Symmetry-forbidden Transitions Research Articles

Strong emission and plant-growth effects of inversion symmetry lowered Mn4+ on Ca2+, Ga3+ and P5+ substituted SrAl12O19: Mn4+

The high-temperature solid-phase method was used to synthesize the co-solid phosphor (Sr, Ca) (Al, P)12O19: Mn4+, Mg2+, Ga3+. Previous studies have already observed that the doping of Ca2+ for Sr2+ site and Ga3+ for Al3+ site enhances the luminescence intensity of SrAl12O19: Mn4+. Here, we observed that the unique doping of P5+ for Al3+ significantly enhances the luminescence intensity. However, it is never known why these cations can enhance the luminescence. We analyzed Mn K-edge pre-edge peak of XANES and EXAFS and conducted Rietveld analyses of XRD. Then, we reached a comprehensive trend that in all cases, the substitution of cation having significantly larger covalency elongates nearby Mn4+/Al3+–O2- bond and shrink Mn4+/Al3+–O2- bond in opposite side in (Mn4+, Al3+)O6 octahedron, significantly lowering inversion symmetry and lowering parity forbidden transition of 1 s → 3d (pre-edge peak) and 3d → 3d (deep red emission) common as a transition of even parity → even parity. Such a deep red phosphor receiving multiple doping had high external quantum efficiency, 53 %, which much increased the yield of lettuce (+40 %) and mini-tomato (+49 %), which is world-top increase for the important growth of tomato series, compared with the growth (+20 %) under the conventional greenhouse-type sunlight converter.

Steric Control of Luminescence in Phenyl-Substituted Trityl Radicals.

Triphenylmethyl (trityl) radicals have shown potential for use in organic optoelectronic applications, but the design of practical trityl structures has been limited to donor/radical charge-transfer systems due to the poor luminescence of alternant symmetry hydrocarbons. Here, we circumvent the symmetry-forbidden transition of alternant hydrocarbons via excited-state symmetry breaking in a series of phenyl-substituted tris(2,4,6-trichlorophenyl)methyl (TTM) radicals. We show that 3-fold phenyl substitution enhances the emission of the TTM radical and that steric control modulates the optical properties in these systems. Simple ortho-methylphenyl substitution boosts the photoluminescence quantum efficiency from 1% (for TTM) to 65% at a peak wavelength of 612 nm (for 2-T3TTM) in solution. In the crystalline solid state, the neat 2-T3TTM radical shows a remarkably high photoluminescence quantum efficiency of 25% for emission peaking at 706 nm. This has implications in the design of aryl-substituted radical structures where the electronic coupling of the substituents influences variables such as emission, charge transfer, and spin interaction.

Revisiting the Spectrum of Co(CN)63–: The Role of Correlation, Solvation, and Vibronic and Spin–Orbit Couplings

In the present work, we revisit the spectrum of the hexacyanocobaltate(III) ion, [Co(CN)6]3-, which has been considered a prototype complex in the coordination chemistry, with modern quantum chemistry methods. The main features have been describing by revealing the role of different effects, such as vibronic coupling, solvation and spin-orbit coupling. The UV-vis spectrum is composed by two bands (1A1g → 1T1g and 1A1g → 1T2g), characterized by singlet-singlet metal-centered transitions, and a more intense third one, characterized by charge transfer transition. There is also a small band shoulder. The first two are symmetry-forbidden transitions in the Oh group. Their intensity can only be explained by a vibronic coupling mechanism. For the band shoulder, additional to vibronic coupling, spin-orbit coupling is also necessary, since the transition is characterized as singlet to triplet, 1A1g → 3T1g.

