Low Temperature Optical Spectroscopy Research Articles

PHOTOCHEMISTRY OF RADICAL CATIONS OF SOME ORGANIC OXYGEN-CONTAINING COMPOUNDS IN FROZEN MATRICES

The review is dedicated to the photochemical reactions of radical cations (RC) of various organic oxygen-containing compounds stabilized in low-temperature matrices. The nature of RC and the products of their photochemical reactions has been established via quantum chemistry, electron paramagnetic resonance (EPR) and low-temperature optical spectroscopy. Depending on the structure of the precursor molecule, various mechanisms of photoconversion arise for these RC: charge transfer to matrix molecules, hydrogen atom and proton transfer, isomerization, dissociation. This study allowed us to posit that there is no correlation between the structure of the molecule of the precursor molecule and the variety of available phototransformation channels for the corresponding RC in frozen matrices.

Magnetic field tuning of crystal field levels and vibronic states in the spin ice compound Ho2Ti2O7 observed with far infrared reflectometry

Low temperature optical spectroscopy in applied magnetic fields provides clear evidence of magnetoelastic coupling in the spin ice material Ho$_2$Ti$_2$O$_7$. In far-IR reflectometry measurements, we observe field dependent features around 30, 61, 72 and 78~meV, energies corresponding to crystal electronic field (CEF) doublets. The calculations of the crystal-field Hamiltonian model confirm that the observed features in IR spectra are consistent with magnetic-dipole-allowed excitations from the ground state to higher $^5$I$_8$ CEF levels. We present the CEF parameters that best describe our field-dependent IR reflectivity measurements. Additionally, we identify a weak field-dependent shoulder near one of the CEF doublets. This indicates that this level is split even in zero-field, which we associate with a vibronic bound state. Modeling of the observed splitting shows that the phonon resides at slightly lower energy compared to the CEF level that it couples to, which is in contrast with previously published inelastic neutron measurements. The magnetic field dependence of the vibronic state shows a gradual decoupling of the phonon with the CEF level as it shifts. This approach should work in pyrochlores and other systems that have magnetic dipole transitions in the IR spectroscopic range, which can elucidate the presence and the ability to tune the nature of vibronic states in a wide variety of materials.

Exploring event horizons and Hawking radiation through deformed graphene membranes

Analogue gravitational systems are becoming an increasing popular way of studying the behaviour of quantum systems in curved spacetime. Setups based on ultracold quantum gases in particular, have been recently harnessed to explore the thermal nature of Hawking’s and Unruh’s radiation that was theoretically predicted almost 50 years ago. For solid state implementations, a promising system is graphene, in which a link between the Dirac-like low-energy electronic excitations and relativistic quantum field theories has been unveiled soon after its discovery. This link could be extended to the case of curved quantum field theory when the graphene sheet is shaped in a surface of constant negative curvature, known as Beltrami’s pseudosphere. Here we provide numerical evidence that energetically stable negative curvature graphene surfaces can be realized. Owing to large-scale simulations, our geometrical realizations are characterized by a ratio between the carbon–carbon bond length and the pseudosphere radius small enough to allow the formation of an analog of a black hole event horizon. Additionally, from the energy dependence of the spatially resolved density of states, we infer some thermal properties of the corresponding gravitational system, which could be investigated using low temperature scanning tunnelling microscopy or optical near field spectroscopy. These findings pave the way to the realization of a solid-state system in which the curved spacetime dynamics of quantum many body systems can be investigated.

Non equilibrium anisotropic excitons in atomically thin ReS2

We present a systematic investigation of the electronic properties of bulk and few layer ReS2 van der Waals crystals using low temperature optical spectroscopy. Weak photoluminescence emission is observed from two non-degenerate band edge excitonic transitions separated by ∼20 meV. The comparable emission intensity of both excitonic transitions is incompatible with a fully thermalized (Boltzmann) distribution of excitons, indicating the hot nature of the emission. While DFT calculations predict bilayer ReS2 to have a direct fundamental band gap, our optical data suggests that the fundamental gap is indirect in all cases.

