Vibronic coupling and intersystem crossing in aromatic amines

The fluorescence and phosphorescence spectra of the aromatic amines acridan, iminobibenzyl, and carbazole have been measured in Shpolskii matrices at 10 K. Under these conditions the emission exhibits a detailed vibrational structure which has been analyzed. The change of the polarization degree observed within the fluorescence spectra at 77 K, particularly pronounced in acridan and iminobibenzyl, is attributed to vibronic interaction between the closely lying S₁(¹A₁) and S₂(¹B₁) excited states. This process activates a b₁ vibration with a frequency of 1200 cm⁻¹ in the ground state. The appearance of a long-axis (b₁) polarized vibration (700 cm⁻¹) following the out-of-plane polarized 0-0 band of the phosphorescence of these amines at 77 K is suggested to arise from vibronic interactions in the triplet manifold. This second-order spin-orbit coupling (soc) process is superimposed upon the dominant first-order electronic soc mechanism, which couples the lowest π, π^* triplet with high-energy (σ, π)^* singlet states.     

Calculation of the third depolarization virial coefficient for monoatomic gases

The temperature dependence (10–160 K) of the phosphorescence lifetimes and intensities of 2,4,5-trimethylbenzaldehyde (TMB) in crystalline durene have been investigated. Two phosphorescence bands (407.5 and 411.2 nm) are observed to grow in with increasing temperature, reach a maximum intensity, and decrease in intensity at higher temperatures. Exponential phosphorescence decays are observed in the 10 to 95 K and 110 to 160 K ranges, while non-exponential decays are observed in the 95 to 110 K region. Ultra-violet photolysis, particularly at higher temperatures, is seen to decrease the intensity of the infra-red carbonyl stretching band at 1703 cm^-1, but not the one at 1684 cm^-1. A model in which two inequivalent sets of TMB molecules in the durene lattice undergo their own unique photochemical and photophysical processes is suggested to explain the results. It is proposed that within each site the lower, predominantly ³ππ* state of the anti TMB conformer is in thermal equilibrium with the higher-lying ³nπ* state of the syn conformer which is formed by out-of-plane rotation of the aldehyde group in the excited state. In one site, intramolecular photoenol formation occurs via thermal activation from the ³nπ* of the syn conformer, while in the other site intermolecular hydrogen abstraction occurs, leading to the simultaneous production of a photoenol and a duryl radical. This model is shown to be consistent with previous knowledge of the primary photochemical processes of TMB in durene.     

The vibrational structure of the electronic spectra of quinoxaline in shpolskii-type hydrocarbon matrices

Quasilinear spectrum of the S₁(n, π¹, ¹B₁) ← S₀ transition of quinoxaline in crystalline methylcyclohexane matrix and quasilinear phosphorescence spectra of quinoxaline in high- and low-temperature modifications of isooctane matrix were obtained at 77 K. The results of a full vibrational analysis of the spectra are presented and discussed. The diffuseness of the absorption spectrum in the range 3200-2900 Å is ascribed essentially to a small energy gap between the S₂ and S₃ states. The activity of out-of-plane vibration, 865 cm^-1, in the phosphorescence and the influence of matrices on the structure of the spectra suggest the possibility of out-of-plane distortion of the quinoxaline molecule in the T₁ state.     

Observation of the distorted form of Pd-porphin in single site spectra at low temperatures

Highly resolved phosphorescence and excitation of phosphorescence spectra for palladium porphin (PdP) in polycrystalline Shpol'skii matrices at liquid helium temperatures were recorded. Two non-interactive forms of PdP in the ground state have been identified. The energy splitting between the two forms amount up to 78 cm⁻¹ for PdP in an n-octane at 4.2 K. The short-wavelength spectral form is attributed to the structure, where the central Pd(II) ion is in plane of the porphyrin macrocycle, while the long-wavelength form is associated with the nonplanar saddle-type conformation of the PdP. The frequencies of the normal vibrations in the ground electronic state have been measured separately for both forms and the differences in the normal modes of two macrocycle conformations are discussed. The set of temperature activated bands in phosphorescence spectra were found. Analysis of phosphorescence spectra at elevated temperatures and excitation of phosphorescence spectra under direct excitation in the S₀→T₁ channel make possible the value of zero-field splitting of quasi-degenerate T_1,2 state for two forms to be determined. The splitting value in n-octane matrix amount to 40 and 57 cm⁻¹ for planar and nonplanar conformations of PdP, respectively.     

