Single crystal growth, optical absorption and luminescence properties under VUV-UV synchrotron excitation of type III Ce3+:KGd(PO3)4, a promising scintillator material

Scintillator materials have gained great interest for many applications, among which the medical applications stand out. Nowadays, the research is focused on finding new scintillator materials with properties that suit the needs of each application. In particular, for medical diagnosis a fast and intense response under high-energy radiation excitation is of great importance. Here, type III Ce3+-doped KGd(PO3)4 single crystals with high crystalline quality are grown and optically characterized as a new promising scintillator material. The 4f → 5d electronic transitions of Ce3+ are identified by optical absorption. The optical absorption cross section of Ce3+ for the electronic transition from the 2F5/2 to the 5d1 level is 370 × 10−20 cm2. The luminescence of KGd0.996Ce0.004(PO3)4 crystal by exciting the 5d levels of Ce3+ with VUV-UV synchrotron radiation shows down-shifting properties with strong emissions at 322 and 342 nm from the 5d1 to 2F5/2 and 2F7/2 levels of Ce3+ with a short decay time of ~16 ns, which is very suitable for scintillator applications. Moreover, these intense emissions are also observed when Gd3+ is excited since an energy transfer from Gd3+ to Ce3+ exists.

The crystal growth rates were generally higher than 3 × 10 −3 g·h −1 . In previous reports, the crystal growth rate was around 8-9 × 10 −3 g·h −1 for undoped KGdP 25 and around 7 × 10 −3 g·h −1 for Nd-doped KGdP 18 . Several reasons can induce this slower crystal growth, which are the different composition of the solution with a reduced amount of solute in it, the total mass of the solution which in the present work is lower, and finally, the presence of cerium in the solution. However, it is significant to point out the difficulty of the growing high quality crystals due to the high viscosity of the solution, around 19 Pa·s at 950 K, and how by choosing and designing accurate crystal growth conditions, such as appropriate stirring of the solution by the use of stirrer and cooling ramps, high crystalline quality crystals have been obtained. Crystal growth is very difficult in highly viscous solutions, since due to the low molecular mobility inside them, the growth units find difficulties to reach the crystal surface.
As an example, Fig. 1 shows an as-grown Ce 3+ :KGdP single crystal with the platinum stirrer and a morphological scheme with the crystalline faces observed in this crystal. The crystal scheme corresponds to the crystalline habit of type III phase of KGdP.
The distribution coefficient of Ce 3+ in KGdP was calculated using the EPMA results according to the formula:  Table 2. The cerium distribution coefficient is larger than one; this means that the Ce 3+ distribution inside the crystal may be not homogeneous. Besides, the distribution coefficient of Ce 3+ is larger than the observed one for Yb and similar to the one reported for Nd 18,24 . This last fact can be related to the different ionic radii of the lanthanide doping ions.
In addition, X-ray powder diffraction analysis was made to study the effect on the unit cell parameters of KGdP when it is doped with Ce 3+ ions. For undoped KGdP, with non-centrosymmetric crystalline structure, the unit cell parameters are a = 7.2510(4) Å, b = 8.3498(2) Å, c = 7.9240(2) Å, β = 91.823(3)°, with Z = 2 and the unit cell volume is 479.51(3) Å 3 . In the case of KCe 0.026 Gd 0.974 (PO 3 ) 4 crystals, the unit cell parameters are a = 7.2520(4) Å, b = 8.3524(2) Å, c = 7.9265(2) Å, β = 91.826(3)°, with Z = 2 and the unit cell volume is 479.88(3) Å 3 . As can be seen, the unit cell parameters and the unit cell volume slightly increase when KGdP is doped with Ce 3+ ions. This is expected since the ionic radius of Ce 3+ with coordination VIII is higher than the ionic radius of Gd 3+ with the same coordination (1.143 Å and 1.053 Å, respectively) 26 . Moreover, by acquiring the X-ray diffraction pattern for obtaining the unit cell parameters of the Ce 3+ -doped crystals, it is proved that the crystalline phase of the crystals is the type III one (space group: P2 1 ).
