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A velocity map imaging (VMI) spectrometer to investigate photodissociation dynamics was designed and fabricated. This spectrometer combines a skimmed molecular beam with a multi-stage time-of-flight mass spectrometer equipped with position-sensitive detection. Investigations on the 266 nm photodissociation of o-xylene leading to {sp2}C–C{sp3} bond cleavage resulting in CH3 and C6H4CH3 fragments reveal that the kinetic energy partitioning in the centre-of-mass frame is almost identical in both the fragments indicating a single dissociation mechanism with CH3 radical produced almost exclusively in the ν = 0 level of the ground electronic state. The weakening of the {sp2}C–C{sp3} bond by about 17 kcal mol−1 in the cationic ground state relative to the neutral ground state of o-xylene facilitates the dissociation, propelled by the three-photon absorption process. A velocity map imaging (VMI) spectrometer to investigate photodissociation dynamics was designed and fabricated. This spectrometer combines a skimmed molecular beam with a multi-stage time-of-flight mass spectrometer equipped with position-sensitive detection.

The dynamics of chlorine atom (2Pj) formation in the photodissociation process of halogen substituted pyrimidines, namely, 2,4,6-trichloropyrimidine and 5-chloro-2,4,6-trifluoropyrimidine have been...

The characterization of various intermediates in the ultrafast deactivation of photoexcited ( Z, Z)-bilirubin-IXα was carried out using different computational methods. Various excited states of ( Z, Z)-bilirubin-IXα and their respective vertical excitation energies were calculated using time-dependent density functional theory (TD-DFT) employing the Coulomb-attenuating method (CAM) combined with the B3LYP functional, which is known to predict accurate results on the charge transfer excitation process. Optimized geometries and absorption spectra were determined in chloroform solvent using the polarizable continuum model incorporating the integral equation formalism. The optimized geometries of different conformers of bilirubin ( ZZ, ZE, EZ, and EE) along with their relative energies and vertical excitation energies were obtained. The geometry of the first excited state, S1, for the ZZ conformer was optimized using TD-DFT. The computational study suggests that excited-state intramolecular proton transfer (ESIPT) plays a major role in the deactivation process of ( Z, Z)-bilirubin-IXα on a shorter time scale. The lactam-lactim tautomerism that arises from the ESIPT process gives rise to various intermediates of ( Z, Z)-bilirubin-IXα. The computational results nicely corroborate the experimental findings available in the literature.

The reaction of dimethyl ether (DME) with OH radical was studied using a laser photolysis-laser induced fluorescence technique in the temperature range of 257–333 K at 30 Torr. The temperature dependent rate coefficient is represented by the expression, k T = 9.7 ± 1.0 × 10 - 12 exp - 380 ± 25 / T c m 3 m o l e c u l e s - 1 s - 1 and atmospheric lifetime was found to be 4.7 days. To elucidate the reaction mechanism and complement the experimental work, the energies and structures of the reactants and transition states involved were determined theoretically using the G3 method. The radiative forcing and the global warming potential for 100 years time horizon of DME were estimated to be 0.03 W m−2 ppb−1 and 0.02, respectively.

Kinetic studies for the gas phase reaction of OH radical with 3,4-Dichloro-1,2,5-thiadiazole has been performed using the laser photolysis – laser induced fluorescence technique. The rate coefficients were measured over the temperature range of 265–353 K and at a total pressure of~ 40 Torr of N2. The bimolecular rate coefficients obtained was fitted to the expression k(T)expt=(1.38 ± 0.91) × 10−13 exp[(903 ± 128)/T] cm3 molecule−1 s−1. Theoretical rate coefficients were also evaluated using the energy grained master equation solver which uses energies and structures of transition states and complexes calculated by ab initio method at G4MP2//M06-2X/6-311++G(d,p) level of theory. The reaction shows weak inverse temperature dependence.

The photodissociation dynamics of 4-bromo-2,3,5,6-tetrafluoropyridine (BTFP) have been studied in a supersonic molecular beam around 234 nm, which prepares the molecule in its ππ* state. The dynamics of the C Br bond dissociation have been investigated using resonance-enhanced multiphoton ionization coupled with time-of-flight mass spectrometer (REMPI-TOFMS) by detecting the nascent spin orbit states of the primary bromine atoms, namely, Br (2P3/2) and Br*(2P1/2). State specific polarization dependent TOF profiles were obtained, from which the translational energy distributions and recoil anisotropy parameters, βi, were extracted using forward convolution method. A strong polarization dependence of TOF profiles suggests anisotropic distributions of the Br and Br* fragments. Two components, namely, the fast and the slow, are observed in the translational energy distribution of Br and Br* atoms, formed from different potential energy surfaces. The average translational energies released into the Br and Br* channels for the fast component are 17.3 ± 2.0 and 11.1 ± 2.0 kcal/mol, respectively. Similarly, for the slow component, the average translational energies imparted into the Br and Br* channels are 2.6 ± 1.0 and 1.5 ± 1.0 kcal/mol, respectively. The relative quantum yields of Br and Br* are 0.87 ± 0.15 and 0.13 ± 0.06. The anisotropy parameters for Br and Br* are characterized by a similar value of 0.60 ± 0.05. The energy partitioning into the translational modes is interpreted with the help of various models. The experimental studies revealing the nature of translational energies distribution for Br and Br* and its average value along with the theoretical calculations employing Time-Dependent Density Functional Theory (TD-DFT) and Multi Configuration Quasi Degenerate second order Perturbation Theory (MCQDPT2) suggest that the initially prepared ππ* state crosses over to a nearby σ* repulsive state along the C Br bond, mainly, πσ* state from where the dissociation takes place. The results also indicate the process of fast internal conversion to ground state from the initially prepared ππ* state, which eventually forms slow Br atoms after C-Br bond dissociation process on the ground state. During the course of experiments, the absorption spectrum was also obtained with its absolute absorption cross section. Non-adiabatic curve crossing plays an important role in the C Br bond dissociation of BTFP.

