ULTRAFAST PHOTOEXCITATION STUDIES OF CONCENTRATED SOLUTIONS OF ALKALI METAL HALIDES. Udaya Indike Rodrigo. A Thesis

Transcription

1 ULTRAFAST PHOTOEXCITATION STUDIES OF CONCENTRATED SOLUTIONS OF ALKALI METAL HALIDES Udaya Indike Rodrigo A Thesis Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE December 2006 Committee: Michael Rodgers, Adviser Thomas Kinstle John Cable.

2 ii Michael A.J. Rodgers, Adviser ABSTRACT The primary photochemical processes that occur via a two photon mechanism in pure water during high intensity excitation with a femtosecond pulse at 267 nm were studied by using a white light continuum as probe pulse. Two photon femtosecond laser studies of the excitation and relaxation of pure water showed that, geminate lifetimes were comparable with reported data but the recorded absorption spectrum was blue shifted. Experiments carried out for highly concentrated lithium compounds with different anion indicated that these compounds produced their maximum transient absorption between the wavelengths of 550nm to 700nm. The time profiles of these compounds fell into two major categories. LiI and LiOH showed different kinetic profiles from the others by the initial presence of a short lived species along with longer decay kinetics, where as LiCl, LiBr and LiClO 4 shows typical decay kinetics of long lived species only. The fitted experimental data on single exponential function for absorption kinetics of LiCl showed that the absorption rise time falls into same time regime as reported for the purported three body complex of NaCl. This leads to the thought that LiCl may follow the same mechanism. The concentration dependence study of LiCl showed its absorption maxima were red shifted as the concentration was changed from high to low. The kinetic data revealed that rate of absorption for low concentrated solution of LiCl is higher than that of high concentrated LiCl solution. The comparison of LiCl with KCl and NaCl showed that there is a significant contribution from the cation for higher concentrated solution to alter their kinetic and transient absorption. But in the case of low concentrated solutions that effect is not significant.

3 To my mother, late father, all my teachers and friends iii

4 iv ACKNOWLEDGEMENT I would like to express my sincere gratitude to my research Adviser, Dr. Michael Rodgers for accepting of me as a master s research student, and providing continuous guidance and close supervision throughout this research work. Also, I would like to express my deep appreciation to Dr. Paul Endres, for his continuous encouragement and guidance during the project. My sincere thanks are also due to Dr. Eugene Danilov, who was the research coordinator, for teaching me how to use facilities of Ohio Laboratory for Kinetic Spectrometry research laboratory. I am grateful to, Department of Chemistry, Bowling Green State University, giving me unrestricted laboratory facilities during day and night. I am also grateful to the technical and office staff of the Department of Chemistry, for their assistance given to me during my technical and administrative issues. I take this opportunity to thank my colleagues, who shared good and bad moments with me during my research career at bowling green. Finally, I am grateful to the Ohio Laboratory for Kinetic Spectrometry for instrumental support to make this research project a success.

5 v TABLE OF CONTENTS Chapter 1 INTRODUCTION Solvated Electrons Method of Formation of Solvated Electrons Hydrated Electrons Structure of Hydrated Electrons Two photon excitation and femtosecond lasers... 9 Primary Events during the Excitation of Water Metal-solvent systems Ultrafast Dynamics of Aqueous Hydroxide Flash Photolysis of Halides Photolysis of Halides in Aqueous Solution Chapter 2 MATERIALS AND METHODS Materials Experimental Section Instrumentation and Method UV- Visible Absorption Spectroscopy Femtosecond Transient Absorption Spectroscopy Chapter 3 RESULTS Pure Water Concentrated Alkali Metal Halide Solutions LiCl Solutions; Effect of Concentration Chapter 4 DISCUSSION AND CONCLUSION References... 44

6 vi LIST OF FIGURES Figure 1.1 The structure of the nearest solvation shell of hydrated... 6 electron in glassy water Figure 1.2 Shows the lowest electronic transition in the hydrated electron... 7 (a) Absorption spectrum. The smooth solid curve shows experimentally measured absorption at room temperature. Squares depict the result of quantum molecular simulations The dashed curves correspond to the individual absorption components originating from three non-degenerate s p transitions. (b) Electronic wave function plots for typical ground, s-like, state and lowest three excited, p-like, states Figure 1.3 Model of primary events in pure liquid water at two photon excitation. Figure 2.1 Experimental layout of the transient absorption Spectrometer. Figure 3.1 Transient absorption spectrum of distilled deionized water (at 80ps delay time) in 0.09mm calcium fluoride(caf 2 ) thin window cell. Figure 3.2 Transient absorption spectrum of distilled deionized water (at 114ps delay time) in 2mm quartz cuvette. Figure 3.3 Transient absorption kinetic data for a 0.09 mm cell of neat water excited with 267nm femtosecond pulses (maximum absorption at 650 nm wavelength.). Figure 3.4 Plot of Transient Absorption (ΔA) vs Wavelength (nm) of different lithium compound(in 2 mm flow through cuvette) excited with 267nm femtosecond pulses at room temperature. Figure 3.5 Kinetic plots of data for different lithium compounds (in mm flow through cuvette) excited with 267nm femtosecond pulses (at their respective maximum absorption wavelength).

