Photoaquation mechanism of hexacyanoferrate(II) ions: ultrafast 2D UV and transient visible and IR spectroscopies

Ferrous iron(II) hexacyanide in aqueous solutions is known to undergo photoionization and photoaquation reactions depending on the excitation wavelength. To investigate this wavelength dependence, we implemented ultrafast two-dimensional UV transient absorption spectroscopy, covering a range from 280 to 370 nm in both excitation and probing, along with UV pump/visible probe or time-resolved infrared (TRIR) transient absorption spectroscopy and density functional theory (DFT) calculations. As far as photoaquation is concerned, we find that excitation of the molecule leads to ultrafast intramolecular relaxation to the lowest triplet state of the [Fe(CN)6]4– complex, followed by its dissociation into CN– and [Fe(CN)5]3– fragments and partial geminate recombination, all within <0.5 ps. The subsequent time evolution is associated with the [Fe(CN)5]3– fragment going from a triplet square pyramidal geometry, to the lowest triplet trigonal bipyramidal state in 3–4 ps. This is the precursor to aquation, which occurs in ∼20 ps in H2O and D2O solutions, forming the [Fe(CN)5(H2O/D2O)]3– species, although some aquation also occurs during the 3–4 ps time scale. The aquated complex is observed to be stable up to the microsecond time scale. For excitation below 310 nm, the dominant channel is photooxidation with a minor aquation channel. The photoaquation reaction shows no excitation wavelength dependence up to 310 nm, that is, it reflects a Kasha Rule behavior. In contrast, the photooxidation yield increases with decreasing excitation wavelength. The various intermediates that appear in the TRIR experiments are identified with the help of DFT calculations. These results provide a clear example of the energy dependence of various reactive pathways and of the role of spin-states in the reactivity of metal complexes

Multiphoton-Excited Luminescent Lanthanide Bioprobes: Two- and Three-Photon Cross Sections of Dipicolinate Derivatives and Binuclear Helicates

Multiphoton excited luminescent properties of water-soluble EuIII and TbIII complexes with derivatives of dipicolinic acid functionalized with a polyoxyethylene pendant arm and terminal groups, [Eu(LOMe)3]3-, [Eu(LNH2)3]3-, and [Tb(LOH)3]3-, as well as of binuclear helicates with overall composition [Ln2(LCX)3] (X ) 2, 5) are investigated. Characteristic emission from the 5D0 and 5D4 excited levels of EuIII and TbIII, respectively, upon ≈800 nm excitation results from three-photon absorption (3PA) for [Eu(LOMe)3]3-, [Eu(LNH2)3]3-, [Tb(LOH)3]3-, and [Ln2(LC2)3], while luminescence from [Eu2(LC5)3] is induced by two-photon absorption (2PA) owing to its 1PA spectrum extending further into the visible. The 3PA cross sections have been determined and are the first ones reported for lanthanide complexes: (i) those of EuIII and TbIII bimetallic helicates [Ln2(LC2)3] are 20 times larger compared to the corresponding values for tris(dipicolinates); (ii) derivatization of dipicolinic acid for TbIII complexes has almost no influence on the 3PA cross section; however, for EuIII complexes a ∼2 times decrease is observed. The feasibility of [Eu2(LC5)3] as multiphoton luminescence bioprobe is demonstrated by two-photon scanning microscopy imaging experiments on HeLa cells incubated with this bimetallic helicate

ELECTRONIC SPECTRA OF TRIPLET Rb2_{2} MOLECULES ON HELIUM NANODROPLETS. EFFECTS ON THE SPIN-ORBIT STATES

Author Institution: Institute of Experimental Physics, TU Graz, 8010 Graz, AustriaA beam of cold (0.4\,K) superfluid helium droplets, each consisting of several thousand He atoms, is used for production and spectroscopic investigation of cold Rb$_{2}$ molecules in their lowest triplet state. We have investigated the $^{3}\Pi_{\mathrm{g}}\leftarrow{^{3}\Sigma^{+}_{\mathrm{u}}}$ electronic absorption spectrum of Rb$_{2}$ at $\approx$13500\,cm$^{-1}$, as well as its dispersed emission spectra. The absorption spectrum only shows two unstructured peaks separated by $\approx$200\,cm$^{-1}$, and is well modeled with gas-phase potentials if an overall droplet-induced blue shift is included ($\approx$53\,cm$^{-1}$). Of three accessible spin-orbit components the one with $\Omega=1$ seems to be missing. This is confirmed by the emission spectra: The latter are sharp (confirming that emission occurs from the gas phase after separation from the droplet) and consequently much more detailed. We observe that such spectra can be modeled with emission from the $\Omega=0,2$ components alone and, for any given excitation frequency, only one component is needed. Vibrational populations in the excited states are more evenly distributed ($v^{\prime}=0..9$) when emission occurs from $\Omega =2$. In contrast, emission from $v^{\prime}=0$ is predominant in the $\Omega=0$ state, where the vibrational relaxation rate must therefore be higher. Atomic emission, corresponding to dissociation, is also present. Unlike the case of KRb, presented in the companion talk, no blue-shifted (singlet) emission is observed

Electron Dynamics in Anatase TiO2 Nanoparticles by Ultrafast Broadband Deep-Ultraviolet Spectroscopy

The optical bandgap of anatase TiO2 nanoparticles is dominated by bulk absorption bands in the deep-ultraviolet due to strongly bound excitons. These spectral features can be utilized as a sensitive probe of carrier and lattice dynamics inside the TiO2 nanoparticles. Here, we implement ultrafast broadband spectroscopy tuned to the exciton resonances in order to track the electron cooling in the conduction band of bare anatase nanoparticles and monitor the electron injection dynamics from an external dye in the case of sensitized anatase nanoparticles

MAGNETIC DICHROISM OF POTASSIUM ATOMS ON THE SURFACE OF HELIUM NANODROPLETS

Author Institution: Institute of Experimental Physics, TU Graz, Petersgasse 16, 8010 Graz, Austria/EUWe measured laser induced fluorescence spectra of potassium atoms on the surface of superfluid helium droplets, with and without a moderately strong external magnetic field ($\approx 3$\,kG). With no magnetic field we demonstrate saturation of the D lines with a few hundred mW of laser power. With magnetic field, under saturation, we observe a greater signal for linear polarization but no difference between the two states of (circular) polarization of the exciting laser. We take this as evidence that the two spin sublevels of the ground-state K atom are equipopulated, despite a Zeeman splitting comparable in magnitude to $kT$ (at the temperature of the droplet, $T=0.38$\,K). We estimate that the rate of spin relaxation induced by the droplet, if any, must be $< 1000$/s. Our measurements already show that, by selective depletion, it is possible to create a beam of He droplets doped with spin-polarized alkali atoms. We are in the process of further improving the accuracy of the experiment, and of testing our conclusions with one-color two-laser experiments. Further measurements on the potassium triplet dimer do show magnetic circular dichroism, suggesting that for this molecule the droplet does induce a fast spin relaxation

Electron Dynamics in Anatase TiO

The optical bandgap of anatase TiO2 nanoparticles is dominated by bulk absorption bands in the deep-ultraviolet due to strongly bound excitons. These spectral features can be utilized as a sensitive probe of carrier and lattice dynamics inside the TiO2 nanoparticles. Here, we implement ultrafast broadband spectroscopy tuned to the exciton resonances in order to track the electron cooling in the conduction band of bare anatase nanoparticles and monitor the electron injection dynamics from an external dye in the case of sensitized anatase nanoparticles