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ABSTRACT: We present a novel analysis of time-resolved extended x-ray absorption fine structure (EXAFS) spectra based on the fitting of the experimental transients obtained from optical pump/x-ray probe experiments. We apply it to the analysis of picosecond EXAFS data on aqueous [Fe(II)(bpy)(3)](2+), which undergoes a light induced conversion from its low-spin (LS) ground state to the short-lived (tau approximately 650 ps) excited high-spin (HS) state. A series of EXAFS spectra were simulated for a collection of possible HS structures from which the ground state fit spectrum was subtracted to generate transient difference absorption (TA) spectra. These are then compared with the experimental TA spectrum using a least-squares statistical analysis to derive the structural change. This approach reduces the number of required parameters by cancellation in the differences. It also delivers a unique solution for both the fractional population and the extracted excited state structure. We thus obtain a value of the Fe-N bond elongation in the HS state with subpicometer precision (0.203+/-0.008 A).
The Journal of chemical physics 04/2009; 130(12):124520. · 3.09 Impact Factor
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G Aeppli,
R Allemann,
A Almond,
D Angal-Kalinin,
M Ashfold,
V Averbukh,
C Bagshaw,
M Barahona,
P Barker,
R Bartolini, [......],
A Wolf,
J Womersley,
P Woodruff,
N Woolsey,
S Yaliraki,
E Yates,
A Zeitler,
M Zepf,
A Zholents,
B Martl
01/2009;
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ABSTRACT: We report a femtosecond pump-probe study on the photochemistry of concentrated aqueous solutions of [RuII(bpy)3]2+, as a function of pump power (up to 2 TW/cm2) at 400 nm excitation. The transient absorption spectra in the 345-660 nm range up to 1 ns time delay enable the observation of the following photoproducts: the triplet 3MLCT (metal-to-ligand-charge-transfer) excited state, the reduced form [RuII(bpy)3]+, the oxidized species [RuIII(bpy)3]3+, and the solvated electron e(aq). The 3MLCT state is formed within the excitation pulse and undergoes vibrational relaxation in 3-5 ps, as evidenced by the shift of the ligand-centered (LC) absorption band below 400 nm. Even at the highest pump powers, the majority of e(aq) originates from multiphoton ionization of [RuII(bpy)3]2+ and not from the solvent, generating [RuIII(bpy)3]3+ as a byproduct. At 10 ps time delay, the total concentration of the three product species is balanced by the depleted concentration of [RuII(bpy)3]2+, even at the highest fluences used, indicating that no further reaction products significantly contribute to the overall photochemistry. On the 100 ps time scale, most probably diffusion-controlled reduction of ground-state [RuII(bpy)3]2+ by solvated electrons occurs, next to recombination between e(aq) and [RuIII(bpy)3]3+.
The Journal of Physical Chemistry B 01/2007; 110(51):26497-505. · 3.70 Impact Factor
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ABSTRACT: We describe an experimental setup, which measures transient chemical changes of photoexcited solutes in disordered systems via time-resolved X-ray Absorption Spectroscopy (XAS) with picosecond temporal resolution. The setup combines a femtosecond amplified laser with picosecond x-ray pulses at beamline 5.3.1 at the Advanced Light Source in Berkeley, USA. New results on time-resolved XAS used for probing both the electronic and the geometric modifications of a photoexcited tris-(2,2'-bipyridine) ruthenium (II), [RuII(bpy)3]2+ in water solution are presented.
