Vibronic Intensities Research Articles

Sky-blue tri-cyclometalated heteroleptic iridium (III) phosphors: Influence of ancillary ligand

Two sky-blue tri-cyclometalated heteroleptic iridium complexes with the Ir(C^N)2(C1^N1) framework, namely M1 and M2, were developed with (3,5-bis(trifluoromethyl)phenyl)pyridine (cf3ppy) as the main cyclometalating ligand and 3-(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine (pt) or 2′,6′-difluoro-2,3′-bipyridine (dfbpy) as the ancillary cyclometalating ligand. The effect of the C1^N1 ancillary ligand on the photophysical, thermal, electrochemical properties as well as the device performances were studied comparatively. It was revealed that the ancillary ligand pt favored to suppress the longer-wavelength vibronic band intensity and thus enhanced the blue color purity of the tris-cyclometalated iridium complex M1. Sky-blue phosphorescent organic light-emitting diodes (OLEDs) were fabricated with these iridium phosphors as doped emitter. The M1-based OLED exhibited better performance with improved color purity and reduced intensity at long-wavelength vibronic band, and a maximum current efficiency and external quantum efficiency of 28.2 cd/A and 12.5 %. This study provides a valid molecular design strategy to control the intensity of the shoulder vibronic sub-band for optimized color purity of cyclometalated iridium phosphors.

Vibronic Splitting of the Electronic Origin in Two Conformers of the 3-Tolunitrile Dimer.

3-tolunitrile (3-TN) can form three different dimers, which differ in the relative orientation of the methyl groups. We determined the geometry changes of two of these conformers of 3-TN upon electronic excitation via a Franck-Condon fit of the vibronic intensities in the fluorescence emission spectra. Both aromatic rings expand upon electronic excitation, showing a delocalized one-photon excitation of the cluster. The conformer with the smaller center-of-mass distance shows an unusual order of the split components of the electronic origin. We attribute this changed order to the stronger charge transfer contributions to the splitting and a partial breakdown of the point dipole approximation, made in the Frenkel type interaction.

Similarity of Near-Threshold Energy Dependence of Positronium Formation and Photoionization in Molecules.

High-resolution measurements of the positronium formation cross sections for positron energies from threshold to 10eV are presented for aniline (C_{6}H_{5}NH_{2}), pyridine (C_{5}H_{5}N), and cyclopentane (C_{5}H_{10}). The data reveal that the measured energy dependence of the cross sections on the excess energy in the near-threshold region (1-2eV) is nearly identical to that of the corresponding photoionization cross sections. This similarity occurs despite the difference between the basic threshold laws for processes without and with a Coulomb interaction between the final-state particles. It is proposed here that the near-threshold behavior of these two different ionization processes is governed by the vibrational dynamics of the final-state cation. This is supported by comparison of the data with the calculated spectrum of vibronic intensities for the pyridine cation [Trofimov etal., J. Chem. Phys. 153, 164307 (2020)JCPSA60021-960610.1063/5.0024446].

Observation of Renner-Teller and predissociation coupled vibronic intensity borrowing in dissociative electron attachment to OCS.

We use a time-of-flight-based velocity map imaging method to look into the dissociative electron attachment to a linear OCS molecule at electron beam energies ranging from 4.5 to 8.5eV. The conical time-gated wedge slice imaging method is utilized to extract fragments' slice images, kinetic energy (KE), and angular distributions, which provide a complete kinematic understanding of this experiment on the dissociative electron attachment process. We observe that the formation of S- is relatively higher than the O- product. Three distinct dissociative KE bands of S-/OCS have been observed for the 5.0 and 6.5eV resonance positions. We notice a prominent rovibrationally coupled bimodality for each KE band in the variation of the most probable KE values. When the electron energy is changed from 5.5 to 6.0eV, we observed vibronic intensity borrowing in the highest momentum band of S- via the Σ → Π symmetric dipole-forbidden transitions within the 1.5eV energy gap. Multiple peaks in the angular distributions of S- and their modeling indicate the presence of Renner-Teller vibronic splitting. Using Q-Chem's implemented complex absorbing potential-equation of motion-electron affinity coupled cluster singles and doubles aug-cc-pVDZ+4s3p level of multireference-based electronic structure theory, we confirm the presence of OCS temporary negative ion bending vibrations and Renner-Teller vibronic splittings for the Π symmetric states. Additionally, we notice the presence of a non-radiative predissociation continuum (bringing down the rotational spectrum) and speed-dependent angular anisotropy in the S- fragmentation. Our findings at the resonance of OCS at 6.5eV closely align with the prediction of vibronic intensity borrowing by Orlandi and Siebrand [J. Chem. Phys. 58, 4513 (1973)].

