Uli Schmidhammer

Compact autocorrelator for the online measurement of tunable 10 femtosecond pulses

An optical pulse autocorrelator is described which allows online measurement of femtosecond pulse profiles of tunable light sources having 100 Hz–10 kHz repetition rates. The device is capable of dispersion-free noninterferometric measurement of pulse durations ranging from about 100 to sub-10 fs. The applicability of the device is demonstrated in the wavelength range from 420 to 1460 nm. The time delay is scanned with a simple open-loop piezoceramic translator, which is interferometrically calibrated. The accuracy of the device is verified by comparing an autocorrelation trace of a 10 fs pulse with the autocorrelation computed from a zero-additional-phase-spectral phase interferometry for direct electric field reconstruction (ZAP-SPIDER) measurement in the visible region. Its compact design and high sensitivity make the autocorrelator an ideal tool for the diagnostics of tunable ultrashort pulses.

Distance dependence of the reaction rate for the reduction of metal cations by solvated electrons: a picosecond pulse radiolysis study.

The decay of the solvated electron generated by picosecond electron pulse radiolysis is studied by broad-band transient absorption measurements in ethylene glycol solutions containing decimolar concentrations of Cu(2+), Ni(2+), and Pb(2+) metal cations. Analysis of the nonexponential kinetics of the decays reveals molecular parameters of the electron transfer reaction. It is found that the reaction occurs at long distance for Cu(2+) solutions and is only limited to contact distance in the case of Ni(2+) solutions. The distribution of reaction distance strongly depends on the free enthalpy change of the reactions.

Time-Dependent Radiolytic Yield of OH Radical Studied by Picosecond Pulse Radiolysis

Picosecond pulse radiolysis measurements using a pulse-probe method are performed to measure directly the time-dependent radiolytic yield of the OH(•) radical in pure water. The time-dependent absorbance of OH(•) radical at 263 nm is deduced from the observed signal by subtracting the contribution of the hydrated electron and that of the irradiated empty fused silica cell which presents also a transient absoption. The time-dependent radiolytic yield of OH(•) is obtained by assuming the yield of the hydrated electron at 20 ps equal to 4.2 × 10(-7) mol J(-1) and by assuming the values of the extinction coefficients of e(aq)(-) and OH(•) at 782 nm (ε(λ=782 nm) = 17025 M(-1) cm(-1)) and at 263 nm (ε(λ=263 nm) = 460 M(-1) cm(-1)), respectively. The value of the yield of OH(•) radical at 10 ps is found to be (4.80 ± 0.12) × 10(-7) mol J(-1).

Picosecond pulse radiolysis study of highly concentrated nitric acid solutions: formation mechanism of NO3• radical.

The formation of nitrate radical, NO(3)(•), is observed for the first time directly by picosecond pulse radiolysis of highly concentrated nitric acid solutions. The experimental yield of NO(3)(-) ionization is deduced from the pulse-probe transient absorption measurements in the visible region where this radical absorbs. On the basis of the value of the extinction coefficient of nitrate radical at 640 nm equal to 1300 M cm(-1), the experimental yield of NO(3)(•) at 20 ps is found to be around 0.36 × 10(-7), 1.33 × 10(-7), and 2.85 × 10(-7) mol J(-1) for 1, 3.5, and 7 M nitric acid solutions, respectively. Relative to the dose absorbed by nitric acid by the direct effect, we find an unexpected high formation yield of the nitrate radical within the electron pulse. Therefore, we suggest that the trapping of the positive hole, H(2)O(•+), by NO(3)(-) also contributes to the formation of NO(3)(•) within the electron pulse. Moreover, after the pulse and within 4 ns, the beginning of the reaction of OH(•) radical with undissociated nitric acid is observed for the most concentrated nitric acid solution.

Picosecond Pulse Radiolysis of Direct and Indirect Radiolytic Effects in Highly Concentrated Halide Aqueous Solutions

Recently we measured the amount of the single product, Br(3)(-), of steady-state radiolysis of highly concentrated Br(-) aqueous solutions, and we showed the effect of the direct ionization of Br(-) on the yield of Br(3)(-). Here, we report the first picosecond pulse-probe radiolysis measurements of ionization of highly concentrated Br(-) and Cl(-) aqueous solutions to describe the oxidation mechanism of the halide anions. The transient absorption spectra are reported from 350 to 750 nm on the picosecond range for halide solutions at different concentrations. In the highly concentrated halide solutions, we observed that, due to the presence of Na(+), the absorption band of the solvated electron is shifted to shorter wavelengths, but its decay, taking place during the spur reactions, is not affected within the first 4 ns. The kinetic measurements in the UV reveal the direct ionization of halide ions. The analysis of pulse-probe measurements show that after the electron pulse, the main reactions in solutions containing 1 M of Cl(-) and 2 M of Br(-) are the formation of ClOH(-•) and BrOH(-•), respectively. In contrast, in highly concentrated halide solutions, containing 5 M of Cl(-) and 6 M of Br(-), mainly Cl(2)(-•) and Br(2)(-•) are formed within the electron pulse without formation of ClOH(-•) and BrOH(-•). The results suggest that, not only Br(-) and Cl(-) are directly ionized into Br(•) and Cl(•) by the electron pulse, the halide atoms can also be rapidly generated through the reactions initiated by excitation and ionization of water, such as the prompt oxidation by the hole, H(2)O(+•), generated in the coordination sphere of the anion.

