Kenneth L. Stevenson

Nanosecond UV laser photoionization of aqueous tryptophan: temperature dependence of quantum yield, mechanism, and kinetics of hydrated electron decay

Abstract The 266xa0nm photoionization of aqueous tryptophan (Trp) solution at natural pH has been studied by laser flash photolysis using 3–18xa0mJ, 7xa0ns, 266xa0nm pulses. The transient absorption method was employed to detect hydrated electrons. The photoionization mechanism is a combination of mono and biphotonic processes. Results from measurements of the power dependence of photoionization and of triplet (T 1 ) state production suggest that both processes involve the same one-photon and two-photon singlet intermediate state. The hydrated electron yield increases with temperature, and this can be attributed primarily to the monophotonic photoionization component. Hydrated electrons decay by a non-exponential rate law that is consistent with a decay mechanism involving their bimolecular scavenging by tryptophan radical cations with a rate constant of (7.2±0.6)×10 10 xa0M −1 xa0s −1 .

Low-temperature specific heat of the molecule-based magnet M[N(CN)2]2 (M=Co, Ni, Cu, Zn) series

Abstract We report the specific heat as a function of temperature in zero applied magnetic field for three magnetic systems (α-Co[N(CN)2]2, β-Co[N(CN)2]2, Ni[N(CN)2]2) and two nonmagnetic systems (Cu[N(CN)2]2, Zn[N(CN)2]2). The anomalies revealed by the magnetic contribution to the specific heat are associated with the onset of long-range order. The transition entropies imply that the magnetic ordering originates from a ground-state doublet ( J= 1 2 ) for the Co2+ systems and a ground-state triplet (J=1) for the Ni2+ system. In addition, we extract and compare the exchange couplings J using several theoretical models: interacting spin waves, high-temperature series expansions, total magnetic energy and mean field. The J-values and the Ising-like anisotropy obtained here, are in quantitative agreement with earlier magnetization and neutron diffraction results. The comparison of the zero-field and in-field (8xa0T) specific heat of the Cu2+ system ( J= 1 2 ) demonstrates the previously unknown ferromagnetic order at very low temperature ( ⩽1.7 K ).

Photoinduced electron transfer and luminescence in aqueous bromocuprate(I) complexes

Abstract Luminescence, laser flash photolysis and continuous photolysis studies of equilibrated solutions of CuBr 2 − and CuBr 3 2− were carried out in the UV region. Excitation of the absorption band at 279 nm in CuBr 3 2− results in emission centered at 475 nm, with a lifetime of 710 ns in neutral solution, and quenched by hydronium ions with a rate constant of 6.2 × 10 8 M −1 s −1 . Neutral solutions of the complexes produce hydrated electrons when they absorb 15 ns pulses of laser light at 266 nm. The electrons are scavenged by the copper(I) species itself with a second-order rate constant of 7.5 × 10 9 M −1 s −1 , and by hydronium ions with a second-order rate constant of 1.3 × 10 10 M −1 s −1 at 0.5 M ionic strength. Individual quantum yields of electron production, determined at 1 M ionic strength, are 0.67 for CuBr 2 − and 0.34 for CuBr 3 2− . Continuous photolysis of acidic solutions of the complexes reveals a dependnece on hydronium ion concentration which is different from that for the scavenging of electrons, a dependence on Br − concentration and an action spectrum consistent with the 279 nm absorption band as the photoactive state. These plus other observations and arguments support a mechanism for dihydrogen evolution, involvin the formation of a steady state hydride intermediate which reacts with H + to form dihydrogen.

Mechanism of electron ejection induced by mono- and biphotonic excitation of Cu(CN)2−–halide ion systems

Abstract This contribution reviews recent studies devoted to understanding the mechanism of electron ejection in aqueous solutions of dicyanocuprate(I) in the presence of halide ions of high concentration using steady state and time resolved luminescence and absorption techniques. It has been demonstrated that electrons originate from two excited states: the lower energy level achieved by one-photon absorption, and the high energy state populated by biphotonic excitation.

Electronic Structure of Transition-Metal Dicyanamides Me(N(CN)2)2 (Me = Mn, Fe, Co, Ni, Cu)

The electronic structure of Me[N(CN)

Prompt and delayed photoejection of hydrated electrons in the UV photolysis of aqueous solutions of copper(I) complexes

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Noncollinear antiferromagnetic structure of the molecule-based magnet Mn [ N ( CN ) 2 ] 2

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The triamminecopper(I) ion in aqueous solution. Spectral and photochemical evidence for CTTS behavior

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Electronic structure of the molecule-based magnet Mn[N(CN)2]2 from theory and experiment

(Me=Mn, Fe, Co, Ni, Cu) molecular magnets has been investigated using x-ray emission spectroscopy (XES) and x-ray photoelectron spectroscopy (XPS) as well as theoretical density-functional-based methods. Both theory and experiments show that the top of the valence band is dominated by Me 3d bands, while a strong hybridization between C 2p and N 2p states determines the valence band electronic structure away from the top. The 2p contributions from non-equivalent nitrogen sites have been identified using resonant inelastic x-ray scattering spectroscopy with the excitation energy tuned near the N 1s threshold. The binding energy of the Me 3d bands and the hybridization between N 2p and Me 3d states both increase in going across the row from Me = Mn to Me = Cu. Localization of the Cu 3d states also leads to weak screening of Cu 2p and 3s states, which accounts for shifts in the core 2p and 3s spectra of the transition metal atoms. Calculations indicate that the ground-state magnetic ordering, which varies across the series is largely dependent on the occupation of the metal 3d shell and that structural differences in the superexchange pathways for different compounds play a secondary role.

Photochemistry of iodocuprate(I) complexes

Abstract The ejection of hydrated electrons from 266-nm laser-photoexcited solutions containing Cu(NH 3 ) + 3 , CuCl 2− 3 , or CuBr 2− 3 occurs through two pathways on the nanosecond time scale: a prompt ejection ( τ τ >laser pulsewidth) which follows a first-order rate law. This behavior is consistent with electron ejection from two excited states: the primary CTTS state, and longer-lived triplet species consisting of an exciplex and its precursor. The quantum yields for both prompt and delayed ejection are quite high, in the 0.15–0.4 range.