Temperature Dependence of Mn4+ Emission Intensity and Lifetime in TriGain® Phosphor (K2SiF6:Mn4+)

The temperature dependence of the Mn4+ photoluminescence and lifetime has been measured in the red emitting K2SiF6:Mn4+ phosphor that is sold under the trade name, TriGain®. In the range 12−450 K, the 2E → 4A2 emission intensity increases considerably while the lifetime decreases. Above 450 K, rapid decrease in the intensity and lifetime is observed. The results are interpreted in terms of dynamically induced increases in the emission intensity and decay rate coupled with an Arrhenius-type thermal quenching. The parameters of our fitting to physical models are compared with those reported on laboratory synthesized K2SiF6:Mn4+ phosphor. This comparative study sheds light on factors responsible for non-radiative relaxation processes, which is fundamental to the understanding of phosphor quantum efficiency and performance. The temperature dependence of the Stokes to anti-Stokes vibronic line intensity ratios is also measured and analyzed. Due to low thermal conductivity of K2SiF6, it was necessary to develop protocols to ensure that the surface temperature of the phosphor was close to that of the cold finger in the closed cycle He cryostat.

The Solvent-Dependent Photophysics of Diphenyloctatetraene.

Despite many decades of study, the excited state photophysics of polyenes remains controversial. In diphenylpolyenes with conjugated backbones that contain between 2 and 4 double carbon-carbon bonds, the first two excited electronic states are nearly degenerate but of entirely different character, and their energy splitting is strongly dependent on solvent polarizability. To examine the interplay between these different states, steady-state and time-resolved fluorescence spectroscopies were used to undertake a comprehensive investigation of diphenylocatetraene's (DPO) excited state dynamics in 10 solvents of different polarizabilities and polarities, ranging from weakly interacting alkanes to polar hydrogen-bonding alcohols. These data revealed that photopreparation of the optically bright 1Bu state resulted in fast (<170 ps) internal conversion to the lower-lying optically dark 2Ag state. The 2Ag state is responsible for almost all the observed DPO fluorescence and gains oscillator strength via vibronic intensity stealing with the near-degenerate 1Bu state. The fluorescence lifetime associated with the 2Ag state decayed monoexponentially (4.2-7.2 ns) in contrast to prior biexponential decay kinetics reported for similar polyenes, diphenylbutadiene and diphenylhexatriene. An analysis combining the measured fluorescence lifetimes and fluorescence quantum yields (the latter varying between 7 and 21%) allowed for a 190 cm-1 Herzberg-Teller vibronic coupling constant between the 1Bu and 2Ag states to be determined. The analysis also revealed that the ordering of electronic states remains constant in all the solvents studied, with the 2Ag state minimum always lower in energy than that of the 1Bu state, thus making it a relatively simple polyene compared to structurally similar diphenylhexatriene.

Intensity of cooperative vibronic transitions in the Eu(III), Gd(III) and Tb(III) formates spectra.

The luminescence and absorption spectra of the lanthanide ions in solids and coordination compounds are characterized by sharp pure electronic lines, which are accompanied by much weaker lines of vibronic transitions. The vibronic spectroscopy is a good probing tool for investigations of the properties of surrounding ion ligands. The lanthanides formates are efficient luminescent crystals and can be viewed as the elementary type in the whole class of the oxygen-containing lanthanide coordination compounds. The intensity of vibronic transitions in spectra of luminescence and excitation europium (5 D0 →7 F2 , 7 F0 →5 D2 ), terbium (7 F6 →5 D4 ), gadolinium (6 P7/2 →8 S7/2 ) in anhydrous formates of the type Ln(HCOO)3 (Ln = Eu, Tb, Gd) and Y(HCOO)3 .2H2 O doped with Eu3+ and Tb3+ (C ~1 mol%) are reported. Also, the infrared and Raman spectra were obtained for the same compounds. Related integral intensity vibronic sidebands depend on the type of electronic transition of the same ion and varies for the same electronic transitions in different crystals. The obtained experimental data referring to the rate constants of vibronic transitions and intensity distribution in vibronic spectra on normal vibrations of the formate groups are in agreement with the predictions based on the Stavola-Dexter theory of cooperative vibronic transitions.

