Investigating the stability of a laser-based plasma x-ray source

The use of laser based plasma x-ray sources (LPXS) has quickly expanded during the past decades due to rapid development of ultrafast laser systems. These sources are used in many research applications such as emission, absorption and particularly time-resolved spectroscopy. From the LPXS x-ray pulses are generated when an intense laser pulse is focused onto a liquid or solid interface in gas or vacuum. In this thesis we investigate the source stability which we define as the x-ray flux and spectrum reproducibility from each generated x-ray pulse. Understanding the stability is of great importance for its research applications. A basic theory of the LPXS is introduced describing relevant parameters for the source stability. Two parameters of relevance for the LPXS stability were investigated: The laser pointing and pulse energy fluctuations. These were experimentally determined using a beam profiler, capturing the beam profile of each laser pulse at 1 kHz. The relevance of these parameter fluctuations to the source stability is discussed based on these measurements. An existing LPXS setup was reconstructed in preparation for shot to shot stability measurements. A synchronized chopper system was built to decrease the laser pulse frequency for single pulsed mode and a program was developed for analyzing the x-ray photons captured by an x-ray CCD camera. Problems with the laser system prevented successful gathering of data within the time of the project. Future measurements based on these preparations will reveal the stability of the LPXS.Det mänskliga ögat observerar världen i formen av synligt ljus med olika färger. Allt ljus kan beskrivas som elektromagnetiska vågor med en viss våglängd, längre våglängder ger rött och kortare ger violett ljus. Utanför det för ögat synliga ljuset finner man röntgenstrålning med våglängder mycket kortare än både violett och ultraviolett ljus. Ända sedan dess upptäckt för över 100 år sedan har man funnit mängder av användningsområden för denna typen av ljus. Röntgenstrålning passerar oförhindrat genom många fasta material och används därför inom sjukvården för att se inuti kroppen. Den korta våglängden gör det även möjligt att studera mycket små saker som annars är osynliga med vanligt ljus. Röntgenstrålning är därför ett utmärkt verktyg inom forskning för att undersöka egenskaper hos molekyler och atomer. Det finns många olika metoder idag för att producera röntgenstrålning. I detta arbete har en mycket speciell typ av röntgenkälla undersökts, nämligen en s.k.laser-plasma-röntgenkälla. Denna röntgenkälla genererar korta intensiva röntgenpulser likt en stark kamera blixt. Röntgenkällan fungerar genom att fokusera en stark laserpuls med en enorm effekt på 1.5 biljoner watt och en kortvarighet på 40 femtosekunder (femtonde decimalen av en sekund). Laserpulsen fokuseras på en vattenstråle inte mycket bredare än ett hårstrå. Då den intensiva laserpulsen träffar vattenytan värms ytan upp så pass kraftigt att molekylerna och atomerna slits isär, vilket bildar ett plasma likt solen, bestående utav fria joner och elektroner. Under den kortvariga, men mycket våldsamma interaktionen accelereras och kolliderar elektroner, vilket resulterar i en skarp blixt av röntgenstrålning. Denna skarpa röntgenblixt kan riktas mot ett material som skall undersökas. När röntgenblixten interagerar med materialet kan ljuset som sänds ut från materialet detekteras av en mycket känslig detektor, likt den i en digitalkamera, där bilden kan återskapas. Den informationen som sensorn fångar in kan då analyseras för att ta reda på egenskaper hos materialet. Ju snabbare och intensivare röntgenblixten är destu bättre blir bildkvaliteten. Den extremt kortvariga röntgenblixten gör det möjligt att observera egenskaper hos molekyler och atomer med en mycket god upplösning. Likt en digitalkamera är det mycket viktigt att bildkvaliten är stabil och inte ändras mellan varje bild. I detta arbete undersöks stabiliteten hos denna röntgenkällan genom att bestämma laserpulsens träffsäkerhet och styrka på den hårtunna vattenstrålen. Information om dess stabilitet kan användas för att i slutändan förbättra stabiliteten hos röngenkällan och på så vis förbättra bildkvaliteten med denna speciella kamera för framtida forskning