Physica Scripta 11/2006; 2005(T115):102. · 1.20 Impact Factor
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ABSTRACT: The mean value and the distribution of the penetration depth of F atoms is determined from samples composed of three layers with controlled thickness in the monolayer range and with a test of the compactness of the films via the intensity of surface excitons. F atoms with an average kinetic energy of 4.3 eV are generated in the top layer (Ar doped with F2) by photodissociation of F2 with 10.15 eV in a spin forbidden repulsive state. The F atoms are injected into an Ar spacer layer of variable thickness. Those reaching the interface to the Kr bottom layer are monitored via the intensity of the Kr2F fluorescence at a wavelength of 444 nm, which allows one to discriminate between F in Ar (439 nm), in Kr (453 nm), and at the Kr/Ar (444 nm) interface. The F content at the interface is kept below 1/20th of a monolayer to suppress recombination, and the detection sensitivity is increased to 1/1000th of a monolayer by excitation via Kr exciton energy transfer. The probability for F atoms to penetrate the Ar spacer layer decreases exponentially with increasing thickness down to 10% for a thickness of 23 monolayers, and an average penetration depth of 10 monolayers is derived. These very large penetration depths exceed those of F+ and F− ions by more than one order of magnitude. They are consistent with those molecular dynamics calculations, which predict a rather rectilinear motion in channels of the Ar lattice. An average length of travel of up to 27 monolayers with a mean-free path (large angle scattering) up to four monolayers is compatible with the results. © 1997 American Institute of Physics.
The Journal of Chemical Physics 12/1997; 107(23):10268-10277. · 3.33 Impact Factor
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ABSTRACT: The spectroscopy of argon fluoride and krypton fluoride exciplexes in rare gas matrices is investigated using synchrotron radiation over the range of 50 to 600 nm. The diatomic Ar+F− and Kr+F− species are observed in excitation and emission spectra. Extensive vibrational progressions are observed in the excitation spectra and are used to determine the spectroscopic parameters of the ArF B state (Te=50233 cm−1,ωe=415.5 cm−1, ωexe=3.1 cm−1) and D state (Te=51738 cm−1,ωe=476.7 cm−1,ωexe=3.8 cm−1) and of the KrF B state (Te=39024 cm−1,ωe=342.4 cm−1,ωexe=2.0 cm−1) and D state (Te=44479 cm−1,ωe=331.6 cm−1,ωexe=1.4 cm−1). Lifetimes of 4 to 8 ns are measured for the diatomic emissions. A perturbed KrF species is observed which is identified in analogy to a similarly perturbed XeF species. Excitation spectra of the triatomic exciplexes Kr+2F− and Ar+2F− in neon are presented and described in terms of previous ab initio calculations. Emission and excitation of argon fluoride and krypton fluoride species in other matrices are presented. Large stimulated emission cross sections on the order of 10−16 cm2 are determined for the diatomic D→X and B→X emissions, which, together with nonradiative relaxation processes within the B and D states of ArF that efficiently populate the B(v=0) regardless of initial excitation, make ArF in neon a viable solid‐state laser candidate. © 1996 American Institute of Physics.
The Journal of Chemical Physics 12/1996; 105(23):10178-10188. · 3.33 Impact Factor
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ABSTRACT: The interaction of an electronically excited F atom with the neon matrix is investigated using absorption, excitation, and fluorescence spectroscopy. Upon vacuum ultraviolet excitation of a neon matrix doped with fluorine atoms, emissions are observed at 12.80, 13.08, and 15.48 eV. The emissions at 13.08 and 15.48 eV have a lifetime of 0.7(0.2) ns and are assigned to the radiative relaxation of the 3s and 3s′ Rydberg F atomic states. The emission at 12.80 eV has a detection limited lifetime less than 0.4 ns and is identified as emission from the Ne+F− charge transfer complex. Absorption and excitation spectra of each of the observed emissions are used to identify the Rydberg absorptions of the F atoms perturbed by the neon matrix at 13.99, 16.27, 16.49, 16.94, 17.22, and 19.02 eV. The Rydberg states belong to ns, ns′, and nd progressions with the same quantum defect as in the gas phase and a blueshift of the vertical ionization energies of 0.8 eV. The Stokes shift of 910 and 790 meV for the 3s and 3s′ states and the large linewidth are attributed to a strong electron phonon coupling with Huang–Rhys factors of about 70. A two‐dimensional configuration coordinate model explains the observed absorption, excitation and emission spectra, and the branching ratios of emission from Rydberg and charge transfer states. © 1995 American Institute of Physics.
