Formation of hydroxyl radical from the photolysis of salicylic acid

Photodissociation dynamics of salicylic acid (SA) in the gas phase at different photolysis wavelengths (266, 315-317 nm) is investigated by probing the nascent OH photoproduct employing the single-photon laser-induced fluorescence (LIF) technique. At all the photolysis wavelengths it is found that the nascent OH radicals are produced mostly in a vibrationally ground state (υ' = 0) and have similar rotational state distributions. The two spin-orbit and Λ-doublet states of the OH fragment formed in the dissociation are measured to have a nonstatistical distribution at each photolysis wavelength. The LIF signal of the OH could be observed upon photolysis at 317 nm but not at 317.5 nm. The threshold of OH formation from SA photodissociation is estimated to be 98.2 ± 0.9 kcal/mol. The effect of the phenolic OH group on the dissociation of SA is discussed.     

单光子LIF方法研究亚硝基苯266 nm光解动力学

用266 nm激光解离亚硝基苯（C6H5NO) 产生光解碎片NO,并利用单光子激光诱导荧光(LIF)技术（X2Πν″=0→A2Σ+ν′=0）测得初生态光解产物NO的振转光谱。根据计算所得的模拟光谱对光解碎片NO（X,ν″＝0)的转动量子数J″进行了归属,得到量子数最大到J″＝50.5的各转动能级的相对布居,这表明光解碎片NO具有较高的转动激发。提出了C6H5NO在266 nm下可能的光解机理。     

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.     

NO2 在第二电子吸收带的光解

运用单光子激光诱导荧光方法,研究了NO2分子在第二吸收带的光解反应动力学.首次报道了NO2(B2B2态)光解初生态产物NO自由基的v″=1,2的转动分布.发现了NO自由基v″=1的明显双模式分布.进而提出了可能有两种竞争机理控制该反应.     

Molecular photofragment orientation in the photodissociation of H2O2 at 193 nm and 248 nm

Angular momentum orientation has been observed in the OH(X(2)Π, v = 0) fragments generated by circularly polarized photodissociation of H(2)O(2) at 193 nm and 248 nm. The magnitude and sign of the orientation are strongly dependent on the OH(X) photofragment rotational state. In addition to conventional laser induced fluorescence methods, Zeeman quantum beat spectroscopy has also been used as a complementary tool to probe the angular momentum orientation parameters. The measured orientation at 193 nm is attributed solely to photodissociation via the ?(1)A state, even though at this wavelength H(2)O(2) is excited near equally to both the ?(1)A and B(1)B electronic states. This observation is confirmed by measurements of the photofragment orientation at 248 nm, where access to the ?(1)A state dominates. Several possible mechanisms are discussed to explain the observed photofragment orientation, and a simple physical model is developed, which includes the effects of the polarization of the parent molecular rotation upon absorption of circularly polarized light. Good agreement between the experimental and simulation results is obtained, lending support to the validity of the model. It is proposed that photofragment orientation arises mainly from the coupling of the parent rotational angular momentum with that induced during photofragmentation.     

Photodissociation dynamics of H2O: effect of unstable resonances on the B̃(1)A1 electronic state

We report a tunable vacuum ultraviolet photodissociation study of H(2)O from different unstable resonances in the B?(1)A(1) electronic state, using the H-atom Rydberg tagging technique. The quantum state resolved OH product translational energy distributions and angular distributions have been measured. Experimental results illustrate, for the first time, that excitation to the different unstable resonances has very different effect on the OH(X) and OH(A) product channels. The OH(X) product rotational distributions vary only slightly, while the OH(A) product rotational distributions and state-resolved angular distributions change dramatically as the photolysis energy increases. Effect of parent rotational excitation on the OH(A) product has also been observed. Through careful simulations to the experimental spectra, OH(A)∕OH(X) branching ratios have been determined at five photolysis wavelengths. The general agreement between theory and experiment in the branching ratios is good. The branching ratios for the OH(A) product from different parent rotational levels are close to the nuclear spin-statistics value, which is also consistent with the extremely low rotational temperature of the H(2)O beam in the current experiment.     