7 vii Figure 3.6 Plot of transient absorption kinetic data for different lithium compounds (in 2 mm flow through cuvette) excited with 267nm femtosecond pulses(at their respective maximum absorption wavelength). Figure 3.7 Plot of Absorption vs. Wavelength for LiCl before and after the experiment. Figure 3.8 Transient absorption spectra of LiCl at different concentrations (in 2 mm flow through cuvette) excited with 267nm femtosecond pulses at room temperature. Figure 3.9 Kinetic plots of highest (a) and lowest (b) concentrated solutions of LiCl in deionized water (in 2 mm flow through cuvette) excited with 267nm femtosecond pulses and solid line is the two-exponential decay fit for the experimental data. Figure 3.10 Transient absorption spectra of highest concentrated solutions KCl, NaCl and LiCl in deionized water (in mm flow through cuvette) excited with 267nm femtosecond pulses at room temperature. Figure 3.11 Transient absorption spectra of one molar (1M) solutions KCl, NaCl and LiCl in deionized water (in 2 mm flow through cuvette excited with 267nm femtosecond pulses at room temperature. Figure 3.12 Kinetic plots of highest concentrated solutions of LiCl, KCl and NaCl. (in 2 mm flow through cuvette) excited with 267nm femtosecond pulses

8 viii LIST OF TABLES Table 3.1 Table of lithium compounds with their respective highest concentrations. Table 3.2 First order fitting parameters for the transient absorption kinetic data. Table 3.3 Fitting parameters of highest concentrated solutions of LiCl, KCl and NaCl.

9 1 CHAPTER 1: INTRODUCTION 1.1 Solvated Electrons Upon ejection of an electron into a liquid, it can be captured and relocated in a potential energy well formed by neighboring molecules of the liquid. Such an electron is known as solvated electron. Solvated electrons were first observed in liquid ammonia in 1864 (1); since then the study of excess, or solvated, electrons in liquids has attracted much interest in the fields of chemistry, physics and biology. The existence of such electrons in water (known as hydrated electrons) was first postulated independently in 1952 by Stein (1, 2) and Platzman (1, 3). After decades of research and indirect evidence, the hydrated electron was finally observed in 1962 by Boag and Hart (4), who used visible and near infrared (NIR) absorption spectrometry in an electron pulse radiolysis experiment in water. Subsequently observations of solvated electrons produced by ionization radiation were expanded to include other solvents such as ethers, hydrocarbons and metal ion (alkali and alkali earth) solutions (5). This newly developed capability of electron pulse radiolysis for studying the solvated electron in a wide variety of solvents created a new surge of interest in the whole field. A variety of different approaches and combinations have been employed which has resulted in hundreds of absolute rate constants of solvated electron reactions as well as many physical properties of the solvated electron (5). There have also been developments in the study and understanding of metal ion-solvent systems. These discoveries have led to concomitant growth in theoretical research in the physical and chemical properties of the solvated electron.

10 2 1.2 Method of Formation of Solvated Electrons Radiolysis is the most common way for the production of solvated electrons. Radiationinduced (or laser) ionization of liquid phase chemical species generates an excess electron in the delocalized conduction band; the electron subsequently undergoes rapid localization in a microcavity that exists among the solvent molecules (1,6). The localized electron undergoes nuclear relaxation and becomes an equilibrated solvated (hydrated) electron. There are two factors that can affect the solvation process of the free electron in solution 1. There should be a physical interaction existing between solvent molecule and the electron which is trapped in the potential energy well. 2. The chemical reactivity between the solvent and the electron must be low enough to permit time for the solvation to occur. The solvated electrons that form during radiolysis can react with other transient products formed during the process (5). The hydrated electron was the first of the radiolytically produced species to be observed directly through its visible/nir optical absorption in a pulse radiolysis experiment (5, 6). Since reactions of solvated electrons have been studied more extensively in water and other solvents, it important to first discuss the processes during aqueous radiolysis. Whenever a charged particle travels very rapidly (kev MeV) through a solution, it tends to ionize and excite molecules along its path. Secondary electrons produced by such a process are lower energy and may cause further ionization and excitation in small volumes. Most primary products of radiolysis are inhomogeneously distributed along the primary track of the electron in these small volumes (spurs). A faster moving electron can produce spurs that are widely separated by several hundreds of nanometers. Heavier particles such as -particles, which are in the same energy

11 3 range, travel much more slowly and lose more energy per unit path length. Therefore they produce an essentially continuous track of primary products. These primary species are produced in times less than sec for water (1, 5). Since these primary species are highly reactive and have high local concentrations, some of them react with each other to produce secondary products as they diffuse out of the spurs to form a homogeneous solution. According to the literature the complete spur reaction duration is about 10-7 sec for water (1, 5).