The Journal of Chemical Physics 12/1994; 102(1):48-56. · 3.33 Impact Factor
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W. Gawelda,
V.-T. Pham,
A. El Nahhas,
M. Kaiser,
Y. Zaushitsyn,
S L Johnson,
D. Grolimund,
R. Abela,
A. Hauser, C. Bressler,
M. Chergui
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ABSTRACT: We describe an advanced setup for time-resolved x-ray absorption fine structure (XAFS) spectroscopy with picosecond temporal resoln. It combines an intense femtosecond laser source synchronized to the x-ray pulses delivered into the microXAS beamline of the Swiss Light Source (SLS). The setup is applied to measure the short-lived high-spin geometric structure of photoexcited aq. Fe(bpy)3 at room temp. [on SciFinder (R)]
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ABSTRACT: Environmental effects on electronic states of small molecules in solid rare gases and in liquids have severe consequences on the photochemistry; examples for valence, Rydberg and ionic states as well as the connection to photochemical dry etching are treated. The repulsive interaction of Rydberg states and the solvation of valence and ionic states are related to spectroscopic properties like gas-to-matrix shifts. Stokes shifts and linewidths. Differential shifts of Rydberg and valence states are illustrated for NO in rare gas matrices and Fano-type line shapes are attributed to a coherent interaction of broad Rydberg multiphonon continua with valence zero phonon lines. For F2 in Ne a deperturbation of ion-pair and Rydberg states is demonstrated again to originate from a differential shift. For Cl2 the formation of additional states due to intermolecular charge transfer from the matrix to the guest molecules is observed. H2O serves as an example to show that repulsive surfaces are strongly modified if the character of the state changes from Rydberg type to valence type with internuclear distance. Information on the dynamics in the femtosecond time scale is derived from intensities in overtone progressions of resonance Raman spectra and it is demonstrated that the solvent induces predissociation of the B 3Π0u + state of I2 within 300 fs in CCl4, and within 40 fs in liquid Xe.
Chemical Physics.
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ABSTRACT: The F2 laser emission in Ne matrices at 160.6 nm is reported for the first time. Its excitation spectrum in the 11–16 eV range has been recorded by means of tunable synchrotron radiation. Anomalous envelopes of vibrational progressions occuring in the gas phase due to avoided crossings of ion-pair1Σ+u states and Rydberg1Σ+u states are shown to disappear in Ne matrices. The matrix spectra are discussed in terms of a deperturbed C1Σ+u ion-pair state and strongly blue-shifted I1Σ+u and H1Пu Rydberg states.
Journal of Luminescence.
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ABSTRACT: The authors present a new technique to record single x-ray pulses in time-resolved x-ray absorption spectra spectroscopy (XAS) expts. at low frequencies around 1 kHz compared to the multibunch repetition rate at synchrotron radiation storage rings of several hundred MHz. This scheme of electronic gating of the multibunch train is applied to record high-quality EXAFS spectra. Further measurements show that this scheme permits the authors to record high-quality x-ray absorption spectra with a noise level <3 * 10-3. The achieved results imply the utility of this method with current synchrotron radiation sources. [on SciFinder (R)]
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ABSTRACT: Environmental effects on electronic states of small mols. in solid rare gases and in liqs. have severe consequences on the photochem.; examples for valence, Rydberg and ionic states as well as the connection to photochem. dry etching are treated. The repulsive interaction of Rydberg states and the solvation of valence and ionic states are related to spectroscopic properties like gas-to-matrix shifts, Stokes shifts and linewidths. Differential shifts of Rydberg and valence states are illustrated for NO in rare gas matrixes and Fano-type line shapes are attributed to a coherent interaction of broad Rydberg multiphonon continua with valence zero phonon lines. For F2 in Ne a