Femtosecond photoelectron and photoion spectrometer with vacuum ultraviolet probe pulses

We describe a setup to study ultrafast dynamics in gas-phase molecules using time-resolved photoelectron and photoion spectroscopy. The vacuum ultraviolet (VUV) probe pulses are generated via strong field high-order harmonic generation from infrared femtosecond laser pulses. The band pass characteristic in transmission of thin indium (In) metal foil is exploited to isolate the $9^{\text{th}}$ harmonic of the 800 nm fundamental (H9, 14 eV, 89 nm) from all other high harmonics. The $9^{\text{th}}$ harmonic is obtained with high conversion efficiencies and has sufficient photon energy to access the complete set of valence electron levels in most molecules. The setup also allows for direct comparison of VUV single-photon probe with 800 nm multi-photon probe without influencing the delay of excitation and probe pulse or the beam geometry. We use a magnetic bottle spectrometer with high collection efficiency for electrons, serving at the same time as a time of flight spectrometer for ions. Characterization measurements on Xe reveal the spectral width of H9 to be $190\pm60$ meV and a photon flux of $\sim1\cdot10^{7}$ photons/pulse after spectral filtering. As a first application, we investigate the S$_1$ excitation of perylene using time-resolved ion spectra obtained with multi-photon probing and time-resolved electron spectra from VUV single-photon probing. The time resolution extracted from cross-correlation measurements is $65\pm10$ fs for both probing schemes and the pulse duration of H9 is found to be $35\pm8$ fs

Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy

We report on an element-selective study of the fate of charge carriers in photoexcited inorganic CsPbBr3 and CsPb(ClBr)3 perovskite nanocrystals (NCs) in toluene solutions using time-resolved X-ray absorption spectroscopy with 80 ps time resolution. Probing the Br K-edge, the Pb L3-edge and the Cs L2-edge, we find that holes in the valence band are localized at Br atoms, forming small polarons, while electrons appear as delocalized in the conduction band. No signature of either electronic or structural changes are observed at the Cs L2-edge. The results at the Br and Pb edges suggest the existence of a weakly localized exciton, while the absence of signatures at the Cs edge indicates that the Cs+ cation plays no role in the charge transport, at least beyond 80 ps. These results can explain the rather modest charge carrier mobilities in these materials.Comment: 19 pages, 3 figure

A comparison of sensitized Ln(III) emission using pyridine- and pyrazine-2,6-dicarboxylates - part II

The synthesis, X-ray structures and photophysical properties of several new Ln(iii) complexes with the dianion of pyrazine-2,6-dicarboxylic acid (H 2PYZ) that demonstrate excellent stability and solubility in non-aqueous solution are reported, and compared to structurally analogous complexes derived from pyridine-2,6-dicarboxylic acid (H2DPA). The Eu(iii) and Yb(iii) complexes demonstrate efficient metal centered luminescence in the visible and Near Infra-Red (NIR) regions, respectively. Low temperature (77 K) phosphorescence measurements using the corresponding Gd(iii) complex has allowed the photophysical properties of the sensitizer to be rationalized, together with corresponding TD-DFT studies for a model complex. Lastly, we have evaluated the sensitization efficiencies for these complexes, and have undertaken femtosecond transient absorption (TA) measurements in order to evaluate the relative importance of the intersystem crossing and energy transfer processes involved with sensitized Ln(iii) emission via the antennae effect

Ultrafast excited–state dynamics of radical ions in liquid solution

The spectral and dynamic features of photoexcited radical ions have been studied by ultrafast spectroscopy. The excited state absorption spectrum, lifetimes and the influence of parameters like excess excitation energy, solvent or temperature on the photophysical behavior has been investigated for a series of chemically distinct systems chosen such as to span a range of chemical diversity, excited state energies (1 to 2 eV) and oxidation state (cationic or anionic). The radical ions have been generated either chemically or electrochemically in a home-built flow cell. The results reveal that the lifetime of the lowest electronic excited state, D1, is on the order of picoseconds only. Wurster's salts, for example, have a D1 lifetime of 0.2 ps at room temperature irrespective of solvent and nitrogen substituent but influenced by temperature and, thus, fluorescence can be detected below 120 K. By contrast, temperature does not play a role for perylene radical cation whereas the solvent does. In general, the data clearly shows that the energy gap law, often invoked to explain the lack of fluorescence of radical ions, is not obeyed. Instead conical intersections are involved whose accessibility is governed by subtle effects. The always weak the excited state absorption might explain why they have not been observed in highly exergonic photoinduced charge separation reactions associated with the Marcus inverted region, the absence of which is debated