12 4 1.3 Hydrated Electrons The high degree of interest in the hydrated electron from both theoretical and experimental points of view is due to the importance of the hydrated electron as the transient species in aqueous systems. Photosynthesis, charge transport through bio-membranes and long distance charge transport in nerves are important examples (7-9). Also, the hydrated electron is generally the key intermediate in radiation chemistry and electrochemistry (4, 10-13). Unlike free electrons that are delocalized, electrons in polar solvents become self-trapped because of their interactions with the solvent environment. Owing to strong solute solvent coupling, the evolution of the electronic structure is completely determined by the rearrangement of the solvent molecules. Therefore a detailed study of hydrated electrons is particularly interesting from the point of view of the solvent which is involved in the process of experiment. Being the natural environment of such important biological molecules as nucleic acids and peptides, water is present in all living organisms (1, 14-16). Therefore a detailed understanding of charge transfer reactions initiated by light absorption in water is of significant importance (14, 17). The study of solvated electrons is also very interesting from the point of view of the solvent involved. Among all solvents, water occupies an extraordinary position regarding its specific role in nature, and as such it may influence the results of many reactions because of its large dipole moment and its strong hydrogen bonding tendency (1). The structural and dynamical properties of water are of long standing interest in science. As a result many theoretical and experimental studies have been carried out to study the properties of water as a solvent as well as its solvation dynamics (1, 17). The detailed understanding of solute solvent interaction has a number of practical implications including the dynamics of chemical reactions. All chemical reactions involve the rearrangement of electrons and this is affected

13 5 by the motion of surrounding solvent molecules which are coupled to the reactant s energy levels. The time scale of reactions where the solvent acts to stabilize the new charge distribution of the reacting species can determine how fast the particular reaction crosses into its transition state. During the last several decades, molecular dynamic simulations and ultrafast studies on dye solutions have been used to obtain the basic picture of the solvation process and the relevant time scales (18-23). Because of most of these dye solution in water showed the initial solvation processes to be exceptionally fast and the studies lacked time resolution, many important aspects of early dynamics remain unexplored and unresolved (24). The other motivation for a detailed study of the hydrated electron is the fact that this species is ideally suited for quantum molecular dynamics stimulation in the liquid phase. In this regard it is important to understand the extent of the quantum-mechanical character of the electron interaction with its nearest neighboring water molecules. Since there is no internal degree of freedom in the electron itself, the hydrated electron is ideal for verifying the model potential that describes the interaction between the molecules of liquid water.

14 6 1.4 Structure of Hydrated Electrons Numerous computational studies have been performed to investigate the quantum mechanical status of the hydrated electron and its surrounding microscopic structure. The structure of this species is revealed in an electronic-spin-echo study by L. Kevan (25). According to his study, it was shown that each electron is surrounded by six water molecules each with an OH bond directed towards the electron as shown in the figure (1.1). This idea is confirmed by recent computational studies on hydrated (solvated) electron in water, which show that the first solvation shell is composed of approximately six water molecules (26, 27). There are other hypotheses which Figure 1.1: The structure of the nearest solvation shell of hydrated electron in glassy water (adapted from Ref [1, 25] ) suggest that the electron might be attached closer to one of the dangling protons that are not involved in the hydrogen bonding of the molecule that forms the solvent cage (28). Therefore the details of the exact structure are still under discussion.

15 7 The localization of a hydrated electron in the solvent cavity gives rise to bound eigenstates. These eigenstates are broadened since they are modulated by the coupling to the fluctuations of surrounding water molecules. It has been reported that the high sensitivity of the electronic states of the hydrated electron to the aqueous environment results an intense broad electronic absorption spectrum with a peak at 720nm. Also molecular dynamic stimulations (29) and computational studies have shown that the lowest energy eigenstate of the hydrated electron is nearly spherical and corresponds to an s-like state and the first excited state was found to consist of three non-degenerate p-like orbitals. Figure 1.2 Shows the lowest electronic transition in the hydrated electron.(a) Absorption spectrum. The smooth solid curve shows experimentally measured absorption at room temperature. Squares depict the result of quantum molecular simulations (adapted from ref [1, 30]).The dashed curves correspond to the individual absorption components originating from three non-degenerate s p transitions. (b) Electronic wave function plots for typical ground, s-like, state and lowest three excited, p-like, states (reproduced from ref. [1, 31]). The existence of the s-state indicates that on average the potential energy well created by the molecules surrounding the electron is close to spherical. However, the dynamic nature of liquid

16 8 water causes some asymmetries, therefore the potential energy surface does not have a perfect spherical shape and the excited states were considered as three non-degenerate p-states. Numerous attempts have been made to fit the experimental spectra with various line shapes and superposition of lines (1, 32, 33). Over the last three decades, there are a few outstanding problems about homogeneous / inhomogeneous broadening in the optical absorption spectrum of the excess electron in the water (and other fluids) as well as the explanation of asymmetry and extraordinary spectral width (34-38). Computational and computer simulation studies have provided a fruitful path to address this issue. Computer simulation studies have been used to pictorially describe the shapes of the spectra. The absorption spectrum produced in these computational studies was used to discuss the superposition of three non- degenerate s-p transitions and contributions of the transition from the higher delocalized states (29). Interestingly, it also revealed the charge distribution upon the promotion to the p-state. These models were used to study the dynamic behavior of the hydrated electron and to modulate the energy of relaxation after the instantaneous s-p excitation (26, 27). Also most of these studies show that the solvation dynamics are bimodal and the initial decay is responsible for half of the total energy of relaxation. This relaxations initially occurs at fs time scale and followed by a slower decay around fs (1, 26). The slower decay appeared to be associated with the diffusional motion of water molecules into and out of the first solvation shell. The microscopic nature of the first initial decay is still under considerable debate.

