Formation of furan along with HO2 during the OH-initiated oxidation of 2,5-DHF and 2,3-DHF: an experimental and computational study.

Experimental characterization of products during OH-initiated oxidation of dihydrofurans (DHF) confirms the formation of furan accompanied by the formation of HO2 to be a significant channel in 2,5-DHF (21 ± 3%), whereas it is absent in 2,3-DHF. Theoretical investigations on the reaction of OH with these molecules are carried out to understand this difference. All possible channels of reaction are studied at M06-2X level with 6-311G* basis set, and the stationary points on the potential energy surface are optimized. The overall rate coefficients calculated using conventional TST with Wigner tunneling correction for 2,5-DHF and 2,3-DHF are 2.25 × 10(-11) and 4.13 × 10(-10) cm(3) molecule(-1) s(-1), respectively, in the same range as the previously determined experimental values. The branching ratios of different channels were estimated using the computed rate coefficients. The abstraction of H atom, leading to dihydrofuranyl radical, is found to be a significant probability, equally important as the addition of OH to the double bond in the case of 2,5-DHF. However, this probability is very small in the case of 2,3-DHF because the rate coefficient of the addition reaction is more than 10 times that of the abstraction reaction. This explains the conspicuous absence of furan among the products of the reaction of OH with 2,3-DHF. The calculations also indicate that the abstraction reaction, and hence furan formation, may become significant for OH-initiated oxidation of 2,3-DHF at temperatures relevant to combustion.

Photodissociation dynamics of benzoyl chloride at 235 nm: resonance-enhanced multiphoton ionization detection of Cl and HCl.

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.

Photodissociation dynamics of enolic 1,2-cyclohexanedione at 266, 248, and 193 nm: mechanism and nascent state product distribution of OH.

The photodissociation dynamics of 1,2-cyclohexanedione (CHD), which exists in enolic form in gas phase, is studied using pulsed laser photolysis (LP)-laser induced fluorescence (LIF) "pump-and-probe" technique at room temperature. The nascent state distribution of the OH radical, formed after initial photoexcitation of the molecule to it is (π, π*) and Rydberg states, is determined. The initial (π, π*) and Rydberg states are prepared by excitation with the fourth harmonic output of Nd:YAG (266 nm)/KrF (248 nm) and ArF (193 nm) lasers, respectively. The ro-vibrational distribution of the nascent OH photofragment is measured in collision-free conditions using LIF. The OH fragments are formed in the vibrationally cold state at all the above wavelengths of excitation but differ in rotational state distributions. At 266 nm photolysis, the rotational population of OH shows a curvature in Boltzmann plot, which is fairly described by two types of Boltzmann-like distributions characterized by rotational temperatures of 3100 ± 100 and 900 ± 80 K. However, at 248 nm photolysis, the rotational distribution is described by a single rotational temperature of 950 ± 80 K. The spin-orbit and Λ-doublets ratios of OH fragments formed in the dissociation process are also measured. The average translational energy in the center-of-mass coordinate, partitioned into the photofragment pairs of 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 193 nm excitation, respectively. The energy partitioning into various degrees of freedom of products is interpreted with the help of different models, namely, statistical, impulsive, and hybrid models. To understand the nature of the dissociative potential energy surface involved in the OH formation channel, detailed ab initio calculations are performed using configuration interaction-singles (CIS) method. It is proposed that at 266 nm photolysis, the OH fragment is formed from two different excited state structures, one with a strong H bonding, similar to that in the ground state, and another without effective H bonding, whereas, at 248 nm photodissociation, it seems that the OH formation occurs mainly from the excited state, which lacks effective H-bonding. At 193 nm excitation, the initially prepared population in the Rydberg state crosses over to a nearby σ* repulsive state along the C-O bond, from where the dissociation takes place. The exit barrier for the OH dissociation channel is estimated to be 14 kcal/mol. The existence of dynamical constraint due to strong hydrogen bond in the ground state is effectively present in the dissociation process at 266 and somewhat deficient at 248 nm photolysis.

Photodissociation dynamics of halogenated thiophenes at 235 nm: a resonance enhanced multiphoton ionization-time-of-flight (REMPI-TOF) study.