Zeeman spectroscopy of the 4 Eu → 2 B 1g phosphorescence of copper porphin in an n-alkane single crystal

The phosphorescence spectrum of the metastable ⁴ E_u state of copper porphin in single crystals of n-octane (C₈) and n-decane (C₁₀) has been studied between 2·3 and 35 K, with and without a magnetic field B. The crystal field splitting between the orbital components observed at 35 K is δ = 30·3 ± 0·3 (C₈), 13·8 ± 0·2 cm^-1 (C₁₀). From the Zeeman shifts we derive the effective orbital angular momentum Λ′ = 0·8 ± 0·2 (C₁₀), the spin-orbit coupling parameter |Z′| = 1·5 ± 1·0 cm^-1 (C₁₀), the spin-spin dipolar interaction parameters D = -0·1 ± 0·2 cm^-1 (C₈, C₁₀) and |E| = 0·31 ± 0·03 cm^-1 (C₈, C₁₀), and the g-factors g _∥ = 2·14 ± 0·04 (C₈, C₁₀) and g _⊥ = 2·00 ± 0·03 (C₈, C₁₀).     

Optical and Zeeman studies of the first excited singlet state of zinc porphin in a single crystal of n-octane: Evidence for Jahn-Teller instability

The absorption and fluorescence spectra of Zn porphin in an n-octane single crystal at 4·2 K are reported in the region between 17 400 and 18 500 cm^-1. A strong peak appears in both spectra at 17 961 cm^-1 and is assigned to the origin of one component (|x, 0>) of the nearly degenerate Q-band. In absorption a second strong line occurs at a frequency δ = 109 cm^-1 above the first; a corresponding line is almost totally absent in the emission spectrum at 4·2 K, but it appears as a hot band of appreciable intensity when the temperature is raised to 80 K. This feature is assigned to the origin of the other component (|y, 0>) of the Q-band. The lifting of the degeneracy of the Q-band is interpreted as a crystal field splitting of the Jahn-Teller unstable ¹ E_u state.

The Zeeman effect is investigated for the 0–0 transition of the phosphorescence spectrum and the |x, 0> and |y, 0> components of the Q-band absorption spectrum. From the phosphorescence experiment it is concluded that the great majority of the ZnP guests are oriented in the host with an angle of about 25° between the out-of-plane molecular axes and the crystal a-axis. The analysis of the Zeeman effect in absorption (H//crystal a-axis) is complicated by the Jahn-Teller instability which causes two additional unknowns to appear in the problem: the frequency ν and the nuclear displacement parameter α of the active mode. When not making an assumption about these parameters one can only derive a lower limit for the matrix element of the orbital angular momentum between the two electronic components: Λ > 4·6. If is identified with the low-frequency mode of 180 cm^-1 appearing in the absorption spectrum, then it follows that Λ = 6·1 ± 0·6 with α = 1·4 ± 0·1.     

Temperature dependence of the dual phosphorescence from xanthone in n-hexane matrices

The phosphoresence spectrum and lifetime of xanthone embedded in an n-hexane matrix has been investigated as a function of temperature (1·6-100 K). Vibrational analyses of the spectra reveal that emission occurs from three sites, two of which are dominant. Emission from one site (B) occurs from the ³ nπ* state of a planar xanthone molecule and is characterized by strong totally symmetric carbonyl stretching vibrations and a short lifetime (2·4 ms). Emission from the other site (C) is shown to arise from the ³ππ* state of an out-of-plane distorted xanthone molecule. It displays a vibrational structure rich in modes of a ₁, b ₁, and b ₂ (C _2v notation) symmetry and a long lifetime (115 ms). Both direct spin orbit coupling via configurational mixing of the nπ* and ππ* states due to the non-planarity of the molecule in its ³ππ* state and spin-orbit vibronic interaction involving ³ A ₁(ππ*)-¹ A ₂(nπ*) spin-orbit and ¹ A ₂(nπ*)-¹ B ₂(ππ*) vibronic interaction via out-of-plane b ₁ vibrations are shown to be responsible for the C site emission intensity. Vibronic mixing between the ³ππ* and ³ nπ* states is not important. With increasing temperature, the phosphorescence intensity from the B site (³ nπ*) emitters increases at first, reaches a maximum at ～20 K and then decreases. The C site (³ππ*) intensity simply decreases with rising temperature. At a given temperature, the phosphorescence lifetimes are identical and exponential for all emission bands regardless of site origin. These observations indicate an equilibrium between emitters in the two sites throughout the lattice. A phonon-assisted energy transfer mechanism is proposed to account for these observations.     