17 9 1.5 Two photon excitation and femtosecond lasers In the past two decades increasing attention has be been paid towards a detailed understanding of charge transfer reactions initiated in water by the absorption of light. According to the literature it is known that intense light at λ > 190 nm is able to excite a water molecule via a two photon absorption (TPA) mechanism and initiate its chemical decomposition (14). It was observed that high intensity picosecond pulse UV excitation at λ = 266 nm can cause the water molecule to undergo two-photon absorption with subsequent ionization and dissociation (14-16). H 2 O + 2hν H 2 O* ~~>e aq -, H 3 O +, OH, H It is thus possible to investigate the reaction dynamics of the electron in pure water using high intensity femtosecond laser systems. In this regard most experimental groups use pump and probe techniques with two photon excitation in the UV and probing in the visible and near IR ranges. They have studied electron trapping, solvation and geminate recombination with time resolution down to 20 fs (14, 16). In many of these investigations a colliding pulse mode locked dye laser with second harmonic generation was employed which yields femtosecond UV pulses with corresponding two photon energies (14, 15). Later a few groups directed their studies towards femtosecond kinetic spectroscopy with pumping at 282 nm and probing in the visible, near IR and the ultraviolet ranges. This allowed them to monitor the two photon absorption process in time with subsequent measurement of the UV pump pulse duration. This leads to the determination of the two photon absorption coefficients and quantum yields of hydrated electrons in water.

18 10 Primary Events during the Excitation of Water H 2 O * H 2 O *(1) τ tr τ hyd e - wet e - eq, H 3 0 +,OH,H T j geminate and volume recombination H 2 O Figure 1.3 Model of primary events in pure liquid water at two photon excitation (14) With the absorption of two light quanta (λ= 267 nm) a water molecule acquires an energy of 9.2 ev, more than sufficient to promote the water molecule to the ionization and dissociation threshold energies represented in the scheme above(14,16). Therefore as a result a water molecule may either undergo ionization or dissociation. the ionization channel as shown in the figure 3 (14). Two-photon excitation promotes the water molecule to an excited state of H 2 O *. During the ionization channel, it proceeds via H 2 O *(1) and includes the processes of electron detachment and localization (with time constant τ tr ) forming the so-called wet electron in the pre-hydrated state (e - wet). This is followed by hydration with characteristic time τ hyd producing the hydrated electron (e - eq).then the geminate recombination with positive ion H 2 O + (5) occurs inside a solvent cage with appropriate geminate recombination time. Ions that escape the solvent cage will undergo volume recombination (kinetically second order) over a longer time.

19 11 Other researchers have used alternative approaches to carry out the time resolved studies of fluids rather than the use of conventional time resolved studies of hydrated electrons generated by multi photon ionization of neat water and the observation of the transient absorption of super continuum probe. Recently, a few group of researchers tried to excite the hydrated electron, which is already in the equilibrated form, from its ground s-state to the p-state using a short pulse and the resulting solvation dynamics was probed as a function of time with another delayed pulse (39). By following this route researchers were able to explain how these excited electrons relax to the ground state via not yet equilibrated hot ground state, state where the solvation took place before relaxation back to the ground state, by using the so called three-state model (40).

20 Metal-solvent systems Because of substantial reactivity of the secondary products which are produced in the spurs during the ionization process, researchers have tended to examine other possibilities such as competition from cations and anions by introducing them into the system. Most have focused their interest on metal ions in solutions with different concentrations. In this regard the nature of metal ions, especially alkali and alkaline earth, in different solvents and properties of such solutions have been extensively studied. The solubility of alkali and alkaline earth metals in liquid ammonia is well known and also it is known that in dilute solutions one finds mainly ammoniated metal ions and ammoniated electrons which are independent from each other. As the concentration of the metal ions in the solutions increased the association becomes appreciable and the solutions tend to show the properties of liquid metal. Therefore initial concerns were drawn towards the dilute solutions in which the solvated electrons predominant. During such experiments, two absorption bands are found in metalammonia solutions. One of these found in the infrared region and is nearly independent of the metal solute and it is assigned to the solvated electron. This absorption peak can also be found in the solutions in which the metal ions are absent. The other band is at higher energy is due to the metal ion and the position of the peak depends on the metal. Several groups have examined the solubilities and the properties of the absorption spectra of metal ions in different solvents (5). It has been concluded that photochemical formation of solvated electrons should be possible at least in the more polar solvents where it solvates with suitable absorption bands (due to its charge transfer to the solvent). At the same time it was found that aqueous solutions of inorganic ions such as Cl, Br, I - OH and CN can be photoionized to give e aq. Also, there are a few organic solvents with aromatic ring structure (with low π π* energies) capable of producing intermediates that can release an