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, ß, 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, ß(ι). The TOF profiles for Cl, Cl*, Br, and Br* are found to be independent of laser polarization; i.e., the ß 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.

Photodissociation dynamics of 3-bromo-1,1,1-trifluoro-2-propanol and 2-(bromomethyl) hexafluoro-2-propanol at 234 nm: resonance-enhanced multiphoton ionization detection of Br (2P(j)).

The photodissociation dynamics of 3-bromo-1,1,1-trifluoro-2-propanol (BTFP) and 2-(bromomethyl) hexafluoro-2-propanol (BMHFP) have been studied at 234 nm, and the C-Br bond dissociation investigated using resonance-enhanced multiphoton ionization coupled with time-of-flight mass spectrometer (REMPI-TOFMS). Br formation is a primary process and occurs on a repulsive surface involving the C-Br bond of BTFP and BMHFP. Polarization dependent time-of-flight profiles were measured, and the translational energy distributions and recoil anisotropy parameters extracted using forward convolution fits. A strong polarization dependence of time-of-flight profiles suggest anisotropic distributions of the Br((2)P(3/2)) and Br((2)P(1/2)) fragments with anisotropy parameter, ß, of respectively 0.5 ± 0.2 and 1.2 ± 0.2 for BTFP, and 0.4 ± 0.1 and 1.0 ± 0.3 for BMHFP. The measured velocity distributions consist of a single velocity component. The average translational energies for the Br((2)P(3/2)) and Br((2)P(1/2)) channels are 9.2 ± 1.0 and 7.4 ± 0.9 kcal/mol for BTFP, and 15.4 ± 1.8 and 15.1 ± 2.0 kcal/mol for BMHFP. The relative quantum yields of Br((2)P(3/2)) and Br((2)P(1/2)), which are 0.70 ± 0.14 and 0.30 ± 0.06 in BTFP and 0.81 ± 0.16 and 0.19 ± 0.04 in BMHFP, indicate that the yield of the former is predominant. The measured anisotropy parameters for the Br((2)P(3/2)) and Br((2)P(1/2)) channels suggest that the former channel has almost equal contributions from both the parallel and the perpendicular transitions, whereas the latter channel has a significant contribution from a parallel transition. Non-adiabatic curve crossing plays an important role in the C-Br bond dissociation of both BTFP and BMHFP. The estimated curve crossing probabilities suggest a greater value in BTFP, which explains a greater observed value of the relative quantum yield of Br((2)P(1/2)) in this case.

Laser-induced UV photodissociation of 2-bromo-2-nitropropane: dynamics of OH and Br formation.

Photoexcitation of 2-bromo-2-nitropropane (BNP) at 248 and 193 nm generates OH, Br, and NO(2) among other products. The OH fragment is detected by laser-induced fluorescence spectroscopy, and its translational and internal state distributions (vibration, rotation, spin-orbit, and Λ-doubling components) are probed. At both 248 and 193 nm, the OH fragment is produced translationally hot with the energy of 10.8 and 17.2 kcal∕mol, respectively. It is produced vibrationally cold (v" = 0) at 248 nm, and excited (v" = 1) at 193 nm with a vibrational temperature of 1870 ± 150 K. It is also generated with rotational excitation, rotational populations of OH(v" = 0) being characterized by a temperature of 550 ± 50 and 925 ± 100 K at 248 and 193 nm excitation of BNP, respectively. The spin-orbit components of OH(X(2)Π) are not in equilibrium on excitation at 193 nm, but the Λ-doublets are almost in equilibrium, implying no preference for its π lobe with respect to the plane of rotation. The NO(2) product is produced electronically excited, as detected by measuring UV-visible fluorescence, at 193 nm and mostly in the ground electronic state at 248 nm. The Br product is detected employing resonance-enhanced multiphoton ionization with time-of-flight mass spectrometer for better understanding of the dynamics of dissociation. The forward convolution analysis of the experimental data has provided translational energy distributions and anisotropy parameters for both Br((2)P(3∕2)) and Br∗((2)P(1∕2)). The average translational energies for the Br and Br∗ channels are 5.0 ± 1.0 and 6.0 ± 1.5 kcal∕mol. No recoil anisotropies were observed for these products. Most plausible mechanisms of OH and Br formation are discussed based on both the experimental and the theoretical results. Results suggest that the electronically excited BNP molecules at 248 and 234 nm relax to the ground state, and subsequently dissociate to produce OH and Br through different channels. The mechanism of OH formation from BNP on excitation at 193 nm is also discussed.

