Andreas N. Unterreiner

Excited state dynamics of metastable phthalocyanine-tetrasulfonate tetra-anions probed by pump/probe photoelectron spectroscopy.

Femtosecond time-resolved pump-probe photoelectron spectroscopy was used to study elementary relaxation processes occurring in isolated phthalocyanine-tetrasulfonate tetra-anions ([MPc(SO3)4]4-, M=Cu,Ni, and free-base [H2Pc(SO3)4]4-) following Q band excitation by one-photon absorption at 775 nm. Whereas the Cu and Ni systems decay rapidly by means of internal conversion without electron loss, the free-base phthalocyanine primarily undergoes excited state tunneling electron emission. This reflects less efficient coupling to lower lying states within the corresponding spin manifold. Results are interpreted in terms of (time-dependent) density functional theory calculations of ground and electronically excited states and kinetically modeled to yield the associated rates.

Femtosecond UV Excitation in Imidazolium-Based Ionic Liquids

Femtosecond pump-probe absorption spectroscopy was employed to investigate ultrafast dynamics in various room temperature ionic liquids (RTILs) based on imidazolium cations, i.e., 1,3-dimethylimidazolium iodide ([DMIM]I), 1-butyl-3-methylimidazolium iodide ([BMIM]I), 1-hexyl-3-methylimidazolium iodide ([HMIM]I), 1-hexyl-3-methylimidazolium chloride ([HMIM]Cl), and 1-methyl-3-octylimidazolium chloride ([MOIM]Cl). Immediately after photoexcitation, an induced absorption was observed at various probe wavelengths (555-1556 nm). Afterward, the decay of the induced absorption was found to be independent of the alkyl chain length and viscosity of the ionic liquids. Two alternative mechanisms were proposed to explain the dynamics. In a first scenario excess electrons are generated through one-photon photodetachment of halides analogous to aqueous halide photodetachment. The dynamics in this case were analyzed with the help of a competing kinetic model proposed for geminate recombination in aqueous chloride photodetachment. Alternatively, imidazolium cations may be subject to photoionization. The transient NIR absorption can then be assigned to imidazolium dimer radical cations and/or excess electrons which may be formed upon association of imidazolium radicals with their parent cations. Both scenarios suggest that a thorough explanation of the ultrafast dynamics probably requires the implication of cooperative effects in the ionic liquids upon photoexcitation.

Substituent Effects on Dynamics at Conical Intersections: Cycloheptatrienes

Using selective methyl substitution, we study the effects of vibrational dynamics at conical intersections in unsaturated hydrocarbons. Here, we investigate the excited state nonadiabatic dynamics of cycloheptatriene (CHT) and its relation to dynamics in other polyenes by comparing CHT with 7-methyl CHT, 7-ethyl CHT, and perdeuterated CHT using time-resolved photoelectron spectroscopy and photoelectron anisotropy. Our results suggest that, upon ππ*-excitation to the bright 2A state, we observe an early intersection with the dark 2A state close to the Franck-Condon region with evidence of wavepacket bifurcation. This indicates that the wavepacket evolves on both states, likely along a planarization coordinate, with the majority of the flux undergoing nonadiabatic transition via conical intersections within 100 fs following light absorption. In CHT, large amplitude motion along the planarization coordinate improves the intra-ring π-overlap, yielding a delocalized electronic density. However, substitutions in 7 position, chosen to modify the inertia of the planarization motion, did not markedly alter the first step in the sequential kinetic scheme. This suggests that there is a crossing of potential energy surfaces before planarization is achieved and, thus, nonadiabatic transition likely takes place far away from a local minimum.

Femtosecond spectroscopy of solvated electrons from sodium-ammonia-d3 solutions: Temperature jump versus local density jump

The relaxation dynamics of solvated electrons from sodium-ammonia-d3 solutions was studied by femtosecond time-resolved near-infrared spectroscopy. The experimental pump-probe data reveal a pulse-width limited pump-induced redshift of the absorption spectrum of the ammoniated electron and a subsequent slower blueshift on a time scale of roughly 200 fs. The spectrotemporal response is interpreted using the nonadiabatic relaxation mechanism for cavity-bound solvated electrons in condensed phases. In particular, we develop a local density-jump model, which traces the dynamic spectrum back to a sequence of a pump-induced cavity expansion due to Pauli repulsion and a succeeding cavity contraction upon nonadiabatic return of the electron back to its ground state. Using the existing thermodynamic data of the solvent and experimental temperature and density-dependent absorption spectra of metal-ammonia solutions, an overall increase in the interparticle distance within the solvent cavity of 25% is crudely estimated. The density-jump model is compared to the temperature-jump model we proposed previously for the femtosecond relaxation dynamics of metal-NH(3) solutions.