Phosphorescence of Hydrogen-Capped Linear Polyyne Molecules C8H2, C10H2 and C12H2 in Solid Hexane Matrices at 20 K

Laser-ablated polyyne molecules, H(C≡C)nH, were separated by size in solutions and co-condensed with excess hexane molecules at a cryogenic temperature of 20 K in a vacuum system. The solid matrix samples containing C8H2, C10H2, and C12H2 molecules were irradiated with UV laser pulses and the phosphorescence 0–0 band was observed at 532, 605, and 659 nm, respectively. Vibrational progression was observed for the symmetric stretching mode of the carbon chain in the ground state with increments of ~2190 cm−1 for C8H2, ~2120 cm−1 for C10H2, and ~2090 cm−1 for C12H2. Temporal decay analysis of the phosphorescence intensity revealed the lifetimes of the triplet state as ~30 ms for C8H2, ~8 ms for C10H2, and ~4 ms for C12H2. The phosphorescence excitation spectrum reproduces UV absorption spectra in the hexane solution and in the gas phase at ambient temperature, although the excitation energy was redshifted. The symmetry-forbidden vibronic transitions were observed for C10H2 by lower excitation energies of 25,500–31,000 cm−1 (320–390 nm). Detailed phosphorescence excitation patterns are discussed along the interaction of the polyyne molecule and solvent molecules of hexane.

Accurate prediction and measurement of vibronic branching ratios for laser cooling linear polyatomic molecules.

We report a generally applicable computational and experimental approach to determine vibronic branching ratios in linear polyatomic molecules to the 10-5 level, including for nominally symmetry-forbidden transitions. These methods are demonstrated in CaOH and YbOH, showing approximately two orders of magnitude improved sensitivity compared with the previous state of the art. Knowledge of branching ratios at this level is needed for the successful deep laser cooling of a broad range of molecular species.

Некоторые особенности спектра и кинетики фосфоресценции трифенилена в бромбензоле при 77 К

The spectrum and kinetics of triphenylene phosphorescence under monochromatic and broadband excitation in bromobenzene at 77 K were studied. It was found that the intensity distribution in the phosphorescence spectrum depends on the spectral composition of the exciting light. In the transition from excitation by monochromatic radiation of a nitrogen laser ($\lambda_{ex} = 337$ nm) to broadband excitation by a high-pressure xenon lamp with a filter of 313 nm, a 0--0 band gain is observed relative to the most intense vibronic band of a half-symmetric oscillation ($\nu =1605$ cm$^{-1})$. In this case, a broadening of the 0--0 band and a shift of its maximum to the short-wave region by 1.8 nm are observed. The decay of triphenylene phosphorescence under these conditions is two-exponential with characteristic decay times $\tau_{1} = 0.40$ s and $\tau_{2} = 0.13$ s. The contribution of the component damping with $\tau_{1}$ to the total intensity increases with the allocation of the short-wave section of the 0--0 band. The contribution of the component damping with $\tau_{2}$ increases with the allocation of the long-wave portion of the 0-0 band. An increase in the contribution of the component with $\tau_{2}$ is also observed with monochromatic laser excitation in comparison with broadband excitation. Based on these results, it was concluded that there are two spatially separated emission centers with different probabilities of deactivation of triplet excitations of triphenylene molecules. The interpretation of the results is based on the generally accepted literature provisions that halogen atoms (chlorine, bromine, iodine) increase the probability of spin forbidden and symmetry transitions in highly symmetric molecules such as triphenylene (symmetry type D$_{3h})$ and coronene (symmetry type D$_{6h})$. Based on a comparison of changes in the relative intensity of the 0-0 band of centers with $\tau_{1}$ and $\tau_{2}$, it was concluded that external heavy atoms (bromine) to a greater extent enhance the symmetry-forbidden transitions of centers with $\tau_{1}$ than with $\tau_{2}$. The faster attenuation of the phosphorescence of centers with $\tau_{2}$ than of centers with $\tau_{1}$ is due to a greater degree to an increase in their triplet - singlet transition forbidden along the spin.

