Pulsed Photolysis Research Articles

Vibrationally Excited Ozone Relaxation by CO

Time profiles of ozone concentration after pulsed UV laser photolysis in the O2- O3-Ar-CO mixture, measured using time-resolved absorption spectroscopy, are presented. The experimental results show the dominance of the stabilization channel over the reactive one for the interaction between the vibrationally excited ozone molecule O3(υ) and carbon monoxide CO. The rate constant of the process O3(υ)+CO→O3 + CO, obtained by processing experimental data by the kinetic modeling method is (1.5 ± 0.2) · 10−13 cm3/s.

A novel multiplex absorption spectrometer for time-resolved studies.

A Time-Resolved Ultraviolet/Visible (UV/Vis) Absorption Spectrometer (TRUVAS) has been developed that can simultaneously monitor absorption at all wavelengths between 200 and 800 nm with millisecond time resolution. A pulsed photolysis laser (KrF 248 nm) is used to initiate chemical reactions that create the target species. The absorption signals from these species evolve as the composition of the gas in the photolysis region changes over time. The instrument can operate at pressures over the range ∼10-800 Torr and can measure time-resolved absorbances <10-4 in the UV (300 nm) and even lower in the visible (580 nm) 2.3 × 10-5, with the peak of sensitivity at ∼500 nm. The novelty of this setup lies in the arrangement of the multipass optics. Although appearing similar to other multipass optical systems (in particular the Herriott cell), there are fundamental differences, most notably the ability to adjust each mirror to maximise the overlap between the probe beam and the photolysis laser. Another feature which aids the sensitivity and versatility of the system is the use of 2 high-throughput spectrographs coupled with sensitive line-array CCDs, which can measure absorbance from ∼200 to 800 nm simultaneously. The capability of the instrument is demonstrated via measurements of the absorption spectrum of the peroxy radical, HOCH2CH2O2, and its self-reaction kinetics.

Experimental and theoretical studies of the reactions of ground-state sulfur atoms with hydrogen and deuterium.

The gas-phase kinetics of S(3P) atoms with H2 and D2 have been studied via the laser flash photolysis-resonance fluorescence technique. S atoms were generated by pulsed photolysis of CS2 at 193 nm and monitored by time-resolved fluorescence at 181 nm. The rate coefficients for H2 (k1) and D2 (k2), respectively, are summarized as k1(600-1110 K) = 3.0 × 10-9 exp-1.317×105-2.703×107K/T8.314 T/K cm3 molecule-1 s-1 and k2(770-1110 K) = 2.2 × 10-14 (T/298 K)3.55 exp(-5420 K/T) cm3 molecule-1 s-1. Error limits are discussed in the text. The rate coefficients for formation of SH(SD) + H(D) on a newly developed triplet potential energy surface were characterized via ring polymer molecular dynamics and canonical variational transition-state theory. There is excellent agreement above about 1000 K between theory and experiment. At lower temperatures, the experimental rate coefficient is substantially larger than the results computed for the adiabatic reaction, suggesting a significant role for intersystem crossing to the singlet potential energy surface at lower temperatures.

Measurement of line strengths in the à 2A’ ← [formula omitted]2A” transition of HO2 and DO2

The absorption spectrum of the à 2A’ ← X˜2A” 000–000 band of HO2 and DO2 radicals has been measured in the range 6941–7077 cm−1 with a resolution of around 0.005 cm−1. HO2 and DO2 radicals were generated from the reaction of Cl-atoms with CH3OH and CD3OD, respectively, whereby Cl-atoms were generated by pulsed photolysis of (COCl)2 at 248 nm. One time-resolved absorption curve was measured by cw-Cavity Ring Down Spectroscopy (cw-CRDS) for each wavelength. The relative absorption coefficient for each wavelength was then obtained from the ring down time τ, extrapolated to t = 0 s with respect to the photolysis pulse, and the ring down time τ0 before the photolysis pulse. Absolute absorption coefficients were quantified for several selected lines relative to a well characterized absorption line of HO2 at 6638.21 cm−1 by a second cw-CRDS absorption path. The maximum absorption cross section at 50 Torr Helium was for HO2 the transition N Ka Kc J = 11 0 11 11.5 ← 11 1 11 11.5 with σ7000.28 cm−1 = 2.12 × 10−19 cm2 and for DO2 the doublet transition N Ka Kc J = 18 1 18 18.5 ← 18 0 18 18.5 and 18 1 18 17.5 ← 18 0 18 17.5 with σ7019.83 cm−1 = 2.97 × 10−19 cm2. The full spectrum has been very well reproduced by employing spectroscopic data from earlier works.