NV−–N+ pair centre in 1b diamond

With the creation of nitrogen (NV) in 1b diamond it is common to find that the absorption and emission is predominantly of negatively charged NV centres. This occurs because electrons tunnel from the substitutional nitrogen atoms to NV to form NV−–N+ pairs. There can be a small percentage of neutral charge NV0 centres and a linear increase of this percentage can be obtained with optical intensity. Subsequent to excitation it is found that the line width of the NV− zero-phonon has been altered. The alteration arises from a change of the distribution of N+ ions and a modification of the average electric field at the NV− sites. The consequence is a change to the Stark shifts and splittings giving the change of the zero-phonon line (ZPL) width. Exciting the NV− centres enhances the density of close N+ ions and there is a broadening of the ZPL. Alternatively exciting and ionizing N0 in the lattice results in more distant distribution of N+ ions and a narrowing of the ZPL. The competition between NV− and N0 excitation results in a significant dependence on excitation wavelength and there is also a dependence on the concentration of the NV− and N0 in the samples. The present investigation involves extensive use of low temperature optical spectroscopy to monitor changes to the absorption and emission spectra particularly the widths of the ZPL. The studies lead to a good understanding of the properties of the NV−–N+ pairs in diamond. There is a critical dependence on pair separation. When the NV−–N+ pair separation is large the properties are as for single sites and a high degree of optically induced spin polarization is attainable. When the separation decreases the emission is reduced, the lifetime shortened and the spin polarization downgraded. With separations of <12 A0 there is even no emission. The deterioration occurs as a consequence of electron tunneling in the excited state from NV– to N+ and an optical cycle that involves NV0. The number of pairs with the smaller separations and poorer properties will increase with the number of nitrogen impurities and it follows that the degree of spin polarization that can be achieved for an ensemble of NV− in 1b diamond will be determined and limited by the concentration of single substitutional nitrogen. The information will be invaluable for obtaining optimal conditions when ensembles of NV− are required. As well as extensive measurements of the NV− optical ZPL observations of Stark effects associated with the infrared line at 1042 nm and the optically detected magnetic resonance at 2.87 GHz are also reported.

A critical size for emergence of nonbulk electronic and geometric structures in dodecanethiolate-protected Au clusters.

We report on how the transition from the bulk structure to the cluster-specific structure occurs in n-dodecanethiolate-protected gold clusters, Au(n)(SC12)m. To elucidate this transition, we isolated a series of Au(n)(SC12)m in the n range from 38 to ∼520, containing five newly identified or newly isolated clusters, Au104(SC12)45, Au(∼226)(SC12)(∼76), Au(∼253)(SC12)(∼90), Au(∼356)(SC12)(∼112), and Au(∼520)(SC12)(∼130), using reverse-phase high-performance liquid chromatography. Low-temperature optical absorption spectroscopy, powder X-ray diffractometry, and density functional theory (DFT) calculations revealed that the Au cores of Au144(SC12)60 and smaller clusters have molecular-like electronic structures and non-fcc geometric structures, whereas the structures of the Au cores of larger clusters resemble those of the bulk gold. A new structure model is proposed for Au104(SC12)45 based on combined approach between experiments and DFT calculations.

Optical and luminescence spectroscopy studies of electronic structure of Li6GdB3O9 single crystals

This article presents the study of electronic structure of Li6GdB3O9 single crystals and radiative relaxation of electronic excitations in them. The investigation was performed by the means of low-temperature optical and luminescence far-ultraviolet spectroscopy upon excitation by synchrotron radiation. On the basis of the low-temperature (T=10 and 30K) spectra of the reflection, recorded in the present research and the dispersions of the complex optical functions of εˆ(E),nˆ(E) and μ(E), calculated in the framework of the oscillator model, we have determined the parameters of the electronic structure of the Li6GdB3O9 crystals as follows. The value of the minimum energy for the interband transitions in the boron–oxygen framework is Eg=9.42eV, the energy position of the first excitonic peak in the excitation spectra for anionic excitons is En=1=7.46eV, the minimum threshold energy for excitation of excitons in linear chains of the Gd3+ cations is Ec=6.80eV. The excitation spectra of an intrinsic luminescence of Li6GdB3O9 crystals, recorded at 10K in the range of the optical charge-transfer transitions O–Gd have a band with the maximum at ECT=6.57eV.