Ce 3+ spectroscopy in KGd(PO 3 ) 4 single crystals. Optical absorption. As it is known, the 5d electrons present a strong interaction with the crystal field, which determines the position of the 5d energy levels and its high dependence with the crystal host [27][28][29] . These bands are also wide due to the 5d electron interaction with the lattice phonons. In the non-centrosymmetric KGdP crystalline phase, Ce 3+ substitutes the Gd 3+ ions in a C 1 position inside a GdO 8 distorted dodecahedra. The 5d level splits in five non-degenerated crystal-field levels.    Figure 2 shows the unpolarised optical absorption coefficient of KGd 0.996 Ce 0.004 (PO 3 ) 4 at room temperature and the inset displays the unpolarised optical absorption cross section of 2 F 5/2 →5d 1 transition of Ce 3+ in the same crystal at room temperature. As can be seen in this Figure, the strongest bands are assigned to the 4f→5d vibronic transitions. The wavelengths assigned to each 4f→5d 1 , 5d 2 , 5d 3 , 5d 4 and 5d 5 absorptions are resumed in Table 3.
The wavelength values for the 4f→5d electronic transitions in other hosts are also summarised in Table 3.
The broad band centred at 302.5 nm (33058 cm −1 , 4.10 eV) (see Fig. 2) corresponds to the first spin-allowed 4f→5d transition from ground state to the first 5d level (5d 1 ) of Ce 3+ in the KGdP host. This value is very close to the reported values in other crystalline phosphates, such as 310 nm for K 3 La(PO 4 ) 2 30 , and 307 nm for the centrosymmetric KGdP (space group: C2/c) 14 . It is worth to point out that the photons at 194, 244 and 302.5 nm excite both Ce 3+ and Gd 3+ ions; whereas the other identified wavelengths, 214 and 226 nm, excite predominantly only Ce 3+ ions.
The centroid position of the 5d energy levels of Ce 3+ in a free ion is 51230 cm −1 28 . Due to the large interaction of the 5d electron with the crystal field of the host A, the first dipole allowed 4f→5d transition is shifted in relation with the free ion value; this shift is defined as the spectroscopic redshift, D(A) 28 . The spectroscopic redshift of Ce 3+ due to the type III KGdP host is D(KGdP) = 16282 cm −1 . As mentioned by Dorenbos et al., this depression is expected to be the same for each Ln 3+ ion used as scintillator centre in the same KGdP host 31 .
As described by Dorenbos et al. 32 , the polyphosphate compounds have a large fraction of strongly bonding phosphate atoms, which cause that the D(A) is amongst the smallest, and the crystal field splitting, ε cfs , and the bandgap amongst the largest of all oxide compounds. By comparing the values of the D(A) of Table 3, it can be observed how this value decreases as smaller is the alkaline ion (K > Na > Li); this tendency in the phosphates was already observed by Dorenbos 31 . Table 3 shows the spectroscopic redshift, D(A), as well as the centroid shift, ε c , crystal field splitting, ε cfs , effective distance of Gd-O bond, R eff , and spectroscopic polarizability, α sp , for the type III KGdP host and others. D(A), ε c and ε cfs were calculated by using the optical absorption spectrum in Fig. 2; R eff and α sp were calculated considering the ε c , as in the reference 14 , and the crystallographic data of the undoped non-centrosymmetric KGdP 22 .   Table 3. Spectroscopic properties and crystallographic space groups of Ce 3+ doped phosphates. Note: R av = average distance, CF = coordination figure, ddh = dodecahedron, λ 5 , λ 4 , λ 3 , λ 2 , λ 1 = absorption bands of the 5d levels, D(A) = spectroscopic redshift, ε c = centroid shift , ε cfs = crystal field splitting, R eff = effective distance of Gd-O, and α sp = spectroscopic polarizability.
The crystal field splitting, ε cfs (the energy difference between the 5d 1 and the 5d 5 ) for the non-centrosymmetric KGdP is smaller than in the centrosymmetric one. This fact does not follow the expected behaviour since ε cfs is determined by the strength of the crystal field, related to the shape and the size of the coordination polyhedron, and considering the same polyhedral shape, the smaller the bond length and larger distortion, the higher the crystal field splitting 28 . Comparing the two hosts, the non-centrosymmetric KGdP crystal presents shorter bond length than the centrosymmetric one, and they have similar distortion in the cation site. This different crystal field suffered by the Ce 3+ ions may be related to effects in the second coordination sphere.