Ab initio investigation of ground and excited states of ScH molecule

The theoretical study of the electronic structure of Scandium Hydride ScH has been carried out using ab-initio methods. By employing (MRCI-SD/SA-CASSCF) and by using basis sets involving contribution of all electrons of both Scandium and Hydrogen atoms, 18 singlet and 15 triplet low-lying electronic states have been calculated below 28803 cm−1. Potential energy curves have been plotted and the term values at equilibrium Te, the vibrational constants ωe and ωeχe of ScH electronic states have been fitted. In addition, we calculated the permanent electric dipole moments for all these predicted states, the transition dipole moments TDMs within states at a range of internuclear distance around the equilibrium, the vibronic intensities FCF and the radiative lifetime. The calculated spectroscopic constants are in excellent consistency with the available experimental results and with other previous theoretical works. These calculations also predicted many excited states unobserved experimentally. The observed perturbations in D1Π and C1Σ+ have been investigated in this work.

Herzberg-Teller Effect on the Vibrationally Resolved Absorption Spectra of Single-Crystalline Pentacene at Finite Temperatures.

The line shape of an electronic spectrum conveys the coupling between electronic and vibrational degrees of freedom. In the present study, the light absorption spectra of single-crystalline pentacene were measured by polarized UV-vis microscopy at 77, 185, and 293 K. The vibronic coupling encoded in each spectrum was resolved by the Herzberg-Teller theory that considers the contributions from the Franck-Condon (FC) factor, Franck-Condo/Herzberg-Teller (FC/HT) interference, and Herzberg-Teller (HT) coupling. Specifically, excitation energies, electronic transition dipole moments, and their nuclear gradients were evaluated by the GW method to ensure numerical accuracy, while the computationally efficient density function theory was employed to determine the optimized structures and vibrational normal modes. For every pair of electronic transition and normal mode that gives rise to a strong vibronic transition intensity, we examined their spatial characteristics by projecting them onto the three crystal axes. It was found that all normal modes strongly coupled to the lowest-lying a-polarized electronic transitions oscillate along axis a, whereas none of their counterparts for the lowest-lying b-polarized electronic transitions is predominantly along axis b. This notable difference on the alignment between the electronic transition and molecular vibration could help the directional control of charge dissociation and/or spin separation. Moreover, a significant variance of the destructive FC/HT interference was discovered with increasing temperatures that can well explain the a-polarized fading tableland near 650 nm. Finally, the importance of HT coupling was corroborated by comparing its intensity with those of FC factor and FC/HT interference. Taken all together, the vibrational dependence of the electronic wave function is critical to resolve the light absorption spectra of single-crystalline pentacene and its temperature effects, facilitating the systematic design of functional optical materials based on pentacene and its derivatives.

Ab Initio Study of Vibronic and Magnetic 5f-to-5f and Dipole-Allowed 5f-to-6d and Charge-Transfer Transitions in [UX6]n- (X = Cl, Br; n = 1, 2).

The absorption spectra of the octahedral [UX6]n- (X = Cl, Br; n = 1, 2) complexes in the near-infrared (NIR) and UV-vis spectral regions were studied theoretically, using a relativistic restricted active space second-order perturbation theory (RASPT2) wavefunction framework, with the spin-orbit (SO) coupling treated by state interaction, in conjunction with Kohn-Sham density functional theory calculations for determining the vibrational normal modes. The electric-dipole-allowed ligand-to-metal charge-transfer (LMCT) and 5f-to-6d transitions, and the electric-dipole-forbidden 5f-to-5f ligand field (LF) transitions, are thereby obtained within the same theoretical framework. For the 5f-to-5f LF transitions, the observed absorption intensity is mostly due to vibronic coupling with low-energy electric-dipole-allowed transitions, but in some cases, the magnetic dipole intensity of the purely electronic transition has comparable intensity to the vibronic transitions. Experimental LF spectra of 5f2 open-shell systems have been reported decades back, but ab initio calculations of their vibronic intensity have not yet been reported in the literature. Although the LF spectra for the 5f2 systems can be assigned in detail, based on the calculations, the spectra are very complex and the underlying electronic states are strongly multiconfigurational. Therefore, the usefulness of the LF spectra beyond serving as a "fingerprint" of the LF and the metal oxidation state appears to be limited.

