Kinetics of the reactions of the CHBr2 and CHBr2O2 radicals with O2 and NO

When bromoform (CHBr3) is photolyzed at 266 or 303 nm in the presence of O2 and NO, the formation of secondary Br atoms is observed. By following the rate of growth of this secondary Br atom signal as a function of conditions, rate constants have been determined for the reactions CHBr2 + O2, CHBr2 + NO (both pressure-dependent), and CHBr2O2 + NO (k(2a) = (1.74 +/- 0.16) x 10(-11) cm3 molecule(-1) s(-1) at 23 degrees C). By measuring the amplitude of the secondary Br signal compared to the primary Br formed in the initial photolysis, it is established that the CHBr2O radical spontaneously decomposes to form CHBrO + Br at least 90%, and probably 100%, of the time, in agreement with previous work and with recent ab initio calculations. A survey of four other polybrominated methanes, CH2Br2, CHClBr2, CF2Br2, and CBr4, shows that they all generate secondary Br atoms when photolyzed at 266 nm in the presence of O2 and NO, suggesting that their reaction sequences are similar to that of bromoform.     

Photodissociation of polycrystalline and amorphous water ice films at 157 and 193 nm

The photodissociation dynamics of amorphous solid water (ASW) films and polycrystalline ice (PCI) films at a substrate temperature of 100 K have been investigated by analyzing the time-of-flight (TOF) mass spectra of photofragment hydrogen atoms at 157 and 193 nm. For PCI films, the TOF spectrum recorded at 157 nm could be characterized by a combination of three different (fast, medium, and slow) Maxwell-Boltzmann energy distributions, while that measured at 193 nm can be fitted in terms of solely a fast component. For ASW films, the TOF spectra measured at 157 and 193 nm were both dominated by the slow component, indicating that the photofragment H atoms are accommodated to the substrate temperature by collisions. H atom formation at 193 nm is attributed to the photodissociation of water species on the ice surface, while at 157 nm it is ascribable to a mixture of surface and bulk photodissociations. Atmospheric implications in the high latitude mesopause region of the Earth are discussed.     

State-resolved dynamics of the CH(A2Delta) channels from single and multiple photon dissociation of bromoform in the 10-20 eV energy range

Single photon dissociation of bromoform using synchrotron radiation has been investigated by Fourier transform visible fluorescence spectroscopy (FTVIS). The photodissociation of bromoform in the 12-18 eV energy range results in several products, among which are the CH(A2Delta) and CH(B2Sigma) radicals. Vibrational and rotational state distributions of the CH(A2Delta) are determined from their fluorescence spectra. From the threshold photon energy above which emission from the CH(A2Delta) radicals is observed, the most likely process leading to CH(A) formation is CHBr3 --> CH + 3Br rather than CHBr3 --> CH + Br + Br2. The rotational Boltzmann temperatures in the CH(A --> X) emission spectra for v' = 0 and v' = 1 range between 1570 and 3650 K, depending on the excitation photon energy. From the high rotational excitation, the results suggest that the mechanism for the loss of three bromine atoms is most likely sequential. A small negative emission anisotropy of the CH(A) radicals [(Ipar - Iper)/(Ipar + 2Iper) = -0.024 +/- 0.005] is constant across the action spectrum; a small net absorption dipole of CHBr3 in the vacuum ultraviolet is parallel to the 3-fold symmetry axis of the CHBr3 molecule. The state distributions of the CH(A2Delta) radicals from multiphoton dissociation of bromoform using the 266 nm output (three photons) of a femtosecond laser (Boltzmann temperatures: T(v'=0)(rot)= 4250 +/- 300 K; T(v'=1)(rot)= 3100 +/- 550 K) are compared to those from the single photon dissociation results (Boltzmann temperatures: T(v'=0)(rot)= 3650 +/- 150 K; T(v'=1)(rot)= 2400 +/- 200 K) at the same total excitation energy under collision free conditions. The analysis of the CH(A) rotational populations shows hotter rotational populations for the femtosecond experiment, also suggesting sequential dissociation of the bromoform in the femtosecond experiment. The duration of the femtosecond laser pulse is approximately 180 fs, setting a limit on the time scales for the multiple dissociations.     

