Flash Photolysis Methods Research Articles

Dynamic Behaviors of the Electron Donor-Acceptor Complex in its Lowest Excited Singlet State

Abstract Dynamic behaviors of the weak electron donor-acceptor (EDA) complex in its lowest excited singlet state have been investigated by the nsec flash photolysis method and measurements of fluorescence and transient photocurrent. In polar solvents the weak EDA complex in its lowest excited singlet state dissociates spontaneously into ions. The dynamic behaviors of s-tetracyanobenzene(TCNB)-benzene and TCNB-toluene complexes were studied in some solvents and the existence of the direct radiationless process at the excited Franck-Condon (FC) state as well as in the course of relaxation process from the excited FC state to the fluorescent state has been confirmed in the case of 1,2-dichloroethane solution. The nature of the intra-complex radiationless processes was discussed on the basis of this result. It has been pointed out that the lifetime of excited states is of crucial importance for understanding the dynamic behaviors of excited weak EDA complexes.

A FLASH PHOTOLYSIS STUDY OF INTERMEDIATES IN PHOTOCHEMICAL REACTIONS OF CHLOROPHYLL

Abstract— The photochemical reactions of chlorophyll intermediates in vitro have been studied by the flash photolysis method. The flash excitation of pigment solutions has been shown to involve the population of a chlorophyll triplet state where the oxidation‐reduction processes occur. The mechanism and kinetics of pigment triplet decay have been investigated from 20°to — 50°C and the ability of chlorophyll molecules to carry out triplet‐triplet energy transfer has been established. The latter phenomenon has been used to show up the role of chlorophyll triplets in the reversible photooxidation reaction with P‐quinone. There have been studied initial products of pigment photoreduction with ascorbic acid and phenylhydrazine. Experimental data of the mechanism of the initial oxidation and reduction in chlorophyll photosensitized reactions have been analysed. There have been also obtained the differential spectra of chlorophyll triplets and radicals. A calculation has been made of rate constants for a few elementary reactions.

Preparation and Properties of α- and β-Chains from Human Hemoglobin

Abstract A new procedure is described using carboxymethyl cellulose and diethylaminoethyl cellulose columns for the preparation of native α and β chains from human hemoglobin. The chains appeared to be more stable than those obtained by earlier methods and permitted the preparation of modified chains containing meso- and deuterohemes. The kinetics of carbon monoxide binding was determined for proto-, meso-, and deutero-α and β chains by both stopped flow and flash photolysis methods. The kinetic properties of β—SH chains with protoheme are similar to those reported for hemoglobin H, and their rate of reaction with carbon monoxide is more than twice as great as that for α—SH chains.

Flash Photolysis of Hydrogen Peroxide I: The Reaction of Ozonide with Hydrogen Peroxide

AbstractThe photochemistry of basic aqueous solutions containing hydrogen peroxide and oxygen has been investigated by the flash photolysis method. The decay of O3− was followed at 4300 Å over the pH range 13 – 13.7 as a function of hydrogen peroxide and oxygen concentrations. The decay was found to be of the first order under all conditions −[d(O3)−/dt] = kobs[O3−]. The pseudo first‐order rate constant kobs was found to be dependent on H2O2 and O2 concentrations. A mechanism which is consistent with the experimental results is suggested.

Experimental study of the viscous sublayer in turbulent pipe flow

AbstractA flash photolysis method for nondisturbing turbulent flow measurements has been applied to study conditions in the viscous sublayer in a square smooth pipe. Experimental results are obtained that are extremely difficult to obtain by any other known technique.The results show that, adjacent to the wall, there is a layer of very small thickness y+ = 1.6 ± 0.4 in which a linear velocity gradient occurs virtually at all times, but the slope of the gradient changes with time. Beyond y+ = 34.6 essentially turbulent flow exists. In the transition region in between, very disturbed flow conditions prevail. However, the technique used clearly indicates the instantaneous velocity distribution in the flow field bewteen the wall and the fully turbulent region.Statistical distributions of the instantaneous wall shear stress (evaluated from the slope of the velocity gradient), the laminar sublayer thickness, and the velocity at the edge of the laminar sublayer are obtained. It follows that the partial turbulence mechanism whereby the flow at a point is turbulent only a fraction of the time is, at least to an extent, responsible for the reduction of eddy diffusivity by kinematic viscosity close to the wall.Experimental velocity profile obtained in the present investigation is compared with the proposed and experimental velocity profiles of other authors.