Self Referencing Heterodyne Transient Grating Spectroscopy with Short Wavelength

Heterodyning by a phase stable reference electric field is a well known technique to amplify weak nonlinear signals. For short wavelength, the generation of a reference field in front of the sample is challenging because of a lack of suitable beamsplitters. Here, we use a permanent grating which matches the line spacing of the transient grating for the creation of a phase stable reference field. The relative phase among the two can be changed by a relative translation of the permanent and transient gratings in direction orthogonal to the grating lines. We demonstrate the technique for a transient grating on a VO2 thin film and observe constructive as well as destructive interference signals

Excited-State Dynamics of Wurster's Salts

The excited-state dynamics of a series of Wurster's salts (p-phenylenediamine radical cations) with different subtituents on the nitrogen atoms was investigated under a variety of experimental conditions using a combination of ultrafast spectroscopic techniques. At room temperature, the lifetime of the lowest excited state of all radical cations is on the order of 200 fs, independently of the solvent, that is, water, nitriles, alcohols, and room-temperature ionic liquid. On the other hand, all cations, except that with the bulky nitrogen substituents, become fluorescent below 120 K. The observed dynamics can be accounted for by the presence of a conical intersection between the D1 and D0 states. For the cations with a small nitrogen substituent, this conical intersection could be accessed through a twist of one amino group, as already suggested for Wurster's Blue. However, this coordinate cannot be invoked for the cation with bulky nitrogen subtituents, and more probably, pyramidalization of the nitrogen center and/or deformation of the phenyl ring play an important role. Consequently, the excited-state dynamics of these structurally very similar Wurster's salts involves different decay mechanisms

Ultrafast Long-Distance Excitation Energy Transport in Donor–Bridge–Acceptor Systems

The excited-state dynamics of two energy donor–bridge–acceptor (D–B–A) systems consisting of a zinc tetraphenylporphyrin (ZnP) and a free base tetraphenylporphyrin (FbP) bridged by oligo-p-phenyleneethynylene units with different substituents has been investigated using ultrafast spectroscopy. These systems differ by the location of the lowest singlet excited state of the bridge, just above or below the S2 porphyrin states. In the first case, Soret band excitation of the porphyrins is followed by internal conversion to the local S1 state of both molecules and by a S1 energy transfer from the ZnP to the FbP end on the 10 ns time scale, as expected for a center-to-center distance of about 4.7 nm. On the other hand, if the bridge is excited, the energy is efficiently transferred within 1 ps to both porphyrin ends. Selective bridge excitation is not possible with the second system, because of the overlap of the absorption bands. However, the time-resolved spectroscopic data suggest a reversible conversion between the D*(S2)–B–A and D–B*(S1)–A states as well as a transition from the D–B*(S1)–A to the D–B–A* states on the picosecond time scale. This implies that the local S2energy of the ZnP end can be transported stepwise to the FbP end, i.e., over about 4.7 nm, within 1 ps with an efficiency of more than 0.2

Excited-state Dynamics of Radical Ions in Liquids

Thomas Bally has acquired international recognition for his work on the photochemistry of reactive intermediates, which include radical ions. Here, we present a brief overview of our investigations of the excited-state dynamics of radical ions in liquids at room temperature, which are still poorly documented. A better understanding of these dynamics is most relevant, as open-shell ions in the excited state are being increasingly used in redox photochemistry and have been proposed to play a key role in highly exergonic photoinduced electron transfer reactions.</p