Br2 molecular elimination in 248 nm photolysis of CHBr2Cl by using cavity ring-down absorption spectroscopy

Elimination of molecular bromine is probed in the B (3)Pi(ou) (+)<--X (1)Sigma(g) (+) transition following photodissociation of CHBr(2)Cl at 248 nm by using cavity ring-down absorption spectroscopy. The quantum yield for the Br(2) elimination reaction is determined to be 0.05+/-0.03. The nascent vibrational population ratio of Br(2)(v=1)Br(2)(v=0) is obtained to be 0.5+/-0.2. A supersonic beam of CHBr(2)Cl is similarly photofragmented and the resulting Br atoms are monitored with a velocity map ion-imaging detection, yielding spatial anisotropy parameters of 1.5 and 1.1 with photolyzing wavelengths of 234 and 267 nm, respectively. The results justify that the excited state promoted by 248 nm should have an A(") symmetry. Nevertheless, when CHBr(2)Cl is prepared in a supersonic molecular beam under a cold temperature, photofragmentation gives no Br(2) detectable in a time-of-flight mass spectrometer. A plausible pathway via internal conversion is proposed with the aid of ab initio potential energy calculations. Temperature dependence measurements lend support to the proposed pathway. The production rates of Br(2) between CHBr(2)Cl and CH(2)Br(2) are also compared to examine the chlorine-substituted effect.     

Photodissociation dynamics of CBr4 at 267 nm by means of ion velocity imaging

The photodissociation dynamics of CBr4 at 267 nm has been studied using time of flight (TOF) mass spectrometry and ion velocity imaging techniques. The photochemical products are detected with resonance enhanced multiphoton ionization (REMPI) as well as single-photon vacuum ultraviolet ionization at 118 nm. REMPI at 266.65 and 266.71 nm was used to detect the ground Br(2P32) and spin-orbit excited Br(2P12) atoms, respectively. The translational energy and angular distributions are consistent with direct dissociation from an excited triplet state and indirect dissociation from high vibrational levels on the singlet ground state surface. Br2+ ions are also observed in the TOF spectra with a focused 267 nm laser. The counter fragment, CBr2+, is observed when this photolysis laser is unfocused, and photons at 118 nm are used to ionize the radical products. The translational energy distributions of the CBr2+ and Br2+ products can be momentum matched, which indicates that molecular Br2 elimination is one of the primary dissociation channels.     

Vacuum ultraviolet photodissociation and surface morphology change of water ice films dosed with hydrogen chloride

Time-of-flight (TOF) spectra of photofragment H atoms from the photodissociation of water ice films at 193 nm were measured for amorphous and polycrystalline water ice films with and without dosing of hydrogen chloride at 100-145 K. The TOF spectrum is sensitive to the surface morphology of the water ice film because the origin of the H atom is the photodissociation of dimerlike water molecules attached to the ice film surfaces. Adsorption of HCl on a polycrystalline ice film was found to induce formation of disorder regions on the ice film surface at 100-140 K, while the microstructure of the ice surface stayed of polycrystalline at 145 K with adsorption of HCl. The TOF spectra of photofragment Cl atoms from the 157 nm photodissociation of neutral HCl adsorbed on water ice films at 100-140 K were measured. These results suggest partial dissolution of HCl on the ice film surface at 100-140 K.     

二溴代烷烃的紫外光解动力学研究

利用飞行时间质谱装置研究了234和267nm激光作用下二溴甲烷、二溴乙烷、二溴丙烷和二溴丁烷分子的光解离过程．研究表明二溴代烷烃分子在紫外激光的作用下主要是断裂C—Br键解离出一个Br原子,并且存在两种可能的布居：基态Br(^2P3/2^0)和激发态Br^*(^2P1／2^0)．通过共振增强多光子电离技术探测两种光解产物布居的分支比．对比得到了分子构型对称性不同的二溴代烷烃的分支比,提出了两种假设的光解离模型．     

Adsorption, desorption, and diffusion of nitrogen in a model nanoporous material. I. Surface limited desorption kinetics in amorphous solid water