The centroid shift, ε c (energy difference between the centroid value of the 5d of the free electron and the one inside the crystal) may be related to the ligand polarization and has no dependence on the crystal field splitting value. As it can be seen in the same table, the ε c for the non-centrosymmetric KGdP in comparison with the centrosymmetric one is higher. Taking into account the ligand polarization model 29 , the fact that the non-centrosymmetric KGdP has a ε c value larger than the centrosymmetric one, should be attributed to the major contribution of the effective distances, R eff , in the value of centroid shift (ε c ∝ R eff

−6
). All discussed parameters until now are dependent of the crystalline host, so they are also related to the Ce 3+ concentration in the crystal and temperature. The spectroscopic values for non-centrosymmetric KGdP have been calculated for KGd 0.996 Ce 0.004 (PO 3 ) 4 and at room temperature.
Moreover, as 4f→5d transitions are parity allowed transitions, it was also expected a high value of optical absorption cross section. The observed value is 370 × 10 −20 cm 2 at 302.5 nm for Ce 3+ in the type III KGdP crystal. The 4f→4f transitions in lanthanide ions are very weakly influenced by the crystal field and for this reason the position of such absorption transitions of Gd 3+ are the expected ones and have been labelled by using the Dieke's diagram 10 .
Optical emission. Figure 3 shows the emission spectra of KCe 0.004 Gd 0.996 (PO 3 ) 4 under λ exc = 244 nm (5.09 eV), 226 nm (5.49 eV), 214 nm (5.80 eV) and 194 nm (6.40 eV), which correspond to the excitation to the 5d 2 , 5d 3, 5d 4 and 5d 5 energy levels, respectively, of Ce 3+ in the KGdP host. The intensities have been corrected by the excitation photon flux values. From this Figure, it can be seen an intense doublet peak centred at 322 and 342 nm corresponding to the 5d 1 → 2 F 5/2 and 5d 1 → 2 F 7/2 transitions, respectively, of Ce 3+ in KGdP. The energy difference between these two emission peaks is consistent with the energy difference between the 4f levels of Ce 3+ , 2 F 5/2 and 2 F 7/2 levels, as can be seen in the Dieke's diagram 10 . This emission appears by exciting the sample under all these four different excitation wavelengths, in which the emission intensities are in accordance with the values of absorption coefficient for each excitation wavelength. The intensity of emission when the sample is excited at 194 nm is lower than when it is excited at 214 nm, despite their similar absorption coefficient values. This behaviour might be attributed to a depopulation of the 5d 5 level of Ce 3+ due to energy transfer to the 4f levels of Gd 3+ .
It is worth saying that the 6 P 7/2 → 8 S 7/2 transition of Gd 3+ which should appear as a sharp emission around 311 nm, being common in many hosts [33][34][35][36] , is not present in the non-centrosymmetric KGd 1−x Ce x (PO 3 ) 4 crystals, as observed in Fig. 3. The same behaviour is observed in the KGd 0.981 Ce 0.019 (PO 3 ) 4 and KGd 0.974 Ce 0.026 (PO 3 ) 4 crystals, referring to the emissions of Ce 3+ and Gd 3+ under the wavelength excitation 244, 226, 214 and 194 nm. By comparing the emission spectrum obtained under VUV excitation and that obtained under X-ray excitation of four highly similar hosts 2,14,37 to our compound, it has been observed that the 6 P 7/2 → 8 S 7/2 transition of Gd 3+ is not present under VUV excitation, and it does not appear by exciting the compound with X-rays, either. Thus, in type III Ce:KGdP, since the above mentioned 4f→4f transition of Gd 3+ is not observed (Fig. 3), it is expected that it will not appear under X-ray excitation. This is a favourable feature for scintillator use because the emissions which are of interest in this application are based on 5d→4f transitions, instead of the 4f→4f transitions, since the 5d levels have very short decay times, in the order of nanoseconds. However, it should be noted that similar spectra are not always obtained when the compound is excited with VUV radiation and with X-rays.