deperturbation of ion-pair and Rydberg states is demonstrated again to originate from a differential shift. For Cl2 the formation of addnl. states due to intermol. charge transfer from the matrix to the guest mols. is obsd. H2O serves as an example to show that repulsive surfaces are strongly modified if the character of the state changes from Rydberg type to valence type with internuclear distance. Information on the dynamics in the femtosecond time scale is derived from intensities in overtone progressions of resonance Raman spectra and it is demonstrated that the solvent induces predissocn. of the B3P0 +u of I2 state within 300 fs in CCl4, and within 40 fs in liq. Xe. 62 Refs. [on SciFinder (R)]
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ABSTRACT: Calcns. are presented of the optimum conditions for performing a laser-pump x-ray probe time-resolved x-ray absorption expt. The results concerning sensitivity and feasibility for implementing the method are illustrated for the case of the nascent I radical environment following I- photolysis in H2O. [on SciFinder (R)]
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ABSTRACT: Time-resolved x-ray absorption fine structure (XAFS) spectroscopy with picosecond temporal resoln. is a new method to observe electronic and geometric structures of short-lived reaction intermediates. It combines an intense femtosecond laser source synchronized to the x-ray pulses delivered into the microXAS beamline of the Swiss Light Source (SLS). We present key expts. on charge transfer reactions as well as spin-crossover processes in coordination chem. compds. next to solvation dynamics studies of photogenerated at. radicals.
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ABSTRACT: Dizzy cooling: Femtosecond excitation of the singlet states of aqueous [FeII(bpy)3]2+ (bpy=2,2-bipyridine) leads to the formation of a vibrationally hot quintet state that exhibits wave-packet dynamics arising from a chelate-ring and bending mode. The vibrational relaxation involves at least two modes: the FeN stretching mode (see picture) and the coherently excited chelate-ring and bending mode, which relax on different time scales.
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ABSTRACT: The light-induced spin and structure changes upon excitation of the singlet metal-to-ligand charge transfer (1MLCT) state of Fe(II)-polypyridine complexes are investigated in detail in the case of aqueous iron(II)-tris-bipyridine ([FeII(bpy)3]2+) by a combination of ultrafast optical and X-ray spectroscopies. Polychromatic femtosecond fluorescence up-conversion, transient absorption studies in the 290-600nm region and femtosecond X-ray absorption spectroscopy allow us to retrieve the entire photocycle upon excitation of the 1MLCT state from the singlet low-spin ground state (1GS) as the following sequence: 1,3MLCT→5T→1GS, which does not involve intermediate singlet and triplet ligand-field states. The population time of the HS state is found to be ∼150fs, leaving it in a vibrationally hot state that relaxes in 2-3ps, before decaying to the ground state in 650ps. We also determine the structure of the high-spin quintet excited state by picosecond X-ray absorption spectroscopy at the K-edge of Fe. We argue that given the many common electronic (ordering of electronic states) and structural (Fe-N bond elongation in the high-spin state, Fe-N mode frequencies, etc.) similarities between all Fe(II)-polypyridine complexes, the results on the electronic relaxation processes reported in the case of [FeII(bpy)3]2+ are of general validity to the entire family of Fe(II)-polypyridine complexes. © 2009 Elsevier B.V.
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R. Abela,
A. Aghababyan,
M. Altarelli,
C Altucci,
G. Amatuni,
P. Anfinrud,
P Audebert,
V. Ayvazyan,
N. Baboi,
J. Baehr, [......],
J. Wojtkiewicz,
A Wolf,
M Wulff,
M. Yurkov,
I. Zagorodnov,
P. Zambolin,
K. Zapfe,
P Zeitoun,
V. Ziemann,
A. Zolotov
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G Aeppli,
R Allemann,
A Almond,
D Angal-Kalinin,
M Ashfold,
V Averbukh,
C Bagshaw,
M Barahona,
P Barker,
R Bartolini, [......],
J Wishart,
A Wolf,
J Womersley,
P Woodruff,
N Woolsey,
S Yaliraki,
E Yates,
A Zeitler,
M Zepf,
A. Zholents
