Ground–state splittings can not only be probed by the methods described in the two preceding sections, but also by electronic Raman spectroscopy, as shown schematically in Figure 1a.
A modern example for the application of this technique is the electronic Raman spectroscopy of severalvanadium(III) alums in Figure 11.27 The experimental results were obtained on large single crystals of the highest quality. In perfectly octahedral coordination, the electronic ground state for d2 ions such as vanadium(III) is 3T1g, with spin–orbit levels separated by less than 300 cm1.38 In the low-temperature electronic Raman spectra in Figure 11, broad bands are observed between 1,600 cm1 and 2,200 cm1 for the transition to the higher-energy ground state components. These energy differences are larger by almost an order of magnitude than the spin–orbit splittings, and they quantitatively reveal the low-symmetry effects of the crystal lattice on the ground state electronic structure of the chromophore.Figure 12compares ground- state splittings observed in both luminescence and electronic Raman spectra for V(urea)6I3, another vanadium(III) complex with a distorted octahedral coordination sphere consisting of oxygen atoms.43 Its electronic Raman band coincides in energy with the luminescence band observed for the transition from the emitting state to the higher-energy ground-state components, clearly indicating a ground-state splittingE of approximately 1,500 cm1, significantly smaller than measured for the vanadium(III) alums inFigure 11. The luminescence spectrum shows bands at energies lower than 8,400 cm1, not present in the Raman spectrum and most likely correspond- ing to vibronic origins. Comparisons as shown inFigure 12 have become straightforward with modern instruments. Ground-state splittings larger by more than a factor of five than those described for the first-row transition metalcompounds above are observed for second- and third- row complexes.105
Externalperturbations, such as pressure35,36 or electric and magnetic fields, discussed in Chapters 2.23 and 2.25, often lead to dramatic optical spectroscopic effects. An example is the luminescence of samarium(II) ions doped into SrFCl shown inFigure 13, where pressure induces a large change in the spectrum.35Between ambient pressure and 67 kbar, the expected intracon- figurational 5D0!7F1 luminescence transitions are observed as narrow lines. They show a pressure-dependent red shift of 5 cm1/kbar, a value higher than the shift of the R1 line of ruby, the longstanding pressure calibration standard. At higher pressures, shown inFigure 13up to a limit of 310 kbar, a broad, unstructured transition dominates the spectrum, clearly corres- ponding to another category of electronic transition. Its final state has been assigned as arising from the lowest level of the 4f55d1 electron configuration. The energies of the 5d orbitals are expected to change strongly with pressure, allowing the crossover to occur. Luminescence lifetimes as a function of pressure for the same solid show a decrease, as expected for increasing mixing of 5d character into the emitting state electron configuration.106 Such spectroscopic experiments illustrate the potential for tuning luminescence properties over a wide range and give quantitative information on bonding and vibronic effects. Of particular recent interest have been systems where the emitting state changes with pressure, as illustrated in Figure 13. Many
such studies have been reported for chromium(III) compounds.35 More recently, other first-row metalcompounds, such as octahedralVCl63
, have been investigated.107The interactions between electronic ground and excited states have been probed for compounds of the heavier transition elements.108 The magnetic exchange interaction in chromium(III) compounds similar to the example inFigure 10 has been studied as a function of externalpressure, and shifts of electronic origins that are larger by an order of magnitude than those of the ruby R1 line have been observed.109,110A recent development is the study of vibronic band shapes and resolved vibronic structure as a function of external pressure, applied, for example, to luminescence spectra of Jahn–Teller active titanium(III) ions doped into Al2O3111
and to chromium(III) ions doped into chloride lattices,112where the effect of pressure on the Jahn–Teller effect in the4T2gemitting state was determined. Room-temperature luminescence intensities from square planar complexes of palladium(II) and platinum(II) have been shown to increase significantly with pressure.113 Pressure-induced changes of the high-frequency vibronic structure in trans-dioxo complexes of rhenium(V) have also been reported.108
Materials with multiple emitting states have become an important area where a variety of novel non-linear processes can be probed by optical spectroscopy.6,7A clear-cut example is illustrated in Figures 14 and15.114 Luminescence from multiple excited states is observed from molybdenum (III) ions doped into several chloride and bromide elpasolite lattices. These ions have the samed3 electron configuration as chromium(III), but due to their stronger crystal field, all quartet excited states are at very high energies, above the lowest sets of doublet states, as illustrated in the energy level scheme in Figure 14. Figures 14 and15 show that luminescence is observed from both the
RbV(SO4)2.12H2O
RbV(SO
4)
2.12D
2O RbV(SO4)2.12H2O
ca. 4.2 K
79 K
81 K v(OD)
v2(HOHIII)
v2(HOHI)
Wavenumber [cm–1]
2,400 2,200 2,000 1,800 1,600 1,400
Intensity
Figure 11 Polarized electronic Raman spectra of three different vanadium alums (reproduced by permission of the American ChemicalSociety fromInorg. Chem.1996,35, 5730–5736).