Nonadiabatic Decomposition of N2O in the Deactivation of O(1D) by N2

An extension of a flash-photolysis method described previously was used to measure the following absolute rate constants (all in liter per mole·sec): O(1D)+O3→2O2k2=2.0±1.0×1011,O(1D)+N2→O(3P)+N2k4a=1.3±0.6×1010,O(1D)+Xe→O(3P)+Xek4b=3.5±1.7×1010,O(1D)+Ar→O(3P)+Ark4c∼5×107. The efficiency with which the O(1D)→O(3P) transition is induced by collisions depends specifically on the added gas. The results for deactivation by nitrogen are discussed in terms of the formation of an N2O* intermediate in a high level of vibrational excitation. Dissociation to N2+O(3P) at this energy level (85.2 kcal) is compared with thermal decomposition of N2O where predissociation to a state which correlates with O(3P) occurs at a lower energy level (〈ε〉=61.8 kcal).

Quantum yields of triplet formation in solutions of chlorophyll

The method of flash photolysis has been used for direct measurement of the quantum yields of triplet formation in dilute chlorophyll solutions at 23 °C. In ether solution, the triplet yields were found to be 0⋅64 and 0⋅88 for chlorophyll a and b , respectively. In very dry hydro­carbon solution, the yields decrease by at least a factor of 5. It is suggested that the low yields of fluorescence and intersystem crossing in dry solvents are due to dissociation of excited singlet dimer chlorophyll into ground state monomer species.

A new method for non-disturbing turbulent flow measurements very close to a wall

A general flash photolysis method suitable for various non-disturbing turbulent flow measurements very close to a wall of any shape has been developed. A very dilute solution of 2-(2, 4-dinitro-benzyl)-pyridine in 95 per cent ethyl alcohol is employed. A tracer, in the form of a blue line, is introduced upon the exposure of the solution to the high intensity light beam formed by a parabolic mirror. The colour is due to a tautomeric shift in the structure of the dissolved substance. The tracer is recorded photographically after an increment of time has elapsed since its formation.

Tracer introduction by flash photolysis

A flash photolysis method for introducing colored tracer into flowing water without disturbing its flow patterns is developed. The method employs a lightly colored dilute solution which undergoes a chemical reaction similar to the blueprint reaction upon exposure to high intensity light. A dark blue permanent tracer, easily observed or detected by colorimetry, is produced. As a demonstration and test, axial dispersion is measured in laminar flow in a round tube. Other flow visualization studies where it is important to avoid disturbing the flow should benefit from such methods.

The infrared detection of free radicals using flash photolysis methods

A rapid-scan infrared spectrometer has been constructed and successfully applied to the gas phase study of short-lived intermediates produced by flash photolysis. A zinc-doped germanium semiconductor detector and a Littrow mirror rotating at 10 000 rpm permit scan rates of 1 000 cm–1 /100 microseconds. Detector sensitivity extends over a wide spectral range, 2-40 μ. Two intermediates of short lifetime, chloroformic acid and CF2 , have been identified and rough estimates of their half-lives have been obtained, 50-70 microseconds and 2,5 milliseconds, respectively. The possibilities of infrared emission studies are suggested by the emission spectrum from a Cl2 — H2 explosion initiated by flash photolysis.