Far-red light photoactivatable near-infrared fluorescent proteins engineered from a bacterial phytochrome

Ability to modulate fluorescence of optical probes can be used to enhance signal-to-noise ratio for imaging within highly autofluorescent environments, such as intact tissues and living organisms. Here we report two phytochrome-based photoactivatable near-infrared fluorescent proteins, named PAiRFP1 and PAiRFP2. PAiRFPs utilize heme-derived biliverdin, ubiquitous in mammalian tissues, as the chromophore. Initially weakly fluorescent PAiRFPs undergo photoconversion into a highly fluorescent state with excitation/emission at 690 nm/717 nm following a brief irradiation with far-red light. After photoactivation, PAiRFPs slowly revert back to initial state, enabling multiple photoactivation-relaxation cycles. Low-temperature optical spectroscopy reveals several intermediates involved in PAiRFP photocycles, which all differ from that of the bacteriophytochrome precursor. PAiRFPs can be photoactivated in a spatially selective manner in mouse tissues, and optical modulation of their fluorescence allows for substantial contrast enhancement, making PAiRFPs advantageous over permanently fluorescent probes for in vivo imaging conditions of high autofluorescence and low signal levels.

Unambiguous identification of the role of a single Cu atom in the ZnO structured green band

High quality and purity single crystal ZnO samples doped with single isotopes of 63Cu and 65Cu, with equal concentrations of both these isotopes, and with natural Cu using a wet chemical atomic substitution reaction and anneal were studied using low temperature optical spectroscopy. Our data on the zero phonon line of the structured green band in ZnO confirm unambiguously the involvement of a single Cu atom in this defect emission. These data allow us to confirm the main features of the assignment proposed by Dingle in 1969 and to comment further on the defect structure.

A Man for Single Molecules

Single-molecule methods have developed into a mature and powerful research approach that is adding new insights into many different areas within the physical and life sciences. The ability to detect and perform experiments on single molecules opens up new experimental approaches to investigating the physical world. One is no longer limited by ensemble averaging, making it possible to detect subpopulations directly, to measure dynamic processes without synchronization of the sample, to visualize rare events and to gather information over the heterogeneity of the sample. Single molecules can be detected and investigated using electrophysiological approaches, force microscopy, electron microscopy or fluorescence microscopy. Pioneering work in the fluorescence-based detection of single molecules was performed by Rotman in the 1960’s and 1970’s, where he trapped individual enzymes in microdroplets and watched the generation of the fluorescent product. Hirschfeld was the first to directly detect fluorescently labeled molecules diffusing in solution, using antibodies labeled with 80–100 fluorophores. In 1989 and 1990, individual fluorophores were detected for the first time using low-temperature optical spectroscopy, initially with absorption spectroscopy, shortly thereafter with fluorescence spectroscopy and at room temperature. Historical accounts of the development of single-molecule detection can be found in refs. [5, 6] . ChemPhysChem, European in origin but international in scope, deals with all aspects of the overlapping areas between chemistry, physics, biology, and materials science. It is co-owned by Chemistry Publishing Society Europe (ChemPubSoc Europe) and published by Wiley-VCH. Contributions in ChemPhysChem cover a wide range of topics including atmospheric science, hard and soft matter, femtochemistry, nanoscience, complex biological systems, single-molecule research, clusters, colloids, catalysis, and surface science; experimental and theoretical studies can be published. ChemPhysChem publishes short Communications and long Articles, as well as Reviews, Minireviews, Highlights, Concepts, Essays, Book Reviews, and occasionally Conference Reports. Authors can submit manuscripts to ChemPhysChem online through our homepage (see left) by clicking on “Submit an Article” and following the simple instructions.