Temperature dependent steady state UV/vis absorption technique is combined with ab initio calculations to investigate the ground-state intramolecular proton transfer (GSIPT) process in 1,4-dihydroxy-9,10-anthraquinone (quinizarin). The relative energy for various tautomers is calculated at MP2/6-311++G(d,p) level of theory. The next higher energy tautomer for quinizarin is identified as 9,10-dihydroxy-1,4-anthraquinone which is around 7.7 kcal/mol higher in energy. The experimental Gibbs free energy of the equilibrium between two tautomers is evaluated to be 4.5±1.5 kcal/mol. The transition state for GSIPT process leading to the higher energy tautomer is theoretically characterized which lies ~11.5 kcal/mol above the ground state of quinizarin.

Rationale 1,2,5-Thiadiazoles are an important class of compounds mostly used in synthetic chemistry, and as herbicides, insecticides, drugs, organic conductors, etc. Recently, they have been used as a source for the generation and study of nitrile N-sulfides, RCNS, and its isomers. In this study, we monitor the fragmentation pattern of ionic halogenated 1,2,5-thiadiazoles, namely, 3,4-dichloro-1,2,5-thiadiazole, which generates the nitrile sulfides, to establish its various dissociation mechanisms. Methods The molecular cation of 3,4-dichloro-1,2,5-thiadiazole was prepared using multiphoton excitation using a laser at 235 nm. Various product ions upon fragmentation of the molecular ion were mass analyzed using time-of-flight mass spectrometry. Laser power dependence studies were conducted for various product ions to arrive at the dissociation mechanism. Theoretical calculations were performed for the estimation of the ΔH values for various reactions to support the experimental data. Results The most abundant product ion was observed to be the NS+ radical cation followed by the S+ ion and the SCl+ radical cation. The other product ions such as the CNS+ radical cation and the ClCNS+ and ClCN+ cations were also observed to a lesser extent in the fragmentation pattern of the parent molecular ion. Various dissociation channels were identified and supported with ab initio calculations. Conclusions In conclusion, we have studied the fragmentation pattern of the molecular cation of 3,4-dichloro-1,2,5-thiadiazole and the formation mechanisms of various product ions have been assigned. It has been also observed that most of the product ions are nitrile N-sulfides. Finally, it is inferred that there are two primary paths for the fragmentation of the parent molecular cation, namely, (1) Cl atom migration and subsequent ring opening by N–S bond cleavage and (2) direct ring opening by N–S bond cleavage. The ionization energies were accurately predicted for various species using ab initio calculations. Copyright © 2016 John Wiley & Sons, Ltd.

Photodissociation pathways for 3,4-dichloro-1,2,5-thiadiazole was investigated in its ground state at various wavelengths in the UV region. The photolysis products were identified by the UV absorption and emission spectroscopy. Molecular chlorine (Cl 2 ) elimination was observed using absorption methods in the UV region. UV absorption and quantum calculation confirms the migration of chlorine atom and subsequent ring opening to form NC C(Cl 2 ) N S isomer. Multiphotonic excitation leading to the UV emission in the vicinity of 389 nm was observed due to the formation of CN(B 2 Σ + ) moiety. New UV absorption bands observed at ∼255 and ∼290 nm were assigned to the its isomer NC C(Cl 2 ) N S.

The photodissociation dynamics of halogen substituted pyridines, namely, 3-chloropyridine (ClPy) and 3-chloro-2,4,5,6-tetrafluoropyridine (ClFPy), has been studied around 235 nm by detecting chlorine atoms in their spin orbit states Cl( 2 P 3/2 ) and Cl ∗ ( 2 P 1/2 ) using the REMPI-TOF technique. We have determined the translational energy distribution, the recoil anisotropy parameter, β , and the spin–orbit branching ratio, for chlorine atom elimination channels. The TOF profiles for Cl and Cl ∗ are found to be independent of laser polarization suggesting a zero value for β , within the experimental uncertainties. For 3-chloropyridine, the average translational energies for Cl and Cl ∗ elimination channels are determined to be 3.7 ± 1.0 and 7.0 ± 1.5 kcal/mol, respectively. Similarly, for 3-chloro-2,4,5,6-tetrafluoropyridine, the average translational energies for Cl and Cl ∗ elimination channels are determined to be 8.0 ± 1.5 and 9.0 ± 1.5 kcal/mol, respectively. The theoretical calculation suggests that the fluorine substitution increases the possibility of cross over to the π – σ ∗ state from the initially prepared π – π ∗ state.

In one colour experiment, the photodissociation dynamics of halogen substituted thiadiazole, namely, 3,4-dichloro-1,2,5-thiadiazole, has been studied in a supersonic molecular beam around 235 nm, which prepares the molecules in its π–π* state. The study is conducted using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique, by detecting the nascent state of the primary chlorine atoms in their spin orbit states Cl(2P3/2) and Cl*(2P1/2). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin–orbit branching ratio, for chlorine 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, βi. The TOF profiles for Cl and Cl* 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. The translational energy distributions is well explained with only one component for both Cl and Cl* atom elimination channel. The average translational energies for Cl and Cl* channel are 5.2 ± 1.5 and 7.9 ± 1.5 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 experimental studies revealing the nature of translational energies distribution for Cl and Cl* and its average value along with the theoretical calculations employing Time-Dependent Density Functional Theory (TD-DFT) and ab initio molecular orbital (MO) theory suggest that the initially prepared π–π* state crosses over to a nearby σ* repulsive state along the C Cl bond, from where the dissociation takes place. The results also indicates the role of fast internal conversion to ground state from the initially prepared π–π* state, from where various dissociation channels occurs. During the course of experiments, the absorption spectrum was also obtained with its absolute absorption cross section.