Vibrationally resolved valence and core photoionization and photoexcitation spectra of an electron-deficient trivalent boron compound: the case of catecholborane.

Compounds containing trivalent boron (TB) as the electron-deficient site(s) find numerous practical uses ranging from Lewis bases in organic synthesis to high-tech industry, with a number of novel applications anticipated. We present an experimental and theoretical study of the gas-phase valence photoionization (VUV-PES), core photoionization (XPS) and photoexcitation (NEXAFS) spectra of a representative TB compound catecholborane (CB). For modelling and assigning the spectra we used the ΔDFT and restricted single excitation space TD-DFT methods for the XPS and NEXAFS, and OVGF and EOM-CCSD for the VUV-PES. The vibrationally resolved structure was computed in the Franck-Condon (FC) and Herzberg-Teller (FCHT) approximations generally resulting in a good agreement with the observed spectral features. For the prediction of core-electron binding energies (CEBEs) several density functionals were tested. The best performance overall was furnished by ωB97X-D suggesting that including the dispersion correction is beneficial. The FCHT vibronic intensities are in clear discrepancy with the B 1s NEXAFS spectrum if the harmonic approximation is used for the B-H wagging mode both in the ground and in the first core-excited state. Instead, a much better agreement is obtained if the excited state potential is approximated to a symmetric double-well. The observed vibronic pattern could be a general fingerprint of the presence of TB centre(s), specifically, the transfer of the (core) density to the vacant boron p-orbital in the excited state.

Evidence of Increased Hydrophobicity and Dynamics inside the Tail Region of Glycolipid Self-Assemblies Using 2-n-Alkyl-Pyrene Derivatives to Probe Different Locations.

New designer biofluorophores are being increasingly used in the investigation of complex cellular processes. In this study, we utilized new derivatives of pyrene (Py), i.e., 2-n-alkyl-pyrenes (Py-C4 and Py-C8), in order to probe different regions inside the hydrophobic tail of n-dodecyl β-d-maltoside (βMal-C12) in two different phases (cubic ↔ lamellar). Although the sensitivity to the local environment is reduced compared to that of Py, attaching C4 and C8 at the 2-position of Py can provide a possible means to probe the local hydrophobicity in different parts of the tail region. The absence of excimer fluorescence and the ratio of the vibronic fluorescence peak intensities (I1/I3) in a lipid environment indicate the existence of Py as monomers in the hydrophobic region, similar to hydrophobic solvation, yet close to the headgroup region. When Py is replaced by Py-C4 and Py-C8, there is a small increase in hydrophobicity (reduction in I1/I3) as the Py moiety is pulled deeper inside the tail region of both cubic and lamellar phases. The larger space of the tail region in the lamellar phase is reflected as more local hydrophobicity measured by the probes which can penetrate deep inside, whereas the curved structure of the cubic phase limits the available space for the probes. Three fluorescence lifetime components were measured in lipid, indicating the heterogeneous nature of the hydrophobic region. In the lamellar phase, a large reduction in the average lifetime value, led by the long decay component, was measured for Py-C4 (reduction by 25%) and Py-C8 (45%) compared to that of the parent Py. This observation suggests the presence of a mechanism of interaction more collisional than static between the Py moiety and the tail region of the bilayer unit due to the ample space provided by the lamellar phase as the probe is buried deeper inside the hydrophobic region. A much smaller effect was observed in the cubic phase and was correlated with the tight environment around the probes, which stems from the increased curvature of the cubic phase. The current results provide a deeper understanding of the hydrophobic region during phase transition of lipid self-assembly which is important for better control during the process of membrane-protein crystallization.

Anomalous Linear Dichroism in Bent Chromophores of π-conjugated Polymers: Departure from the Franck-Condon Principle.

We examine the influence of bending of π-conjugated chromophores on photoluminescence (PL) by spectrally resolving the depolarization of fluorescence on the single-molecule level. The effect of excited-state mixing mediated by molecular vibrations is manifested in the departure from the usual achromatic linear dichroism of fluorescence, with the polarization anisotropy decreasing in the vibronic progression. Bent chromophores reveal an overall increase in vibronic PL intensity with polarization orthogonal to the molecular long axis. This manifestation of the Renner-Herzberg-Teller (RHT) effect illustrates the breakdown of the Franck-Condon principle in macromolecules used in organic electronics, providing information on the orientation of transition-dipole moments and the origin of spectral broadening. While some of the spectral signatures of the RHT effect appear similar to those of H aggregation in molecular dimers, discrimination between the two phenomena is straightforward since H aggregation does not induce anomalous linear dichroism.

