Elmars Krausz

Two-laser spectral hole burning in a colour centre in diamond

Abstract Using two high-resolution lasers a short lived (∼1 ms) hole burning spectrum has been observed in the 637 nm zero-phonon transition associated with the nitrogen-vacancy centre in diamond. Amongst the prominent antiholes the ones at ±2.88 GHz coincide in frequency with that obtained in EPR for a spin-doublet-singlet splitting of a metastable 3 A state. In this work it is claimed that 3 A is the ground state and this is supported by the observation of temperature-dependent magnetic circular dichroism signals. Details in the hole burning spectrum are then interpreted in terms of a strain split 3 A→ 3 E transition.

The Creutz-Taube complex revisited

Les structures cristallines obtenues par rayons X et spectres Mossbauer ont ete enregistres pour 3 complexes binucleaires du type [(NH 3 ) 5 Ru(pyz)Ru(NH 3 ) 5 ] n+ avec n=4, 5, 6 et pyz=pyrazine. Pas plus la structure que les spectres Mossbauer ne peuvent etre decrits comme une moyenne des especes oxydees (n=6) et reduites (n=4). Spectres RPE, electroniques IR et Raman

High resolution MCD spectroscopy of transition metal ions in fluoride crystals

A brief description of a high-resolution circular dichrograph is given. For MCD measurements a 0·52 T permanent magnet and silica flow tubes are used to obtain data with variable sample temperatures as low as 4·2 K. The MCD results provide an unambiguous assignment of the 4 A 1g and 4 Eg a states of Mn2+ in KMgF3 and KZnF3. The 4 Eg a state lies 90–100 cm-1 below 4 A 1g . A theoretical analysis of the MCD results confirms the assignment of the 4 A 1g state, while an analysis of the spin-orbit splitting of the 4 Eg a state has been carried out using all 252 |αLSJMJ > functions of the d5 configuration. The observed positions of the three components of 4 Eg a are accurately accounted for by Dq values appropriate to KMgF3 and KZnF3 and a spin-orbit coupling constant of approximately 300 cm-1.

Electronic spectroscopy of M(bpy)2+3 (M = Fe, Ru, Os), Cr(bpy)3+3 and related compounds

Abstract Absorption and luminescence spectroscopy of the title compounds have been studied in single crystals, PVA foils and alcohol solutions. Particular attention has been given to crystal sites with defined symmetry (D 3 and C 2 for Fe, Ru, Os). The effect of 4,4′ substitution in bpy has been studied for Fe, Ru, and OS complexes and significant intensity flow out of the ligand absorption bands into the metal to ligand charge transfer bands has been found for Ru and Os, consistent with back π-bonding. CD, CPL, MCD, MCPL and time resolved luminescence spectroscopy have been used to help make assignments. All of these data support a delocalized description of the luminescent states, except in fluid solutions where a viscosity dependent relaxation to a localized charge transfer state occurs. The first detailed investigation of the excited states of chromium (III) trisbipyridine in crystals is reported.

Assignment of the Q -Bands of the Chlorophylls: Coherence Loss via Q x − Q y Mixing

We provide a new and definitive spectral assignment for the absorption, emission, high-resolution fluorescence excitation, linear dichroism, and/or magnetic circular dichroism spectra of 32 chlorophyllides in various environments. This encompases all data used to justify previous assignments and provides a simple interpretation of unexplained complex decoherence phenomena associated with Qx → Qy relaxation. Whilst most chlorophylls conform to the Gouterman model and display two independent transitions Qx (S2) and Qy (S1), strong vibronic coupling inseparably mixes these states in chlorophyll-a. This spreads x-polarized absorption intensity over the entire Q-band system to influence all exciton-transport, relaxation and coherence properties of chlorophyll-based photosystems. The fraction of the total absorption intensity attributed to Qx ranges between 7% and 33%, depending on chlorophyllide and coordination, and is between 10% and 25% for chlorophyll-a. CAM-B3LYP density-functional-theory calculations of the band origins, relative intensities, vibrational Huang-Rhys factors, and vibronic coupling strengths fully support this new assignment.

The assignment of the luminescent states of Ru(bpy)2+3. MCPL and time-resolved luminescence at 2.0 K

The luminescent states of Ru(bpy)2+3 are shown to be not in thermal equilibrium below ≈6 K. Time-resolved luminescence spectra and MCPL spectra show that the emission comes from E states. The assignments preclude the possibility of static localization of charge in the luminescent states.

Density-Functional Theory Investigation of the Geometric, Energetic, and Optical Properties of the Cobalt(II)tris(2,2'-bipyridine) Complex in the High-Spin and the Jahn-Teller Active Low-Spin States.