First Evidence of Vibrationally Driven Bimolecular Reactions in Solution: Reactions of Br Atoms with Dimethylsulfoxide and Methanol.

We present evidence for vibrational enhancement of the rate of bimolecular reactions of Br atoms with dimethylsulfoxide (DMSO) and methanol (CH3OH) in the condensed phase. The abstraction of a hydrogen atom from either of these solvents by a Br atom is highly endoergic: 3269 cm-1 for DMSO and 1416 or 4414 cm-1 for CH3OH, depending on the hydrogen atom abstracted. Thus, there is no thermal abstraction reaction at room temperature. Broadband electronic transient absorption shows that following photolysis of bromine precursors Br atoms form van der Waals complexes with the solvent molecules in about 5 ps and this Br•-solvent complex undergoes recombination. To explore the influence of vibrational energy on the abstraction reactions, we introduce a near-infrared (NIR) pump pulse following the photolysis pulse to excite the first overtone of the C-H (or O-H) stretch of the solvent molecules. Using single-wavelength detection, we observe a loss of the Br•-solvent complex that requires the presence of both photolysis and NIR pump pulses. Moreover, the magnitude of this loss depends on the NIR wavelength. Although this loss of reactive Br supports the notion of vibrationally driven chemistry, it is not concrete evidence of the hydrogen-abstraction reaction. To verify that the loss of reactive Br results from the vibrationally driven bimolecular reaction, we examine the pH dependence of the solution (as a measure of the formation of the HBr product) following long-time irradiation of the sample with both photolysis and NIR pump beams. We observe that when the NIR beam is on-resonance, the hydronium ion concentration increases fourfold as compared to that when it is off-resonance, suggesting the formation of HBr via a vibrationally driven hydrogen-abstraction reaction in solution.

Laser Measurements of the H Atom + Ozone Rate Constant at Mesospheric Temperatures.

The exothermic H + O3 reaction produces OH(v) Meinel band emissions, used to derive mesospheric H concentrations and chemical heating rates. We remeasured its rate constant to reduce its uncertainty and extended the measurements to lower mesospheric temperatures using modern laser-induced fluorescence (LIF) techniques. H atoms were produced by pulsed ultraviolet laser trace photolysis of O3, followed by reaction of O((1)D) with added H2. A second, delayed, frequency-mixed dye laser measured the reaction decay rate with the remaining ozone using LIF. We monitored either the H atom decay by two photon excitation at 205 nm and detection of red fluorescence, or the OH (v = 9) product time evolution with excitation of the B(2)Σ(+)-X(2)Π (0,9) band at 237 nm and emission in the blue B(2)Σ(+)-A(2)Σ(+) (0,7) band. By cooling the enclosed low pressure flow cell we obtained measurements from 140 to 305 K at 20 to 200 Torr in Ar. Small kinetic modeling corrections were made for secondary regeneration of H atoms. The results are consistent with the current NASA JPL recommendation for this rate constant and establish its extrapolation down to the lower temperatures of the mesosphere.

Low Temperature Chlorine-Initiated Oxidation of Small-Chain Methyl Esters: Quantification of Chain-Terminating HO2-Elimination Channels.

Cl-initiated oxidation reactions of three small-chain methyl esters, methyl propanoate (CH3CH2COOCH3; MP), methyl butanoate (CH3CH2CH2COOCH3; MB), and methyl valerate (CH3CH2CH2CH2COOCH3; MV), are studied at 1 or 8 Torr and 550 and 650 K. Products are monitored as a function of mass, time, and photoionization energy using multiplexed photoionization mass spectrometry coupled to tunable synchrotron photoionization radiation. Pulsed photolysis of molecular chlorine is the source of Cl radicals, which remove an H atom from the ester, forming a free radical. In each case, after addition of O2 to the initial radicals, chain-terminating HO2-elimination reactions are observed to be important. Branching ratios among competing HO2-elimination channels are determined via absolute photoionization spectra of the unsaturated methyl ester coproducts. At 550 K, HO2-elimination is observed to be selective, resulting in nearly exclusive production of the conjugated methyl ester coproducts, methyl propenoate, methyl-2-butenoate, and methyl-2-pentenoate, respectively. However, in MV, upon raising the temperature to 650 K, other HO2-elimination pathways are observed that yield methyl-3-pentenoate and methyl-4-pentenoate. In each methyl ester oxidation reaction, a peak is observed at a mass consistent with cyclic ether formation, indicating chain-propagating OH loss/ring formation pathways via QOOH intermediates. Evidence is observed for the participation of resonance-stabilized QOOH in the most prominent cyclic ether pathways. Stationary point energies for HO2-elimination pathways and select cyclic ether formation channels are calculated at the CBS-QB3 level of theory and assist in the assignment of reaction pathways and final products.