The photodissociation dynamics of enolic-acetonylacetone (H3C C(O) CH2 CH C(OH) CH3), in gas phase, is studied using Laser Photolysis–Laser Induced Fluorescence (LP–LIF) technique at room temperature. The nascent state distribution of the OH radical is probed formed after photo-excitation of the parent molecule to its π C = C − π C = C * state using 193 nm laser light. The ro-vibrational distribution of the nascent OH photofragment is measured using LIF in collision free conditions. The rotational distribution is Boltzmann-like, and characterized by a single rotational temperature of 650 ± 50 K. The spin-orbit and Λ-doublet ratios of OH fragment formed in the dissociation process are also determined. For OH producing channel, the average translational energy partitioned into the photofragment pairs in the center-of-mass co-ordinate is found to be 25.0 ± 4.0 kcal/mol, corresponding to a fT value of 0.43. The experimental studies along with theoretical calculations employing time-dependent density functional theory (TD-DFT) along with ab initio molecular orbital (MO) theory suggest that the initially prepared π C = C − π C = C * state crosses over to a nearby σ* repulsive state along the C O bond, from where the dissociation takes place. The dynamics of OH formation is compared with that of acetylacetone photodissociation and effect of H-bonding is discussed.

Dynamics of OH radical formation in the 193 nm photolysis of 2-methoxyethanol is studied using Laser Photolysis–Laser Induced Fluorescence technique. The nascent state distribution of the OH radical is measured. The OH fragments are formed vibrationally cold, characterized by a Boltzmann-like single rotational temperature of 450 ± 100 K. The spin–orbit and Λ -doublet ratios of OH fragments are measured. The relative average translational energy of the OH channel is determined to be 17.0 ± 3.0 kcal/mol. The experimental studies along with theoretical calculations suggest a complex mechanism for OH formation consisting of at least three pathways. The prominent pathway at shorter timescale ( 1 ms) involves a series of reaction with initial H3C–OCH2CH2OH bond cleavage, followed by rearrangement of OCH2CH2OH to CH2OCH2OH, and a final concerted step to generate OH and ethylene epoxide.

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 ν ″ = 1 to that in ν ″ = 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 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 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 ∼200...

In one-color REMPI-TOF experiment, the photodissociation dynamics of tetrachloroethylene has been studied by probing the chlorine atom photofragments, namely, Cl (2P3/2) and Cl∗ (2P1/2), using 2 + 1 REMPI scheme in the 234–236 nm region. We have determined the centre-of-mass photofragment speed distribution, recoil anisotropy parameter (β), and the spin–orbit branching ratio for chlorine atom elimination channels. The β is well characterized by a value of ∼0.0, within the experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distributions of Cl and Cl∗. The average translational energies for the Cl and Cl∗ channels for the fast components are 17.6 ± 1.9 and 14.0 ± 1.7 kcal/mol, while, that for the slow components are 2.2 ± 1.0 and 3.2 ± 1.0 kcal/mol, respectively. To understand the nature of the dissociative potential energy surface involved in the chlorine atom formation channel, detailed theoretical calculations are performed using Time-dependent Density Functional Theory (TD-DFT) method.

The photodissociation dynamics of 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane) have been studied near 234 nm, in a molecular beam environment, employing resonance-enhanced multiphoton ionization with time-of-flight mass spectrometer (REMPI-TOF-MS). Both bromine and chlorine atoms are detected in both the ground and spin–orbit excited states. The C–Br bond scission is observed predominantly due to direct σ∗(C–Br) ← n(Br) transition, with translational energy distribution described with a soft impulsive model. The recoil anisotropy parameter (β) of 0.6 ± 0.2 for this channel vindicates the impulsive nature of dissociation. The diabatic crossing from the nσ∗(C–Br) surface to nσ∗(C–Cl) diabatic surface is responsible for chlorine formation via the C–Cl bond fission, which is supported by the excited state molecular orbital calculations. The velocity distribution of bromine atom has one component, however, that of chlorine atom is bimodal. Both fast and slow chlorine atom channels, with the β value of 0.3 ± 0.1, are produced impulsively from the nσ∗(C–Cl) repulsive surface.

Employing ultraviolet laser flash photolysis in combination with pulsed vacuum ultraviolet laser induced-fluorescence spectroscopy room-temperature rate constants k Q,H 2 = (5.8 ± 0.6) × 10−11 cm3 s−1, k Q, D 2 = (5.6 ± 0.6) × 10−12 cm3 s−1, and k Q,HD = (2.7 ± 0.3) × 10−11 cm3 s−1 for electronically non-adiabatic quenching of spin–orbit excited Cl(2P1/2) atoms in collisions with H2, D2, and HD gases were determined, which allow for comparison with results of recent quantum-scattering calculations [14] , [15] .