The adsorption and desorption kinetics of N2 on porous amorphous solid water (ASW) films were studied using molecular beam techniques, temperature programed desorption (TPD), and reflection-absorption infrared spectroscopy. The ASW films were grown on Pt(111) at 23 K by ballistic deposition from a collimated H2O beam at various incident angles to control the film porosity. The experimental results show that the N2 condensation coefficient is essentially unity until near saturation, independent of the ASW film thickness indicating that N2 transport within the porous films is rapid. The TPD results show that the desorption of a fixed dose of N2 shifts to higher temperature with ASW film thickness. Kinetic analysis of the TPD spectra shows that a film thickness rescaling of the coverage-dependent activation energy curve results in a single master curve. Simulation of the TPD spectra using this master curve results in a quantitative fit to the experiments over a wide range of ASW thicknesses (up to 1000 layers, approximately 0.5 microm). The success of the rescaling model indicates that N2 transport within the porous film is rapid enough to maintain a uniform distribution throughout the film on a time scale faster than desorption.     

Direct observation of an isopolyhalomethane O-H insertion reaction with water: picosecond time-resolved resonance Raman (ps-TR3) study of the isobromoform reaction with water to produce a CHBr2OH product

Picosecond time-resolved resonance Raman (ps-TR3) spectroscopy was used to obtain the first definitive spectroscopic observation of an isopolyhalomethane O-H insertion reaction with water. The ps-TR3 spectra show that isobromoform is produced within several picoseconds after photolysis of CHBr3 and then reacts on the hundreds of picosecond time scale with water to produce a CHBr2OH reaction product. Photolysis of low concentrations of bromoform in aqueous solution resulted in noticeable formation of HBr strong acid. Ab initio calculations show that isobromoform can react with water to produce a CHBr2(OH) O-H insertion reaction product and a HBr leaving group. This is consistent with both the ps-TR3 experiments that observe the reaction of isobromoform with water to form a CHBr2(OH) product and photolysis experiments that show HBr acid formation. We briefly discuss the implications of these results for the phase dependent behavior of polyhalomethane photochemistry in the gas phase versus water solvated environments.     

Crystallization kinetics and excess free energy of H2O and D2O nanoscale films of amorphous solid water

Temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) are used to investigate the crystallization kinetics and measure the excess free energy of metastable amorphous solid water films (ASW) of H(2)O and D(2)O grown using molecular beams. The desorption rates from the amorphous and crystalline phases of ASW are distinct, and as such, crystallization manifests can be observed in the TPD spectrum. The crystallization kinetics were studied by varying the TPD heating rate from 0.001 to 3 K/s. A coupled desorption-crystallization kinetic model accurately simulates the desorption spectra and accurately predicts the observed temperature shifts in the crystallization. Isothermal crystallization studies using RAIRS are in agreement with the TPD results. Furthermore, highly sensitive measurements of the desorption rates were used to determine the excess free energy of ASW near 150 K. The excess entropy obtained from these data is consistent with there being a thermodynamic continuity between ASW and supercooled liquid water.     

Surface Photocatalysis-TPD Spectrometer for Photochemical Kinetics

A surface photocatalysis-TPD apparatus devoted to studying kinetics and mechanism of pho-tocatalytic processes with various signal crystal surfaces has been constructed. Extremely high vacuum ( 0.2 nPa) in the ionization region is obtained by using multiple ultrahigh vacuum pumps. Compared with similar instruments built previously by others, the H₂, CH₄ background in the ionization region can be reduced by about two orders of magnitude, and other residual gases in the ionization region can be reduced by about an order of magnitude. Therefore, the signal-to-noise ratio for the temperature programmed desorption (TPD) and time of flight (TOF) spectra is substantially enhanced, making experimental studies of pho-tocatalytic processes on surfaces much easier. In this work, we describe the new apparatus in detail and present some preliminary studies on the photo-induced oxygen vacancy defects on TiO₂(110) at 266 nm by using the TPD and TOF methods. Preliminary results suggest that the apparatus is a powerful tool for studying kinetics and mechanism of photochemical processes.     