In the inset of Fig. 3, it can be seen how several weak peaks appeared in the spectral range from 560 to 640 nm when the KCe 0.004 Gd 0.996 (PO 3 ) 4 sample is excited at 194 nm. These four weak peaks centred at 578, 592, 613 and 633 nm correspond to 4f→4f transitions of Gd 3+ , being the 6 G 11/2,9/2,5/2 → 6 P 7/2 , 6 G 7/2 → 6 P 7/2 , 6 G 11/2,9/2,5/2 → 6 P 3/2 and 6 G 7/2 → 6 P 3/2 transitions, respectively. This may occur since the 8 S 7/2 → 6 G 13/2 absorption transition of Gd 3+ about 194 nm is overlapped with the 2 F 5/2 →5d 5 of Ce 3+ , as can be seen in Fig. 2. The energy levels of Gd 3+ and Ce 3+ in KGdP and the possible energy transfer mechanism are shown in Fig. 4. Returning to Figs 2 and 3, since the absorption from 2 F 5/2 ground state of Ce 3+ to its 5d 1 level appears at 302.5 nm and the emission corresponding to the 5d 1 → 2 F 5/2 transition appears at 322 nm, the Stokes shift in the type III KGdP host is ΔS = 2002 cm −1 . By observing the work done by Shalapska et al. 12 , the absorption peak corresponding to 2 F 5/2 →5d 1 transition of Ce 3+ in Ce:NaPrP 4 O 12 (space group: P2 1 /n) appear at 295 nm, and the 5d 1 → 2 F 5/2 and 5d 1 → 2 F 7/2 transitions appear as emissions at 310 and 328 nm. The characteristics of Ce 3+ -luminescence in LiYP 4 O 12 host have also been studied by Shalapska et al. 38 . In this host, the absorption peaks corresponding to 2 F 5/2 →5d 1 transition of Ce 3+ appear at 295 and the 5d 1 → 2 F 5/2 and 5d 1 → 2 F 7/2 transitions appear at 312 and 333 nm. ] than the calculated in the three hosts mentioned above, but still slightly below the most frequent Stokes shift measured in 240 different compounds, being 2200 cm −1 27 . The Stokes shift is induced by lattice relaxation at the excited states. Hence, the higher the value of Stokes shift is, the larger the relaxation of the electron at the excited state is before emitting electromagnetic radiation, resulting in higher non-radiative losses 39,40 . The Stokes shift is not dependent of the lanthanide ion, but only depends on the host. So, it can be extrapolated for future doping ions in the KGdP crystal 31 .
Moreover, it is also interesting to observe the intensity ratios of the peaks corresponding to 5d 1 → 2 F 5/2 and 5d 1 → 2 F 7/2 transitions of Ce 3+ , since it varies with the doping level. As the Ce 3+ doping is higher, the peak corresponding to the transition from 5d 1 to 2 F 5/2 becomes less intense compared to the transition to 2 F 7/2 . The evolution of this ratio of intensities versus Ce 3+ concentration can be seen in Fig. 5. This behaviour may be due to the presence of reabsorption of the emission corresponding to the 5d 1 → 2 F 5/2 transition by the neighbouring Ce 3+ atoms. Figure 6 shows the excitation spectrum of KGd 0.981 Ce 0.019 (PO 3 ) 4 at room temperature for the emission wavelengths λ emi = 342 nm (5d 1 → 2 F 7/2 of Ce 3+ ) and λ emi = 592 nm ( 6 G 7/2 → 6 P 7/2 of Gd 3+ ) with the electronic transitions labelled. In the case of the excitation spectrum for the emission of Ce 3+ , it can be observed that these emissions take place when Ce 3+ is excited and also when Gd 3+ is excited. Therefore, this reaffirms that an energy transfer from Gd 3+ to Ce 3+ occurs. In addition, from the same spectrum (Fig. 6a), the complete band of the 5d 5 level of Ce 3+ has been observed complementing the assignment of 5d levels energy values in the absorption measurements. From Fig. 6b, it can be seen how the emission at 592 nm is produced mainly when Gd 3+ cations are directly excited. Similar behaviours have been observed for the emissions centred at 578, 613 and 633 nm, all corresponding to 4f→4f electronic Gd 3+ transitions.
The energy of the exciton creation (E ex ), that is, bound electron and hole pairs, in type III KGdP has been predicted that appears at 164 nm (7.57 eV) by the calculations explained below. First, the energy of the onset of the   41 . Then, the estimated value of E ex has been obtained by adding 0.35 eV to E fa since these measurements have been performed at room temperature 42 . Hence, the band corresponding to E ex could appear centred at 164 nm in the excitation spectra (Fig. 6), although it would not be appreciated due to an overlapping with the 8 S 7/2 → 4 F 9/2 transition of Gd 3+ .
From the value of E ex , it can be calculated the approximate energy difference from the bottom of the conduction band to the top of the valence band (E VC ); being approximately 1.08 times the energy of the exciton creation 42 . Therefore, the estimated value for type III KGdP is E VC = 8.17 eV (152 nm).
