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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.

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.

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.

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 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.