Quenching of SO2 phosphorescence by a magnetic field

Phosphorescence excitation spectra of the ã³B₁← [Xtilde]¹A₁ transition of SO₂ were measured in the absence and presence of a magnetic field (B = 0?44 T², P(SO₂) = 0?7 Torr). The absorption and phosphorescence excitation spectra of the ã³B₁← [Xtilde]¹A₁ transition of SO₂ measured in the absence of a magnetic field show that the relative intensity of the bands of the phosphorescence excitation spectrum is smaller than the relative intensity of the corresponding bands of the absorption spectrum beginning with the (0, 2, 0) band. In the presence of a magnetic field, the intensity of the phosphorescence excitation band falls, for ν_exc> 26400 cm^-1. Under the direct excitation of the ã³B₁← [Xtilde]¹A₁ transition, the dependence of the magnetic quenching of the SO₂ phosphorescence on the excitation frequency (ν_exc) was studied at P(SO₂) = 0?7 Torr and B = 0?44 T. The dependence of the magnetic field effect on ν_excshows that only the vibrationally excited levels of the ã³B₁ state are sensitive to an external magnetic field. The magnetic field strength and the pressure dependence of the magnetic field effects were studied under indirect excitation of the ã³B₁← [Xtilde]¹A₁ transition at λ_exc = 308 nm. The magnetic field and the pressure dependence were investigated for pure SO₂ and for SO₂ + RH (RH n-C₅H₁₂) mixtures. It was found that the magnetic field effect was saturated at B ? 0?25 T. The saturation value (Gr = l(0?3 T)/l(0)) increases with increasing gas pressure. The magnetic field, the pressure and the excess vibrational energy (ν_excess) dependence of the magnetic quenching of SO2 phosphorescence show that the data observed can be explained by an indirect mechanism within the framework of a low level density approximation.     

Influence of temperature on phosphorescence spectra of Pd-porphine in Shpol’skii matrices

We have studied the temperature dependence of the radiative deactivation of the Pd-porphine triplet states in Shpol’skii matrices in the temperature range 1.2–210 K. A substantial transformation of the phosphorescence spectra is observed as the temperature increases and is due to the inclusion of thermally activated Pd-porphine states in the radiative deactivation processes. The activation energy E_a of these Pd-porphine states is measured in matrices of n-octane and n-nonane. The splitting of the lowest quasidegenerate triplet state ΔE(T₂−T₁) is determined for planar and distorted conformations of the Pd-porphine macrocycle in the n-octane matrix as 40 and 57 cm⁻¹, respectively. The ability to use the temperature dependence of the phosphorescence properties of Pd-porphine to fabricate molecular thermometers for the low-temperature range is analyzed. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 4, pp. 460–464, July–August, 2007.     

The ground-state rotational constants of silane

From thirty-nine combination difference equations we have determined three significant ground-vibronic state constants of silane: β ⁰/hc=2·85941 cm^-1, γ ⁰/hc=-3·82×10^-5 cm^-1 and ε ⁰/hc=-7·97×10^-7 cm^-1 or in Hecht's notation B ₀=2·85941 cm^-1, D _s=3·82×10^-5 cm^-1 and D _t=2·436×10^-6 cm^-1.     

Autoionization spectra of Sm atom

Spectra of autoionizing states of the Sm atom in an energy region between 45948.9 cm-1 and 46943.6 cm-1 are systematically investigated by a three-color multi-step resonant excitation scheme with three different excitation paths.The three intermediate states,4f 66s7s 7F3,4f 66s7s 7F4,and 4f 66s7s 9F5 are employed for the three paths,respectively.Based on precise calibration of wavelength,the level energies of 112 autoionizing states are determined with the line widths and the relative line intensities of the related transitions.The possible influence of configuration interaction on the line shape of autoionizing state is also discussed.In addition,a unique value of J,the total angular momentum,is assigned to all detected states by comparing the three spectra obtained with the different excitation paths.     

