Time-and angle-resolved photoelectron spectroscopy with 13 fs temporal resolution is used to follow the different stages in the formation of a Fermi-Dirac distributed electron gas in graphite after absorption of an intense 7 fs laser pulse. Within the first 50 fs after excitation a sequence of time frames is resolved which are characterized by different energy and momentum exchange processes among the involved photonic, electronic, and phononic degrees of freedom. The results reveal experimentally the complexity of the transition from a nascent non-thermal towards a thermal electron distribution due to the different timescales associated with the involved interaction processes. 63.20.kd, 81.05.ue, 81.05.uf The extraordinary nonlinearities and optical response times of graphitic materials suggest useful applications in photonics and electronics including light harvesting [1, 2], ultrafast photodetection [3,4], THz lasing [5,6], and saturable absorption [7,8]. Both characteristics are closely linked to the ultrafast dynamics of photoexcited carriers which for this material class is governed by weakly screened carrier-carrier scattering and carrier-phonon interaction. Fundamental aspects related to these processes were addressed in different time-domain studies in the past [9][10][11][12][13]. Because of limitations in the time resolution, most of these studies were restricted, however, to the characteristic timescales of electron-lattice equilibration, i.e., timescales ranging from ≈100 fs to ≈10 ps. The primary processes directly after photoexcitation are, in contrast, still largely unexplored and were investigated experimentally only in a few studies so far [14][15][16]. The dynamics in this strongly non-thermal regime is determined by phenomena such as transient population inversion, carrier multiplication, Auger recombination, but also phonon-mediated carrier redistribution [17][18][19][20]. The challenge is to decode the relative importance and temporal sequence of these processes that drive the electronic system from a nascent non-thermal distribution as generated by photoexcitation towards a Fermi-Dirac (FD) distribution within only ≈ 50 fs [14,15]. It is obvious that such investigations rely on experiments capable of sampling this time window at an adequate time resolution of the order of 10 fs, as well as high energy and momentum resolution. This letter reports on the non-thermal carrier dynamics in highly-oriented pyrolytic graphite (HOPG) as probed in a time-and angle-resolved photoemission spectroscopy (trARPES) experiment that is operated near the transform limit at a resolution of 13 fs (FWHM of the pumpprobe cross correlation) [21]. Over the first 100 fs, we monitor the different stages in the temporal evolution of an initially non-thermal carrier distribution generated by the absorption of a 7 fs near-infrared pulse. We are able to dissect the non-thermal to thermal transition into FIG. 1. (a) WL-pump/XUV-probe cross correlation signal of the experiment. For details see Refs. [21] and [24]. (b) ...

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Photoluminescence spectra and luminescence kinetics of pure CaMoO(4) and CaMoO(4) doped with Ln(3+) (Ln = Pr or Tb) are presented. The spectra were obtained at high hydrostatic pressure up to 240 kbar applied in a diamond anvil cell. At ambient pressure undoped and doped samples exhibit a broad band emission extending between 380 and 700 nm with a maximum at 520 nm attributed to the MoO(4)(2-) luminescence. CaMoO(4) doped with Pr(3+) or Tb(3+) additionally yields narrow emission lines related to f-f transitions. The undoped CaMoO(4) crystal was characterized by a strong MoO(4)(2-) emission up to 240 kbar. In the cases of CaMoO(4):Pr(3+) and CaMoO(4):Tb(3+), high hydrostatic pressure caused quenching of Pr(3+) and Tb(3+) emission, and this effect was accompanied by a strong shortening of the luminescence lifetime. In doped samples, CaMoO(4):Pr(3+) and CaMoO(4):Tb(3+), quenching of the emission band attributed to MoO(4)(2-) was also observed, and at pressure above 130 kbar this luminescence was totally quenched. The effects mentioned above were related to the influence of the praseodymium (terbium) trapped exciton PTE (ITE-impurity trapped exciton) on the efficiency of the Pr(3+) (Tb(3+)) and MoO(4)(2-) emissions.

In this contribution, an extensive spectroscopic study of Y2O2S doped with Eu(3+) and Tb(3+) is presented. Steady-state luminescence and luminescence excitation spectra as well as the time-resolved spectra and luminescence kinetics were obtained at high hydrostatic pressures up to 240 kbar. It was found that pressure quenches the luminescence from the (5)D3 excited state of Tb(3+) and recovers additional luminescence related to transitions from the (5)D3 state of Eu(3+). These effects are related to the pressure-induced increases in the energies of the ground electronic manifold 4f(n) of Eu(3+) and Tb(3+) ions with respect to the band edges. Analysis of the emission and excitation spectra allowed the estimation of the energies of the ground states of all lanthanide (Ln) ions (Ln(3+) and Ln(2+)) with respect to the valence and conduction bands edges of the Y2O2S host. The bandgap energy and difference between energies of the ground states of Ln(2+) and Ln(3+) have been calculated as functions of pressure. The experimental high-pressure spectroscopy results allow the calculation of the absolute values (calculated with respect to the vacuum level) of the energies and pressure-induced shifts of the conduction and valence band edges and the ground states of Ln(3+) and Ln(2+) ions in Y2O2S.

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