Método DPPH

Table 4-1 - Summary of samples used to test system.

CW XEOL Spectroscopy

4.9.1

Collection of a CW XEOL spectrum is achieved using the synapse CCD detector with the 150 grating, and with this configuration, the entire spectrum from 200 to 900 nm is

Sample list

Ref. Name /description Details References Element of

interest RT1020 one quarter of a 2” wafer with

a nominally un-doped GaN epilayer of 4.4 micrometres thickness on a 430 micrometres thick sapphire substrate

density of threading dislocations measured at 3e8 cm- 2 as determined by AFM (Kappers M.J., 2011)

RT1013 single crystal chrome rich tourmaline

Gem specimen Cr

MISI single crystal alkali feldspar Microcline Fe R1-11a Rapakivi Granite suite of South

Greenland it is from the Prins Christians Sund granite

(Harrison et al., 1990, Finch and Klein, 1999)

Fe

RT1001 Pyrope garnet Gem specimen Fe

RT1002 Andalusite Gem specimen Mn

RT1003 Tsavorite garnet Gem specimen V

RT1004 Grossular garnet Gem specimen V

RT1005 Yellow chrysoberyl Gem specimen Cr

RT1006 Ceylon sapphire Gem specimen Ti

RT1007 Yellow chrysoberyl displaying chatoyancy

Gem specimen Ti

RT1008 Kashmir sapphire Gem specimen Ti

RT1010 Jadeite Gem specimen Ti

RT1011 Kunzite Gem specimen Cr

RT1012 Rhodolite garnet Gem specimen Cr

Dose dependence of a sample is determined through a sequence of CW XEOL spectra collected from the same spot. The CW XEOL spectrum is collected and repeated multiple (e.g. 1000) times the experiment explores how the luminescence changes with time. The doses experienced by the sample can be estimated as the product of time and X-ray flux. Beamline I18 has a range of attenuators upstream of the final focussing mirrors

(Mosselmans et al., 2009), principally aluminium foils of different thicknesses. CW XEOL was also explored as a function of dose rate by inserting combinations of these foils into the X-ray beam upstream of the experiment (Table 4-2). The estimated attenuation of the X-rays for a particular energy was calculated using the X-ray transmission data available at http://henke. lbl. gov/optical_constants/. All of the CW experiments in this section were completed on a Na rich feldspar called cleavelandite (CLBR).

Al foil thickness (μm) Attenuation (%)

250 99 150 94 100 84 50 61 30 45 15 25 0 0

Table 4-2 - Summary of attenuation foil thicknesses and relative attenuation for 7 keV X-rays.

TR XEOL

4.9.2

Using the hybrid mode of the synchrotron and examining the timing of photon arrival events following excitation, the decay profile of the XEOL is determined Figure 4-17. A prerequisite in TCSPC is to model the system response to enable its subtraction from the data collected. Most systems have some inherent distortion to the decay profile, which can be determined and therefore corrected. In TR PL, the primary laser profile is

measured directly using a medium that scatters the primary beam and has no significant lifetime. For our TR XEOL system, analysis of the primary beam profile using scattering is not possible since the X-rays used for excitation are not detected by the CCD or MCP PMT. I therefore sought a material which has an ultrafast (e.g. fs) luminescence decay during X-irradiation as a substitute for a scattering medium. I obtained a sample of a GaN thin film RT1020 that from pulsed PL measurements is known to have a lifetime less than 10 ps. Personal communication (Kappers M.J., 2011) data is also collected from a Fe-rich microcline (ordered potassium feldspar) MISI.

As an experimental procedure, It proved valuable to collect CW XEOL spectrum using the CCD before each TR XEOL experiment so that the lifetime component could be

understood within the context of the overall spectrum. The time window between the single bunch and the main bunch in hybrid and alpha modes is ~ 200 ns, (Thomas et al., 2006) and hence lifetimes longer than ~400 ns cannot be resolved. However, the presence of longer lifetimes can be inferred from the intensity of the ‘background’ signal.

profiles. The experiment was completed using 4.5 keV excitation, an entrance slit width of 2 mm and the 150 groove 500 blazed grating centred at 300, 350, 375, 400, 425, 450 and 500 nm on 2 mm side exit slits to MCP PMT with 60 s integration.

XEOL Spatial Mapping

4.9.3

This experimental methodology explores the capabilities of TR XEOL in heterogeneous samples. The beamline has an existing micron resolution XRF mapping facility that can be utilised with this system. To test the viability spectra were collected from different points in a multi-phase sample R1-11a using 4.02 keV X-rays, with an entrance slit width of 1 mm using the 1200 groove 500 blazed grating centred at 500 nm on 2 mm side exit slits giving a spectral width of ~200 nm onto the MCP PMT. If developed this is a

valuable technique to investigate texture and can reveal concealed and overlapping emissions on a micron scale spatial resolution. Larcheri et al. (2008) have investigated scanning near-field optical microscopy (SNOM) to detect CW XEOL with success at high spatial resolutions but the technique is primarily a surface technique. Rosenberg et al. (2012) has reported on CW XEOL imaging using a conventional microscope to collect the signal, achieving resolutions in μm. Sham et al. (2010) have reported on the combined collection of XEOL and XANES for imaging but this is primarily a CW technique. Reiger et al. (2010) describe a new system at the Canadian Light Source (CLS) incorporating a streak camera capable of single collection combined TR and wavelength resolved data. A XEOL /XANES mapping system has been developed at the ESRF described in (Martinez- Criado et al., 2007)

OD XAS

4.9.4

Beamline I18 delivers X-rays from 2-20 keV and I have completed experiments to measure OD XAS in this harder X-ray regime. TR XEOL spectra were collected from a suite of mineral samples (Figure 4-17); RT1003, RT1004, RT1002, RT1001, RT1005, RT1006, RT1008, RT1007, RT1011, RT1010, RT1012, and RT1013, with reported inter-ion absorption-relaxation centres with known PL characteristics. Spectra were collected from above and below the K (or L3) absorption edge of interest to identify samples

showing a TR-OD-XAS response.