In one-color REMPI-TOF experiment, the photodissociation dynamics of POCl 3 has been studied by photolyzing POCl 3 and probing the chlorine atom photofragments, namely, Cl( 2 P 3/2 ) and Cl ∗ ( 2 P 1/2 ) using 2 + 1 REMPI scheme, in the 234–236 nm region. We have determined the centre-of-mass photofragment speed distribution, recoil anisotropy parameter, and the spin–orbit branching ratio for chlorine atom elimination channels. The anisotropy parameters for Cl and Cl ∗ are the same, and characterized by a value of 0.0 ± 0.05. Two components, namely, the fast and the slow, are observed in the translational energy distributions of Cl and Cl ∗ . The average translational energies for the Cl and Cl ∗ channels for the fast components are 12.5 ± 1.5 and 16.8 ± 1.5 kcal/mol, while, for the slow components, the average translational energies are 1.5 ± 1.0 and 2.5 ± 1.0 kcal/mol, respectively. Apart from the chlorine atom elimination channel, Cl 2 elimination is also observed in the photodissociation of POCl 3 .

The photodissociation dynamics of 2-furoyl chloride at 235 nm has been investigated, employing resonance-enhanced multiphoton ionization technique and time-of-flight mass spectrometry. Both the Cl fragments, Cl( 2 P J= 3/2 , relative quantum yield 0.85 ± 0.11) and Cl ∗ ( 2 P J =1/2 ), have the recoil anisotropy parameter ( β ) value close to zero, and show bimodal translational energy distributions. The branching ratio of the high kinetic energy C Cl bond scission to the low energy C Cl scission is 0.78/0.22. The dominant high kinetic energy channel arises mainly because of electronic pre-dissociation. But, the low energy channel results from the ground electronic state of 2-furoyl chloride, formed subsequent to non-radiative relaxation of the initially prepared excited state.

The photodissociation dynamics of trichloroethylene was investigated near 235 nm, (π,π*) transition, by detecting the nascent products, Cl (2P3/2) and Cl* (2P1/2), via 2 + 1 resonance enhanced multiphoton ionization. The photofragment speed distribution, the recoil anisotropy parameter β 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.

Photodissociation of 2-(bromomethyl)hexafluoro-2-propanol (BMHFP) and 3-bromo-1-propanol (BP), involving σ C – Br * ← n Br transition at 193 nm, has been investigated by measuring laser-induced fluorescence spectra of the expected OH product. The OH channel is a minor dissociation pathway with a quantum yield of 0.17 ± 0.05 in BMHFP, whereas it was not observed in BP. Partitioning of the available energy into translation, rotation, and vibration of the photoproducts has been measured by state selective detection of the nascent OH product in BMHFP. OH is produced mostly in the ground vibrational level ( v ″ = 0), with a rotational distribution being characterized by a temperature of 465 ± 25 K. But, a significant fraction of the available energy of 30.2 kcal mol −1 is partitioned into translation of OH (14.6 kcal mol −1 ). The OH( v ″ = 0, J ″) populations in the spin-orbit states as well as in the Λ-doublet states are statistical. A plausible mechanism of OH formation on excitation of BMHFP at 193 nm is suggested, with the primary reaction channel being elimination of Br atom by direct C–Br bond dissociation from a repulsive surface. The Br radical is detected using (2 + 1) resonance-enhanced multiphoton ionization (REMPI) at ∼234 nm. It is produced in both the ground ( 2 P 3/2 ) and the excited ( 2 P 1/2 ) spin-orbit states with the relative quantum yield of the latter to be 0.36. The co-fragment of Br undergoes secondary C–O bond dissociation to produce OH and F 3 C–C( CH 2 )–CF 3 , with the reaction having a barrier located in the exit channel. In this two-step three-body dissociation process, a major fraction of the available energy is released into translation (〈 f T 〉 ∼ 0.75), resulting from an impulsive C–Br bond dissociation in the primary step and presence of an exit barrier in the secondary process. Experimental results combined with theoretical calculations provide a clear picture of the dynamics of OH formation from BMHFP at 193 nm. In addition, the energetics of another channel, competing with OH, have been calculated from the primary product F 3 C–C(CH 2 )(OH)–CF 3 . In contrast to BMHFP, the OH product could not be observed from the photolysis of 3-bromo-1-propanol (another saturated halogenated propanol) at 193 nm under the detection limit of the present experimental condition, although it has a higher absorption cross-section at 193 nm.

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.

The reaction of O((3)P) with HCl at hyperthermal collision energies (45-116 kcal mol(-1)) has been investigated with crossed-molecular beams experiments and direct dynamics quasi-classical trajectory calculations. The reaction may proceed by two primary pathways, (1) H-atom abstraction to produce OH and Cl and (2) H-atom elimination to produce H and ClO. The H-atom abstraction reaction follows a stripping mechanism, in which the reagent O atom approaches the HCl molecule at large impact parameters and the OH product is scattered in the forward direction, defined as the initial direction of the reagent O atoms. The H-atom elimination reaction is highly endoergic and requires low-impact-parameter collisions. The excitation function for ClO increases from a threshold near 45 kcal mol(-1) to a maximum around 115 kcal mol(-1) and then begins to decrease when the ClO product can be formed with sufficient internal energy to undergo secondary dissociation. At collision energies slightly above threshold for H-atom elimination, the ClO product scatters primarily in the backward direction, but as the collision energy increases, the fraction of these products that scatter in the forward and sideways directions increases. The dependence of the angular distribution of ClO on collision energy is a result of the differences in collision geometry. Collisions where the H atom on HCl is oriented away from the incoming reagent O atom lead to backward-scattered ClO and those where the H atom is oriented toward the incoming O atom lead to forward-scattered ClO. The latter trajectories do not follow the minimum energy path and involve larger translational energy release. Therefore, they become dominant at higher collision energies because they lead to lower internal energies and more stable ClO products. The H-atom abstraction and elimination reactions have comparable cross sections for hyperthermal O((3)P) + HCl collisions.