21 13 electron to the solvent on flash photolysis (5). Later, quantum yields were determined, tabulated and interpreted further according to the yield of radicals escaping cage recombination in aqueous halide solutions. It was found that the initial yield of (I aq +e aq ) cage is higher than the yield shown in the data tables (5), presumably because I has a substantial fraction recombining geminately. Since then people have been studying and reviewing the process of photolysis of simple inorganic anions in aqueous solution and the spectral nature of the charge transfer to solvent as well as to check whether there is a reasonable photochemical efficiency for hydrated electron production (5). The detailed studies on the absorption spectra of dilute solutions of alkali metals in ammonia and some organic solvents are revealing and have resulted in the characterization of the absorption spectrum of the solvated electron (1, 5, 41). In a liquid consisting of polar molecules with very low (or no) electron affinity, such as water and alcohols, the loss of an electron from a photoexcited anion frequently involves a short lived mediating state which is unique to the particular polar liquid. In a charge transfer to solvent state (CTTS), the excess charge resides in a diffuse orbital that protrudes from the anion into a cavity which consists of solvent molecules. (This diffuse orbital of anions such as halide and hydroxide, where they promote electron from its p-orbital has the primary s-like character). It is found that this type of state is stabilized by the pre-existing orientation of the solvent dipoles and electrostatic attraction is known to stabilize solvated electrons in the bulk liquid. It is also known that a fully detached, thermally relaxed electron is formed as the CTTS state dissociates and the resulting species equilibrates with the solvent (1, 5, 41). Later it was shown that a large fraction of solvated electrons reside in close proximity to their geminate partner and weakly interact with it by means of an attractive potential; they are essentially indistinguishable from the bulk species ( 1, 5, 41). It is still uncertain what fractions of those close pairs are in actual contact or separated by

22 14 a solvent molecule, or what is the barrier between these forms. The Schwartz group interpreted their observation in terms of populations of contact and solvent-separated pairs (5, 41). They interpreted their observation as being due to both types of these pairs, generated in the photoexcitation of small inorganic anions and corresponding branching ratio of some of these anions upon excitation energy (5). In the 1960 s it was found that the absorption spectrum of the hydrated electron is modified in concentrated solutions of ammonia (4, 41) where it shifts its absorption spectrum of the hydrated electron to longer wavelength. This aroused the curiosity and created the interest to investigate the effect of other solutes on the absorption spectrum of e aq. This led researchers to think about the generation of solvated electron in concentrated aqueous electrolytes. In contrast to the case of ammonia, changing the solvents affected the absorption spectrum of e aq shifting its position to shorter or longer wavelength (4, 42, 43). Anbar and Hart were the first to report the properties of electron pulse-generated solvated electrons in concentrated aqueous electrolytes (43). During their experiment, the absorption spectra of solvated electrons in ethylenediamine and in concentrated aqueous solutions were measured by determining the absorption spectra of the transients formed on pulsed radiolysis. They found that the maximum of the absorption band of the hydrated electron shifted from 720 nm in pure water is to shorter wavelengths in concentrated solutions of MgCl 2, KF, NaOH, KOH, NaCLO 4 and LiCl. Further they found that the variation of the rate constant of e aq in different salt solution differed significantly from the corresponding rate constant in water (42, 43).

23 Ultrafast Dynamics of Aqueous Hydroxide Photoinduced detachment of an electron from aqueous hydroxide anion is one of the less studied CTTS reactions in this field (41). Photochemically it yields a geminate pair of hydroxyl radical (OH. ) and hydrated electron ( e aq ). OH OH + e aq.( 1 ) Although the structure of the aqueous hydroxide is simply written as OH, it is a strongly bonded structure where on average 3-4 water molecules are bound to each other though oxygen atoms leading to a strong solvent ordering. Also there is a rapid proton transfer along the hydrogen bonding network. Therefore these structural and dynamical properties of aqueous hydroxide solution offer a new opportunity for the current research (41, 44-46). Species formed during the photoexcitation of OH rapidly recombine both geminately and in the bulk to either reform the parent anion or by deprotonation of the hydroxyl radical with parent anion to yield the O radical anion. The possible reactions in the aqueous hydroxide solution are OH + e aq OH (a) OH + OH O +H 2 O.... (b) O + e aq + H 2 O 2 OH... (c) It was found that reaction (a) is one of the fastest reactions for the hydrated electron and reaction (c) also happens nearly as rapid as the OH radical in reaction (a).

24 16 Later, researchers from Argonne National Laboratory and Department of Chemistry at University of Southern California have studied the charge transfer to solvent for hydroxide ions (41). They used either monophotonic or biphotonic excitation of hydroxide anion in aqueous solution by means of pump-probe ultrafast laser spectroscopy. They studied the transient absorption of hydrated electrons as function of anion (hydroxide) concentration and temperature. They were able to observe that the geminate decay kinetics are bimodal, with a fast exponential component and a slower power tail due to diffusive escape of the electron. They also found that for biphotonic excitation the fraction of escaped electrons is twice that for the monophotonic excitation because of the broadening of the electronic distribution. According to their observation, biphotonic electron detachment is very insignificant and it shows no concentration dependence for the time profile of concentration of the solvated electron between hydroxide concentrations of 10mM to 10M. Further, it also shows that at higher temperatures the escape fraction of electrons increases and the recombination and diffusion controlled dissociation of the close pair become faster (41).