Влияние интерфейсных эффектов на электронный спектр структур GaAs/AlGaAs, используемых для создания фотоприемных устройств среднего ИК-диапазона

The effect of transition processes arising in the growth chamber of the molecular beam epitaxy unit on the interfaces structure and electronic spectrum of GaAs /AlxGa1-xAs quantum wells used for MID IR photodetectors was investigated. It is shown that such processes lead to (1) low-frequency shift of the photosensitivity spectrum; (2) reduction of energy shift between the fundamental and first excited hole levels in quantum wells; (3) the appearance of the symmetry forbidden transition in the absorption spectra. These effects provide a simple approach for non-destructive assessment of the interfaces quality in GaAs/AlxGa1-xAs heterostructures for MID IR photodetectors.

A2Πu and 14Σu- States of Br2+ Studied by [1+1] Two-Photon Dissociation Spectroscopy in a Cold Ion Beam.

The A2Πu-X2Πg and 14Σu--X2Πg electronic transition spectra of Br2+ have been studied in the 500-720 nm wavelength range in a cold ion beam using a cryogenic cylindrical ion trap velocity map imaging spectrometer. The cryogenic ion trap produces a rotationally and vibrationally cold mass selected ion beam of Br2+, which simplifies the experimental spectra from vibrational hot bands and bands of mixed isotopic species. Vibrationally resolved photofragment excitation spectra are recorded for individual isotopologues of Br2+ (79Br2+, 79Br81Br+, 81Br2+) by [1+1] two-photon dissociation spectroscopy. Velocity map imaging of the photofragmented Br+ ions provides complementary information in the determination of spin-orbit states involved in corresponding electronic transitions. An experimental identification of the 14Σu- state has becomes possible based on the present experimental results and previously reported theoretical calculations. Vibrational analyses of the photofragment excitation spectra have yielded spectroscopic parameters, including state origins, harmonic frequencies, and anharmonic constants, for both A2Πu and 14Σu- states. The observed A2Πu state spin-orbit splitting and the "spin-forbidden" 14Σu--X2Πg transition band intensities indicate considerable spin-orbit couplings between the 14Σu- and A2Πu states. In addition, two groups of weak vibrational bands are also observed in the experimental spectra of 79Br81Br+, which may be due to symmetry-forbidden transitions from the X2Πg ground state to low-lying gerade states.

Quantum confined stark effect in wide parabolic quantum wells: real density matrix approach

We show how to compute the optical functions of wide parabolic quantum wells (WPQWs) exposed to uniform electric F applied in the growth direction, in the excitonic energy region. The effect of the coherence between the electron-hole pair and the electromagnetic field of the propagating wave including the electron-hole screened Coulomb potential is adopted, and the valence band structure is taken into account in the cylindrical approximation. The role of the interaction potential and of the applied electric field, which mix the energy states according to different quantum numbers and create symmetry forbidden transitions, is stressed. We use the real density matrix approach (RDMA) and an effective e-h potential, which enable to derive analytical expressions for the WPQWs electrooptical functions. Choosing the susceptibility, we performed numerical calculations appropriate to a GaAs/GaAlAs WPQWs. We have obtained a red shift of the absorption maxima (quantum confined Stark effect), asymmetric upon the change of the direction of the applied field (F → −F), parabolic for the ground state and strongly dependent on the confinement parameters (the QWs sizes), changes in the oscillator strengths, and new peaks related to the states with different parity for electron and hole.

Allotropic halogens.

