Set-up for broadband Fourier-transform multidimensional electronic spectroscopy.

We present a compact passively phase-stabilized ultra-broadband 2D Fourier transform setup. A gas (argon)-filled hollow core fiber pumped by an amplified Ti:Al2O3 laser is used as a light source providing spectral range spanning from 420 to 900 nm. Sub-10-fs pulses were obtained using a deformable mirror-based pulse shaper. We probe the nonlinear response of Rhodamine 101 using 90 nm bandwidth and resolve vibrational coherences of 150 fs period in the ground state.

Reinvestigation of the Rb2 (2)3Pi(g)-a 3Sigma(u)+ band on helium nanodroplets.

It is well known that alkali-metal molecules are preferentially observed in the weak van der Waals-bound high spin states by helium droplet isolation spectroscopy. In [F. R. Brühl et al., J. Chem. Phys. 115, 10275 (2001)] the Rb(2) (2)(3)Pi(g)-a (3)Sigma(u)(+) band on He droplets was investigated by laser-induced fluorescence and dispersed emission spectroscopy. At that time no information on the magnitude of spin-orbit coupling was available for the (2)(3)Pi(g) state which connects to the atomic 5s+4d asymptote and it was neglected. In this work we reinvestigate the observed spectra. The dispersed emission spectra, which resulted from free molecules, are consistent with state-of-the-art nonrelativistic potential energy surfaces and effective spin-orbit coupling matrix elements obtained from resonance-enhanced multiphoton ionization spectroscopy of cold Rb dimers [J. Lozeille et al., Eur. Phys. J. D 39, 261 (2006)]. Having validated the theoretical description of the free molecule state, we use the laser-induced fluorescence spectrum to discuss the influence of the He droplet on the excitation band.

Observation of relativistic E x e vibronic coupling in Rb3 and K3 quartet states on helium droplets.

Spectroscopy of alkali-metal complexes on helium droplets (He(N), N=1000-10 000) provides the unique opportunity to observe high-spin species, which exhibit prominent spin-orbit (SO) effects. In this work we present laser-induced-fluorescence and magnetic-circular-dichroism (MCD) spectra of the (2)(4)E(')<--(1)(4)A(2) (') band of Rb(3) and K(3) on He(N). For Rb(3) we find a progression of four SO split bands, weakly perturbed by linear vibronic [Jahn-Teller (JT)] coupling. The K(3) transition was previously observed and interpreted in terms of linear and quadratic JT effects in the (2)(4)E(') state [J. H. Reho et al., J. Chem. Phys. 115, 10265 (2001)]. The structure of the MCD spectrum, however, clearly reveals the importance of SO coupling also for the K(3) transition and suggests a different assignment with weak linear JT and nonsignificant quadratic JT coupling. The observed strong C-type MCD spectra arise from different populations of the ground-state Zeeman sublevels. Thus a quantitative analysis allows a determination of the spin temperature, and since alkali-metal complexes are situated on the surface of the helium droplet this corresponds to a determination of the surface temperature. Our results from the trimer spectra are consistent with the value of approximately 0.4 K, which is found in the interior of a droplet and which we already confirmed for the surface from the spectra of triplet-state alkali-metal dimers in a previous work.

Strongly bound excitons in anatase TiO2 single crystals and nanoparticles.

Anatase TiO2 is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron-hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier-Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy.Here the authors combine steady-state angle-resolved photoemission spectroscopy, ellipsometry and ultrafast two-dimensional ultraviolet spectroscopy to examine the role of many-body correlations in anatase TiO2, revealing the existence of strongly bound excitons in single crystals and nanoparticles.