamics of trichloroethylene was investigated near 235 nm, (p,p*) transition, by detecting the nascent products, Cl (2 P 3/2) and Cl* (2 P 1/2), via 2 ? 1 resonance enhanced multiphoton ionization. The photofragment speed distribution, the recoil anisotropy parameter b and the spin-orbit branching ratio for chlorine atom elimination channels were determined from time of flight profiles in polarization experiments. Based on the dynamical information, the features of potential energy surface along the reaction coordinate were discussed.
Chemical Physics Letters, 2016
Abstract The dynamics of the C⿿Br bond dissociation on UV excitation of methyl 2-bromopropionate ... more Abstract The dynamics of the C⿿Br bond dissociation on UV excitation of methyl 2-bromopropionate mainly to the 1 (nϿ * ) state, repulsive in the C⿿Br bond, has been investigated, employing resonance-enhanced multiphoton ionization. Both the ground state and spin-orbits excited bromine atoms were detected, with the former being the major channel. Bromine fragments show bimodal translational energy distributions, with slow and fast (major) bromine atoms arising mainly from the ground and excited electronic states, respectively. The measured recoil anisotropy suggests isotropic angular distributions of bromine atoms. Molecular orbital calculations reveal an important role of avoided curve crossing on C⿿Br bond dissociation dynamics.
Chemical Physics, 2014
ABSTRACT Photodissociation of 2-chloro-6-nitrotoluene (ClNT) at 193, 248 and 266 nm and nitrocycl... more ABSTRACT Photodissociation of 2-chloro-6-nitrotoluene (ClNT) at 193, 248 and 266 nm and nitrocyclopentane (NCP) at 193 nm leads to the formation of OH, as detected by laser-induced fluorescence (LIF). The nascent OH produced from the photolysis of ClNT at all the wavelengths is vibrationally cold, with the Boltzmann type rotational state distributions. However, the nascent OH product from NCP is in the ground and vibrationally excited states with the measured average relative population in nu '' = 1 to that in nu '' = 0 of 0.12 +/- 0.03, and these levels are characterized by rotational temperatures of 650 +/- 180 K and 1570 +/- 90 K, respectively. The translational energy partitioned in the OH fragment has been measured for photodissociation of both ClNT and NCP. On the basis of both the experimental results and the ground state molecular orbital (MO) calculations, a plausible mechanism for the OH formation has been proposed.
The Journal of Physical Chemistry C, 2014
The effects of various compositions in mixed surfactant solutions of sodium dodecyl sulfate (SDS)... more The effects of various compositions in mixed surfactant solutions of sodium dodecyl sulfate (SDS), an anionic surfactant, and cetyltrimethylammonium bromide (CTAB), a cationic surfactant, at an air−water interface have been investigated using vibrational sum-frequency generation (VSFG) nonlinear optical spectroscopy. The work is focused on understanding the temporal evolution of aggregation behavior of the catanionic system of CTAB and SDS at the charged air−water interface. For the mixed surfactants, the VSFG intensity of the OH stretching bands decreases, whereas that of the CH stretching of alkyl chains increases with time. For the 1:1 ratio of surfactants, the VSFG intensity of the OH stretch vanishes much earlier than the complete growth of the CH stretching modes. Thus, the polar ordering of interfacial water molecules is faster than the time-evolution of alkyl chains of the catanionic system. The temporal growth of the complex between surfactants CTAB and SDS exhibited an induction time (up to ∼2000 s), followed by a rapid growth (∼30 s) and then a slow growth for hours. The effects of different compositions of the surfactants on the induction time and the adsorption kinetics have been investigated. Our results on VSFG are supported by measurements, employing the surface pressure-time (π−t) kinetics and Brewster angle microscopy (BAM). The nature of π−t curves and its dependence on composition of surfactants is qualitatively similar to VSFG temporal profiles, except for the absence of the rapid adsorption growth in the former. This difference in the adsorption kinetics is explained based on formation of surfactant domains, as detected by BAM, at the air−water interface.