Decay time measurements. Ce 3+ -doped crystals were excited at 302.5 nm ( 2 F 5/2 →5d 1 of Ce 3+ ), 194 nm ( 8 S 7/2 → 6 G 13/2 of Gd 3+ and 2 F 5/2 →5d 5 of Ce 3+ ) and 174 nm ( 8 S 7/2 → 6 H 15/2 of Gd 3+ ). It should be noted that in order to gain in photon flux reaching to the sample, the excitation beam had a bandwidth of around 7%, being not purely monochromatic. Figure 7a shows the luminescence decay curves for the emission at λ emi = 322 nm of Ce 3+ -doped KGdP crystals with different doping concentrations and at different excitation wavelengths. As can be seen in the Figure, the decay curves can be fitted by a single exponential decay. The lifetime obtained for 5d 1 level of Ce 3+ in this host is around 16 ns. No significant changes in the lifetime value were obtained either due to the change of the wavelength of excitation or the different Ce 3+ concentration in the crystals.
This measured lifetime of the 5d 1 level of Ce 3+ in the KGdP crystal is similar or even shorter than the measured ones in other scintillator materials 3,12,14,43 . By comparing this value with the obtained in the centrosymmetric KGdP (space group: C2/c, 21.1 ns), the lifetime of the 5d 1 level of Ce 3+ in type III KGdP (space group: P2 1 ) is significantly lower 14 .
It has been also observed that in type III Ce 3+ :KGdP crystals, the 5d 1 → 2 F 5/2 , 2 F 7/2 decay is not only composed by a fast component, but also by a slow component. This fact was observed when the intensity of the emission does not reach zero value because of the short interpulse time in the synchrotron measurements. To measure in the appropriate magnitude this slow component, the lifetime was measured in a conventional fluorimeter. . In the first case, it was only observed at high energy excitations and it was attributed to trapping effect which makes the host-cerium energy transfer process more difficult, but in the case of Ce:CsGd(PO 3 ) 4 , the slow component was observed when solely the Gd 3+ ion was excited and not the Ce 3+ ions. The long component in the Ce 3+ -doped non-centrosymmetric KGdP crystal has been observed when exciting at short wavelength values (Fig. 7a) and also when exciting directly to the 5d 1 level  (Fig. 7b). Therefore, the origin of this slow component could be attributed to some contribution of the trapping effect, but the contribution of the Gd 3+ emission corresponding to the electronic transition 6 P 7/2 → 8 S 7/2 cannot be disregarded since the lifetime of this emitting 6 P 7/2 level of Gd 3+ is of the same order (4.9 ms in NaY 0.80 Gd 0. 20

Conclusions
Type III Ce 3+ -doped bulk single crystals have been grown by Top Seeded Solution Growth-Slow cooling technique from self-fluxes in high crystalline quality. Till a Ce 3+ doping level of 2 at% substituting Gd 3+ in solution, no effect of the cerium doping has been observed in the crystalline quality of the single crystals. The maximum optical absorption cross section has been measured for the electronic transition from the ground state of Ce 3+ until the 5d 1 level, being the value at 302.5 nm around 370 × 10 −20 cm 2 . The spectroscopic redshift of the 5d energy levels of Ce 3+ in the type III KGdP host is 16282 cm −1 . Under ultraviolet excitation, a doublet emission peak centred at 322 and 342 nm has been observed in all grown crystals, corresponding to the 5d 1 → 2 F 7/2 and 5d 1 → 2 F 5/2 transitions of Ce 3+ , respectively. No important Gd 3+ 4f→4f emissions have been clearly observed by exciting at this range. By last, the lifetime of the 5d 1 level of Ce 3+ in type III KGdP has been measured by setting the doublet emission peak and a lifetime around 16 ns has been obtained, which is similar or even shorter than the lifetime of the same level of Ce 3+ in other hosts. In addition, the presence of a slow component has been observed by setting the same emission peak. The reported results confirm the potentiality of the Ce 3+ -doped type III KGd(PO 3 ) 4 crystals for scintillation applications.  25 , the mixing of the solution was produced by using high axial thermal gradient (3.0 K·mm −1 in depth), with the coolest point at the center of the solution surface (the solution density decreases with increasing the temperature), accompanied by the use of a platinum stirrer (20 mm in diameter) located at about 12 mm below the surface of the solution, rotating at 55 rpm. The growth was carried out on an undoped KGdP seed with a * orientation with the b crystallographic direction tangential to the rotation direction, because in previous works it has been proved that this crystalline orientation gives excellent results in terms of crystal quality 16 . The seed, located in contact with the solution surface at about 10 mm from the rotation axis, was joined with the stirrer and rotated together 25 .