Ultrafast Excited-State Dynamics of Rhenium(I) Photosensitizers [Re(Cl)(CO)_(3)(N,N)] and [Re(imidazole)(CO)_(3)(N,N)]^+: Diimine Effects

Femto- to picosecond excited-state dynamics of the complexes [Re(L)(CO)_(3)(N,N)]^n (N,N = bpy, phen, 4,7-dimethyl-phen (dmp); L = Cl, n = 0; L = imidazole, n = 1+) were investigated using fluorescence up-conversion, transient absorption in the 650−285 nm range (using broad-band UV probe pulses around 300 nm) and picosecond time-resolved IR (TRIR) spectroscopy in the region of CO stretching vibrations. Optically populated singlet charge-transfer (CT) state(s) undergo femtosecond intersystem crossing to at least two hot triplet states with a rate that is faster in Cl (~100 fs)^(−1) than in imidazole (~150 fs)^(−1) complexes but essentially independent of the N,N ligand. TRIR spectra indicate the presence of two long-lived triplet states that are populated simultaneously and equilibrate in a few picoseconds. The minor state accounts for less than 20% of the relaxed excited population. UV−vis transient spectra were assigned using open-shell time-dependent density functional theory calculations on the lowest triplet CT state. Visible excited-state absorption originates mostly from mixed L;N,N^(•−) → Re^(II) ligand-to-metal CT transitions. Excited bpy complexes show the characteristic sharp near-UV band (Cl, 373 nm; imH, 365 nm) due to two predominantly ππ*(bpy^(•−)) transitions. For phen and dmp, the UV excited-state absorption occurs at 305 nm, originating from a series of mixed ππ* and Re → CO;N,N•− MLCT transitions. UV−vis transient absorption features exhibit small intensity- and band-shape changes occurring with several lifetimes in the 1−5 ps range, while TRIR bands show small intensity changes (≤5 ps) and shifts (~1 and 6−10 ps) to higher wavenumbers. These spectral changes are attributable to convoluted electronic and vibrational relaxation steps and equilibration between the two lowest triplets. Still slower changes (≥15 ps), manifested mostly by the excited-state UV band, probably involve local-solvent restructuring. Implications of the observed excited-state behavior for the development and use of Re-based sensitizers and probes are discussed

EXAFS Structural Determination of the Pt2(P2O5H2)44– Anion in Solution

We present the first structural determination of the Pt2(P2O5H2)44– anion in solution by analyzing the extended X-ray absorption fine structure (EXAFS) spectrum of the Pt LIII edge. The data could be fit with a simple model involving single and multiple scattering paths to near and far P-atoms, bridging O-atoms, and the other Pt-atom in the binuclear complex. A Pt–Pt distance of 2.876(28) Å and a Pt–P bond length of 2.32(4) Å are obtained. These values are in line with distances found in previous X-ray diffraction studies. The assignment of the EXAFS spectrum of the Pt2(P2O5H2)44– anion in its ground state is required for future time-resolved X-ray absorption measurements with the goal of determining the structure and dynamics of the complex in the 1,3A2u excited states

Retrieving photochemically active structures by time-resolved EXAFS spectroscopy

Describing the nature and structure of molecular excited states is important in order to understand their chemical reactivity and role as intermediates in photochemical reactions. The recent implementation of x-ray absorption spectroscopy in the ultrafast time domain allows studying the electronic and structural dynamics of photochemically active molecules in solutions. In this work we present the structural determination of a photoexcited diplatinum molecule, [Pt-2(P2O5H2)(4)](4-), which plays a photocatalytic role in important chemical conversions. A novel analysis of time-resolved EXAFS spectra based on the fitting of the experimental transients obtained from optical pump/x-ray probe experiments has been performed to derive a contraction of 0.31(5) angstrom of the two Pt atoms and a ligand expansion of 0.010(6) angstrom. The former is assigned to the formation of a transient Pt-Pt bond in the excited state, while the latter indicates a concomitant weakening of the Pt-ligand coordination bonds

Time-Resolved Optical and X-Ray Spectroscopy of Rhenium Based Molecular Complexes