Vacuum Ultraviolet Photoionization of Complex Chemical Systems.

Tunable vacuum ultraviolet (VUV) radiation coupled to mass spectrometry is applied to the study of complex chemical systems. The identification of novel reactive intermediates and radicals is revealed in flame, pulsed photolysis, and pyrolysis reactors, leading to the elucidation of spectroscopy, reaction mechanisms, and kinetics. Mass-resolved threshold photoelectron photoion coincidence measurements provide unprecedented access to vibrationally resolved spectra of free radicals present in high-temperature reactors. Photoionization measurements in water clusters, nucleic acid base dimers, and their complexes with water provide signatures of proton transfer in hydrogen-bonded and π-stacked systems. Experimental and theoretical methods to track ion-molecule reactions and fragmentation pathways in intermolecular and intramolecular hydrogen-bonded systems in sugars and alcohols are described. Photoionization of laser-ablated molecules, clusters, and their reaction products inform thermodynamics and spectroscopy that are relevant to astrochemistry and catalysis. New directions in coupling VUV radiation to interrogate complex chemical systems are discussed.

Low Temperature Rate Constants for the Reactions of O(1D) with N2, O2, and Ar

The kinetics of the gas-phase quenching reactions O((1)D) + N2, O((1)D) + O2, and O((1)D) + Ar have been studied over the 50-296 K temperature range using the Laval nozzle method. O((1)D) atoms were created in situ by the pulsed photolysis of O3 precursor molecules at 266 nm. Rate constants for these processes were measured directly, following the decay of O((1)D) atoms through vacuum ultraviolet laser-induced fluorescence at 115.215 nm. For the O((1)D) + N2 and O((1)D) + O2 reactions, the quenching efficiencies are seen to increase as the temperature falls. For the O((1)D) + N2 system, this indicates the likely influence of the intermediate complex lifetime on the quenching rate through nonadiabatic processes. For the O((1)D) + O2 system, which is considerably more complex, this behavior could result from the interactions between several potential energy surfaces.

Incomplete ozone recovery effect in the presence of active oxygen species

The degree of ozone recovery after pulsed UV laser photolysis of O2-O3-Ar mixture is measured using time-resolved absorption spectroscopy. The effect of incomplete ozone recovery in the presence of molecular singlet oxygen O2(a1Δ) and atomic oxygen is detected. This effect is caused by the reaction between the vibrationally excited ozone molecule formed due to recombination and atomic oxygen. The branching factor for the chemical channel of reaction products is γr = 0.81 ± 0.13.

Photodissociation of bulk nitrobenzene at 250, 266, and 280 nm using a picosecond laser

We have applied the technique of picosecond laser spectroscopy to study the photodissociation of nitrobenzene at room temperature and 100mTorr. Three different photolysis pulses, λ2=250, 266, and 280nm, with a duration of 20–25ps, were used. The NO photofragment was detected via LIF between λ1=220 and 250nm. The profile of rotational population distribution shows a dependency on the photolysis wavelength and the delay time. The observed rotational population distributions are non-Boltzmann and bimodal for v″=0, 1, and 2.

High-temperature kinetics of the reaction between chlorine atoms and hydrogen sulfide

The rate constant k1 for Cl+H2S has been measured over 290–914K using the laser flash photolysis/resonance fluorescence technique. Atomic chlorine was generated by pulsed 193nm photolysis of CCl4 and C6H5Cl and monitored by time-resolved resonance fluorescence at 130–140nm. The data are summarized as k1=(2.90±0.10)×10−11exp(2.38±0.11kJmol−1/RT)cm3molecule−1s−1 where the uncertainties in the Arrhenius parameters are ±2σ. 95% confidence limits for k1 are ±6%. These extended range measurements confirm a negative activation energy for the reaction.