A combination of beam-surface-scattering, quartz-crystal-microbalance, and surface-recession experiments was conducted to study the effects of various combinations of O atoms [in the O((3)P) ground state], Ar atoms, and vacuum ultraviolet (VUV) light on fluorinated ethylene-propylene copolymer (FEP) Teflon and poly(methyl methacrylate) (PMMA). A laser-breakdown source was used to create hyperthermal beams containing O and O(2) or Ar. A D(2) lamp provided a source of VUV light. O atoms with 4 eV of translational energy or less did not react with a pristine FEP Teflon surface. Volatile O-containing reaction products were observed when the O-atom energy was higher than 4.5 eV, and the signal increased with the O-atom energy. Significant erosion of FEP Teflon ( approximately 20% of Kapton H) was observed when it was exposed to the hyperthermal O/O(2) beam with an average O-atom energy of 5.4 eV. FEP Teflon and PMMA that were exposed to VUV light alone exhibited much less mass loss. Collision-induced dissociation by hyperthermal Ar atoms also caused mass loss, similar in magnitude to that caused by VUV light. There were no observed synergistic effects when VUV light or Ar bombardment was combined with O/O(2) exposure. For both FEP Teflon and PMMA, the erosion yields caused by simultaneous exposure to O/O(2) and either VUV light or Ar atoms could be approximately predicted by adding the erosion yield caused by O/O(2), acting individually, to the erosion yield caused by the individual action of either VUV light or Ar atoms.

Noncovalent interaction of Thioflavin T (ThT) with versatile macrocyclic host molecules, namely, cucurbit[7]uril (CB7) and cucurbit[5]uril (CB5), has been investigated in aqueous solutions by photophysical methods. Steady-state and time-resolved fluorescence studies illustrate significant enhancements/modifications in the ThT fluorescence yield, lifetime, and spectral features on interaction with the CBs and are assigned due to the formation of 1:1 and 2:1 complexes between the CBs and the ThT. The high binding constant values for the 1:1 complex (K(1) approximately 10(5) M(-1)) indicate the strong ion-dipole interaction between the host and guest molecules, whereas the 2:1 complex formation is mainly driven by weaker forces like hydrophobic interaction as evident from the lower binding constants (K(2) approximately 10(3) M(-1)). From the characteristic differences in the photophysical properties of the CB7-ThT and CB5-ThT complexes, it has been adjudged that ThT forms an inclusion complex with CB7 whereas with CB5, the interaction is through an exclusion complex formation. These contentions have been further verified by the rotational relaxation dynamics, NMR, and quantum chemical calculations on CB-ThT systems. The present results have also been compared with those reported for the dye in the presence of cyclodextrin hosts.

Beam-surface scattering experiments were used to probe products that scattered from FEP Teflon surfaces during bombardment by various combinations of atomic and molecular oxygen, Ar atoms, and vacuum ultraviolet (VUV) light. A laser-breakdown source was used to create hyperthermal (translational energies in the range 4-13 eV) beams of argon and atomic/molecular oxygen. The average incidence energy of these beams was tunable and was controlled precisely with a synchronized chopper wheel. A filtered deuterium lamp provided a source of VUV light in a narrow-wavelength range centered at 161 nm. Volatile products that exited the surfaces were monitored with a rotatable mass spectrometer detector. Hyperthermal O atoms with average translational energies above approximately 4 eV may react directly with a pristine FEP Teflon surface, and the reactivity appears to increase with the translational energy of the incident O atoms. VUV light or highly energetic collisions of O2 or Ar may break chemical bonds and lead to the ejection of volatile products; the ejection of volatile products is enhanced when the surface is subjected to VUV light and energetic collisions simultaneously. Exposure to VUV light or to hyperthermal O2 or Ar may increase the reactivity of an FEP Teflon surface to O atoms.

Hyperthermal collisions of ground-state atomic oxygen with H2O have been investigated, with special attention paid to the H-atom elimination reaction, O((3)P) + H(2)O(X (1)A(1)) --HO(2)((2)A') + H((2)S). This reaction was observed in a crossed-beams experiment, and the relative excitation function in the region around its energy threshold (50-80 kcal mol(-1)) was measured. Direct dynamics calculations were also performed at two levels of theory, B3LYP/6-31G(d,p) and MP2/6-31G(d,p). The shape of the B3LYP excitation function closely matches that of the experiment. The calculations provided a detailed description of the dynamics and revealed a striking dependence of the reaction mechanism on collision energy, where the cross section rises from a threshold near 60 kcal mol(-1) to a peak at approximately 115 kcal mol(-1) and then decreases at higher energies as secondary dissociation of the internally excited HO(2) product becomes dominant. The calculations show that the cross section for H-atom elimination (O + H(2)O --HO(2) + H) is about 10-25% that of the H-atom abstraction (O + H(2)O --OH + OH) cross section for collision energies in the 70-160 kcal mol(-1) range.

Photodissociation of 3-mercaptopropionic acid (MPA), in the gas phase, at 193 nm generates OH, as detected by laser-induced fluorescence. Measurements on the nascent OH(v″ = 0, J″) reveal that it is produced mostly vibrationally cold, with a rotational population distribution characterized by a temperature of 925 ± 75 K. A major portion of the available energy (∼30%) is channeled into relative translation of the photofragments, implying the presence of an exit barrier and that dissociation occurs from an excited state of MPA. Both experimental and theoretical results on dissociation of MPA are compared with that of its isomer, thiolactic acid, and almost similar dynamics are observed from the ground electronic states, but different from the excited states.