The explosive oxidation of ammonia and hydrazine studied by kinetic spectroscopy

The explosive oxidation of ammonia and hydrazine are studied by the method of flash photolysis and kinetic spectroscopy. The propagating reactions describing the oxidation of ammonia are concluded to be NH2+ O2= HNO + OH and OH + NH3= NH2+ H2O, nitric oxide being the main product and resulting from the breakdown of HNO. The oxidation of hydrazine is described by two mechanisms for the two separate overall reactions. It is concluded that the minor reaction yielding nitric oxide as the product results from the splitting of N2H3into NH and NH2, the propagating reactions being NH + O2= NO + OH and OH + N2H4= N2H3+ H2O. The major reaction giving rise to nitrogen as the product is described in terms of a mechanism which does not involve the fission of the N—N bond. A successive dehydrogenative mechanism is considered though a scheme analogous to that in the ammonia oxidation involving nitrosamine, NH2NO, via the reaction N2H3+ O2= NH2NO + OH is favoured in the light of the evidence. A general scheme for the oxidation of non-metal hydrides is considered. Preliminary experiments were carried out on the photolyses of ammonia and hydrazine. The thermal nature of the decomposition of ammonia has been illustrated, a strong NH spectrum being observed when the conditions for a temperature rise were chosen. Hydrazine has been detected as a product in this system. A mechanism in agreement with that postulated by Ramsay is reported for the photolysis of hydrazine, the first step being the splitting of the N—N bond followed by abstraction reactions on the parent molecule, fission of the hydrazyl radical occurring to a small extent only when explosion takes place. These preliminary studies served as a basis for the investigation of the combustion reactions.

IV1. Fast Reactions in Solution by Flash Photolysis Methods

AbstractFlash photolysis techniques are particularly suitable for the study of rapid reactions in solution. Thermal effects are then negligible and Beers law is generally valid, even with low dispersion instruments. The time resolution of present equipment is shown to be adequate for the study of most reactions which are second order in concentration and for first order reactions with rate constants less then 105 sec−1. Within this time range flash photolysis can be used for the study of almost every type of fast reaction.

The combustion of hydrogen sulphide studied by flash photolysis and kinetic spectroscopy

The combustion of hydrogen sulphide has been investigated by the method of kinetic spectroscopy and flash photolysis. If no large excess of inert gas is present the reaction produces sulphur dioxide. The reaction has been shown to take place in steps in which the radicals SH and OH participate. Simultaneously with the appearance of the sulphur dioxide; a light emission has been observed which is attributed to the process: SO + O→SO2 + hv . In the presence of a large excess of inert gas the reaction results in S 2 O 2 . It has been shown that the formation of S 2 O 2 is favoured by a low temperature. Flash photolysis of hydrogen sulphide, sulphur dioxide and S 2 O 2 has also been investigated. In the last two cases the absorption spectra disappear temporarily. Inert gas prevents the disappearance of the SO 2 , but not that of S 2 O 2 . From an analysis of our results a mechanism for the combustion of hydrogen sulphide is derived and discussed.

Studies of I Atom and Br Atom Recombination Following Flash Photolysis of Gaseous I2 and Br2

The method of flash photolysis has been used to determine the rate constant, k, in the expression d(X2)/dt = k(X)2M for the combination of iodine atoms in argon, and of bromine atoms in argon, oxygen, and nitrogen. At room temperature nitrogen is approximately 1.3 and oxygen 2.2 times as effective as is argon in serving as a ``third body'' for the combination of bromine atoms. Measurements of the rate constants at a series of temperatures up to 160° give apparent activation energies of —1.4 kcal/mole for I in A, —2.0 kcal/mole for Br in A and —1.2 kcal/mole for Br in O2. The (I2)/(A) ratios used in the iodine atom studies were in the range of the ``tail'' of the (I2)/(A) vs k plot of the Cambridge group and the k values are in agreement with their results. Estimates of the maximum ``thermal effect'' possible in the bromine atom studies, compared with the experimental results, indicate that such effects were probably negligible. Experimental tests and calculations of the effect of nonuniform illumination of the reaction cell by the flash lamp indicate that the observed rate constants are quite insensitive to this factor.