25 Flash Photolysis of Halides Although the halogens (halides and pseudo-halides anions) are well known as efficient electron acceptors in charge transfer complexes with various organic and inorganic substrates, they can also act as electron donors in many ion pairs (47). This dual property accounts for the existence of poly-halide radical anions through the interaction of halogen atom and anion. The existence of polyhalide radical anions as an intermediate in many redox reactions was postulated many years ago (47-49) but the developments in flash photolysis and pulse radiolysis give a new ability to find direct evidence for these species. In conventional flash photolysis polychromatic light is used and the resulting time resolution is of the order of few microseconds which limit the ability to study the reactivity of the radical. Recent developments in pulsed laser photolysis allow researchers to use monochromatic excitation and to reduce of the period of observation to sub - picoseconds. There is no corresponding problem regarding the technique of pulse radiolysis since the electrons are absorbed by the solvent. Most of the studies that have been conducted so far concern the polyhalide radical anion in polar solvents such as water and various alcohols (47). Other low polarity solvents have been used to a lesser extent since they do not favor the solvation of charged species. Researchers ultimately found that whatever the experimental method used to produced polyhalide radicals, in nearly all cases the primary photochemical or radiochemical processes corresponds to the formation of a halogen atom or pseudo-halogen followed by a dark equilibrium reaction with a halide or a polyhalide present in the medium (47).

26 Photolysis of Halides in Aqueous Solution Alkali and alkali earth halides in aqueous solutions have been studied on numerous occasions using conventional flash photolysis (47, 50-52). Grossweiner and Matheson were the first people to observe the optical transient absorption in the near UV due to the unstable species produced by the conventional flash photolysis of KCl, KBr and KI (42, 45). During their experiment they found absorption maximums at 340, 350 and 370 nm respectively and assigned the transient absorption to. Cl. 2, Br. 2 and I. 2. They were able to carry out detailed investigation which revealed that both I 2. and Br 2 show some concentration dependence of their respective decay time. It is found that in. the case of I 2 the decay kinetics was second order and became faster when the concentration of. iodide ion decreases. By contrast Br 2 shows the same decay kinetics but the rate constant decreases as concentration of bromide ion increases. Further experiments have confirmed their decay kinetics and UV band assignments, and revealed a new absorption in the red spectral region which was. assigned to X 2. The simplest mechanism proposed for the production of radical anion is X X * X. + e aq (d) X. + X X 2.. (f) and it was assumed that formation of photo induced X. is due to electron transfer from halide anion to the solvent( 47). Also they proposed decay processes to explain the dependence of radical anion disappearance on the halide ion concentration. To extend the scope of the proceeding investigations, researchers have studied various inorganic compounds containing halide anions in order to understand the electron solvation in water (51). It was found that in all cases transient absorptions were in the UV region and show similar kinetics confirming the assignments and mechanism mentioned above. Further, there were some experiments carried out with mixtures of

27 19 halide anions but there is no evidence of formation of interhalide radical anions. Up to now people have been making numerous attempts to study these so called I /I. system in aqueous and non aqueous solvents. Later research is directed towards different solvents, various types of halide such as trihalide and so on. A paper by M. Anbar and Edwin J. Hart in 1968 (43), was found to be one of the few studies in the area of solvated electrons in highly concentrated electrolytes. In it they discussed how solvent and solute affect the absorption spectrum (43). Other researchers found that ultra fast dynamics of the electrolyte solution offers opportunities for some direct observation of elementary oxidation and reduction reactions with charged reactants (54). It was also found that short time dynamics of charge transfer process between and electron donor and acceptor (cationic) could be influenced by nonequilibrium electronic configurations taking place in solvent bridged ion pairs (54, 55). When the solute-solvent caging effects are strong, early cage back geminate recombination may take place which is the case with the transient charge transfer to solvent state of aqueous halide ions (54, 56). Therefore attention was drawn toward high concentrated alkali and alkali earth metal solutions with different anions. Thus, set out to examine how such solutes could affect the behavior of the hydrated electron, specifically the kinetics and spectral shifts (change in absorption spectra).

28 20 CHAPTER 2: MATERIALS AND METHODS 2.1 Materials Lithium Hydroxide (98%, Sigma-Aldrich, Reagent grade), Sodium Chloride (>99.5%, Fluka & Fisher, Reagent grade), Potassium chloride (>99.5%, Fluka & Fisher, Reagent grade), Lithium Chloride (99%, Sigma-Aldrich, Sigma ultra minimum), Lithium Bromide, Anhydrous (99%, Matheson Coleman & Bell, Reagent grade), Lithium Perchlorate (99.99%, Aldrich, ReagentPlus Grade) were used as received to prepare experimental solutions. A micro pump (Micropump, Inc) and Teflon tubing, 2mm Quartz flow though cuvettes (Uvonic Instruments, Inc), 10mm Quartz Flourometer cells (Sterna cell, Inc) and a calcium fluoride (CaF 2 ) thin window cell were used during the experiments. All the sample solutions were prepared in deionized water with a ph of 6.70.