A C-C bond-breaking reaction has been observed when a beam containing hyperthermal oxygen was directed at a continuously refreshed saturated hydrocarbon liquid (squalane) surface. The dynamics of this C-C bond-breaking reaction have been investigated by monitoring time-of-flight and angular distributions of the volatile product, OCH3 or H2CO. The primary product is believed to be the methoxy radical, OCH3, but if this radical is highly internally excited, then it may undergo secondary dissociation to form formaldehyde, H2CO. Either the primary or the secondary product may scatter directly into the gas phase before thermal equilibrium with the surface is reached, or they may become trapped on the surface and desorb in thermal equilibrium with the surface. Direct, single-collision scattering events that produce a C-C bond-breaking product are described with a kinematic picture that allows the determination of the effective surface mass encountered by an incident O atom, the atom-surface collision energy in the center-of-mass frame, and the fraction of the center-of-mass collision energy that goes into translation of the scattered gaseous product and the recoiling surface fragment. The dynamical behavior of the C-C bond-breaking reaction is compared with that of the H-atom abstraction reaction, which was the subject of an earlier study. Another reaction, H-atom abstraction by O2 (which is present in the hyperthermal beam), has also been observed, and the dynamics of this reaction are compared with the inelastic scattering dynamics of O2 and the dynamics of H-atom abstraction by O. The dynamics involving direct inelastic and reactive scattering of O2 are also described in terms of a kinematic picture where the incident O2 molecule is viewed as interacting with a local region of the surface.

The photodissociation of allyl and propargyl alcohols at 193 nm, which involves π−π* electronic transition and leads to the formation of OH, has been studied by using a laser photolysis−laser-induced fluorescence technique. The nascent OH radicals formed from both these molecules are found to be rotationally and vibrationally excited. The vibrational distributions are found to be similar, described by vibrational temperatures of 2070 ± 380 and 2130 ± 440 K for allyl and propargyl alcohols, respectively. The rotational temperatures of both v‘ ‘ = 0 and 1 levels of OH radicals from allyl alcohol are found to be almost the same, viz., 1960 ± 150 and 1900 ± 250 K, respectively, and close to the vibrational temperature. On the other hand, the rotational temperatures of OH radicals, 1760 ± 130 K at the v‘ ‘ = 0 level and 690 ± 120 K at the v‘ ‘ = 1 level, are very different from each other, in the case of propargyl alcohol. In both molecules, a significant part of the available energy is partitioned into the re...

The photodissociation dynamics of difluoroacetic acid (DFA) is investigated in both its ground and the first excited electronic states employing, respectively, pulsed CO2 and ArF (193 nm) lasers. DFA undergoes facile infrared multiphoton dissociation (IRMPD) on irradiation with a pulsed CO2 laser to open up various dissociation channels from the ground electronic state. Energetically the most preferred primary dissociation channel of DFA is 1,3-HF elimination, as demonstrated by ab initio molecular orbital (MO) calculations and the unimolecular rates for all molecular channels using Rice−Ramsperger−Kassel−Marcus theory. Contrary to an assumed dissociation mechanism for fluorinated carboxylic acids, the decarboxylation reaction is observed as a primary dissociation channel in the IRMPD of DFA. The vibrationally excited photoproducts CO, CO2, COFH, and HF have been detected by measurement of time-resolved infrared fluorescence. Photoexcitation of DFA to the electronically excited S1 state at 193 nm leads to...

The dynamics of the formation of OH radical upon 193 nm excitation of pyruvic acid has been investigated by the laser-photolysis laser-induced-fluorescence technique. OH radicals were generated in the ground electronic state, with no vibrational excitation. The estimated rotational temperature is 720±90 K, and the translational energy is 18.7±6.5 kcal mol−1. Ab initio calculations on excited electronic states were performed at the configuration interaction with single electronic excitation level with 6-31+G(d,p) basis function. All low-lying electronic excited states (S1–S3 and T1–T6) were characterized and the transitions were identified. A transition state for the C–OH dissociation channel has been obtained from the T1 state with a late exit barrier. A mechanism for the formation of OH radicals involving internal conversion and intersystem crossing from the initially populated S3 state to T1 state and the dissociation from the T1 potential energy surface with the calculated barrier is proposed, which re...

In this review article, recent studies on the photodissociation dynamics of carboxylic acids carried out in our laboratory are presented. The dynamics are investigated by mapping the energy partitioning in the nascent photoproduct OH using laser-induced fluorescence spectroscopy. To understand the effect of the nature of the CC bond on the dissociation dynamics, both saturated (acetic) as well as unsaturated (acrylic and propiolic) carboxylic acids are investigated. In all of the carboxylic acids studied, a high percentage of the available energy is partitioned into the product translational state, indicating the presence of an exit barrier in the dissociative potential energy surface. Based on the energy partitioning, the quantum yield and the OH formation rate, the photoexcitation dynamics of carboxylic acids are revealed.

The (n,π*) and (π,π*) transitions in acrylic acid (H2C=CHCOOH) are excited by KrF (248 nm) and ArF (193 nm) laser pulses, respectively, and the dynamics of its photodissociation to give OH fragments is studied using laser induced fluorescence technique. At both the photolysis wavelengths, the OH fragments produced are vibrationally cold, but have different rotational state distributions. To get an insight into the potential energy surface involved in the dissociation process, spin–orbit and Λ-doublets ratios are also measured. Average relative translational energy partitioned into the photofragments is determined using linewidth of the Doppler profiles to be 13.2±3.1 and 10.2±2.8 kcal/mol at 193 and 248 nm excitations, respectively. High percentage of translational energy released into the photofragments suggests the presence of an exit barrier for the dissociation. On 248 nm excitation, the OH radicals are formed instantaneously during the laser pulse, while on 193 nm excitation, a risetime of ∼2 μs is seen. Another difference between the photodissociation at 193 nm and 248 nm is the observation of an intense fluorescence in UV–visible region at the former, and no fluorescence at the later wavelength. Our experimental results are compared with those obtained by recent ab initio calculations by Fang and Liu. It is concluded that when (π,π*) transition of acrylic acid is excited at 193 nm, the initially prepared S2 state undergoes nonradiative transitions to S1 and T2 states, and from where the molecule subsequently dissociates, while excitation to (n,π*) transition at 248 nm leads to dissociation solely from the initially prepared S1 state.