State-to-state correlated study of CD3I photodissociation at 266 and 304 nm

High-resolution photofragment translational spectroscopy is used in this work to measure the translational and internal energy distributions in the CD3 and iodine fragments produced from the photodissociation of CD3I at 266 and 304 nm. Channel selected detection, via resonantly enhanced multiphoton ionization, combined with one-dimensional core sampling provides detailed information about vibrational state distributions of the CD3 fragments. The vibrational state distributions of CD3 fragments in the I*(2P12) channel have a propensity of nu2 ' umbrella bending mode with a maximum at nu2 ' = 1 for 266 nm photodissociation. For I*(2P12) channel at 304 nm photodissociation, vibrational state distributions of CD3 fragment have a maximum in the vibrational ground state. For the I(2P32) channel (1Q1 <-- 3Q(0+)), nu2 ' umbrella bending vibrational distribution is measured as the predominant vibrational mode but has a much broader distribution when compared to that of the I* channel. The vibrational state distributions of the CD3 fragment produced from the perpendicular transition, i.e., 3Q1, which was determined at 304 nm photodissociation, has a maximum at nu2 ' = 1. The curve crossing possibility between the 1Q1 and 3Q(0+) adiabatic potentials is determined as 0.19 for 266 and 0.85 for 304 nm. The trend in reaction dynamics in 266 and 304 nm photodissociation of CD3I is compared with theoretical calculations. A bond dissociation energy D0(C-I) = 56.60+/-0.5 kcal/mol was derived by applying laws of energy conservation.     

Correlated product distributions from ketene dissociation measured by dc sliced ion imaging

Speed distributions of spectroscopically selected CO photoproducts of 308 nm ketene photodissociation have been measured by dc sliced ion imaging. Structured speed distributions are observed that match the clumps and gaps in the singlet CH2 rotational density of states. The effects of finite time gates in sliced ion imaging are important for the accurate treatment of quasicontinuous velocity distributions extending into the thickly sliced and fully projected regime, and an inversion algorithm has been implemented for the special case of isotropic fragmentation. With accurate velocity calibration and careful treatment of the velocity resolution, the new method allows us to characterize the coincident rotational state distribution of CH2 states as a smoothly varying deviation from an unbiased phase space theory (PST) limit, similar to a linear-surprisal analysis. High-energy rotational states of CH2 are underrepresented compared to PST in coincidence with all detected CO rotational states. There is no evidence for suppression of the fastest channels, as had been reported in two previous studies of this system by other techniques. The relative contributions of ground and first vibrationally excited singlet CH2 states in coincidence with selected rotational states of CO (upsilon=0) are well resolved and in remarkably good agreement with PST, despite large deviations from the PST rotational distributions in the CH2 fragments. At 308 nm, the singlet CH2 (upsilon2=0) and (upsilon2=1) channels are 2350 and 1000 cm(-1) above their respective thresholds. The observed vibrational branching is consistent with saturation at increasing energies of the energy-dependent suppression of rates with respect to the PST limit, attributed to a tightening variational transition state.     

Vector and scalar correlations in the photodissociation of NCCN

The CN photofragments from the photodissociation of NCCN at 193 nm have been measured by high-resolution transient absorption spectroscopy. Doppler-broadened profiles of isolated rotational lines in the 2-0 and 3–1 vibrational bands of the CN A---X transition were observed under collisionless conditions with a tunable, single-frequency Ti:sapphire ring laser. Analysis of the Dopple profiles reveals a vector correlation between the translation and rotation of CN photoproducts, with the angular momentum of the high rotational states increasingly perpendicular to the recoil velocity. After correction for vector correlations, the laboratory-frame scalar speed distribution of state-selected photoproducts can be determined. The mean squared laboratory velocity is directly related to the average internal energy of coincident CN fragments. The wings of the Doppler profiles indicate that the available energy for a pair of ground state CN photoproducts following 193 nm dissociation of NCCN at 295 K is 5300±150 cm⁻¹, which includes the average vibrational energy of the parent molecules selected by the photolysis laser. Phase space theory with an optimized available energy of 5300 cm⁻¹ produces laboratory speed distributions that are in qualitatively reasonable agreement with the kinetic energy measurements, but overestimate the total internal energy of the photofragments. The measurements are good enough to warrant comparison with more sophisticated models of unimolecular decomposition.     

Quantum state-selected photodissociation dynamics of H2O: two-photon dissociation via the C̃ electronic state

The photodissociation dynamics of H(2)O via the C? state by two-photon excitation has been investigated using the H atom Rydberg tagging time-of-flight technique. The rotational resolved action spectrum of the C?←X? transition band has been measured. The line widths show a pronounced dependence on the parent rotational excitation in the C? state. The quantum state resolved OH product translational energy distributions and angular distributions have also been obtained. By carefully simulating these distributions, quantum state distributions of the OH product as well as the state-resolved angular anisotropy parameters were determined. The experimental results confirm the variation of two competitive predissociation pathways. A heterogeneous predissociation channel is mediated by rotational coupling to the B??(1)A(1) state associated with the a-axis (k(a)(')), and a homogeneous pathway arises from purely electronic coupling to the A??(1)B(1) state. We have also obtained the branching ratios of the OH(X) and OH(A) products, and related these to the C?→A? and C?→B? pathways. The branching ratios display a strong k(a)(') dependence.     