29 Experimental Section Instrumentation and Method UV- Visible Absorption Spectroscopy A Varian Cary 50 Bio (Varian Corporation) single beam spectrophotometer was used to record the UV-visible electronic spectra of the ground state species. All spectra were recorded at room temperature (293K-295K), using 10mm path length quartz cells. Femtosecond Transient Absorption Spectroscopy All the transient absorption experiments were carried out in the Ohio Laboratory for Kinetic Spectrometry at Bowling Green State University. The experimental setup of the transient absorption spectrometer (shown in Fig. 2.1) was used. It employs a Mai Tai diode-pumped mode-locked Ti: Sapphire laser which generates pulses of 60 fs duration at 80 MHz repetition rate with average power of 700 mw. These pulses are used to seed a Ti: Sapphire amplifier pumped by a Q-switched Nd: YLF laser. The fundamental output of the Ti: Sapphire laser (Hurricane, Spectra Physics) is 1W at 800 nm generated as 100-fs pulses in a 1-kHz train. This output was divided into two parts: one was used to excite the sample and the other was employed for generation of the probe light for monitoring the optical absorption of the excited molecules generated by the pump pulse. For pumping, 92% of the 800nm laser output was sent through a X3 telescope onto a second harmonic generator (SHG) then to a BBO crystal (2 mm length, cut at θ =32 o for type I phase matching), where it provided vertically polarized pulses at 267nm. (The typical energy conversion efficiency of BBO crystal is 25%)

30 22

31 23 A white light continuum was used as the probe pulse. Prior to continuum generation the 800 nm beam goes to an optical variable delay stage, which provides an experimental time window of 1.6 ns with a step resolution of 6.6 fs. An iris diaphragm is used to modulate the beam cross-section to get the most stable white light. In order to avoid dipole-dipole interaction between the probe pulse and transition dipoles of the excited molecules, the polarizer was set up at the magic angle (54.7 o ) with respect to the pump beam polarization. A half-wave plate was used in front of the polarizer to adjust the intensity of the beam and a spherical (f = 10 cm) mirror was used to focus the beam on a 3 mm thick sapphire plate where the white light continuum was generated. Another spherical (f = 5 cm) mirror was used after the sapphire plate to collimate and focus the white light beam onto the sample cell. The pump and probe beams overlap in the sample and the white light experiences an absorption dependence on the presence of the pump. A 750 nm short-pass filter was used to eliminate the remaining fundamental beam and a neutral density filter was used to adjust the intensity of the probe light. An f = 15 cm lens focused the white light beam on to a 400 μm fiber optic cable which serves as the input into a CCD based spectrometer for time- resolved spectral information ( nm).

32 24 CHAPTER 3: RESULTS 3.1 Pure Water The initial stage of the experiment studied the photo detachment of electrons from water using two-photon excitation. Various types of laboratory available waters were tried and finally came to a conclusion that deionized water was the best option. Therefore, throughout our experiments distilled deionized water was used. A calcium fluoride (CaF 2 ) thin window cell with optical path of 0.09mm was used throughout the experiment. During the measurement the water cell was continuously monitored and kept perpendicular to the pump beam in order to avoid any air bubbles due to undesirable thermal effects caused by the pump beam. A first attempt was to reproduce the absorption spectrum of the hydrated electron in water which has maximum absorption at 720nm (38). The absorption spectrum obtained by irradiating pure (distilled deionized) water with 267nm pulses is shown in the figure 3.1; the maximum absorption occurred at 650nm. The difference in the reported value and that recorded here may be due to the calcium fluoride (CaF 2 ) window cell reacting in some manner with the solvent water. To check this, the experiment was repeated using a 2 mm quartz cuvette. Figure 3.2 shows the maximum absorption at 665 nm and another smaller absorption at around 720 nm.

33 Transient Absorption delta A (a.u.) Wavelength / nm Figure 3.1: Transient absorption spectrum of distilled deionized water (at 80ps delay time) in 0.09mm calcium fluoride (CaF 2 ) thin window cell Transient Absorption delta A (a.u.) Wavelength / nm Figure 3.2: Transient absorption spectrum of distilled deionized water (at 114ps delay time) in 2mm quartz cuvette.

34 26 Figure 3.3 shows the time profile of the absorption at 650 nm. The decay of the signal was clearly biphasic delta A Time/ps Figure 3.3 Transient absorption kinetic data for a 0.09 mm cell of neat water excited with 267nm femtosecond pulses (maximum absorption at 650 nm) Data: Data6_B Model: ExpDec2 The signal rose to the maximum value in close to ~3 ps and then decayed over a 20 ps to give a Chi^2/DoF = E-7 R^2 = long lived absorption change. According to the literature this picosecond relaxation is due to the y ± A ± t ± A ± t ± geminate recombination of the hydrated electron with the H ion (12).

35 27 We evaluated the lifetime of the geminate recombination process at 267 nm pump by fitting the data to a double exponential decay. It was found that geminate recombination lifetime of water was 7.6 ± 1.4 ps. The literature records geminate recombination life times between 2.7 ± 5 ps and 12.2 ± 2.2 ps (12). The observed lifetime for the geminate process falls into the range in the literature.

36 Concentrated Alkali Metal Halide Solutions To investigate the behavior of transient species in highly concentrated aqueous ionic halide solutions, lithium compounds with different anions such as lithium hydroxide(lioh), lithium chloride ( LiCl), lithium bromide( LiBr), lithium perchlorate ( LiClO 4 ) and lithium iodide (LiI) were selected. These lithium compounds were used to prepare the highest concentrated solution that could be prepared. Table 3.1: Table of lithium compounds with their respective highest concentrations Lithium Compound Concentration (Mol/ L) Lithium Hydroxide(LiOH) 3 Lithium Chloride( LiCl) 15 Lithium Bromide( LiBr) 15 Lithium Perchlorate ( LiClO 4 ) 10 Lithium Iodide (LiI) 3 Transient absorption spectra were recorded for these lithium compounds at the concentrations as shown in table 3.2. A 267 nm pump beam was used which produced 4 mj/s energy light on the sample as shown in the figure 3.4. A 2 mm quartz flow through cell was employed in order to minimize buildup of photo products. The flow rate was of 2 ml/s. The excitation of LiCl, LiBr and LiClO 4 gave the highest absorption with well defined absorption maxima; other solutions such as LiI and LiOH gave absorption spectra similar to that seen for water. LiCl at 15 M concentration gave an absorption maximum near 590 nm and 15M LiBr near 640 nm (figure 3.4). LiCl showed an absorption maximum around the same wavelength (590nm) as reported by Anbar and Hart in 1968 (38).