The photocycle of rhenium carbonyl complexes, type facial-[Re(L)(CO)3(diimine)]n [L=halide / n=0, L=4-Ethyl-pyridine (Etpy)- Imidazole (ImH) /n=+1, diimine= bpy , 10- phenanthroline (phen), 4,4'-dimethyl-2,2'-bpy (dmb)] was studied using a set of ultrafast spectroscopic techniques: Fluorescence up-conversion, transient absorption and X-ray absorption spectroscopy. Relaxation of the initially excited singlet charge transfer (CT) b1A' state was investigated using fluorescence up-conversion (FlUC). The excited b1A' state undergoes intersystem crossing (ISC) simultaneously to two triplet states (a3A'' and b3A'') with a time constant τ1 in the 85-160 fs range that depends on the ligand L. An internal conversion process between the involved triplet states was found to occur with a solvent dependant lifetime τ2 of ∼500 fs in CH3CN and ∼1.4 ps in DMF. Femtosecond transient absorption measurements of the bpy-containing complexes revealed two processes. The first one, occurring in the same time range as the internal conversion measured using FlUC, also showed a solvent dependence. We attribute it to vibrational and electronic relaxation as well as solvation leading to population of the a3A'' state. The second strongly solvent-dependant process, is manifested mainly by a continuous rise of the bpy- absorption band at 370 nm. We assign it to reorganization within a supramolecular cluster consisting of the [Re(L)(CO)3(bpy)]n chromophore and several strongly interacting local solvent molecules, probably intercalated between the ligands. In this series of complexes, theory predicts that the ligand (L) is important in tuning the character of the lowest excited state. For an electron-rich L ligand, the time-dependent density functional theory (TD-DFT) calculations characterize the lowest CT state as a metal-ligand-to-ligand-charge-transfer (MLLCT) state which leads to the reduction of the diimine ligand. To confirm this prediction and to extract structural information about the excited state, we used X-ray absorption spectroscopy. We first characterized the ground state by static measurements at the Re L3 and the Br K edges of the Re(Br)(CO)3bpy complex. Picosecond X-ray absorption measurements of the excited complex revealed a charge transfer from both the Re and the Br sites, confirming thus the mixed excited state character. These results constitute the first evidence about charge transfer from a monoatomic ligand containing Re complex. The excited state structure was also extracted by an analysis of the XANES and EXAFS spectra at the Re L3 edge. It involved contraction of the Re-N and Re-Br bond distances, and elongation of the Re-C distance, supporting thus the MLLCT character. The structural analysis confirms the prediction from the TD-DFT calculations

Photochemistry of Pheomelanin Building Blocks and Model Chromophores: Excited-State Intra- and Intermolecular Proton Transfer

Pheomelanins, the epidermal pigments of red-haired people responsible for their enhanced UV susceptibility, contain 1,4-benzothiazines and 1,3-benzothiazole as main structural components. Despite the major role played in pheomelanin phototoxicity, the photoreactivity of these species has so far remained unexplored. Static and time-resolved fluorescence spectroscopy was used to identify excited-state reactions of the two main pheomelanin benzothiazole building blocks, namely, the 6-(2-amino-2-carboxyethyl)-4-hydroxy-1,3-benzothiazole (BT) and the 2-carboxy derivative (BTCA) together with model chromophores lacking some of the ionizable functions. The results show that in aqueous buffer solution the OH at 4-position and the benzothiazole nitrogen atom control the photochemistry of both BT and BTCA via excited-state proton transfer to solvent (ESPT) and excited-state intramolecular proton transfer (ESIPT), while the amino acidic groups of the alanyl chain have a minor influence on the photochemistry. The ESPT and ESIPT produce several different excited-state ionic species with lifetimes ranging from similar to 100 ps to similar to 3 ns

Sequential Proton-Coupled Electron Transfer Mediates Excited-State Deactivation of a Eumelanin Building Block

Skin photoprotection is commonly believed to rely on the photochemistry of 5,6-dihydroxyindole (DHI)- and 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-based eumelanin building blocks. Attempts to elucidate the underlying excited-state relaxation mechanisms have been partly unsuccessful due to the marked instability to oxidation. We report a study of the excited-state deactivation of DHI using steady-state and time-resolved fluorescence accompanied by high-level quantum-chemistry calculations including solvent effects. Spectroscopic data show that deactivation of the lowest excited state of DHI in aqueous buffer proceeds on the 100 ps time scale and is 20 times faster than in methanol. Quantum-chemical calculations reveal that the excited-state decay mechanism is a sequential proton-coupled electron transfer, which involves the initial formation of a solvated electron from DHI, followed by the transfer of a proton to the solvent. This unexpected finding would prompt a revision of current notions about eumelanin photophysics and photobiology