Hydrogen atom formation from the photodissociation of water ice at 193 nm

The TOF spectra of photofragment hydrogen atoms from the 193 nm photodissociation of amorphous ice at 90-140 K have been measured. The spectra consist of both a fast and a slow components that are characterized by average translational energies of 2k(B)T(trans)=0.39+/-0.04 eV (2300+/-200 K) and 0.02 eV (120+/-20 K), respectively. The incident laser power dependency of the hydrogen atom production suggests one-photon process. The electronic excitation energy of a branched cluster, (H(2)O)(6+1), has been theoretically calculated, where (H(2)O)(6+1) is a (H(2)O)(6) cyclic cluster attached by a water molecule with the hydrogen bond. The photoabsorption of this branched cluster is expected to appear at around 200 nm. The source of the hydrogen atoms is attributed to the photodissociation of the ice surface that is attached by water molecules with the hydrogen bond. Atmospheric implications are estimated for the photodissociation of the ice particles (Noctilucent clouds) at 190-230 nm in the region between 80 and 85 km altitude.     

Spin-orbit ab initio investigation of the photolysis of o-, m-, and p-bromotoluene

The photodissociations of o-, m-, and p-bromotoluene were investigated by ab initio and spin-orbit ab initio calculations. The possible photodissociation mechanisms at 266 and 193 nm were clarified by multistate second order multiconfigurational perturbation theory (MS-CASPT2) calculated potential energy curves, vertical excitation energies, and oscillator strengths of low-lying states. The dissociation products with spin-orbit-coupled states of Br(*)((2)P(12)) and Br((2)P(32)) were identified by the MS-CASPT2 method in conjunction with spin-orbit interaction through complete active space state interaction (MS-CASPT2/CASSI-SO) calculations. The effects of methyl rotation and substituent on the photodissociation mechanism were discussed.     

Laser photo-induced dissociation using tandem time-of-flight mass spectrometry

A novel tandem time-of-flight (TOF) mass spectrometer has been developed for studying the photo-induced dissociation of large molecules and elemental clusters. It consists of a linear first stage TOF analyser for primary mass separation and precursor ion selection, and a second orthogonal reflecting field TOF analyser for product ion analysis. The instrument is equipped with a large volume throughput molecular beam source chamber allowing the production of jet-cooled molecules and molecular clusters, as well as elemental clusters, using either a pulsed laser vaporisation source (LVS) or a pulsed are cluster ion source (PACIS). A second differentially pumped chamber can be used with effusive sources, or for infrared laser desorption of large molecules, followed by laser ionisation. These primary ions can then be irradiated with a second, high energy laser to induce photodissociation. Detailed information about the fragmentation mechanisms can be deduced from the product ion mass spectra. Preliminary results on the photo-induced dissociation (PID) of the molecule ion of aniline at 266 nm are presented. In this case the molecule ions were generated via two-photon laser ionisation at 266 nm using an effusive source. Results for the collision-induced dissociation (CID) of the aniline molecule ion, using a commercial mass spectrometer equipped with an atmospheric pressure electrospray ionisation interface, are also presented. Copyright 2000 John Wiley & Sons, Ltd.     

Ultrafast ring opening in 1,3-cyclohexadiene investigated by simplex-based spectral unmixing

We use spectral unmixing to determine the number of transient photoproducts and to track their evolution following the photo-excitation of 1,3-cyclohexadiene (CHD) to form 1,3,5-hexatriene (HT) in the gas phase. The ring opening is initiated with a 266 nm ultraviolet laser pulse and probed via fragmentation with a delayed intense infrared 800 nm laser pulse. The ion time-of-flight (TOF) spectra are analyzed with a simplex-based spectral unmixing technique. We find that at least three independent spectra are needed to model the transient TOF spectra. Guided by mathematical and physical constraints, we decompose the transient TOF spectra into three spectra associated with the presence of CHD, CHD(+), and HT, and show how these three species appear at different times during the ring opening.     

Collision-energy-resolved Penning ionization electron spectroscopy of bromomethanes (CH3Br, CH2Br2, and CHBr3) by collision with He*(2(3)S) metastable atoms

