We report experimental results on x-ray diffraction of quantum-state-selected and strongly aligned ensembles of the prototypical asymmetric rotor molecule 2,5-diiodobenzonitrile using the Linac Coherent Light Source. The experiments demonstrate first steps toward a new approach to diffractive imaging of distinct structures of individual, isolated gas-phase molecules. We confirm several key ingredients of single molecule diffraction experiments: the abilities to detect and count individual scattered x-ray photons in single shot diffraction data, to deliver state-selected, e.g., structural-isomer-selected, ensembles of molecules to the x-ray interaction volume, and to strongly align the scattering molecules. Our approach, using ultrashort x-ray pulses, is suitable to study ultrafast dynamics of isolated molecules.
The ultrafast solvation and recombination dynamics of the hydrated electron generated by two-photon ionization of water at 4.65 eV is studied by transient absorption spectroscopy as a function of temperature in the range from 277 K to 355 K. The part of the spectral blue shift which is observed in the absorption spectrum of the hydrated electron after 1 ps is purely continuous and is accurately described by the well known analytical expression for the temperature dependent absorption spectrum of the ground state hydrated electron. This indicates that thermal relaxation or more likely solvation of the hydrated electron predominantly causes the blue shift. The survival probability of the hydrated electron shows a strong temperature dependence, which is satisfactory explained by the temperature dependent mobility and reaction rates of the species involved in the recombination. This implies that the average initial separation between the hydrated electron and the ionization site of 〈r0〉=1.0±0.1 nm does not depend significantly on the bulk water temperature.
The intersystem crossing, internal conversion, and vibrational relaxation of p-nitroanaline (PNA) in water and 1,4-dioxane have been studied using ultrafast transient absorption spectroscopy. Following the photoexcitation of PNA at 400 nm, the transient absorption dynamics were probed from 340 to 960 nm. The measurements were performed on a common, absolute absorption scale, permitting an accurate determination of the temporal evolution of the absorption spectrum. The data reveal that relaxation on the excited singlet state surface, followed by internal conversion to the ground state and intersystem crossing to the triplet state, is extremely rapid (<0.3 ps) in both solvents. The observed intersystem crossing efficiency is Φisc ≈ 0.4 in dioxane and Φisc ≈ 0.03 in water, indicating that the coupling between the excited singlet and triplet states depends strongly on the solvent polarity. With the estimated quantum yield for intersystem crossing in water and dioxane, we find a time constant for intersystem crossing of ≤10 ps in water and ≤0.8 ps in dioxane. The transient absorption features observed in the visible region are assigned to vibrationally excited PNA in the electronic ground state and three triplet−triplet absorption bands.
A coherent anti-Stokes Raman scattering microscope based on a Ti:sapphire femtosecond oscillator and a photonic crystal fiber is demonstrated. The nonlinear response of the fiber is used to generate the additional wavelength needed in the Raman process. The applicability of the setup is demonstrated by imaging of micrometer-sized polystyrene beads.
The photolysis of aqueous ClO2 has been studied with a new femtosecond transient absorption spectrometer, allowing absorbance changes as small as ΔA ≈ 1 × 10-4 to be recorded with a time resolution of 150 fs. ClO2 was photolyzed at 390 nm and the ultrafast formation and decay of photoproducts were monitored at 260, 390, and 780 nm. As expected from earlier studies, Cl atoms are formed with a quantum yield of Φ(Cl) = 0.1. However, the rate of formation is nearly 2 orders of magnitude higher than that reported. Moreover, Cl is the only photoproduct that survives 25 ps after the photolysis pulse. The main photolytic products, ClO + O, formed with a quantum yield of 0.9, disappear through fast geminate recombination, producing vibrational excited ClO2 in the electronic ground state. The vibrational relaxation of this species occurs with a time constant of 10 ps. The vanishing yield of cage escape for ClO + O, which contrasts with the reported result of the photolysis at 355 nm, indicates that the amount of excess energy imparted to these products at 390 nm is insufficient to enable them to separate. The decay of a photoinduced dichroism observed at 390 nm is interpreted as an orientational relaxation of ground-state ClO2 , the time constant (0.6 ps) agreeing with that calculated from the hydrodynamical slip model.
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