Structure-Based Identification of Energy Sinks in Plant Light-Harvesting Complex II

The local S(0) → S(1) transition energies (site energies) and corresponding excitonic couplings of chlorophyll a (Chla) and b (Chlb) pigments bound to trimeric, major light-harvesting complex II (LHCII) of higher plants are calculated on the basis of the two crystal structures (Liu et al. Nature 2004, 428, 287-292; Standfuss et al. EMBO J. 2005, 24, 919-928) by using a combined quantum chemical/electrostatic method (Müh et al. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 16862-16867) that has been modified to cover membrane proteins and to account more realistically for the behavior of protonatable groups under the conditions of low-temperature optical spectroscopy. The obtained exciton levels are in reasonable agreement with experimental information (including linear absorption, linear dichroism, circular dichroism, fluorescence spectra of native as well as wild-type-minus-mutant difference absorption spectra of recombinant LHCII) and differ from earlier treatments based on fitted site energies (Novoderezhkin et al. J. Phys. Chem. B 2005, 109, 10493-10504) mainly by assigning a lower energy level to Chla 604 (in the nomenclature of Liu et al.) and Chlb 608 and a higher energy level to Chlb 605 and 609. The energy sink at cyrogenic temperatures is located at Chla 610 in the stromal layer of pigments, but structural changes at elevated temperatures may change the nature of the terminal emitter domain (including Chla 610/611/612). The site energy red-shift of Chla 610 is calculated to be significantly larger on the basis of the crystal structure of Standfuss et al. compared to that of Liu et al. due to conformational differences in the neighborhood of this pigment. A possible conformational change in the vicinity of Chla 604 involving tyrosine 112 and neoxanthin is found to strongly affect the site energy of this Chla and render it an alternative energy sink in the lumenal layer. A detailed, structure-based analysis of electrostatic pigment-protein interactions is performed to identify amino acid residues that are of interest for future mutagenesis experiments with the aim to further characterize the energy sinks, putative "bottleneck" states for excitation energy transfer, and potential sites of nonphotochemical quenching.

D1 protein variants in Photosystem II from Thermosynechococcus elongatus studied by low temperature optical spectroscopy

In Photosystem II (PSII) from Thermosynechococcus elongatus, high-light intensity growth conditions induce the preferential expression of the psbA 3 gene over the psbA 1 gene. These genes encode for the D1 protein variants labeled D1:3 and D1:1, respectively. We have compared steady state absorption and photo-induced difference spectra at < 10 K of PSII containing either D1:1 or D1:3. The following differences were observed. (i) The pheophytin Q x band was red-shifted in D1:3 (547.3 nm) compared to D1:1 (544.3 nm). (ii) The electrochromism on the Pheo D1 Q x band induced by Q A − (the C550 shift) was more asymmetric in D1:3. (iii) The two variants differed in their responses to excitation with far red (704 nm) light. When green light was used there was little difference between the two variants. With far red light the stable ( t 1/2 > 50 ms) Q A − yield was ∼ 95% in D1:3, and ∼ 60% in D1:1, relative to green light excitation. (iv) For the D1:1 variant, the quantum efficiency of photo-induced oxidation of side-pathway donors was lower. These effects can be correlated with amino acid changes between the two D1 variants. The effects on the pheophytin Q x band can be attributed to the hydrogen bond from Glu130 in D1:3 to the 13 1-keto of Pheo D1, which is absent for Gln130 in D1:1. The reduced yield with red light in the D1:1 variant could be associated with either the Glu130Gln change, and/or the four changes near the binding site of P D1, in particular Ser153Ala. Photo-induced Q A − formation with far red light is assigned to the direct optical excitation of a weakly absorbing charge transfer state of the reaction centre. We suggest that this state is blue-shifted in the D1:1 variant. A reduced efficiency for the oxidation of side-pathway donors in the D1:1 variant could be explained by a variation in the location and/or redox potential of P +.