Compact laser flash photolysis techniques compatible with ultrafast pump-probe setups

Two new transient absorption measurement techniques are described which use commercially available pulsed laser diodes or high-power light-emitting diodes (LEDs) as monitoring beam. The semiconductor devices substitute the probe in a kilohertz-repetition-rate ultrafast pump-probe setup. A fully functional and highly compact laser flash photolysis system reaching the nanosecond to millisecond time scale is thereby added to a state-of-the-art femtosecond system. The sample is excited with UV-Vis tunable femtosecond pulses, and for the electronically synchronized probing light either subnanosecond pulsed laser diodes for selected wavelengths or LEDs covering the visible to near infrared and UV regions are used. The applicability and reliability of the devices are demonstrated for various probe wavelengths in the visible by the investigation of excited-state decay or photoinduced bimolecular reactions. The time resolution is found to be 400 ps for the pulsed laser diodes and a few nanoseconds for the LEDs. Th...

Reactivity of the Strongest Oxidizing Species in Aqueous Solutions: The Short-Lived Radical Cation H2O(•.).

The radical cation H2O(•+) formed under irradiation of liquid water undergoes an ultrafast proton transfer reaction and consequently exhibits an extremely short lifetime. The proton transfer yields an oxidizing OH(•) radical whose reactivity has been extensively studied. By contrast, H2O(•+) reactivity with molecules other than water has not been established experimentally and was subject to controversy. The direct oxidation by H2O(•+) can take place in various situations. In highly concentrated solutions, the radical cation H2O(•+) may also be involved in ultrafast electron transfer reactions. We have applied picosecond pulse radiolysis conducted at the electron accelerator ELYSE on solutions with various H2SO4 concentrations to determine the scavenging yield of H2O(•+). The yield of H2O(•+) at a few tens of femtoseconds is estimated to be around 5.3 × 10(-7) mol J(-1), and its reactivity is quantitatively determined. Moreover, a simple estimation of the reduction potential of this short-lived radical cation shows that it is the most powerful oxidizing species.

The Key Role of Solvation Dynamics in Intramolecular Electron Transfer : Time-Resolved Photophysics of Crystal Violet Lactone

The intramolecular electron-transfer reaction in crystal violet lactone in polar aprotic solvents is studied with femtosecond transient absorption spectroscopy. The initially excited charge transfer state (1)CT A is rapidly converted into a highly polar charge transfer state (1)CT B. This ultrafast electron transfer is seen as a solvent-dependent dual fluorescence in steady-state spectra. We find that the electron-transfer process can be followed by a change from a double-peaked transient absorption spectrum to a single-peak one in the low picosecond range. The transient absorption kinetic curves are multiexponential, and the fitted time constants are solvent dependent but do not reproduce the known solvation times. For 6-dimethylaminophthalide, the optically active constituent of crystal violet lactone, only a small temporal evolution of the spectra is found. To explain these findings, we present a model that invokes a time-dependent electron-transfer rate. The rate is determined by the instantaneous separation of the two charge-transfer states. Because of their differing dipole moments, they are dynamically lowered to a different extent by the solvation. When they temporarily become isoenergetic, equal forward and backward transfer rates are reached. The intrinsic electron-transfer ( (1)CT A --> (1)CT B) reaction is probably as fast as that in the structurally analogous malachite green lactone (on the 100 fs time scale). The key element for the dynamics is therefore its control by the solvent, which changes the relative energetics of the two states during the solvation process. With further stabilization of the more polar state, the final equilibrium in state population is reached.

Ambident Reactivity of the Cyanate Anion

Because the charge density is higher at the more electronegative oxygen center while the larger HOMO coefficient is at nitrogen, the concept of charge and orbital control predicts hard electrophiles to attack at the oxygen side and soft electrophiles to attack at the nitrogen. It was, therefore, expected that alkyl cyanates should be formed in nucleophilic substitution reactions with SN1 character while alkyl isocyanates should be formed in nucleophilic substitution reactions with SN2 character. [4]

Direct Evidence for Transient Pair Formation between a Solvated Electron and H3O(+) Observed by Picosecond Pulse Radiolysis.

The reaction between the solvated electron and hydronium cation H3O(+) in water constitutes a fundamental reaction in chemistry. Due to significant rearrangement of solvent molecules around both the electron and H3O(+), the reaction rate of this process is not controlled by diffusion. The presence of a reaction barrier suggests the formation of an intermediate that has so far not been observed. Here, the time-resolved visible absorption spectra in three concentrated acid solutions, perchloric, sulfuric, and phosphoric, at various concentrations are recorded by the picosecond pulse radiolysis method. In contrast to previous reports, a strong blue shift of the absorption band of the solvated electron in acidic solutions compared to neat water is clearly observed, consistent with formation of a pair between the solvated electron and hydronium cation.