The dynamics of formation of oxygen atoms after UV photoexcitation of SO2 in the gas-phase was studied by pulsed laser photolysis-laser-inducedfluorescence ‘pump-and-probe’ technique in a flow reactor. SO2 at room-temperature was excited at the KrCl excimer laser wavelength (222.4 nm) and O(3Pj) photofragments were detected under collision-free conditions by vacuum ultraviolet laser-induced fluorescence. The use of narrow-band probe laser radiation, generated viaresonant third-order sum-difference frequency conversion of dye laser radiation in Krypton, allowed the measurement of the nascent O(3Pj=2,1,0) fine-structure state distribution:nj=2/nj=1/nj=0 = (0.88 ± 0.02)/(0.10 ± 0.01)/(0.02 ± 0.01). Employing NO2photolysis as a reference, a value of Φ0(3P) = 0.13 ± 0.05 for the absolute O(3P) atom quantum yield was determined. The measured O(3P) quantum yield is compared with the results of earlier fluorescence quantum yield measurements. A suitable mechanism is suggested in which the dissociation proceeds via internal conversion from high rotational states of the initially excited SO2(~C1B2 (1, 2, 2) vibronic level to nearby continuum states of the electronic ground state.

Photolysis of propiolic acid (HC⋮C−COOH) upon π−π* excitation at 193 nm leads to the C−O bond fission generating the primary product OH in a good yield. The partitioning of the available energy into the internal states and the translation of OH(v,J) is evaluated by measuring the relative intensities of ro-vibronic lines employing laser-induced fluorescence and their Doppler profiles, respectively. The vibrational distribution of nascent OH(v‘ ‘=0 and 1) corresponds to the vibrational temperature of 1030 ± 80 K. The rotational population of OH(v‘ ‘=0 and 1) is characterized by rotational temperatures of 800 ± 30 and 620 ± 30 K, respectively. Using widths of the Doppler-broadened lines, the OH(v‘ ‘=0) average translational energy was measured to be 24.4 ± 3.0 kcal/mol, which implies about 75% of the total available energy goes to the translation of the fragments. The observed high translational energy is due to the presence of a barrier in the exit channel, suggesting the C−O bond cleavage occurs on an elec...

In contrast to the photoexcitation of hydroxyacetone (HA) at 193 nm resulting in an instantaneous dissociation of the Rydberg 1 ðn; 3sÞ state, on photoexcitation at 248 nm the singlet 1 ðn; p � Þ excited HA molecule first undergoes intersystem crossing (ISC) to the triplet state, followed by a minor dissociation channel to CH3COCH2 and OH radicals. The real time formation of OH, which is probed by laser-induced fluorescence (LIF), shows a rate constant to be P 10 8 s � 1 . The initial rotational state distribution of OHðX 2 PÞ is found to be Boltzmann-like, characterized by a single rotational temperature Trot of 450� 40 K. The average relative translational energy of the photofragments is determined by Doppler spectroscopy to be 8:7� 2:0 kcal mol � 1 . The observation of OH with a modest rotational energy, no vibrational energy, and a large amount of translational energy suggests significant exit energy barrier with the dissociating surface. 2002 Published by Elsevier Science B.V.

Upon photoexcitation at 193 nm, hydroxyacetone dissociation appears to give CH 3 and COCH 2 OH radicals as primary products, and the latter undergoes further dissociation to OH and ketene. Real time LIF observation of OH formation shows a dissociation rate of COCH 2 OH as (4.6±0.5)×10 6 s −1 . There is no significant population ( ( X 2 Π) observed. The rotational state distribution has a Boltzmann temperature of the OH photofragment 380±40 K . Doppler spectroscopy shows an average translational energy with the OH photofragment as 3.6±1.3 kcal mol −1 . The bimolecular rate constant for OH + hydroxyacetone is (2.8±0.2)×10 −12 cm 3 molecule −1 s −1 .

The rate constants for the reactions of hydroxyl radical (OH) with two unsaturated alcohols (ROH) namely, allyl alcohol (H 2 CCHCH 2 OH) and propargyl alcohol (HCCCH 2 OH) in the gas phase have been measured. The kinetic measurements were carried out using laser photolysis (LP) combined with laser induced fluorescence (LIF) technique at room temperature over a pressure range of 10–20 Torr. The bimolecular rate constants for the reactions OH+ROH→products, are determined at room temperature to be (3.7±0.5)×10 −11 , (9.2±1.4)×10 −12 cm 3 molecule −1 s −1 , respectively, for allyl alcohol and propargyl alcohol. The measured rate constants in combination with ab initio molecular orbital calculation provide a better understanding of the structure-reactivity rules.

The dynamics of furfuryl alcohol (FURFUROL) photodissociation at 193 nm is reported, using laser-induced fluorescence (LIF) of the nascent OH radical and RRKM theory. The nascent OH fragments are probed by LIF under collisionless conditions, to determine the initial product state distributions. There is no significant population ( 780±40 K . The average relative translational energy of the photofragments is determined to be 26±4 kJ mol −1 . The measured rate constant for the FURFUROL dissociation vis-a-vis statistical RRKM theory, suggests a threshold dissociation energy of 357±20 kJ mol −1 .