Ion-Velocity Map Imaging Study of Photodissociation Dynamics of Acetaldehyde

The photodissociation dynamics of acetaldehyde in the radical channel CH3＋HCO has been reinvestigated using time-sliced velocity map imaging technique in the photolysis wavelength range of 275-321 nm. The CH3 fragments have been probed via （2＋1） resonance-enhanced multiphoton ionization. Images are measured for CH3 formed in the ground and excited states （v2=0 and 1） of the umbrella vibrational mode. For acetaldehyde dissociation on T1 state after intersystem crossing from S1 state, the products are formed with high translational energy release and low internal excitation. The rotational and vibrational energy of both fragments increases with increasing photodissociation energy. The triplet barrier height is estimated at 3.8814-0.006 eV above the ground state of acetaldehyde.     

Photodissociation of H2O2 at 248 nm: translational anisotropy and oh product state distributions

Ground-state OH(X²Π) fragments, from the photodissociation of H₂O₂ at 248 nm, have been probed by laser-induced fluorescence. Nascent product rotational distributions and polarisation-dependent Doppler lineshapes are reported. The high translational anisotropy (β = −1 at low N″) indicates a prompt repulsion along the O-O axis. Polarisation-dependent Doppler lineshapes are interpreted as a correlation between the fragment recoil velocity v and angular momentum J_OH, the product rotation being aligned along v. Product rotation is generated by an axial torsion about the O-O axis, consistent with a change in dihedral angle in the upper state.     

Velocity-map imaging study of the photodissociation of acetaldehyde

Velocity-map imaging studies are reported for the photodissociation of acetaldehyde over a range of photolysis wavelengths (317.5-282.5 nm). Images are obtained for both the HCO and CH3 fragments. The mean rotational energy of both fragments increases with photodissociation energy, with a lesser degree of excitation in the CH3 fragment. The CH3 images demonstrate that the CH3 fragments are rotationally aligned with respect to the recoil direction and this is interpreted, and well modeled, on the basis of a propensity for forming CH3 fragments with M approximately K, where M is the projection of the rotational angular momentum along the recoil direction. The origin of the CH3 rotation is conserved motion from the torsional and methyl-rocking modes of the parent molecule. Nonstatistical vibrational distributions for the CH3 fragment are obtained at higher energies.     

Photodissociation dynamics of phenol

The photodissociation of phenol at 193 and 248 nm was studied using multimass ion-imaging techniques and step-scan time-resolved Fourier-transform spectroscopy. The major dissociation channels at 193 nm include cleavage of the OH bond, elimination of CO, and elimination of H(2)O. Only the former two channels are observed at 248 nm. The translational energy distribution shows that H-atom elimination occurs in both the electronically excited and ground states, but elimination of CO or H(2)O occurs in the electronic ground state. Rotationally resolved emission spectra of CO (1 相似文献   

Photofragment Imaging of HNCO Decomposition at 210 nm: the Primary NH(a1￠)+CO(X1§+) Channel

The photodissociation of isocyanic acid (HNCO) on the ˉrst excited singlet state following the excitation at 210 nm was investigated with an ion velocity slice imaging technique by probing the CO fragment. It was found from the (2+1) resonance-enhanced multi-photon ionization (REMPI) spectrum that the CO fragments are rotationally hot with population up to Jmax=50. The velocity imagings of the CO fragments at JCO=30 and 35 indicate that formation of NH(a1￠)+CO(X1§+, v=0) is the predominant dissociation channel at 210 nm. From analysis of the CO fragment translational energy distributions, the NH(a1￠) fragment was observed to be rotationally cold, about half of the available energy was partitioned into the translational motion of fragments after dissociation, and the NH(a1￠)+CO(X1§+) dissociation threshold was determined at 42738§30 cm?1. From analysis of the CO fragment angular distributions, the dissociationanisotropy parameter ˉ was found to be negative, and increasing with the rotational quantum number of the NH fragment, i.e., from ?0.75 at JNH=2-4 to ?0.17 at JNH=11. Impulsive direct and vertical dissociation process of HNCO on the singlet state at 210 nm was conˉrmed experimentally. A classical impact dissociation model was employed to explain the dependence of the ˉ value on the rotational excitation of the NH fragment.     