Sizing of Mo nanoparticles synthesised by Kr–F laser pulse photo-dissociation of Mo(CO)6

In this work, the laser-based synthesis and characterization of Mo nanoparticles formed using Kr–F laser pulse photolysis of the vapor of Mo(CO)6 diluted by argon at room temperature were carried out. The time-resolved laser-induced incandescence (Ti–Re LII) technique was applied for the sizing of aerosolized particle inside of the quartz reactor. The samples of synthesized Mo nanoparticles were analyzed by transmission electron microscopy (TEM). The particle sizes measured by Ti–Re LII were found to be in a range of 3–13 nm in dependence on excimer Kr–F laser pulse energy and number of pulses. The temperature time behavior of laser-heated nanoparticles was observed by two-color pyrometry at the wavelengths 400 and 610 nm. The maximum particle temperature attained at high laser fluences was attributed to evaporation temperature and was found to be around 3,900 K unlike the boiling temperature of bulk Mo (4,800 K). The probable influence of nanoparticle properties on the results of Ti–Re LII sizing and nanoparticle oxidation in air on the results of TEM measurements is discussed.

Spectral and time-resolved properties of novel hetarylazo dyes containing hydrogenated quinolines and triazole moieties

The spectral characteristics of novel hetarylazo dyes containing triazole and hydrogenated quinoline moieties and the spectral and time-resolved parameters of photochemical processes occurring upon their photoexcitation were studied by stationary spectrophotometry and pulse photolysis. All compounds have an intense absorption maximum in the visible spectral range at 470–500 nm with ɛ = 17000–27000 L mol−1 cm−1. Upon excitation with the visible light, reversible trans-cis photoisomerization occurs, which is followed by thermal cis-trans isomerization. The temporal parameters of the isomerization depend on the azo dye structure. The introduction of bulky substituents in the triazole and hydroquinoline moieties results in the increase in characteristic times of the thermal transformation of the generated transient species from milliseconds to seconds and in the partial irreversibility of the process.

Multiplexed time- and photon-energy-resolved photoionization mass spectrometer for kinetics of radical reactions study

A combination of flash photolysis reactor tube and synchrotron radiation photoionization mass spectrometer has been constructed for the study of the gas phase chemical kinetics. The radical-molecule chemical reactions in a side-sampled flow tubular reactor are initiated by a pulsed photolysis laser. Then the gas mixture is sampled via a pinpole and photoionized by tunable synchrotron radiation VUV photons. The constant flux ions are detected by the subsequent time-of-flight mass spectrometer. At the same time, pulsed generator generates a burst of pulses with duration of 40-50 s, which initiate the collection of mass spectrometer. The continuous collection covers the overall reaction time. The time resolution of mass spectrometer is 40 s, fulfilling the detection of transient or stable species on a millisecond time scale. In addition, the reaction products are directly detected by mass spectrometer and different structural isomers can be distinguished by photoionization efficiency curves. The experiment utilizing Cl radical reaction with 1-butene and isobutene obtains the addition and abstraction products as well as their photoionization energies.

Absolute Rate Coefficient of the Gas-Phase Reaction between Hydroxyl Radical (OH) and Hydroxyacetone: Investigating the Effects of Temperature and Pressure

The rate coefficient (k1) of the reaction between hydroxyl radical and hydroxyacetone, which remained so far controversial, was determined over the temperature range 290-500 K using pulsed-laser photolysis coupled to pulsed-laser induced fluorescence (PLP-PLIF). Hydroxyl radical was generated by pulsed photolysis of H2O2 at 248 nm. The results show that at a pressure of 50 Torr He, the rate coefficient obeys a negative temperature dependence k1(T) = (1.77 ± 0.19) × 10(-12) exp((353 ± 36)/T) cm(3) molecule(-1) s(-1) for temperatures between 290 and 380 K, in good agreement with the results of Dillon et al. (Phys. Chem. Chem. Phys. 2006, 8, 236) at 60 Torr He. However, always at 50 Torr He but for the higher temperature range 410-500 K, a positive temperature dependence was found: k1(T) = (1.14 ± 0.25) × 10(-11) exp(-(378 ± 102)/T) cm(3) molecule(-1) s(-1), close to the expression obtained by Baasandorj et al. (J. Phys. Chem. A 2009, 113, 10495) for pressures of 2 and 5 Torr He but at lower temperatures, 280-360 K, where their k1(T) values are well below these of Dillon et al. and of this work. Moreover, the rate coefficient k1(301 K) determined as a function of pressure, from 10 to 70 Torr He, shows a pronounced decrease once the pressure is below ∼40 Torr He, thus explaining the disparity between the higher-pressure data of Dillon et al. and the lower-pressure results of Baasandorj et al. The pressure dependence of k1 and of its temperature-dependence below ∼400 K is rationalized by the reaction proceeding via a hydrogen-bonded prereactive complex (PRC) and a submerged transition state, such that at high pressures collisionally thermalized PRCs contribute additional reactive flux over and through the submerged barrier. The high-pressure rate coefficient data both of Dillon et al. and of this work over the combined range 230-500 K can be represented by the theory-based expression k1(T) = 5.3 × 10(-20) × T(2.6) exp(1100/T) cm(3) molecule(-1) s(-1).