Dynamics of Cl (2Pj) atom formation in the photodissociation of fumaryl chloride (ClCO - CH = CH - COCl) at 235 nm: a resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) study.

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 ((2)P(3/2)) and Cl* ((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, ß, 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, ß(i). The TOF profiles for both Cl and Cl* are found to be independent of laser polarization; i.e., ß 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(E(T)), 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.

Dynamics of N-OH bond dissociation in cyclopentanone and cyclohexanone oxime at 193 nm: laser-induced fluorescence detection of nascent OH (v'', J'').

Cyclohexanone oxime (CHO) and cyclopentanone oxime (CPO) in the vapor phase undergo N-OH bond scission upon excitation at 193 nm to produce OH, which was detected state selectively employing laser-induced fluorescence. The measured energy distribution between fragments for both oximes suggests that in CHO the OH produced is mostly vibrationally cold, with moderate rotational excitation, whereas in CPO the OH fragment is also formed in v'' = 1 (~2%). The rotational population of OH (v'' = 0, J'') from CHO is characterized by a rotational temperature of 1440 ± 80 K, whereas the rotational populations of OH (v'' = 0, J'') and OH (v'' = 1, J'') from CPO are characterized by temperatures of 1360 ± 90 K and 930 ± 170 K, respectively. A high fraction of the available energy is partitioned to the relative translation of the fragments with f(T) values of 0.25 and 0.22 for CHO and CPO, respectively. In the case of CHO, the Λ-doublet states of the nascent OH radical are populated almost equally in lower rotational quantum levels N'', with a preference for Π(+) (A') states for higher N''. However, there is no preference for either of the two spin orbit states Π(3/2) and Π(1/2) of OH. The nascent OH product in CPO is equally distributed in both Λ-doublet states of Π(+) (A') and Π(-) (A'') for all N'', but has a preference for the Π(3/2) spin orbit state. Experimental work in combination with theoretical calculations suggests that both CHO and CPO molecules at 193 nm are excited to the S(2) state, which undergoes nonradiative relaxation to the T(2) state. Subsequently, molecules undergo the N-OH bond dissociation from the T(2) state with an exit barrier to produce OH (v'', J'').

Photodissociation dynamics of phosphorus trichloride (PCl3) at 235 nm using resonance enhanced multiphoton ionization (REMPI) with time-of-flight (TOF) mass spectrometry.

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.

Photodissociation of 3-bromo-1,1,1-trifluoro-2-propanol at 193 nm: laser-induced fluorescence detection of OH(nu'' = 0, J'').

Photodissociation of 3-bromo-1,1,1-trifluoro-2-propanol (BTFP) has been investigated at 193 nm, employing the laser photolysis laser-induced fluorescence technique. The nascent OH product was detected state selectively, and the energy released into translation, rotation, and vibration of the photoproducts has been measured. OH is produced mostly vibrationally cold, with a moderate rotational excitation, which is characterized by a rotational temperature of 640 +/- 140 K. However, an appreciable amount of the available energy of 36.1 kcal mol(-1) is released into translation of OH (15.1 kcal mol(-1)). OH product has no preference for a specific spin-orbit state, Pi(3/2) or Pi(1/2). However, between two Lambda-doublet states, Pi(+) and Pi(-), the OH product has a preference for the former by a factor of 2. A mechanism of OH formation from BTFP on excitation at 193 nm is proposed, which involves first the direct C-Br bond dissociation from a repulsive state (n(Br)sigma*(C-Br)) as a primary process. The primary product, F(3)C-CH(OH)-CH(2), with sufficient internal energy undergoes spontaneous C-OH bond dissociation, through a loose transition state. The formation rate of OH is calculated to be 5.8 x 10(6) s(-1) using Rice-Ramsperger-Kassel-Marcus unimolecular rate theory. Experimental results have been supported by theoretical calculations, and energies of various low-energy dissociation channels of the primary product, F(3)C-CH(OH)-CH(2), have been calculated.