Allotropes are differing structural forms of the elements. The best known example is that of carbon, which comes as diamond and graphite, along with the relatively recently discovered fullerenes and now graphenes. Here I ponder whether any of the halogens can have allotropes. Firstly, I am not aware of much discussion on the topic. But ClF3 is certainly well-known, and so it is trivial to suggest BrBr3, i.e. Br4 as an example of a halogen allotrope. Scifinder for example gives no literature hits on such a substance (either real or as a calculation; it is not always easy nowadays to tell which). So, is it stable? A B3LYP+D3/6-311++G(2d,2p) calculation reveals a free energy barrier of 17.2 kcal/mol preventing Br4 from dissociating to 2Br2.[1] The reaction however is rather exoenergic, and so to stand any chance of observing Br4, one would probably have to create it at a low temperature. But say -78° would probably be low enough to give it a long lifetime; perhaps even 0°. So how to make it? This is pure speculation, but the red colour of bromine originates from (weak, symmetry forbidden) transitions, with energies calculated (for the 2Br2 complex) as 504, 492nm. Geometry optimisation of the first singlet excited state of 2Br2 produces the structure below, not that different from Br4. At least from these relatively simple calculations, it does seem as if an allotrope of bromine might be detectable spectroscopically, if not actually isolated as a pure substance. References Henry S Rzepa., "Br4", 2015. http://dx.doi.org/10.14469/ch/191228

Subwavelength focusing of light with orbital angular momentum.

The spatial structure of light with Orbital Angular Momentum, or "twisted light", closely resembles the shape of atomic wave functions. It could therefore make symmetry-forbidden transitions possible in quantum dots, or "artificial atoms". However, the vanishing intensity in the center of an OAM beam usually makes this effect weak. Here we show a plasmonic approach to focus OAM light to subwavelength dimensions using metallic nanoscale resonant optical antennas. This allows to increase the field intensity of OAM light at the typical dimensions of quantum dots to an intensity larger than a regular Gaussian beam, which corresponds to increasing the interaction strength by 3 orders of magnitude.

Strong enhancement of forbidden atomic transitions using plasmonic nanostructures

We investigate the mediation of symmetry-forbidden atomic transitions using plasmonic nanostructures. We show that the excitation of the electric dipole-forbidden, quadrupole-allowed $6{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}\ensuremath{-}5{\phantom{\rule{0.16em}{0ex}}}^{2}{D}_{5/2}$ transition in cesium may be enhanced by more than 6 orders of magnitude in the intense, inhomogeneous near field of a plasmonic nanoantenna. Using optical reciprocity, the enhancement can be understood to apply to spontaneous emission as well, allowing the fast and efficient optical detection of excited atoms.

Quadratic non-Condon effect in optical spectra of impurity paramagnetic centers in dielectric crystals

Analytical expressions for the absorption and luminescence form functions of impurity paramagnetic centers in dielectric crystals at zero temperature are derived in the adiabatic approximation taking into account the quadratic non-Condon effect. It is proved that, if the optical transition is forbidden due to symmetry selection rules, the non-Condon absorption and luminescence spectra are not mirror symmetric and can contain a zero-phonon line, contrary to the case of linear non-Condon effect. Conditions under which the zero-phonon line is contained in the optical spectra of a symmetry-forbidden transition are determined.

Photoinduced Reaction of Hydrogen-End-Capped Polyynes with Iodine Molecules

Hydrogen-end-capped polyynes, H(C≡C)(n)H (n = 5-7), were photoirradiated in the presence of iodine molecules in nonpolar solvents to find a dramatic change in the UV/vis absorption spectrum. Absorption bands of polyynes in the UV and the band of I(2) in the visible region disappeared, whereas weaker bands of polyynes were intensified in the near UV region. The emerging features are associated with vibronic bands in the symmetry-forbidden transition of the linear polyyne molecule. Stoichiometry for the reaction, i.e., C(12)H(2) + nI(2) → C(12)H(2)I(2n), was determined to be n = 3 from the concentration-dependence experiment. (13)C NMR spectra for 1:3 mixture of polyyne and iodine molecules, namely C(10)H(2)/3I(2) and C(12)H(2)/3I(2), exhibited five and six lines, respectively, all shifted to lower fields compared to those in the case without iodine. After removing excess I(2) by reductive reagent of Na(2)SO(3), recovery of the missing absorption bands for the components of C(10)H(2) and I(2) was observed. These observations strongly support the formation of an unique molecular complex of C(2n)H(2)I(6) (n = 5-7) upon photoabsorption by an I(2) molecule within a cluster of polyyne and iodine molecules, C(2n)H(2)(I(2))(m) (m ≥ 3).