State-of-the-art generalized gradient approximation (GGA) (PBE, OPBE, RPBE, OLYP, and HCTH), meta-GGA (VSXC and TPSS), and hybrid (B3LYP, B3LYP*, O3LYP, and PBE0) functionals are compared for the determination of the structure and the energetics of the D3 [Co(bpy)3](2+) complex in the (4)A2 and (4)E trigonal components of the high-spin (4)T1g([Formula: see text]  [Formula: see text] ) state and in the low-spin (2)E state of octahedral (2)Eg([Formula: see text]  [Formula: see text] ) parentage. Their comparison extends also to the investigation of the Jahn-Teller instability of the (2)E state through the characterization of the extrema of C2 symmetry of this spin states potential energy surface. The results obtained for [Co(bpy)3](2+) in either spin manifold are very consistent among the functionals used and are in good agreement with available experimental data. The functionals, however, perform very differently with respect to the spin-state energetics because the calculated values of the high-spin/low-spin energy difference Δ[Formula: see text] vary between -3212 and 3919 cm(-)(1). Semilocal functionals tend to give too large Δ[Formula: see text] values and thus fail to correctly predict the high-spin state as the ground state of the isolated complex, while hybrid functionals tend to overestimate the stability of the high-spin state with respect to the low-spin state. Reliable results are, however, obtained with the OLYP, HCTH, B3LYP*, and O3LYP functionals which perform best for the description of the isolated complex. The optical properties of [Co(bpy)3](2+) in the two spin states are also analyzed on the basis of electronic excitation calculations performed within time-dependent density functional response theory. The calculated absorption and circular dichroism spectra agree well with experimental results.

Challenges facing an understanding of the nature of low-energy excited states in photosynthesis

While the majority of the photochemical states and pathways related to the biological capture of solar energy are now well understood and provide paradigms for artificial device design, additional low-energy states have been discovered in many systems with obscure origins and significance. However, as low-energy states are naively expected to be critical to function, these observations pose important challenges. A review of known properties of low energy states covering eight photochemical systems, and options for their interpretation, are presented. A concerted experimental and theoretical research strategy is suggested and outlined, this being aimed at providing a fully comprehensive understanding.

The Semiquinone-Iron Complex of Photosystem II: Structural Insights from ESR and Theoretical Simulation; Evidence that the Native Ligand to the Non-Heme Iron Is Carbonate

The semiquinone-iron complex of photosystem II was studied using electron spin resonance (ESR) spectroscopy and density functional theory calculations. Two forms of the signal were investigated: 1), the native g approximately 1.9 form; and 2), the g approximately 1.84 form, which is well known in purple bacterial reaction centers and occurs in photosystem II when treated with formate. The g approximately 1.9 form shows low- and high-field edges at g approximately 3.5 and g < 0.8, respectively, and resembles the g approximately 1.84 form in terms of shape and width. Both types of ESR signal were simulated using the theoretical approach used previously for the BRC complex, a spin Hamiltonian formalism in which the semiquinone radical magnetically interacts (J approximately 1 cm(-1)) with the nearby high-spin Fe(2+). The two forms of ESR signal differ mainly by an axis rotation of the exchange coupling tensor (J) relative to the zero-field tensor (D) and a small increase in the zero-field parameter D ( approximately 6 cm(-1)). Density functional theory calculations were conducted on model semiquinone-iron systems to identify the physical nature of these changes. The replacement of formate (or glutamate in the bacterial reaction centers) by bicarbonate did not result in changes in the coupling environment. However, when carbonate (CO(3)(2-)) was used instead of bicarbonate, the exchange and zero-field tensors did show changes that matched those obtained from the spectral simulations. This indicates that 1), the doubly charged carbonate ion is responsible for the g approximately 1.9 form of the semiquinone-iron signal; and 2), carbonate, rather than bicarbonate, is the ligand to the iron.

Luminescence and excitation line narrowing of [Ru(2,2’‐bipyridine)3−x (2,2’‐bipyridine‐d8)x]2+ (x=0–3) in [Zn(2,2’‐bipyridine)3](CIO4)2. Unequivocal evidence for localized lowest‐excited states

Broadband and narrowed luminescence and excitation spectra at liquid helium temperatures of the title series (abbreviated as h24, d8, d16, and d24 for x=0, 1, 2, and 3, respectively) are reported. The luminescence spectra of the h24, d8, and d24 materials are found to be very similar except that the inhomogeneous broadening varies and a deuteration shift of 37 cm−1 to higher energy is observed for the d24 system. The luminescence of the d16 material is a superposition of the h24 and d24 spectra. Two distinct luminescence lifetimes can be measured in this material. The excitation spectra of the d8 and d16 materials are well represented as superpositions of the h24 and d24 spectra. The crystal structure establishes that there is only one spectroscopic site with two equivalent ligands. The third ligand has a different environment. The lowest‐excited metal‐to‐ligand charge transfer states of the two equivalent ligands are localized on an electronic time scale but very fast intramolecular excitation energy tra...