Studies on photodissociation dynamics of butadiene monoxide at 193 nm.

Butadiene monoxide (BMO) undergoes the S(0)-->S(1) transition, involving the excitation of both pi and n electrons to pi(*) orbital, at 193 nm. After relaxing to the ground electronic state via internal conversion, BMO molecules undergo intramolecular rearrangement and subsequently dissociate to form unexpected OH radicals, which were detected state selectively by laser-induced fluorescence technique, and the energy state distribution was measured. OH is produced vibrationally cold, OH(nu(")=0,J(")), with the rotational population characterized by a rotational temperature of 456+/-70 K. The major portion (approximately 60%) of the available energy is partitioned into internal degrees of the photofragments, namely, vibration and rotation. A considerable portion (25%-35%) also goes to the relative translation of the products. The Lambda doublet and spin-orbit ratios of OH were measured to be nearly unity, implying statistical distribution of these states and, hence, no preference for any of the Lambda doublet (Lambda+ and Lambda-) and spin-orbit (Pi(3/2) and Pi(1/2)) states. Formation time of the nascent OH radical was measured to be <100 ns. Different products, such as crotonaldehyde and methyl vinyl ketone, were detected by gas chromatography as stable products of photodissociation. A reaction mechanism for the formation of all these photoproducts, transient and stable, is proposed. The multiple pathways by which these products can be formed have been theoretically optimized, and energies have been calculated. Absorption cross section of BMO at 193 nm was measured, and quantum yield of OH generation channel was also determined.

Dynamics of OH radical generation in laser-induced photodissociation of tetrahydropyran at 193 nm.

Tetrahydropyran (THP) undergoes photodissociation on excitation with ArF laser at 193 nm, generating OH radical as one of the transient photoproducts. Laser-induced fluorescence technique is used to detect the nascent OH radical and measure its energy state distribution. The OH radical is formed mostly in the ground vibrational level (v"=0), with low rotational excitation. The rotational distribution of OH (v"=0,J) is characterized by a temperature of 433+/-31 K, corresponding to a rotational energy of 0.86+/-0.06 kcalmol. Two Lambda-doublet levels, 2Pi+(A') and 2Pi-(A"), and the two spin-orbit states, the 2Pi(3/2) and 2Pi(1/2), of OH are populated statistically for all rotational levels. The relative translational energy associated with the photoproducts in the OH channel is calculated to be 21.9+/-3.2 kcal mol(-1), from the Doppler-broadened linewidth, giving an ft value of approximately 43%, and most of the remaining 57% of the available energy is distributed in the internal modes of the other photofragment, C5H9. The observed distribution of the available energy is explained well, using a hybrid model of energy partitioning, with an exit barrier of 40 kcal mol(-1). The potential-energy surface of the reaction channel was mapped by ab initio molecular-orbital calculations. Based on experimental and theoretical results, a mechanism for OH formation is proposed. Electronically excited THP relaxes to the ground electronic state, and from there, a sequence of reactions takes place, generating OH. The proposed mechanism first involves C-O bond scission, followed by a 1,3 H atom migration to O atom, and finally, the C-OH bond cleavage giving OH.

Photodissociation dynamics of acetoxime in gas phase.

The dynamics of photodissociation of acetoxime at 193 nm, leading to the formation of (CH3)2C=N and OH fragments, has been investigated. The nascent OH radicals, which are both rotationally and vibrationally excited, were probed by laser photolysis-laser induced fluorescence technique. OH fragments in both v" = 1 and v" = 0 vibrational states were detected with a ratio of population in the higher to lower level of 0.07+/-0.01. The rotational temperatures of v" = 0 and 1 levels of OH radicals are 2650+/-150 K and 1290+/-20 K, respectively. More than 30% of the available energy, i.e., 115+/-21 kJ mol(-1) is partitioned into the relative translational energy of the fragments. The results of excited electronic state and transition state calculations at the configuration interaction with single electronic excitation level suggest that the dissociation takes place with an exit barrier of approximately 126 kJ mol(-1) at the triplet state (T2) potential energy surface, formed by internal conversions/intersystem crossing from the initially populated S2 state. Using the calculated transition state geometry and its energy, the observed energy distribution pattern can be reproduced by the hybrid model within experimental uncertainties. The presence of an exit barrier is further supported by the observation of N-OH dissociation upon 248 nm excitation, where the relative translational energy of the fragments is found to be approximately 96 kJ mol(-1). The photodissociation dynamics of acetoxime is compared with C-OH dissociation in enols and carboxylic acid and N-OH dissociation in nitrous acid. The observed emission (lambda(max)=430 nm) and the N-OH dissociation dynamics indicate crossing of the initially populated state to an emissive state of acetoxime, which is different from the dissociative state.