Effect of esters based on terpenoids and GABA on fluidity of phospholipid membranes

The influence of esters based on gamma-aminobutyric acid (GABA) and mono-/bicyclic terpenoids on membrane structure was investigated. The mechanism of action for terpenoid esters on phospholipids of artificial membranes and lipids isolated from the rat stratum corneum was studied by fluorescence and FT-IR spectroscopy. We report here, that inclusion of monocyclic terpenoid esters in phospholipid liposomes leads to growth of excimer to monomer ratio (IE/IM) indicating a decrease of membrane microviscosity. Another mechanism of influence on biomembranes was proposed for ester of bicyclic borneol - in this case a high ratio of vibronic peak intensities (I1/I3) was revealed. The addition of terpenoid esters appears in the FT-IR spectra as intensity reduction of absorption bands associated with C = O, P = O and P–O–С groups of lecithin phospholipids. Similar results were obtained after esters addition to lipids isolated from stratum corneum indicating a decrease of hydrogen bonds number between polar groups of lipids. Thus, the influence of terpenoid esters on molecular organization of the lipid matrix substantiates the feasibility of their use after transdermal delivery in vivo.

Vibronic Wavepackets and Energy Transfer in Cryptophyte Light-Harvesting Complexes.

Determining the key features of high-efficiency photosynthetic energy transfer remains an ongoing task. Recently, there has been evidence for the role of vibronic coherence in linking donor and acceptor states to redistribute oscillator strength for enhanced energy transfer. To gain further insights into the interplay between vibronic wavepackets and energy-transfer dynamics, we systematically compare four structurally related phycobiliproteins from cryptophyte algae by broad-band pump-probe spectroscopy and extend a parametric model based on global analysis to include vibrational wavepacket characterization. The four phycobiliproteins isolated from cryptophyte algae are two "open" structures and two "closed" structures. The closed structures exhibit strong exciton coupling in the central dimer. The dominant energy-transfer pathway occurs on the subpicosecond timescale across the largest energy gap in each of the proteins, from central to peripheral chromophores. All proteins exhibit a strong 1585 cm-1 coherent oscillation whose relative amplitude, a measure of vibronic intensity borrowing from resonance between donor and acceptor states, scales with both energy-transfer rates and damping rates. Central exciton splitting may aid in bringing the vibronically linked donor and acceptor states into better resonance resulting in the observed doubled rate in the closed structures. Several excited-state vibrational wavepackets persist on timescales relevant to energy transfer, highlighting the importance of further investigation of the interplay between electronic coupling and nuclear degrees of freedom in studies on high-efficiency photosynthesis.

Calculation of Dipole-Forbidden 5f Absorption Spectra of Uranium(V) Hexa-Halide Complexes.

Restricted-active-space wave function calculations including spin-orbit coupling, in combination with Kohn-Sham density functional calculations of vibrational modes, were used to determine the vibronic and electronic absorption intensities of the near-infrared electric dipole-forbidden 5f-5f transitions of representative uranium(V) hexa-halide complex ions. The agreement with experimentally assigned vibronic and electronic transitions measured for powder or solution samples of salts of the complex ions is reasonable overall and excellent for the experimentally best-resolved E5/2u → E5/2u' bands. The intensity of the vibronic transitions may be borrowed from ligand-to-metal charge-transfer excitations as well as 5f-to-6d metal-centered transitions. Magnetically allowed electronic transitions contribute to the two lower-frequency bands of the ligand-field spectrum.

Enhanced Polarization Ratio of Electrospun Nanofibers with Increased Intrachain Order by Postsolvent Treatments

Polarized emission that is beneficial to lighting and display applications can be demonstrated by aligning emissive chromophores, which can be achieved using an electrospinning technique. We investigate the photophysical properties of nanofibers based on poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]/poly(ethylene oxide) blends both with and without postsolvent treatments. Two different solvents were sequentially used in an attempt to extract the insulating electrospinnable polymer and increase the polarization ratio of the nanofiber meshes by molecular reorganization. The polarization ratio of emission from the nanofiber meshes treated with N,N-dimethylformamide (DMF) following treatment with acetonitrile solvents was found to be increased. An increase in the 0-0 emission vibronic intensity relative to that of the 0-1 peak and a reduction in the photoluminescence (PL) bandwidth were found. In addition, the PL decays faster and the parallel component along the nanofiber axis increases after the DMF treatment, indicating that the radiative recombination process becomes faster. Our results consistently show that postsolvent treatment promotes stronger J-aggregate character, with longer coherence lengths of the exciton along the long axis of the nanofibers, due to enhanced intrachain order.