37 water 10M LiClO 4 3M LiOH 15M LiCl 3M LiI 15M LiBr delta A (a.u.) Wavelength / nm Figure 3.4 Plot of Transient Absorption (ΔA) vs Wavelength (nm) of different lithium compound (in 2 mm flow through cuvette) excited with 267nm femtosecond pulses room temperature. Figure 3.5 shows the time profiles obtained by 267nm pump excitation of the above lithium compounds (water is included, for comparison). Qualitatively the kinetics exhibit two major regimes; during the first few picoseconds there is a fast evolution of the photoinduced absorption followed by a decay. The two species LiI and LiOH show different decay kinetics than the other species. Those two traces showed the presence of a short lived (few ps) species (similar to H 2 O), whereas other lithium compounds, such as LiCl, LiBr and LiClO 4, showed decay kinetics in the hundreds of ps range. This type of spectral evolution in the first 2-3 ps was obtained in all the experiments on ionization of neat water (36).

38 30 Figure 3.6 shows the first 3 ps of the kinetic profiles, focusing on the signal rise. The data were fitted to the first order exponential rise using Origin software and the parameters are collected in table 3.2. Transiant Absorpton delta A (a.u.) water 10M LiClO 4 3M LiOH 15M LiCl 3M LiI 15M LiBr Time / ps Figure 3.5 Kinetic plots of data for different lithium compounds (in 2 mm flow through cuvette) excited with 267nm femtosecond pulses (at their respective maximum absorption wavelength).

39 31 Transiant Absorpton delta A (a.u.) water 10M LiClO 4 3M LiOH 15M LiCl 3M LiI 15M LiBr Time / ps Figure 3.6 Plot of transient absorption kinetic data for different lithium compounds (in 2 mm flow through cuvette) excited with 267nm femtosecond pulses (at their respective maximum absorption wavelength). Table 3.2: First order fitting parameters for the transient absorption kinetic data Compound Probing Wavelength ( nm ) T ab (ps) water ± 0.47 LiCl ± 0.07 LiBr ± 0.10 LiI ± 0.13 LiClO ± 0.76 LiOH ± 4.67

40 32 Preliminary work showed a significant permanent difference in the ground state UV spectrum of LiCl solution before and after the radiation as shown the figure 3.7. but the other 1.0 before after Absorption Wavelength / nm Figure 3.7 Plot of Absorption vs. Wavelength for LiCl before and after the experiment. samples did not show such an effect. The UV spectrum, as in the figure 3.7, recorded after the transient experiment showed a new peak at around 256 nm. This gives us clear evidence of formation of a new species during transient experiment. According to the literature an absorption peak around this wavelength may be due to the formation of polychloride (Cl 3 ) ions.

41 33 Reactions that are possible in the aqueous solution of LiCl in the formation of chlorine gas and polychloride ion included: LiCl Cl hν Li + + Cl Cl. + e Cl. + Cl. Cl 2 Cl. + Cl Cl 2. Cl 2. + Cl. Cl 2. + Cl 2. Cl 3 Cl 3 + Cl

42 LiCl Solutions; Effect of Concentration Experiments were carried out with a series of LiCl Solutions at different concentrations. The series of solutions with six different concentrations of LiCl were prepared using distilled deionized water. Transient absorption spectroscopic data (figure 3.7) were recorded along with the ground state absorption spectroscopic data (before and after the experiment). The kinetic data at the maximum absorption of the transient absorption spectrums was recorded at each different concentration. Normalized OD(a.u) M LiCl 2.5M LICl 5.0M LiCl 7.5M LiCl 10.0M LiCl 15.0M LiCl Time / ps Figure 3.8 Transient absorption spectra of LiCl at different concentrations (in 2 mm flow through cuvette) excited with 267nm femtosecond pulses at room temperature.

43 35 The transient absorption spectra of LiCl show (figure 3.7) that the absorption maxima depend upon the concentration. With increasing concentration the absorption maxima are shifted towards shorter wavelengths. Thus 15M LiCl shows a maximum around 590 nm while 1M LiCl shows a maximum around 650nm. Thus, as the concentration of the LiCl decreased, the absorption maxima were red shifted and approaches that of the hydrated electron in pure water (720 nm). Time profiles at the absorption maxima were normalized and fitted to a two exponential decay function. The fitted values for absorption and decay times of the high and low concentrated LiCl solutions were compared. This shows two-photon photoexcitation yield different absorption and decay kinetics for high and low concentrated solutions at nearly the same total excitation energy. These results show a slower rate for absorption growth and decay for the high concentration samples and a higher rate for low concentration samples (Figure 3.8).