Detection of OH radical in laser induced photodissociation of tetrahydrofuran at 193 nm.

On excitation at 193 nm, tetrahydrofuran (THF) generates OH as one of the photodissociation products. The nascent energy state distribution of the OH radical was measured employing laser induced fluorescence technique. It is observed that the OH radical is formed mostly in the ground vibrational level, with low rotational excitation (approximately 3%). The rotational distribution of OH (v"=0,J) is characterized by rotational temperature of 1250+/-140 K. Two spin-orbit states, 2Pi3/2 and 2Pi1/2 of OH are populated statistically. But, there is a preferential population in Lambda doublet levels. For all rotational numbers, the 2Pi+(A') levels are preferred to the 2Pi-(A") levels. The relative translational energy associated with the photoproducts in the OH channel is calculated to be 17.4+/-2.2 kcal mol-1, giving an fT value of approximately 36%, and the remaining 61% of the available energy is distributed in the internal modes of the other photofragment, i.e., C4H7. The observed distribution of the available energy agrees well with a hybrid model of energy partitioning, predicting an exit barrier of approximately 16 kcal mol-1. Based on both ab initio molecular orbital calculations and experimental results, a plausible mechanism for OH formation is proposed. The mechanism involves three steps, the C-O bond cleavage of the ring, H atom migration to the O atom, and the C-OH bond scission, in sequence, to generate OH from the ground electronic state of THF. Besides this high energy reaction channel, other photodissociation channels of THF have been identified by detecting the stable products, using Fourier transform infrared and gas chromatography.

Dissociation dynamics of thiolactic acid at 193 nm: detection of the nascent OH product by laser-induced fluorescence.

Electronically excited thiolactic acid (2-mercaptopropionic acid), H(3)C-CH(SH)-COOH, undergoes the C-OH bond cleavage on excitation to the S(2) state at 193 nm, generating the primary product OH (v,J), which is detected by laser-induced fluorescence technique in a collisionless condition of flow system. The partitioning of the available energy between vibrational, rotational, and translational degrees of freedom of nascent photofragments is obtained from relative intensities of ro-vibronic lines in laser-induced fluorescence spectrum of OH, and their Doppler profiles. The rotational population of OH (v(")=0) is characterized by rotational temperature of 408+/-25 K. OH is produced in a vibrationally cold state, i.e., mostly in v(")=0. The average translational energy of OH (v(")=0,J(")) is found to be 21.5+/-2.0 kcal/mol, which implies 25.6 kcal/mol of energy in relative translation of photoproducts corresponding to the f(t) value of approximately 0.6. The observed high translational energy is due to the presence of a barrier in the exit channel, implying that the C-OH bond scission takes place on an electronically excited potential energy surface. The observed partitioning of the available energy between various degrees of the photofragments is theoretically modeled, and the hybrid model, with 26.0 kcal/mol of barrier in the exit channel, is found to explain the measured data quite well. The experimental results are also supported with ab initio molecular orbital calculations for both the ground and the excited electronic states. Time-dependent density functional theory is used to understand the nature of various electronic transitions connecting the lower excited states. Potential energy curves as a function of the C-OH bond length of thiolactic acid suggest distinct exit barriers in the S(1), T(1), and T(2) states. But, we could locate the transition state structure for OH formation in the S(1) state alone. Thus, although thiolactic acid is excited to the S(2) state at 193 nm, it undergoes internal conversion to S(1) where it dissociates to yield OH. In addition to the OH channel from excited electronic states, we studied theoretically all probable dissociation channels occurring on the ground electronic state of thiolactic acid.