Theory of optical transitions in curved chromophores

Using first order perturbation theory in the Born-Oppenheimer regime of the Frenkel-Holstein model, we develop a theory for the optical transitions in curved chromophores of π-conjugated polymers. Our key results are that for absorption, A, and emission, I, polarized parallel to the 0-0 transition, I01/I00 ≃ A01/A00 = S(N), where S(N) = S(1)/IPR is the effective Huang-Rhys parameter for a chromophore of N monomers and IPR is the inverse participation ratio. In contrast, absorption and emission polarized perpendicular to the 0-0 transition acquires vibronic intensity via the Herzberg-Teller effect. This intensity generally increases as the curvature increases and consequently I01/I00 increases (where I01 is the total 0-1 emission intensity). This effect is enhanced for long chromophores and in the anti-adiabatic regime. We show via DMRG calculations that this theory works well in the adiabatic regime relevant to π-conjugated polymers, i.e., ħ ω/|J| ≲ 0.2.

Zwitterionic betaine transition from micelles to vesicles induced by cholesterol

Vesicle formation by self-assembly in an aqueous mixture of zwitterionic carboxylate betaine surfactant and cholesterol was studied as a potential drug delivery nanocarrier. In this study, pyrene and 1,6-Diphenyle1-1,3,5-hexatrine (DPH) were applied as fluorescence probes that provides information on polarity and fluidity of the microenvironment, and used to monitor the transition of zwitterionic betaine from monomers to micelles, and to vesicles promoted by addition of cholesterol to micellar solutions through the variation of vibronic peak intensity ratio I1/I3 of the pyrene and solubilization with intensity quenching I428 of (DPH). In addition, the steady-state and time-resolved fluorescence anisotropy measurements of the (DPH) probe provides information on the effect of cholesterol on the dynamic properties vesicle membrane and vesicular distribution accompanied by dynamic light scattering (DLS), transmission electron microscopy (TEM), and turbidity measurements. Our results show an abrupt change of the ratio I1/I3 of pyrene and fluorescence intensity I428 of (DPH) in the transition from monomer to micelle with increasing betaine concentration, as well as fluorescence anisotropy results thought the transition from micelle to vesicle promoted by cholesterol and confirmed by TEM result. Furthermore, the fluorescence anisotropy measurements shows that addition of small amount of cholesterol to zwitterionic betaine micelle have a significant influence on the molecular packing leading to vesicle formation and increasing bilayer rigidity with increasing cholesterol concentration up to χchol=0.25.

Vibronics in optical spectra of Yb3+ and Ce3+ in YAG and Y2O3 ceramics

The optical spectra of Yb3+ in YAG and Y2O3 and Ce3+: YAG show complex structure with contributions of vibronic satellites that extend beyond the spectral range delineated by the pure electronic transitions. The rather strong vibronic satellites are connected to the enhanced electron-phonon interaction of the RE3+ ions at the ends of 4fn series. Thus, tentatively we suggest that some of reported satellites in infrared absorption of Ce3+ in YAG crystals could be vibronics. The optical spectra of our Yb-doped YAG and Y2O3 ceramics are similar to those reported for single crystals. The vibronic lines intensities comparable with other lines of Yb makes separation of the zero-phonon lines from vibronics very difficult, particularly for Y2O3 which has two cationic sites. Measurements of optical spectra of Yb- and (Nd,Yb)-doped ceramics under direct excitation in Yb3+ or via energy transfer from Nd3+ and comparison with the phonon spectra enables such separation and identification of Stark level structures. The particularly large intensities of the vibronic lines in vicinity of the zero-phonon lines can be connected to near-resonant effects between the phonon energies and the Stark splitting of manifolds, that are also responsible for the broadening of electronic lines corresponding to transitions to high energy levels of 2F5/2,7/2 manifolds (even at 10 K by phonon emission). This analysis indicates that the observed one-micron spectra of Yb-doped YAG and Y2O3 ceramics belong exclusively to Yb3+.