Lucília Santos

The transition of mature students to higher education: Challenging traditional concepts?

Abstract The Bologna Process, recently implemented in Portugal, has brought many changes to higher education institutions. One of these changes refers to a law that enables mature students (23 years and older) to gain special access to higher education, taking into account their professional experience and other biographical elements. The numbers of non-traditional students are therefore increasing in our country, making our academic population more diverse. We designed a research project to investigate the special circumstances of non-traditional students in our institutions and to provide recommendations that should improve their academic lives. In this article, we describe survey results, focus-group interviews and life histories and use them to understand the transition of mature students into higher education. Our results include interpretations of the factors that students view as barriers to their participation in higher education, the importance of peer support, and reflections on life histories that provide greater insight into the transitional process. Although several barriers were identified by mature students, there is also a positive impact from transition. It is clear that transition today is no longer punctual or linear either in time or space. We should therefore challenge traditional views of the transition concept, in which students are considered to be a problem to higher education institutions, because this diverts attention away from the responsibilities of those institutions towards facilitating change.

Brown diamonds: Long lived visible luminescing centres

Abstract The vibronic structure and decay processes of centres occuring in brown diamonds with zerophonon lines at 2.974, 2.818 and 2.490 eV are analyzed. All have decay times in the μs - ms region, the decay processes being intrinsic to the centres, as they are sample independent.

Luminescence studies in colour centres produced in natural topaz

Luminescence emission bands originating in colour centres produced in natural topaz have been detected. These bands are studied by steady-state and time-resolved photoluminescence spectroscopy. We found that the energy of the emitting bands are different in colourless samples and in samples submitted to treatment and depend on the treatment itself. Also, there is an emission present in long-lived detection regimes, indicating the possible presence of non-radiative processes. These results are analysed and a possible correlation between structural and colour changes resulting from the irradiation and annealing treatment is addressed.

Photoconductivity of natural diamonds

Abstract The photoconductivity of natural brown diamonds was studied between 77 K and room temperature. Centres were revealed, whose presence is hidden in the absorption spectrum by the structureless broad absorption that extends into the visible spectral region in this type of diamond. Apart from ND1, the centres where photoconductivity originates have not been identified in other types of diamond.

Dynamical processes of the 2.818 eV centre in diamond

Abstract In natural Si-rich brown diamonds a luminescence centre with a zero-phonon line at 2.818 eV and a room temperature decay time of 1.7 ms is often found. The vibronic coupling and the decay characteristics of the band are analysed. The luminescence originates mainly in two sublevels of a triplet electronic state that decays to a singlet ground state. The third triplet sublevel reaches thermal equilibrium with the other two above 40 K. The possibility of stimulated tunable emission from this centre is discussed.

Long lived red luminescence in diamond

Abstract Naturally occurring brown diamonds present a widely variety of luminescing centres, several of them with lifetimes of the order of a millisecond. Two of such centres, with zero phonon lines (ZPL) at 2.082 and 2.052 eV, are analysed. The width of the ZPLs is typical of centres in diamond, indicating symmetry-allowed transitions. Although ZPLs are close in energy, the centres are not related, nor are they related with centres with fast decays (ns) whose luminescence occurs in the same spectral region: they have different luminescence excitation spectra and different relative intensities in different samples. The complex, sample-independent, but temperature-dependent, decay processes show the presence of non-radiative states in thermal equilibrium with the emitting states. Non-radiative transitions to the ground state become important above 90 K. The analogies with other slow emissions in brown diamonds are discussed.

Slow emission of the 2.56 eV centre in synthetic diamond

Abstract The 2.56 eV luminescence band in synthetic diamond is studied in detail by time resolved photoluminescence and spin resonance measurements. The temperature dependence of the vibronic band shape indicates that between 2 and 20 K there is a noticeable change in relative intensities of the two dominant lines at 2.56 and 2.54 eV, accompanied by a change in lifetime behaviour. Conventional electron paramagnetic resonance experiments show a considerable amount of substitutional Ni − ( g = 2.032) and N donors in the sample. By optically detected magnetic resonance experiments an isotropic line at g = 2.03 ± 0.01 is detected for the 2.56 eV luminescence. The defect responsible for the luminescence is tentatively related to substitutional Ni − .

Optical Characterization of C 60 Organic Semiconductor and Bilayers

Preparation and characterization either by optical absorption, photoluminescence and micro-Raman spectroscopy of individual components as well as bilayers consisting of organic dye semiconductor Zinc Phthalocyanine (ZnPc) and fullerene, C 60 , thin films are reported. The layers and structures were deposited in vacuum and some fullerene films were also prepared by casting the C 60 solution in benzene. The optical absorption and photoluminescence dependencies on film thickness in bilayers C 60 /ZnPc were observed and may be discussed in a context of interface induced simmetry reduction of C 60 molecules.

Fullerite C60 high-pressure phases. Raman study

Abstract Fullerite C 60 high-pressure phases were studied using micro-Raman spectroscopy. High-pressure phases of C 60 fullerite have been obtained under hydrostatic conditions of 7.8 GPa at 1000–1600 °C. The new phase consists of strongly interacting agglomerates. Compared with pristine C 60 , the spectra of the transformed material reveal irreversible changes depending on the treatment conditions. The Raman spectra of high-pressure phases exhibit some sp 3 - as well as strong sp 2 -hybridized carbon contributions. We discuss these results in terms of a slight shortening of distance between the C 60 molecules.

A Spin Resonance Study of The 2.9 eV Luminescence Centers in Natural and Synthetic Diamond

Time resolved photoluminescence and optically detected magnetic resonance experiments were performed on the 2.985 eV (N 3 ) luminescence band in natural diamond and the 2.964 eV luminescence center in synthetic diamonds. For the N 3 luminescence the resonance occurs in the 2 E excited state of the center. On the 2.964 eV luminescence a strongly anisotropie resonance signal is observed. The frequency response of the magnetic resonance signals confirms the time resolved photoluminescence measurements.