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The photodissociation dynamics of1,2-cyclohexanedione (CHD), whichexists in enolic form in gas phase, is studied using pulsed laserphotolysis (LP)-laser induced fluorescence (LIF) “pump-and-probe”technique at room temperature. The nascent state distribution of theOH radical, formed after initial photoexcitation of the molecule toit is (π, π*) and Rydberg states, is determined. The initial(π, π*) and Rydberg states are prepared by excitationwith the fourth harmonic output of Nd:YAG (266 nm)/KrF (248 nm) andArF (193 nm) lasers, respectively. The ro-vibrational distributionof the nascent OH photofragment is measured in collision-free conditionsusing LIF. The OH fragments are formed in the vibrationally cold stateat all the above wavelengths of excitation but differ in rotationalstate distributions. At 266 nm photolysis, the rotational populationof OH shows a curvature in Boltzmann plot, which is fairly describedby two types of Boltzmann-like distributions characterized by rotationaltemperatures of 3100 ± 100 and 900 ± 80 K. However, at 248nm photolysis, the rotational distribution is described by a singlerotational temperature of 950 ± 80 K. The spin–orbit andΛ-doublets ratios of OH fragments formed in the dissociationprocess are also measured. The average translational energy in thecenter-of-mass coordinate, partitioned into the photofragment pairsof the OH formation channels, is determined to be 12.5 ± 3.0,12.7 ± 3.0, and 12.0 ± 3.0 kcal/mol at 266, 248, and 193nm excitation, respectively. The energy partitioning into variousdegrees of freedom of products is interpreted with the help of differentmodels, namely, statistical, impulsive, and hybrid models. To understandthe nature of the dissociative potential energy surface involved inthe OH formation channel, detailed ab initio calculations are performedusing configuration interaction-singles (CIS) method. It is proposedthat at 266 nm photolysis, the OH fragment is formed from two differentexcited state structures, one with a strong H bonding, similar tothat in the ground state, and another without effective H bonding,whereas, at 248 nm photodissociation, it seems that the OH formationoccurs mainly from the excited state, which lacks effective H-bonding.At 193 nm excitation, the initially prepared population in the Rydbergstate crosses over to a nearby σ* repulsive state along theC–O bond, from where the dissociation takes place. The exitbarrier for the OH dissociation channel is estimated to be 14 kcal/mol.The existence of dynamical constraint due to strong hydrogen bondin the ground state is effectively present in the dissociation processat 266 and somewhat deficient at 248 nm photolysis. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of fumaryl chloride (ClCOCHCH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 translational 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 experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distribution, P(ET), of Cl and Cl* atoms, and assigned to be formed from different potential energy surfaces. The average translational energies for the fast components of the Cl and Cl* channels are 14.9 ± 1.6 and 16.8 ± 1.6 kcal/mol, respectively. Similarly, for the slow components, the average translational energies of the Cl and Cl* channels are 3.4 ± 0.8 and 3.1 ± 0.8 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. Apart from the chlorine atom elimination channel, molecular hydrogen chloride (HCl) elimination is also observed in the photodissociation process. The HCl product has been detected, using a REMPI scheme in the region of 236−237 nm. The observation of the molecular HCl in the dissociation process highlights the importance of the relaxation process, in which the initially excited parent molecule relaxes to the ground state from where the molecular (HCl) elimination takes place. [ABSTRACT FROM AUTHOR]