Photodissociation Dynamics of Nitromethane and Nitroethane at 266 nm

Measurements of the nascent OH product from photodissociation of gaseous nitromethane and nitroethane at 266 nm were performed using the single-photon laser induced °uorescence technique. The OH fragment is found to be vibrationally cold for both systems. The rotational state distribution of nitromethane are Boltzmann, with rotational temperature of Trot=2045§150 and 1923§150 K for both 2|3=2 and 2|1=2 states, respectively. For nitroethane, the rotational state distribution shows none Boltzmann and cannot be well characterized by a rotational temperature, which indicates the di?erent mechanisms in producing OH radicals from photodissociation of nitromethane and nitroethane. The rotational energy is calculated as 14.36§0.8 and 4.98§0.8 kJ/mol for nitromethane and nitroethane, respectively. A preferential population of the low spin-orbit component (2|3=2) is observed for both nitromethane and nitroethane. The dominant population of |+ state in two ¤-doublet states is also observed for both nitromethane and nitroethane,which indicates that the unpaired ? lobe of the OH fragment is parallel to the plane of rotation.     

Molecular elimination of methyl formate in photolysis at 234 nm: roaming vs. transition state-type mechanism

Ion imaging coupled with (2 + 1) resonance-enhanced multiphoton ionization (REMPI) technique is employed to probe CO(v″ = 0) fragments at different rotational levels following photodissociation of methyl formate (HCOOCH(3)) at 234 nm. When the rotational level, J″(CO), is larger than 24, only a broad translational energy distribution extending beyond 70 kcal mol(-1) with an average energy of about 23 kcal mol(-1) appears. The dissociation process is initiated on the energetic ground state HCOOCH(3) that surpasses a tight transition state along the reaction coordinate prior to breaking into CO + CH(3)OH. This molecular dissociation pathway accounts for the CO fragment with larger rotational energy and large translational energy. As J″(CO) decreases, a bimodal distribution arises with one broad component and the other sharp component carrying the average energy of only 1-2 kcal mol(-1). The branching ratio of the sharp component increases with a decrease of J″(CO); (7.3 ± 0.6)% is reached as the image is probed at J″(CO) = 10. The production of a sharp component is ascribed to a roaming mechanism that has the following features: a small total translational energy, a low rotational energy partitioning in CO, but a large internal energy in the CH(3)OH co-product. The internal energy deposition in the fragments shows distinct difference from those via the conventional transition state.     

Dynamics of the gas-liquid interfacial reaction of O(1D) with a liquid hydrocarbon

The dynamics of the gas-liquid interfacial reaction of the first electronically excited state of the oxygen atom, O((1)D), with the surface of a liquid hydrocarbon, squalane (C(30)H(62); 2,6,10,15,19,23-hexamethyltetracosane) has been studied experimentally. Translationally hot O((1)D) atoms were generated by 193 nm photolysis of a low pressure (nominally 1 mTorr) of N(2)O a short distance (mean = 6 mm) above a continually refreshed liquid squalane surface. Nascent OH (X(2)Π, v' = 0) reaction products were detected by laser-induced fluorescence (LIF) on the OH A(2)Σ(+)-X(2)Π (1,0) band at the same distance above the surface. The speed distribution of the recoiling OH was characterized by measuring the appearance profiles as a function of photolysis-probe delay for selected rotational levels, N'. The rotational (and, partially, fine-structure) state distributions were also measured by recording LIF excitation spectra at selected photolysis-probe delays. The OH v' = 0 rotational distribution is bimodal and can be empirically decomposed into near thermal (~300 K) and much hotter (~6000 K) Boltzmann-temperature components. There is a strong positive correlation between rotational excitation and translation energy. However, the colder rotational component still represents a significant fraction (~30%) of the fastest products, which have substantially superthermal speeds. We estimate an approximate upper limit of 3% for the quantum yield of OH per O((1)D) atom that collides with the surface. By comparison with established mechanisms for the corresponding reactions in the gas phase, we conclude that the rotationally and translationally hot products are formed via a nonstatistical insertion mechanism. The rotationally cold but translationally hot component is most likely produced by direct abstraction. Secondary collisions at the liquid surface of products of either of the previous two mechanisms are most likely responsible for the rotationally and translationally cold products. We do not think it likely, a priori, that they could be produced in the observed significant yield via a statistical insertion mechanism for a molecule the size of squalane embedded in a surrounding liquid surface.     

Monte Carlo simulation of O(1D) + H2 and O(1D) + HCl — rotational excitation of product OH radicals

Experimental studies with molecular beam and LIF techniques have independently shown that the reaction O(¹D) + H₂ → OH + H passes through a long-lived complex and gives products with small translational and large rotational excitation. We have previously published a statistical algorithm, based on ordinary RRKM theory with angular momentum restrictions included, which was designed to simulate molecular beam experiments. It has now been modified and applied to simulate the experimental rotational OH distributions from O(¹D)+H₂, measured by Luntz et al. The present study also includes simulation of similar results by Luntz for O(¹D) + HCI → OH + Cl. The purely statistical algorithm successfully simulates the apparently non-statistical experimental rotational distributions. For these reactions the total angular momentum conservation. which is applied at the transition state, proves to be decisive for the product energy distributions.     

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').