Infrared spectroscopic and laser characterization of Tm in disordered double tungstates

The relative energy and characteristics of the Tm 3+ levels in tetragonal double tungstate (DT) and double molybdate (DMo) crystals are investigated by low temperature optical spectroscopy with special emphasis on NaLa(WO 4) 2. In this host the transition bandwidths are intermediate between those found in ordered monoclinic DT and in other disordered tetragonal DT and DMo crystals. This allows for better band resolution of the S 4 site symmetry features in the disordered scheelite-like structure. The potential of such Tm-doped crystals for building infrared tunable lasers is discussed on the basis of the calculated cross sections and their comparison with the experimental photoluminescence. Information on the crystal growth and Tm 3+ spectroscopic details are provided. Tm:NaLa(WO 4) 2 laser operation with the available sample is more efficient for σ-polarized configuration. Up to 200 mW of output power was obtained at λ = 1888 nm and laser tunability extends from 1789 to 1950 nm.

Inhomogeneous broadening of optical transitions in Yb:CaYAlO4

Yb:CaGdAlO4 is a promising laser media for femtosecond impulsions in the infrared, because of a broad emission line around 1μm. This feature is related to the crystal structure where the doping ion, which substitutes Gd3+, is located in a disordered environment where Ca2+ and Gd3+ are randomly distributed on a same crystallographic site. Since Gd3+ ions prevent any EPR measurement to be carried out, CaYAlO4, which is isostructural to CaGdAlO4, has been studied, in the present work. The origin of the broadening of the optical absorption and emission lines has been studied by low temperature optical spectroscopy on CaYAlO4. Two families of sites have been highlighted and are related to the broadening of optical transitions.

Diagnostics of natural and synthetic diamonds with the aid of low-temperature optical spectroscopy

Diagnostics of natural and synthetic diamonds with the aid of low-temperature optical spectroscopy

Ultrafast and Ultraslow Oxygen Atom Transfer Reactions between Late Metal Centers

Oxotrimesityliridium(V), (mes)3Ir=O (mes = 2,4,6-trimethylphenyl), and trimesityliridium(III), (mes)3Ir, undergo extremely rapid degenerate intermetal oxygen atom transfer at room temperature. At low temperatures, the two complexes conproportionate to form (mes)3Ir-O-Ir(mes)3, the 2,6-dimethylphenyl analogue of which has been characterized crystallographically. Variable-temperature NMR measurements of the rate of dissociation of the mu-oxo dimer combined with measurements of the conproportionation equilibrium by low-temperature optical spectroscopy indicate that oxygen atom exchange between iridium(V) and iridium(III) occurs with a rate constant, extrapolated to 20 degrees C, of 5 x 107 M-1 s-1. The oxotris(imido)osmium(VIII) complex (ArN)3Os=O (Ar = 2,6-diisopropylphenyl) also undergoes degenerate intermetal atom transfer to its deoxy partner, (ArN)3Os. However, despite the fact that its metal-oxygen bond strength and reactivity toward triphenylphosphine are nearly identical to those of (mes)3Ir=O, the osmium complex (ArN)3Os=O transfers its oxygen atom 12 orders of magnitude more slowly to (ArN)3Os than (mes)3Ir=O does to (mes)3Ir (kOsOs = 1.8 x 10-5 M-1 s-1 at 20 degrees C). Iridium-osmium cross-exchange takes place at an intermediate rate, in quantitative agreement with a Marcus-type cross relation. The enormous difference between the iridium-iridium and osmium-osmium exchange rates can be rationalized by an analogue of the inner-sphere reorganization energy. Both Ir(III) and Ir(V) are pyramidal and can form pyramidal iridium(IV) with little energetic cost in an orbitally allowed linear approach. Conversely, pyramidalization of the planar tris(imido)osmium(VI) fragment requires placing a pair of electrons in an antibonding orbital. The unique propensity of (mes)3Ir=O to undergo intermetal oxygen atom transfer allows it to serve as an activator of dioxygen in cocatalyzed oxidations, for example, acting with osmium tetroxide to catalyze the aerobic dihydroxylation of monosubstituted olefins and selective oxidation of allyl and benzyl alcohols.

