Robert A. Crowell

Excitation-energy dependence of the mechanism for two-photon ionization of liquid H2O and D2O from 8.3to12.4eV

Transient absorption measurements monitor the geminate recombination kinetics of solvated electrons following two-photon ionization of liquid water at several excitation energies in the range from 8.3 to 12.4 eV. Modeling the kinetics of the electron reveals its average ejection length from the hydronium ion and hydroxyl radical counterparts and thus provides insight into the ionization mechanism. The electron ejection length increases monotonically from roughly 0.9 nm at 8.3 eV to nearly 4 nm at 12.4 eV, with the increase taking place most rapidly above 9.5 eV. We connect our results with recent advances in the understanding of the electronic structure of liquid water and discuss the nature of the ionization mechanism as a function of excitation energy. The isotope dependence of the electron ejection length provides additional information about the ionization mechanism. The electron ejection length has a similar energy dependence for two-photon ionization of liquid D(2)O, but is consistently shorter than in H(2)O by about 0.3 nm across the wide range of excitation energies studied.

Excited state dynamics of liquid water: Insight from the dissociation reaction following two-photon excitation

The authors use transient absorption spectroscopy to monitor the ionization and dissociation products following two-photon excitation of pure liquid water. The primary decay mechanism changes from dissociation at an excitation energy of 8.3 eV to ionization at 12.4 eV. The two channels occur with similar yield for an excitation energy of 9.3 eV. For the lowest excitation energy, the transient absorption at 267 nm probes the geminate recombination kinetics of the H and OH fragments, providing a window on the dissociation dynamics. Modeling the OH geminate recombination indicates that the dissociating H atoms have enough kinetic energy to escape the solvent cage and one or two additional solvent shells. The average initial separation of H and OH fragments is 0.7+/-0.2 nm. Our observation suggests that the hydrogen bonding environment does not prevent direct dissociation of an O-H bond in the excited state. We discuss the implications of our measurement for the excited state dynamics of liquid water and explore the role of those dynamics in the ionization mechanism at low excitation energies.

Transient x-ray absorption spectroscopy of hydrated halogen atom

Time-resolved x-ray absorption spectroscopy has been used to observe the transient species generated by one-photon detachment of an electron from aqueous bromide. The K-edge spectrum of the short-lived Br(0) atom exhibits a resonant 1s-4p transition that is absent for the Br(-) precursor. The strong 1s-4p resonance suggests that there is very little charge transfer from the solvent to the open-shell atom, whereas weak oscillations above the absorption edge indicate that the solvent shell around a neutral Br(0) atom is defined primarily by hydrophobic interactions. These conclusions are in agreement with Monte Carlo and quantum chemical simulations of the solvent structure.

High-repetition-rate femtosecond optical parametric oscillator–amplifier system near 3 μm

An ultrafast laser system for the chemically important 3-μm spectral region has been constructed by means of noncritically phased-matched KTiOPO4 optical parametric gain elements. An optical parametric oscillator, synchronously pumped by a mode-locked Ti:sapphire oscillator, generates high-quality seed pulses for an optical parametric amplifier. The optical parametric amplifier, pumped by a high-repetition-rate Ti:sapphire regenerative amplifier, amplifies the seed pulses by a factor of 520. Pulses with an energy of 550 nJ and a pulse width of 160 fs are produced at a 250-kHz repetition rate in the 3-μm region.

Ultrafast pulse radiolysis using a terawatt laser wakefield accelerator

We report ultrafast pulse radiolysis transient absorption (TA) spectroscopy measurements from the Terawatt Ultrafast High Field Facility (TUHFF) at Argonne National Laboratory. TUHFF houses a 20 TW Ti:sapphire laser system that generates 2.5 nC subpicosecond pulses of multi-mega-electron-volt electrons at 10 Hz using laser wakefield acceleration. The system has been specifically optimized for kinetic TA measurements in a pump-probe fashion. This requires averaging over many shots which necessitates stable, reliable generation of electron pulses. The latter were used to generate excess electrons in pulse radiolysis of liquid water and concentrated solutions of perchloric acid. The hydronium ions in the acidic solutions react with the hydrated electrons resulting in the rapid decay of the transient absorbance at 800 nm on the picosecond time scale. Normalization of the TA signal leads to an improvement in the signal to noise ratio by a factor of 5 to 6. Due the pointing instability of the laser this improvement was limited to a 5 to 10 min acquisition period, requiring periodic recalibration and realignment. Time resolution, defined by the rise time of TA signal from hydrated electron in pulse radiolysis of liquid water, of a few picoseconds, has been demonstrated. The current time resolution is determined primarily by the physical dimensions of the sample and the detection sensitivity. Subpicosecond time resolution can be achieved by using thinner samples, more sensitive detection techniques, and improved electron beam quality.