The saturation temperature of the solution was accurately determined by measuring the growth and dissolution rates of the KGdP seed as a function of the temperature. Beginning at the determined saturation temperature, cooling rates of 0.1 K·h −1 for the firsts 15 K and 0.05 K·h −1 for the next 10-15 K were applied to achieve the required supersaturation of the solution for crystal growth. After finishing the cooling ramps or when the crystal was big enough, it was slowly removed from the solution. To avoid thermal shocks in the crystal, it was cooled to room temperature inside the furnace at a rate of 20-25 K·h −1 .
The composition of the crystals was determined by Electron Probe Microanalysis with Wavelength Dispersive Spectroscopy (EPMA-WDS) using a JEOL JXA-8230. The standard used to measure K, Gd, P and O was a KGdP single crystal, while the Ce content in the crystals was measured using CeO 2 as standard. The measures were made with an accelerating voltage of 20 kV and a current of 20 nA. The measuring time for K, Gd, P and O was 10 s for peak and 5 s for background, while in the case of Ce the measuring time was 120 s for peak and 60 s for background, because of its low concentration in the crystals. K α X-ray lines of K, P and O and L α X-ray lines of Gd and Ce were used for the composition measurements. The dispersive crystals were PETJ for K, LIFH for Gd, PETL for P and Ce and LDE1 for O measurements. Under these conditions, the detection limit of Ce was 195 ppm. X-ray powder diffraction. The unit cell parameters of KGdP with Ce 3+ doping up to 2 at.% in solution were determined by X-ray powder diffraction. The equipment used was a D5000 Siemens X-ray powder diffractometer in a θ-θ configuration with the Bragg-Brentano geometry. The measurements were carried out with step size of 0.03° and a step time of 7 s and recorded in the range 2θ = 10-70°. The unit cell parameters were refined using the TOPAS program 46 .
Optical characterization. The grown crystals were cut in plates perpendicular to the crystallographic a*, b and c* directions with a diamond saw, lapped with Al 2 O 3 suspension with particle size of 9 and 3 μm, successively, and polished with colloidal suspension of amorphous silicon dioxide with a mean particle size of 0.2 μm. These plates were used for optical absorption and emission studies. The optical absorption of Ce 3+ in KGdP has been studied at room temperature using a 0.25 at.% Ce:KGdP plate parallel to the (001) crystallographic plane with a thickness of 0.11 mm and the equipment used was a CARY 5000 UV-Vis-NIR Spectrophotometer.
The emission spectroscopy of Ce 3+ -doped KGdP single crystals under vacuum ultraviolet excitation was carried out in the wavelength range from 120 to 248 nm (10-5 eV). Experiments were performed in the DESIRS beamline at SOLEIL Synchrotron (France, proposal num. 20151215, Standard). The samples were placed in a SCIENtIFIC REPORTS | (2018) 8:11002 | DOI:10.1038/s41598-018-29372-z vacuum chamber which can be evacuated to around 10 −5 bar. A Lithium Fluoride window at the entrance of the vacuum chamber separates it from the synchrotron line. The monochromatic synchrotron light reached perpendicularly to the KGdP plate. The emitted light from the sample was collected at 45° by a silica lens, focused at the entrance of an optical fibre connected to a Jaz spectrometer, Ocean Optics with a minimum spectral resolution of 0.3 nm. The emission spectra were recorded from 192 to 886 nm.
Lifetime measurements were also made in the DESIRS beamline at SOLEIL Synchrotron in a single bunch mode of operation to obtain pulsed radiation with full width at half-maximum pulse duration of 50 ps and interpulse time duration of 1.12 ms (proposal number 20161324, Standard). The crystals were placed at the same vacuum chamber, with the same configuration that in the emission measurements. The output emission of the crystal, focused with the same silica lens, was guided with an optical fibre to an ANDOR Shamrock 193i spectrograph (grating 150 lines/mm) coupled to an iStar Intensified Charge Coupled Device camera with fast response (DH734-18F-03 model). To increase the photon flux reaching the sample, once the vacuum in the chamber was enough high (at least 2 × 10 −5 bars), the optical window between the vacuum chamber and the synchrotron line was removed. Long components of the time decays have been checked and measured by a conventional fluorimeter Cary Eclipse Fluorescence Spectrophotometer.