Interaction of triplet excitons with excited naphthalene and deuteronaphthalene molecules in benzophenone crystals

An increase in the intensity of the exciting light is shown to cause a significant decrease in the quantum yield and rise time of the sensitized phosphorescence of impurity C₁₀H₈ and C₁₀D₈ in benzophenone crystals. Nonlinear quenching of the impurity phosphorescence occurs because one or two excitations are quenched in the interaction of triplet excitons with impurity molecules in the excited state in which they are left after the first exciton capture. Experiments show the nature of the changes in the quantum yield and the rise time of the impurity phosphorescence as the exciting-light intensity is varied from 3.6 · 10¹² to 6 · 10¹⁶ photons/cm² · sec depends strongly on the impurity concentration, the crystal thickness, and the lifetime of the triplet state of the impurity molecules. The experimental results are in good agreement with a proposed theory for the kinetics of the interaction of triplet excitons with impurity molecules in the ground and triplet states.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 8, pp. 12–16, August, 1969.     

A ligand field theory analysis of the spectra of the t 2g 3 levels of IrF6

Calculations and experimental data are presented for the t _2g ³ molecular levels of IrF₆ which imply that a substantial interaction takes place between these 5d³-configuration states and charge transfer states at ca. 20 000 cm^-1. This t _2g ³-charge transfer state configuration interaction is so extensive that the five t _2g ³ levels can only be fit with physically unreasonable values of the Racah (electrostatic) parameters and a spin orbit coupling parameter that is not consistent with expectations for 5d-series transition metal hexafluorides. Parameter values presented for IrF₆ are thus determined from the lowest three levels only, as these should experience the smallest relative shift due to configuration interaction: B = 297 cm^-1, C = 1167 cm^-1, ζ_5d = 4182 cm^-1, and 10Dq = 35 000 cm^-1. In order to corroborate the assertion that t _2g ³-charge transfer configuration interaction is an important factor for the determination of IrF₆ crystal and molecular properties, a number of t _2g ³ spectroscopic properties, which turn out to be quite sensitive to charge transfer state admixture, were studied theoretically and experimentally. These include gas-to-crystal shifts, site splittings, and Jahn-Teller interactions; comparison of calculated and experimental values clearly substantiate the conclusion that there is a strong influence of charge transfer states on the nature of the 5d³-t _2g ³ manifold. Finally, new data for the Γ_8g (² E_g ) and Γ_6g (² T _1g ) states at 1·2 μm are given, completing the data set for the t _2g ³ transitions of IrF₆.     

The phosphorescence of hindered aminopyrimidines

The spectra quantum yields, lifetimes and polarizations of phosphorescence of 4- and 5-aminopyrimidines and their hindered alkyl derivatives were measured in solvents of different polarity at 90 K. The model presented for interpretation of these data allows us to determine the matrix elements of spin-orbit coupling between the emitting singlet and triplet states. These elements are 0.2-0.8 cm^-1 in aminopyrimidines and increase as the amino group becomes more twisted relative to the ring. Spin-orbit coupling of higher ¹(n,π¹) states with emitting triplets is about 10 times stronger than that of the lowest ¹($\text{l}$, a_π¹).     

Vibronic spectra of matrix-isolated lead sulfide

Vibronic spectra are reported for lead sulfide in argon, krypton and SF₆ matrices at low temperatures. Emission stimulated by laser line irradiation of PbS is observed from the v′ = 0 level of three electronic states lying at about 14 500, 18 500 and 21 500 cm^?1 above the ground state. Emission is also observed from an excited state of Pb₂S₂ at about 17 000 cm^?1. In addition, the laser radiation gives rise to the vibrational Raman spectrum of PbS in argon at 423.2 cm^?1 and to a very weak Raman band at 297 ± 2 cm^?1 which we attribute to Pb₂S₂.The effects of temperature on the matrix spectra, of matrix material on the band origins, and of matrix concentration on the vibrational relaxation process, and the apparent degrees of coupling among the electronic states have all been examined. The electronic absorption spectrum of PbS in Ar is reported and the matrix data are compared with available information on gaseous PbS.     