ChemInform Abstract: Advances in Low Temperature Gas‐Phase Kinetics

AbstractReview: 147 refs.

Effect of the excitation wavelength and the structure of nitrated 1,2-dyhydroquinolines on dynamics of primary photophysical and photochemical processes

Dynamics of transformations of excited states and active transient species generated in the photolysis of nitrated 1,2-dihydroquinolines (N-DHQ) has been studied by femto- and nanosecond laser pulse photolysis. Spectral and kinetic parameters of primary photophysical and photochemical processes have been determined, and their dependence on the substituent position at the aromatic ring of 1,2-dihydroquinoline (DHQ) and on the wavelength of excitation light has been established. The lifetime of the excited singlet state S1 in N-DHQ is ca. 100 and 500 fs for 8- and 6-nitro-substituted DHQ, respectively, which is shorter in comparison with DHQ without the nitro group by a factor of 104 and more. The major decay channel of the S1 state is the successive formation of three transient species with lifetimes of 0.5 to 16 ps. A triplet state is generated only upon excitation of the short-wavelength band by UV light. The quantum yield of the triplet state depends on the structure of N-DHQ.

Iron nanoparticle growth induced by Kr–F excimer laser photolysis of Fe(CO)5

In this article the process of nanoparticle formation under the condensation of highly supersaturated atomic vapor produced by the photodissociation of metal-bearing compounds was investigated. The iron nanoparticles were synthesized by Kr–F laser pulse photolysis of Fe(CO)5. The measurements of an optical density of condensed phase were performed using a laser light extinction at a wavelength 633 nm. The particle size during their formation process was measured by a two-color time-resolved laser-induced incandescence. The final iron particle sizes and their structure were analyzed by a transmission electron microscopy. It has been shown that the process of iron particle formation in the investigated conditions could be divided onto three stages: the fast nucleation of iron atoms during 1–2 μs, the surface growth of particles up to the sizes of 1–6 nm with increasing volume fraction of condensed phase during 100–250 μs, and the relatively slow particle coagulation up to the final sizes of 5–9 nm. The effective rate constants of iron clusters and particle growth were extracted using laser light extinction measurements. The essential role of the reactions of iron clusters and particles with the parental Fe(CO)5 molecules was established. The kinetic mechanism of iron nanoparticle growth induced by photo-dissociation of Fe(CO)5 at room temperature based on obtained experimental results and known literature data has been suggested. The results obtained could be used for the developments of methods of synthesis of catalysts, magnetic nanopowders, and others nanomaterials at room temperature. Besides that, the presented experimental data could be useful for the validation of kinetic models of gas-phase condensation of supersaturated vapor of solids.

Advances in low temperature gas-phase kinetics

Rate constants for elementary gas-phase reactions were first measured reliably following the development of pulsed photolysis and flow methods in the 1960's. These techniques have continued to be employed as kinetics experiments have been performed at lower and lower temperatures. Sub-ambient temperatures are reached either by cryogenic methods or by using expansion techniques. In this article, we review the possibilities and limitations of these cooling techniques and the results that have been obtained. These efforts have been driven both by the desire to understand the fundamental factors that control the rates of chemical reactions and also by the wish to provide rate constants that can be used in models of complex environments, such as planetary atmospheres and the interstellar medium. In this review, some emphasis is given to the CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) method which has now been used to determine rate constants for many elementary reactions, including those between neutral species, as well as ion–molecule reactions. This method has provided rate constants for a limited number of reactions to below 10 K. Major efforts are now being made to go to still lower temperatures and we describe some of the results obtained at these very low temperatures in the last section of this article.