Symmetry forbidden vibronic spectra and internal conversion in benzene

The spectra of symmetry-forbidden transitions and internal conversion were investigated in the present work. Temperature dependence was taken into account for the spectra simulation. The vibronic coupling, essential in the two processes, was calculated based on the Herzberg-Teller theory within the Born-Oppenheimer approximation. The approach was employed for the symmetry-forbidden absorption/fluorescence, and internal conversion between 1(1)A(1g) and 1(1)B(2u) states in benzene. Vibrational frequencies, normal coordinates, electronic transition dipole moments, and non-adiabatic coupling matrix elements were obtained by ab initio quantum chemical methods. The main peaks, along with the weak peaks, were in good agreement with the observed ones. The rate constant of the 1(1)A(1g)← 1(1)B(2u) internal conversion was estimated within the order of 10(3) s(-1). This could be regarded as the lower limit (about 4.8 × 10(3) s(-1)) of the internal conversion. It is stressed that the distortion effect was taken into account both in the symmetry-forbidden absorption/fluorescence, and the rate constants of internal conversion in the present work. The distortion effects complicate the spectra and increase the rate constants of internal conversion.

DFT and TD DFT study of isomeric linear benzodifuranones, benzodipyrrolinones and their homologues

Ground state energy and geometry of till unknown 24 mutually isomeric and π-isoelectronic linear polyacenes with two furanone or pyrrolinone rings as the end groups were investigated by density functional theory (DFT). Their 16 symmetrical diphenyl derivatives, among which two were synthesized, were studied, too. Vertical excitation energies were computed by time dependent (TD) DFT and semiempirical ZINDO methods. No tendency to perimeter carbon–carbon bond unification was found, the central hydrocarbon moiety is of quinodimethane character. Both methods predict the similar energies of the lowest allowed spectral transition and its dependence on a size of a conjugated system, but strikingly differ in a prediction of the energies of symmetry forbidden transitions. The comparison with experimental room and low temperature absorption spectra favours the TD DFT results to the ZINDO ones.

Origin of electronic and optical trends in ternaryIn2O3(ZnO)ntransparent conducting oxides(n=1,3,5): Hybrid density functional theory calculations

Ternary oxides formed from zinc and indium have demonstrated potential for commercial optoelectronic applications. We present state-of-the-art hybrid density functional theory calculations for Zn-poor and Zn-rich compositions of the crystalline ${\text{In}}_{2}{\text{O}}_{3}{(\text{ZnO})}_{n}$ compounds. We reveal the origin of the redshift in optical transitions compared to the two component oxides: symmetry forbidden band-edge transitions in ${\text{In}}_{2}{\text{O}}_{3}$ are overcome on formation of the superlattices, with Zn-O contributions to the top of the valence band. Increasing $n$ results in the localization of the conduction-band minimum on the In-O networks. This enhanced localization explains why Zn-poor compounds (lower $n$) exhibit optimal conductivity.

An anomalous linear dichroic effect in the polarized IR spectra of 2-furancarboxylic acid crystals: Proton transfer induced by co-operative interactions involving hydrogen bonds