Luminescence of phenylisocyanide and perdeuterophenylisocyanide at 77° K in polycrystalline matrices

Vibrational analyses are presented for the phosphorescence and fluorescence spectra of C₆H₅NC and C₆D₅NC in polycrystalline methylcyclohexane matrices at 77° K. Polarization measurements indicate that the lowest triplet state is ³A₁. The longest progression found for phosphorescence is that of a totally symmetric C-C stretching mode, and is interpreted as indicative of a planar, non-regular-hexagon benzene ring geometry of the lowest triplet state. The fluorescence spectra show a weak 0-0 band and two strong false origins, both based on one quantum of non-totally symmetric b₂ vibrations. The main progressions here, in the ring breathing mode and C-X stretching mode, suggest a planar slightly expanded regular hexagon ring with a changed C-X bond length for the geometry of S₁.     

Relative intensities of the rotational lines of the (0, 0) and (0, 1) bands of the B2Σ+u-X2Σ+g systems of N+2

A high resolution grating spectrometer and a hollow-cathode lamp were used to study the relative intensities for lines of the transition B²Σ⁺_u→X²Σ⁺_g of the ion N⁺₂. Mulliken's formulae are obeyed with a precision of better than 2% for N^′?30, except for the first two lines, for which the relative intensities of the P lines are 2.5% greater than Mulliken's values and the relative intensities of the R lines 2.5% lower. The differences in the positions of the doublets for the first lines increase linearly (γ′?γ″=0.015±0.002cm^-1 with γ″≈0.010cm^-1) for N′?10. An interpretation of this difference gives an order of magnitude for the perturbation parameters of the state A²Π_u(v′=10)(ξ≈9cm^-1, η≈0.06cm^-1). The effect of this perturbation on the line intensities is negligible for N′?30.     

Assessment of the consistency of H2O line intensities over the near-infrared using sun-pointing ground-based Fourier transform spectroscopy

We report on the consistency of water vapour line intensities in selected spectral regions between 800-12,000 cm⁻¹ under atmospheric conditions using sun-pointing Fourier transform infrared spectroscopy. Measurements were made across a number of days at both a low and high altitude field site, sampling a relatively moist and relatively dry atmosphere. Our data suggests that across most of the 800-12,000 cm⁻¹ spectral region water vapour line intensities in recent spectral line databases are generally consistent with what was observed. However, we find that HITRAN-2008 water vapour line intensities are systematically lower by up to 20% in the 8000-9200 cm⁻¹ spectral interval relative to other spectral regions. This discrepancy is essentially removed when two new linelists (UCL08, a compilation of linelists and ab-initio calculations, and one based on recent laboratory measurements by Oudot et al. (2010) [10] in the 8000-9200 cm⁻¹ spectral region) are used. This strongly suggests that the H₂O line strengths in the HITRAN-2008 database are indeed underestimated in this spectral region and in need of revision. The calculated global-mean clear-sky absorption of solar radiation is increased by about 0.3 W m⁻² when using either the UCL08 or Oudot line parameters in the 8000-9200 cm⁻¹ region, instead of HITRAN-2008. We also found that the effect of isotopic fractionation of HDO is evident in the 2500-2900 cm⁻¹ region in the observations.     

High resolution study of the v 7 + v 8 band of ethylene (C2H4) at 1889 cm-1

The v ₇ + v ₈ A-type band of C₂H₄ has been recorded between 1932 and 1847 cm^-1 with a resolution of 0·06 cm^-1. The transitions with K _-1 ? 8> and J ? 2>5 have been assigned. Although slight Coriolis resonances perturb the band, the analysis has been made easy through the use of an elaborate set of asymmetric top computer programmes. The band centre and a set of upper state constants have been obtained. With these constants, 288 observed upper state energy levels have been fitted with a standard deviation of 0·021 cm^-1.

Using very simple expressions, we have predicted all the resonance effects perturbing the levels of ethylene near 2000 cm^-1. This led us to the identification of the v ₄ + v ₈ and v ₈ + v ₁₀ combination bands in low resolution spectra.