Evidence for laser-induced formation of solvated electrons in room temperature ionic liquids

The photolytic generation of solvated electrons was observed for the first time in two room temperature ionic liquids (RTILs), trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide (IL) and 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide (IL). A 70 fs UV-pulse was used to excite the RTILs, while the transient response was monitored in the visible and near-infrared spectral regions. Immediately after excitation, a pulse duration limited rise of the induced absorption indicated the formation of solvated electrons suggesting the existence of pre-formed traps in RTILs. A broad transient absorption spectrum with a full width at half maximum of about 0.9 eV, typical for solvated electrons, was reconstructed from the transient profiles. Wavelength-independent relaxation dynamics at longer delay times suggest a lifetime of solvated electrons in the ns regime in agreement with results from pulse radiolysis studies. Adding 1,1-dimethylpyrrolidinium iodide to IL led to an increase of the UV absorbance and consequently, to an increase of the yield of solvated electrons. Furthermore, this solute is an efficient electron scavenger causing the transients to decay within about 40 ps.

Relaxation Dynamics upon Ultrashort UV Photo-Excitation of an Iodide Doped Ionic Liquid and of a Pure Lithium Iodide Melt

The ultrafast dynamics of photolytically generated solvated electrons were investigated for the first time in a pure lithium iodide (LiI) melt at 480 °C and in an iodide doped room-temperature ionic liquid. A 70 fs UV-pulse was used to generate excess electrons and their subsequent transient response was monitored in the visible and near-infrared spectral regions. In LiI a pulse-duration limited rise of the induced absorption at all probe wavelengths was attributed to the formation of excess electrons. A minor portion of these electrons (∼20%) were subject to geminate recombination with a time constant of 2 ps. Thereafter, the induced absorption remained unaltered for over 100 ps. The ultrafast dynamics in an iodide doped room-temperature ionic liquid contrasts with the ultrafast response of excess electrons in LiI and in pure room temperature ionic liquids. First, the formation of solvated electrons seems to be delayed by up to 2 ps and second, iodine atoms act as efficient electron scavengers on a tens of ps time scale. As a result, ∼80% of the initially prepared solvated electrons undergo geminate recombination after about 70 ps. Furthermore, an additional component around 720 nm is attributed to the formation of diiodide.

Time-resolved polaron dynamics in molten solutions of cesium-doped cesium iodide

Temperature-dependent investigations of excess electrons in molten solutions of cesium-doped cesium iodide (Cs-CsI) (mole fraction of Cs approximately 0.003) were performed applying femtosecond pump-probe absorption spectroscopy. The pulse-limited induced bleach observed at probe wavelengths from 600 to 1240 nm was attributed to the excitation of equilibrated excess electrons which were initially formed by melting a Cs-CsI mixture. The interpretation of the relaxation process is based on strongly localized polarons that constitute the majority of defect states in this melt. As expected, the bipolaron contribution was insignificant. The time constants (tau1) were found to be temperature dependent confirming our earlier findings in Na-NaI melts that ionic diffusion almost exclusively controls the dynamics of excess electrons in high temperature ionic liquids. Apart from this temperature dependence, the relaxation dynamics of excess electrons do not differ irrespective of the excitation regime (blue or red part of the respective stationary spectra).

Vibrational cooling in the liquid phase studied by ultrafast investigations of cycloheptatriene

Solvent-mediated vibrational relaxation in UV pump–probe experiments has been studied using cycloheptatriene (CHT) and its perdeuterated counterpart (CHTd8) as model systems. Transient spectra containing absolute ε(t) values are obtained by a modified Sulzer–Wieland ansatz and a global fit function for vibrational relaxation is proposed.

Femtosecond Transient Absorption Spectroscopy of Single-Walled Carbon Nanotubes and their Ultrafast Optical Switching Behavior

Two types of individualized single-walled nanotubes (SWNTs) in aqueous surfactant suspensions have been studied by femtosecond two-color absorption spectroscopy. A careful selection of pump and probe wavelengths allows for the determination of the “intrinsic” lifetimes of the lowest excited states which depend on the diameter of the SWNTs. Furthermore, a fast decay component in the ps to sub-ps regime was also observed and tentatively attributed to bundle relaxation. Likewise, this experimental approach can also assess spectral regions with decays faster than 1 ps, which is of great interest for all-optical switching devices near the optical telecommunication wavelength.