Aprendizajes de las validaciones

Different phenomena can be responsible for the sensitivity of the emission spectrum. Two cases are presented here. The first involves closely spaced electronic levels and the second the influence of the crystal field.

The presence of two closely spaced energy levels

Figure 3.5.: Dieke diagram showing Pr:YAG energy levels for transitions between 4f states and locations of 4f5d bands (left) and ratio of1D2→3H4to3Pj →3H4emis- sion versus temperature (right). Reproduced from [2]

The first case is illustrated on Fig. 3.5 where the energy levels of Pr3+are represented. This phosphor was recently used by Rothamer and co-workers [2] to perform gas-phase temperature imaging. The measured ratio has a non-monotonic response to temperature.

For this phosphor, the excitation at 266 nm takes place in the 4f5d configuration fol- lowed by thermalisation and some energy is transferred by crossover relaxation to the3P states and possibly to the1D2 state. This latter is located 4000 cm-1below the lowest 3P state3P0, which is likely to allow non-radiative transfer via multi-phonon emission from 3_{P to}1_D

2. However, the gap between1D2 and the state directly underneath1G4 is 8000 cm-1, so it is far less likely to be bridged by multi-phonon emission. Phosphorescence emissions originate from the1D2and3P states.

A possible explanation for the relative increase in the emission originating from 1D2 with temperature, is the increase in the rate of non-radiative transitions (multi-phonon) 3_P

j →1D2. Simultaneously, higher vibrational levels of the1D2 state become populated, and electrons reach the3P0 state. However due to the relatively large energy gap (4000 cm-1), this mechanism is effective only at high temperature, which results in an inflection point in the ratio of transitions from1_D

2to transitions from3P states at 1100 K, shown in Fig. 3.5.

Both transitions are parity forbidden. The transition1D →3H is also spin forbidden so its lifetime is about 190µs, while the transition3P →3H is spin-allowed and its lifetime is 14µs [65].

Another example is Dy3+and Sm3+. Here the gap between the states involved is 1000 cm-1so the relative increase in the population of the upper state starts at room temperature. The presence of two relatively close energy levels (500 cm-1- 4000 cm-1) above a large gap (>7000 cm-1) seems to be a requirement for a relative temperature dependence be- tween two emission lines. If the levels are close enough, emissions from the upper level increase relatively to those from the lower level as in Dy3+, while if the spacing is larger the opposite is true at low temperature and an inflexion point is expected at elevated tem- perature as in Pr3+.

Crystal field spliting

A second mechanism is observed in Eu2+which has the electronic configuration 5s25p64f7- 6s2. In most hosts, the luminescent transitions are between the 4f7and 4f5d configurations. These are parity allowed but partially spin-forbidden and the lifetime is often about a mi- crosecond. 5d orbitals are more exposed and therefore play a stronger role than 4f orbitals in the chemical bonding between the metal ion and its ligand resulting in a broad excitation and emission spectrum.

Figure 3.6.: Energy separation of the 4f7and 4f65d1bands of Eu2+as a function of cova- lence and ligand field strength. The arrows indicate different emission colours. Reproduced from [3] Host Peak SrB4O7 368 nm Sr2P2O7 420 nm BaMgAl10O17 453 nm Sr4Al14O25 490 nm Ba2SiO4 505 nm SrGa2S4 535 nm Sr2SiO4 575 nm SrS:Eu 615 nm

Table 3.1.: Position of the maximum of the emission band of Eu2+in different hosts

ligand approaches the Eu2+ion, the Coulombic forces between the electrons of the ligand and the electrons of the 5d orbitals increase. The distinct 5d orbitals, originally having the same energy, experience different forces because their wave functions are different, resulting in a splitting of the energy, as shown on Fig. 3.6. Therefore depending on the host, and on the site that the ion occupies in the lattice, the position of the lowest 4f5d component from which the emission originates changes resulting in a wide range of emission peak wavelength, as shown in Table 3.1 (The values of the peaks are extracted from [3]).

With increasing temperature, the thermal lattice dilation weakens the crystal field acting on the Eu2+ ion. This results in a rise of the lowest component of the 5d orbital and therefore a shift of the emission band. In addition, higher vibrational levels of the excited states are populated after thermalisation. Since there is a significant difference in the metal-

Figure 3.7.: Emission spectrum of BAM:Eu2+(see Section 6.1)

ligand distance between the excited and ground state, this results in a broadening of the emission spectrum. The shift and broadening of the emission spectrum with temperature for BAM:Eu2+can be observed on Fig 3.7

Ratio imaging

The spectral response can be used for thermometry by measuring the change of the emis- sion spectrum using two different interference filters, chosen to include emission lines or spectral regions that exhibit relative temperature dependency. The superposition and di- vision of the two images result in an intensity ratio map, which can then be converted to a two-dimensional temperature image using a calibration curve previously obtained at known temperatures. This two-colour technique has been used to measure temperature on surfaces [66, 67] and in sprays [68]. In practice, ratio-based methods are straightforward to apply to planar measurements over a wide temperature range, using only two detectors and a single exposure.

3.4. Thermographic phosphors for measurements in turbulent