The Journal of Physical Chemistry A, 2014
The photodissociation dynamics of benzoyl chloride at 235 nm has been investigated and compared w... more The photodissociation dynamics of benzoyl chloride at 235 nm has been investigated and compared with that of 2-furoyl chloride. Atomic Cl and molecular HCl channels have been detected in benzoyl chloride by employing resonance-enhanced multiphoton ionization technique and time-of-flight mass spectrometry. Both the Cl fragments, Cl((2)PJ=3/2, relative quantum yield 0.70 ± 0.15) and Cl*((2)PJ=1/2), show isotropic angular distribution and bimodal translational energy distributions. The predominant high kinetic energy channel contributes 72% to the C-Cl bond scission and arises from the S1 state having nπ* character of benzoyl chloride. However, the low-energy Cl and HCl channels originate from the ground electronic state. The most plausible mechanism of HCl formation is proposed based on molecular orbital calculations. In contrast to benzoyl chloride, the HCl channel is not observed in 2-furoyl chloride on excitation at 235 nm, and this is attributed to an energy constraint.
The Journal of Physical Chemistry A, 2013
The Journal of Physical Chemistry A, 2012
The photodissociation dynamics of halogen-substituted thiophenes, namely, 2-chlorothiophene and 2... more The photodissociation dynamics of halogen-substituted thiophenes, namely, 2-chlorothiophene and 2-bromo-5-chlorothiophene, has been studied in a supersonic molecular beam around 235 nm, using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique, by detecting the nascent state of the primary halogen atoms. A single laser has been used for excitation of halothiophenes, as well as for the REMPI detection of photoproducts, namely, chlorine and bromine atoms, in their spin-orbit states X((2)P(3/2)) and X*((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio, for chlorine and bromine atom elimination channels. State-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies, β(ι). The TOF profiles for Cl, Cl*, Br, and Br* are found to be independent of laser polarization; i.e., the β is well characterized by a value of ~0.0, within the experimental uncertainties. For 2-chlorothiophene, we have observed two components for the Cl and only one component for the Cl* atom elimination channel in the translational energy distributions. The average translational energies for the fast and the slow components of the Cl channel are 3.0 ± 1.0 and 1.0 ± 0.5 kcal/mol, respectively. For Cl*, the average translational energy is 3.5 ± 1.0 kcal/mol. For 2-bromo-5-chlorothiophene, we have observed only one component for Cl, Cl*, Br, and Br* in the translational energy distributions. The average translational energies for the Cl and Cl* channels are 3.5 ± 1.0 and 5.0 ± 1.0 kcal/mol, respectively, whereas the average translational energies for the Br and Br* channels are 2.0 ± 1.0 and 3.5 ± 1.0 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. The ΔH(f)(298) value for 2-chlorothiophene has been estimated theoretically to be 23.5 kcal/mol.
The Journal of Physical Chemistry A, 2010
The photodissociation dynamics of phosphorus trichloride (PCl(3)) has been studied in a supersoni... more The photodissociation dynamics of phosphorus trichloride (PCl(3)) has been studied in a supersonic beam by resonance enhanced multiphoton ionization (REMPI), using time-of-flight (TOF) mass spectrometry. The study is focused on the nascent state of the primary chlorine atom, formed on excitation of the (n, sigma*) transition of the molecule around 235 nm. Dissociation of PCl(3) and the REMPI detection of chlorine atoms are performed, using the same laser around 235 nm. The photofragments, namely, Cl((2)P(3/2)) and Cl*((2)P(1/2)), are probed, using the 2+1 REMPI scheme in the 234-236 nm region. We have determined the photofragment speed distribution, the recoil anisotropy parameter beta, and the spin-orbit branching ratio for chlorine atom elimination channels. Polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies. The anisotropy parameters for Cl and Cl* are characterized by values of 0.0 +/- 0.05 and 0.20 +/- 0.05, respectively. Two components, namely, the fast and the slow, are observed in the speed distribution (P(v)) of Cl and Cl* atoms, formed from different potential energy surfaces. The average translational energies for the Cl and Cl* channels for the fast component are 29.7 and 30.6 kcal/mol, respectively. Similarly, for the slow component, the average translational energies for the Cl and Cl* channels are 9.5 and 9.1 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of an impulsive model, for the fast component, and a statistical model, for the slow component. Apart from the chlorine atom elimination channel, molecular chlorine (Cl(2)) elimination is also observed in the photodissociation of PCl(3). The observation of the molecular chlorine in the dissociation process and the bimodal translational energy distribution of the chlorine atom clearly indicate the existence of a crossover mechanism from the initially prepared state to the ground state.