Optical properties of GaInNAs/GaAs quantum well structures

The radiative recombination in GaInNAs/GaAs quantum well structures was investigated by low temperature optical spectroscopy. In the temperature region, below 100 K, we found that the observed transition energies strongly depend on the excitation intensity and the temperature, which is indicative of carrier localization. The degree of carrier localization depends on the In-concentration but is not significantly influenced by the N-concentration when the N-concentration exceeds 1.6%. Photoluminescence studies indicate that the degree of the carrier localization decreases with increasing In-concentration at a constant N-concentration. In addition, the experimental results show that carrier localization is strongly correlated to deep level emission. Through post-growth thermal treatment at 650 °C both carrier localization and deep level emission can be eliminated.

Spectroscopic Studies of Photosystem II in Chlorophylld-ContainingAcaryochloris marina

Photosystem II (PSII) electron transfer (ET) in the chlorophyll d-containing cyanobacterium Acaryochloris marina (A. marina) was studied by time-resolved electron paramagnetic resonance (EPR) spectroscopy at room temperature, chlorophyll fluorescence, and low-temperature optical spectroscopy. To maximize the ability to measure PSII ET in the intact cells of this organism, growth conditions were optimized to provide the highest specific O(2) activity and the instrumental parameters for the EPR measurements of tyrosine Z (Y(Z)) reduction were adjusted to give the best signal-to-noise over spectral resolution. Analysis of the Y(Z)(*) reduction kinetics revealed that ET to the oxygen-evolving complex on the donor side of PSII in A. marina is indistinguishable from that in higher plants and other cyanobacteria. Likewise, the charge recombination kinetics between the first plastoquinone acceptor Q(A) and the donor side of PSII monitored by the chlorophyll fluorescence decay on the seconds time scale are not significantly different between A. marina and non-chlorophyll d organisms, while low-temperature optical absorption spectroscopy identified the primary electron acceptor in A. marina as pheophytin a. The results indicate that, if the PSII primary electron donor in A. marina is made up of chlorophyll d instead of chlorophyll a, then there must be very different interactions with the protein environment to account for the ET properties, which are similar to higher plants and other cyanobacteria. Nevertheless, the water oxidation mechanism in A. marina is kinetically unaltered.

Study of indium phosphide wafers treated by long time annealing at high temperatures

High purity InP crystals were grown by liquid encapsulated Czochralski method from undoped InP melt. Wafers from the grown crystals were annealed in phosphorus ambient for 95 hours at 950 °C and cooled slowly. Conversion to semi-insulating state by annealing was studied by temperature dependent Hall measurements and low temperature optical absorption spectroscopy.

On unusual deep-violet microcrystals of diamonds from placers of Ukraine

A distinctive feature of Ukrainian deep-violet diamond microcrystals is a broad absorption band centered at about 560 nm (17850 cm -1 or 2.21 eV). In the Ukrainian samples, the intensity of this band is almost two orders of magnitude greater than in a light- rose diamond from Yakutia. Previously (Taran et al., 1998), the band was attributed to the N-V center because the absorption spectrum of the negative N-V center in irradiated and annealed type Ib diamonds is known to have an absorption band with a maximum at a similar wavelength. However, this assignment is not confirmed by vibrational spectroscopy and low temperature optical absorption spectroscopy in all crystals studied. In addition, at ~77 K the band 17850 cm -1 displays no fine structure that unambiguously evidences that it can not be the band at ~2.2 eV, commonly attributed to the N-V center.