Studies of knock and antiknock by kinetic spectroscopy

The method of flash photolysis has been used to ignite mixtures of acetylene, oxygen and amyl nitrite, the latter serving as fuel and as sensitizer. The course of the explosive combustion was followed by flash spectroscopy and by photoelectric technique. Two phases have been conveniently distinguished in the explosion : (1) An induction period following the immediate disappearance of the spectrum of amyl nitrite during which no radical spectra were visible. (2) The fast build-up of diatomic radical spectra—predominantly OH, with a little NH and CN— at the point of ignition accompanied by strong light emission upon which were superimposed high-temperature emission peaks of only a few microseconds’ duration indicating detonation waves travelling through the reacting gases. On the addition of small amounts of lead tetraethyl the induction period was greatly lengthened, and during that time the waxing and waning of the lead oxide band system was observed. 0∙2 mm (Hg) of lead tetraethyl increased phase 2 up to 200%, and the strong emission caused by heavy detonation was often quenched completely. The entirely different behaviour of aromatics is also shortly discussed. It was established that the antiknock effect of lead tetraethyl is a completely homogeneous gas-phase reaction, and it was concluded that its effect is twofold in nature: first, by reacting with oxygenated intermediates produced in phase 1 of the combustion with the formation of lead oxide, it removes the sources of auto-ignition which lead to knock in phase 2; secondly, the lead monoxide so formed reduces the propagating and branching mechanisms leading to the final explosion by preferential reaction with the free radicals involved, and thus contributes to conditions which do not lead to detonation and knock. This reaction and also the conditions obtaining at the end of the explosion promote the final reduction of the lead monoxide to atomic lead.

Studies of free radical reactivity by the methods of flash photolysis. The photochemical reaction between chlorine and oxygen

Studies of free radical reactivity by the methods of flash photolysis. The photochemical reaction between chlorine and oxygen

Detection of Diatomic Radical Absorption Spectra during Combustion

THE spectra of the CH, C2 and CN radicals which, with OH and NH, form the main part of the emission from hydrocarbon flames and explosions, have hitherto eluded attempts at detection in combustion processes by means of their absorption spectra. This has usually been interpreted as meaning that they are present at very low concentration. We have previously described how the method of flash photolysis and spectroscopy1 may be used for the homogeneous initiation of explosions, the study of the OH radical in the hydrogen/oxygen reaction2 and the detection of the CN and NH radicals in absorption during ethylene/nitrogen dioxide explosions3. The very short life-time of the latter two radicals made it impossible to study their occurrence in detail, and to this end a modified apparatus has been constructed by which shorter times may be investigated. By this means it has been possible to follow the absorption spectra at intervals of 20 microsec. during the explosion from 30 microsec. after the beginning of the initiating flash.

Spectroscopic studies of the hydrogen-oxygen explosion initiated by the flash photolysis of nitrogen dioxide

The method of flash photolysis has been used to initiate the reaction between hydrogen and oxygen, using a small amount of nitrogen dioxide as sensitizer. The dependence of the ignition limits on total pressure, capacity and the pressure of nitrogen dioxide have been studied, as well as the flash photochemical decomposition of nitrogen dioxide alone. The explosive reaction is complete in a few milliseconds, and very short induction periods are found whose duration can be explained in terms of the initial temperature rise. It is shown that the initiation of the explosion in this way is reproducible to less than 10-4s and flash spectroscopy is used to study the development of the reactions with time. The spectra obtained are described and the reactions of the OH radical are investigated by means of its absorption spectrum , whose intensity variations with partial pressure of hydrogen, oxygen, nitrogen and water vapour over time intervals of 0·2 ms throughout the reaction are given. Addition of hydrogen accelerates the reaction, but oxygen in excess has the same effect as an inert gas which is to decrease the rate. It is shown that the important propagation steps are exclusively bimolecular and the main reactions of the OH radical are identified. The mechanism of the overall explosion reaction is discussed and a reaction scheme is given which is in accordance with the results.