Ionization of bromomethanes (CH3Br, CH2Br2, and CHBr3) upon collision with metastable He*(2(3)S) atoms has been studied by means of collision-energy-resolved Penning ionization electron spectroscopy. Lone-pair (nBr) orbitals of Br4p characters have larger ionization cross sections than sigma(C-Br) orbitals. The collision-energy dependence of the partial ionization cross sections shows that the interaction potential between the molecule and the He*(2(3)S) atom is highly anisotropic around CH3Br or CH2Br2, while isotropic attractive interactions are found for CHBr3. Bands observed at electron energies of approximately 2 eV in the He*(2(3)S) Penning ionization electron spectra (PIES) of CH2Br2 and CHBr3 have no counterpart in ultraviolet (He I) photoionization spectra and theoretical (third-order algebraic diagrammatic construction) one-electron and shake-up ionization spectra. Energy analysis of the processes involved demonstrates that these bands and further bands overlapping with sigma(C-Br) or piCH2 levels are related to autoionization of dissociating (He+ - Br-) pairs. Similarly, a band at an electron energy of approximately 1 eV in the He*(2(3)S) PIES spectra of CH3Br has been ascribed to autoionizing Br** atoms released by dissociation of (unidentified) excited states of the target molecule. A further autoionization (S) band can be discerned at approximately 1 eV below the lone-pair nBr bands in the He*(2(3)S) PIES spectrum of CHBr3. This band has been ascribed to the decay of autoionizing Rydberg states of the target molecule (M**) into vibrationally excited states of the molecular ion. It was found that for this transition, the interaction potential that prevails in the entrance channel is merely attractive.     

Acetone Formation from Photolysis of 2-Propanol on Anatase-TiO2(101)

Photocatalysis of 2-propanol on A-TiO₂(101) has been investigated using a temperature programed desorption method with 266 nm laser light. A clear mechanism is proposed for photodissociation of 2-propanol on A-TiO₂(101). Acetone product on five coordinate Ti⁴⁺ sites is formed in a stepwise manner in which the O-H dissociation proceeds first and then followed by secondary C-H dissociation of 2-propanol while H atoms are transferred to the adjacent bridge bond oxygen (BBO) sites. Low temperature water is formed in a thermally driven process via H-atom on BBO in exchange with isopropyl groups of molecule 2-propanol, while isopropyl radical desorbs at high temperature during the TPD process. The observation demonstrates the prospect of TiO₂ as a photocatalyst for degradation of organics.     

Relativistic multireference calculation of photodissociation of o-, m-, and p-bromofluorobenzene

Quantum chemical calculations with relativistic effects were performed on the photodissociation of o-, m-, and p-bromofluorobenzene (o-, m-, and p-BrFPh) at 266 nm. The method of multistate second-order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space state interaction was employed to calculate the potential energy curves for the ground and low-lying excited states of o-, m-, and p-BrFPh along their photodissociation reaction coordinates. The dissociation mechanisms with products of Br((2)P(3∕2)) and Br(?)((2)P(1∕2)) states were clarified with the computed potential energy curves and the surface crossings. The current calculations augmented previous theoretical investigations by including relativistic effects and resolved some differences of experimental assignment regarding the dissociation channels of o-, m-, and p-BrFPh.     

Br(2Pj) atom formation dynamics in ultraviolet photodissociation of tert-butyl bromide and iso-butyl bromide

The photodissociation dynamics of tert-C(4)H(9)Br and iso-C(4)H(9)Br has been studied at 234 and 265 nm using two-dimensional velocity map imaging technique. The translational energy and angular distributions have been analyzed for Br, Br(*), and tert-C(4)H(9) radical. The energy distribution of Br atom in the photodissociation of tert-C(4)H(9)Br is found to consist of two Gaussian components. The two components are correlated to two independent reaction paths on the excited potential energy surfaces: (1) the high-energy component from the prompt dissociation along the C-Br stretching mode and (2) the low-energy component from the repulsive mode along the C-Br stretching, coupled with some bending motions. For the energy distribution of Br(*) atom in the photodissociation of tert-C(4)H(9)Br, a third multiphoton dissociative ionization channel is observed at 265 nm in addition to the two energy components corresponding to channels (1) and (2). The energy distributions of Br and Br(*) atoms in the photodissociation of iso-C(4)H(9)Br can be fitted using only one Gaussian function indicating a single formation channel. Relative quantum yields for Br((2)P(32)) at 234 and 265 nm in the photodissociation of tert-C(4)H(9)Br are measured to be 0.76 and 0.65, respectively. For iso-C(4)H(9)Br, the measured value is Phi(234 nm)(Br)=0.81. The contribution of bending modes to Br and Br(*) is much more obvious in the photodissociation of tert-C(4)H(9)Br than in iso-C(4)H(9)Br.     

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.