Femtosecond Fluorescence and Intersystem Crossing in Rhenium(I) Carbonyl-Bipyridine Complexes

Ultrafast electronic-vibrational relaxation upon excitation of the singlet charge-transfer b1A' state of [Re(L)(CO)3(bpy)]n (L = Cl, Br, I, n = 0; L = 4-Et-pyridine, n = 1+) in acetonitrile was investigated using the femtosecond fluorescence up-conversion technique with polychromatic detection. In addn., energies, characters, and mol. structures of the emitting states were calcd. by TD-DFT. The luminescence is characterized by a broad fluorescence band at very short times, and evolves to the steady-state phosphorescence spectrum from the a3A" state at longer times. The anal. of the data allows us to identify three spectral components. The first two are characterized by decay times t1 = 85-150 fs and t2 = 340-1200 fs, depending on L, and are identified as fluorescence from the initially excited singlet state and phosphorescence from a higher triplet state (b3A"), resp. The third component corresponds to the long-lived phosphorescence from the lowest a3A" state. In addn., it is found that the fluorescence decay time (t1) corresponds to the intersystem crossing (ISC) time to the two emissive triplet states. t2 corresponds to internal conversion among triplet states. DFT results show that ISC involves electron exchange in orthogonal, largely Re-localized, MOs, whereby the total electron momentum is conserved. Surprisingly, the measured ISC rates scale inversely with the spin-orbit coupling const. of the ligand L, but the authors find a clear correlation between the ISC times and the vibrational periods of the Re-L mode, suggesting that the latter may mediate the ISC in a strongly nonadiabatic regime

Ultrafast Excited-State Dynamics ef [Re(L)(CO)(3)(bpy)](n) Complexes: Involvement of the Solvent

Ultrafast excited-state dynamics of [Re(L)(CO)(3)(bpy)](n) (L = Cl, Br, n = 0; L = 4-ethyl-pyridine (Etpy), n = 1+; bpy = 2,2'-bipyridine) have been investigated in dimethylformamide (DMF) solution by fluorescence up-conversion (FIUC) and UV - vis transient absorption (TA) with similar to 100 is time,resolution. TA was also measured in the [1-ethyl-3-methyl-imidazolium]BF4 ionic liquid. The complexes show a very broad fluorescence band at 540-550 nm at zero time delay, which decays with 100-140 Is (depending on L) by intersystem crossing (ISC) to pi pi* intraligand ((IL)-I-3) and a Re(L)(CO)(3) -> bpy charge-transfer ((CT)-C-3) excited states. A second emission decay component (1.1-1.7 ps). apparent in the red part of the spectrum, is attributed to (IL)-I-3 -> (CT)-C-3 conversion, leaving phosphorescence from the lowest (CT)-C-3 state as the only emission signal at longer time delays. The triplet conversion is slower in DMF? than acetonitrile, commensurate with solvation times. Full assignment of the excited-state absorption at long delay times is obtained by TD-DFT calculations on the lowest triplet state, showing,2 that the 373 nm band is the sole diagnostics of bpy reduction in the CT excited state. Bands in the visible are clue to Ligand-to-Metal-Charge-Transfer (LMCT) transitions. Time-resolved UV - vis absorption spectra exhibit a units-of-ps rise of all absorption features attributed to (IL)-I-3 -> (CT)-C-3 conversion as well as electronic and vibrational relaxation, and a similar to 15 ps rise of only the 373 nm pi pi*(bpy(center dot-)) band which slows down to similar to 1 ns in the ionic liquid solvent. It is proposed that this slow relaxation originates mainly from restructuring of solvent molecules that are found very close to the metal center, inserted between the ligands. The solvent thus plays a key role in controlling the intramolecular charge separation, and this effect may well be operative other classes of metal-based molecular complexes