The dynamics of oxygen atom formation in the gas-phase photolysis of nitromethane (CH 3 NO 2 ) was studied using the pulsed laser photolysis/laser-induced fluorescence (LIF) pump-and-probe technique. Room-temperature CH 3 NO 2 molecules were excited at two UV photodissociation laser wavelengths of 248 and 266 nm. Nascent O( 3 P ) photofragments were detected via LIF in the vacuum ultraviolet spectral region under collision-free conditions. Narrow-band probe laser light, tunable over the wavelength range 130.2–130.6 nm, was used to monitor the fine-structure state distribution of nascent O( 3 P J=2,1,0 ) atom product. From Doppler profiles of the O atom, the fraction of the total available energy channeled into product translational energy was determined to be 〈 f T 〉=0.28±0.02 at 248 nm and 〈 f T 〉=0.23±0.04 at 266 nm. These f T values are considerably lower than the value of 〈 f T 〉=0.63 obtained by dynamical simulation of the soft impulsive model for single N–O bond cleavage. The population ratio of the three fine-structure states of the oxygen atoms was found to be close to the statistical ratio at both photolysis wavelengths. The product fine-structure state population distribution measured for the O atoms and the 〈 f T 〉 values indicate that at both photodissociation wavelengths the O atoms are produced mainly via an indirect predissociation mechanism, but at 248 nm there is an additional contribution from a direct predissociation mechanism. The absolute quantum yields for O( 3 P ) atom formation were φ (O)=0.18±0.03 at 248 nm and φ (O)=0.13±0.04 at 266 nm; these values were obtained using a photolytic calibration method that employed NO 2 photodissociation as a reference source of well-defined O atom concentration.

The photodissociation dynamics of acetic acid at 193.3 nm is investigated by probing the nascent photoproduct OH using LIF spectroscopy. The photoproduct OH is formed mainly in the ground vibrational state with a rotational energy of 1.6±0.2 kcal/mol . The energy partitioned as photofragment translational energy is 10.0 −2.8 +5.0 kcal/mol . The present study in combination with the available dynamic information implies that the C–O single bond fission is the dominant OH-producing reaction. The modeling of the observed energy partitioning indicates that about 80% of the excess energy above the barrier is retained in the CO moiety.

The reaction of oxygen atoms [O( 3 P )] with chloroethylenes (RCl) viz. CH 2 CCl 2 , CHClCCl 2 , CCl 2 CCl 2 was studied in a discharge flow set up in the pressure range of 1.2–1.5 Torr, employing the O( 3 P ) chemiluminescence titration method. The bimolecular rate constant for the reactions O( 3 P )+RCl→products, are determined at room temperature to be (6.6±1.1)×10 −13 , (5.9±0.8)×10 −13 and (1.9±0.3)×10 −13 cm 3 molecule −1 s −1 for CH 2 CCl 2 , CHClCCl 2 and CCl 2 CCl 2 , respectively. Detailed ab initio calculations with the MNDO parametrization method 3 (PM3) are used to evaluate the kinetic and thermodynamic parameters. Formation of a biradical involving addition of an oxygen atom to the double bond is proposed as the major channel for the reaction. Unimolecular decomposition of the biradical formed was studied using RRKM theory.

The dynamics of chlorine atom formation after UV photoexcitation of CH3CF2Cl(HCFC-142b) in the gas phase was studied by a pulsed laser photolysis/laser-induced fluorescence (LIF) “pump-and-probe” technique at room temperature. The parent molecule was excited at the ArF excimer laser wavelength (193.3 nm) and nascent ground state Cl(2P3/2) and spin-orbit excited Cl*(2P1/2) photofragments were detected under collision-free conditions via laser induced fluorescence in the vacuum ultraviolet spectral region. Narrow-band probe laser radiation, tunable over the wavelength range 133.5–136.4 nm, was generated via resonant third-order sum-difference frequency conversion of dye laser radiation in Krypton. Using HCl photolysis at 193.3 nm as a source of well-defined Cl(2P3/2) and Cl*(2P1/2) concentrations, values for the total Cl atom quantum yield (ΦCl+Cl*=0.90±0.17) and the [Cl*]/[Cl] branching ratio 0.39±0.11 were determined by means of a photolytic calibration method. From the measured Cl and Cl* atom Doppler pr...

The reaction of atomic hydrogen with HCl was studied in the gas phase using the laser photolysis/vacuum–UV laser-induced fluorescence (LP/VUV–LIF) `pump-and-probe' technique. Translationally energetic H atoms with an average energy of Ecm=1.0 eV in the (H–HCl)–center-of-mass system were generated by laser photolysis of H2S at 248 nm. Doppler profiles of nascent Cl ( 2 P 3/2 ) atoms produced in the reactive collision of the H atoms with room temperature HCl molecules were measured under single collision conditions by VUV–LIF. For the H + HCl → H 2 + Cl ( 2 P 3/2 ) reaction an absolute reaction cross-section σR(1.0 eV)=(0.34±0.05) A2 was determined by means of a photolytic calibration method using the 248 nm laser photolysis of PCl3 as reference for well-defined concentrations of Cl atoms. Small amounts of Cl in the ( 2 P 1/2 ) excited state were observed in the H+HCl experiments. An upper limit of (0.02±0.01) A2 was measured for the Cl ∗ ( 2 P 1/2 ) formation cross-section in the H+HCl reaction at 1.0 eV.