298 Optical (Electronic) Spectroscopy
2,000 1,500
1,000 500
0
Raman shift [cm–1]
Relative intensity
9,500 9,000 8,500 8,000
Wavenumber [cm–1]
∆E
Figure 12 Electronic Raman spectrum (top trace and abscissa scale) and near-infrared luminescence spectrum (bottom trace and abscissa scale) of V(urea)6I3. The energy difference E denoted by the
horizontal double arrow indicates the ground state splitting.
T = RT
4f55d
7F2
7F0
7F1
5D0
4f6
310kbar
217kbar
174kbar
67kbar
600 700 800 900 1,000
Wavelength (nm)
Figure 13 Pressure effect on the room-temperature luminescence spectrum of samarium(II) doped into SrFCl(reproduced by permission of Springer-Verlag fromTop. Curr. Chem.2001,213, 1–94).
lowest energy 2E and 2T1 doublet states and also from the higher energy2T2state, from which no luminescence is observed in chromium(III) compounds. Up-conversion has been achieved in these lattices by using red light to sequentially excite the molybdenum(III) centers to the 2E,2T1states and then to the higher doublet excited states, as schematically illustrated in Figure 1d. Several mechanisms for such phenomena have been discovered and their efficiency has been compared for a number of different transition metaland rare earth compounds,6,7but the area is far from fully explored. Even small structural changes, such as those caused by external pressure, can lead to different up-conversion mechanisms.115 Many other intriguing opticalspectroscopic effects have recently been discovered for materials with multiple emitting states, such as the optical bistability
0 20 40
0 1 2 3 4
Energy/B
Dq/B
2E
4T
1 4T
2 2T2
2T
1
4A2
Γ8
Γ8 Γ8 Γ7 Γ6/Γ8
0 Energy (cm –1)
5,000 10,000 15,000 20,000 25,000
Figure 14 Schematic view of the relevant states and transitions for up-conversion processes and lumines- cence from higher excited states in halide elpasolites doped with molybdenum(III) (reproduced by permission
of the American ChemicalSociety fromJ. Phys. Chem. B2000,104, 10222–10234).
4T1
2T2 2E/2T1
4T
2
(a)
εM o = 5M– 1cm– 1
10,000 12,000 14,000 16,000 18,000 20,000 22,000
Energy (cm– 1) (b)
Figure 15 Absorption and luminescence spectra of molybdenum(III) ions doped into Cs2NaYCl6(a) and Cs2NaYBr6(b), an illustrative system for up-conversion. All spectra were measured at 10 K (reproduced by
permission of the American ChemicalSociety fromJ. Phys. Chem. B2000,104, 10222–10234).
300 Optical (Electronic) Spectroscopy
for ytterbium(III) compounds,116and opticalspectroscopy willcontinue to be a powerfultoolfor the development and characterization of new materials with interesting optical properties.