The polarized IR spectra of the hydrogen bond systems were studied in crystals of 2-furancarboxylic acid C 4H 3O–COOH as well as in crystals of its deuterium-bonded derivative. The spectra were measured by a transmission method at room temperature and at 77 K. Theoretical analysis of the results concerned the linear dichroic effects, the H/D isotopic and the temperature effects observed in the solid-state IR spectra at the frequency ranges of the ν O–H and the ν O–D bands, respectively. The main spectral properties of the crystals can be quantitatively interpreted in terms of the “ strong-coupling” theory for a hydrogen bond dimer model. This model explains satisfactorily not only the two-branch structure of the ν O–H and the ν O–D bands, the temperature-induced evolution of the crystalline spectra, but also the basic linear dichroic effects observed in the band frequency ranges. A vibronic mechanism, responsible for promotion of the symmetry-forbidden transition in the IR and for the excitation of the totally symmetric proton stretching vibrations in centrosymmetric hydrogen bond dimers was also taken into the account. An excess of fine linear dichroic effects, accompanying the splitting of the spectral lines in the crystalline spectra, appeared for the crystal with only one dimer in a unit cell. This fact suggests that the existence of a specific co-operative interaction effect in the lattice, depending on the stimulated rearrangement of the proton positions in the hydrogen bond dimers, occurring in the nearest surrounding of an individual dimer in the lattice. Therefore, regardless of the published X-ray P 1 ¯ space-symmetry of the lattice, the effective unit cell for explaining the crystalline spectra behaved as if it contained two nonequivalent dimers. Re-examination of the crystal X-ray structure supported the hypothesis concerning the lattice transformation predicted by the IR spectroscopic studies.

Observation of Rovibrational Transitions of HCl, (HCl)2, and H2O−HCl in Liquid Helium Nanodroplets

We report the infrared spectra of HCl, (HCl)2, and H2O-HCl in liquid helium nanodroplets in the frequency region between 2680 and 2915 cm(-1). For the HCl monomer a line width of 1.0 cm(-1) (H35Cl) corresponding to a lifetime of 5.3 ps was observed. The line broadening indicates fast rotational relaxation similar to that previously observed for HF. For (HCl)2 the free HCl as well as the bound HCl stretching band has been observed. The nu2+ bands of (HCl)2 could be rotationally resolved, and rotational constants were deduced from the spectra. We observed both the allowed and the symmetry forbidden transition. However, the forbidden "broken symmetry" tunneling transition of the mixed dimer shows an intensity that is considerably enhanced compared to the gas phase. Upon the basis of the present measurements we were able to calculate the tunneling splitting in the excited state. The tunneling splitting is found to be reduced by 28% compared to the gas phase. Transitions from the ground state to the Ka=1 level of the free HCl stretch (nu1) are recorded and show considerable line broadening with a line width of 2 cm(-1). The excited state Ka=1 has an additional rotational energy of about 10 cm(-1), thereby allowing fast rotational relaxation by coupling to helium excitations. In addition we observed the HCl stretch of the HCl-H2O dimer, which exhibits an unusually large width (1.7 cm(-1) for H35Cl)) and large red shift (8.5 cm(-1)), compared to the gas-phase values. The large-amplitude motion originating from the libration mode of the HCl-H2O complex is supposed to act as a fast relaxation manifold.

Spectroscopy of the Ã(B21)-X̃(A11) transition of jet-cooled fluorobenzene: Laser-induced fluorescence, dispersed fluorescence, and pathological Fermi resonances

A detailed study of the S1((1)B2)-S0((1)A1) electronic transition of jet-cooled fluorobenzene has been carried out using laser-induced fluorescence and dispersed fluorescence (DF) spectroscopies. Analysis of over 40 single vibronic level DF spectra resulted in the assignment of 16 fundamental frequencies in the excited electronic state. Progressions in totally symmetric modes, particularly in the ring-breathing mode nu9, feature in both types of fluorescence spectrum. There is also significant activity in non-totally-symmetric modes, with activity in Franck-Condon (FC)-allowed overtones, FC-forbidden combinations induced by Duschinsky mixing, and symmetry-forbidden transitions induced by the same Herzberg-Teller vibronic coupling mechanism that induces the benzene S1-S0 transition. Fermi resonances (FRs) are extensive throughout the spectrum, especially in the important FC-active a1 modes. A consequence of these extensive FRs is that several important previous assignments are shown to be incorrect and have been reassigned here. Ab initio and density functional theory calculations have also been performed to support the experimental assignments.