Electron injection by a nanowire in the bubble regime

The triggering of wave-breaking in a three-dimensional laser plasma wake (bubble) is investigated. The Coulomb potential from a nanowire is used to disturb the wake field to initialize the wave-breaking. The electron acceleration becomes more stable and the laser power needed for self-trapping is lowered. Three-dimensional particle-in-cell simulations were performed. Electrons with a charge of about 100 pC can be accelerated stably to energy about 170 MeV with a laser energy of 460 mJ. The first step towards tailoring the electron beam properties such as the energy, energy spread, and charge is discussed

Pulse radiolysis of liquid water using picosecond electron pulses produced by a table-top terawatt laser system

A laser based electron generator is shown, for the first time, to produce sufficient charge to conduct time resolved investigations of radiation induced chemical events. Electron pulses generated by focussing terawatt laser pulses into a supersonic helium gas jet are used to ionize liquid water. The decay of the hydrated electrons produced by the ionizing electron pulses is monitored with 0.3 μs time resolution. Hydrated electron concentrations as high as 22 μM were generated. The results show that terawatt lasers offer both an alternative to linear accelerators and a means to achieve subpicosecond time resolution for pulse radiolysis studies.

Photo- and radiation-chemistry of halide anions in ionic liquids.

One- and two- photon excitation of halide anions (X(-)) in polar molecular solvents results in electron detachment from the dissociative charge-transfer-to-solvent state; this reaction yields a solvated halide atom and a solvated electron. How do such photoreactions proceed in ionic liquid (IL) solvents? Matrix isolation electron paramagnetic resonance (EPR) spectroscopy has been used to answer this question for photoreactions of bromide in aliphatic (1-butyl-1-methylpyrrolidinium) and aromatic (1-alkyl-3-methyl-imidazolium) ionic liquids. In both classes of ILs, the photoreaction (both 1- and 2-photon) yields bromine atoms that promptly abstract hydrogen from the alkyl chains of the IL cation; only in concentrated bromide solutions (containing >5-10 mol % bromide) does Br2(-•) formation compete with this reaction. In two-photon excitation, the 2-imidazolyl radical generated via the charge transfer promptly eliminates the alkyl arm. These photolytic reactions can be contrasted with radiolysis of the same ILs, in which large yield of BrA(-•) radicals was observed (where A(-) is a matrix anion), suggesting that solvated Br(•) atoms do not occur in the ILs, as such a species would form three-electron σ(2)σ(*1) bonds with anions present in the IL. It is suggested that chlorine and bromine atoms abstract hydrogen faster than they form such radicals, even at cryogenic temperatures, whereas iodine mainly forms such bound radicals. These XA(-•) radicals convert to X2(•-) radicals in a reaction with the parent halide anion. Ramifications of these observations for photodegradation of ionic liquids are discussed.

Femtosecond mid-infrared optical parametric oscillator based onCsTiOAsO 4

A high-repetition-rate Ti:sapphire laser is used to synchronously pump a type-II angle-tuned CsTiOAsO(4) (CTA) optical parametric oscillator. When pumped at 809 nm, the optical parametric oscillator is tunable from 1.007 to 1.180 microm in the signal branch and from 2.590 to 4.120 microm in the idler branch. Powers as high as 235 mW are obtained in the signal branch. Pulse widths as short as 56 fs are generated at 1.115 microm. CTA is shown to have a unique combination of low walk-off and low dispersion that contributes to its high gain and conversion efficiency.

A photon echo study of pentacene in naphthalene at high pressure

Abstract Temperature-dependent (2–10 K) photon echo measurements are presented for pentacene in naphthalene at high pressure (12.5 kbar) in a diamond anvil cell. A decrease in the homogeneous dephasing rate (i.e. a narrowing of the homogeneous linewidth) is observed at high pressure. In addition, the temperature dependence of the homogeneous dephasing rate is used to obtain the effect of pressure on the frequency and lifetime of the pseudolocal phonon associated with a pentacene impurity in a crystalline naphthalene matrix.