Excess Dependence Research Articles

Intramolecular exciplex formation in jet-cooled bichromophoric chain molecules: torsional conformations and excess vibrational energy dependence

Intramolecular exciplex formation and excess vibrational energy dependence were investigated in the bichromophoric EDA systems of several 1-(4-(dialkylamino)phenyl-3-(1- and 9-anthryl)propanes in a supersonic free jet. Fluorescence excitation spectra suggest that two types of ground-state conformers are involved at low He backing pressure (750 Torr). These conformers were tentatively ascribed to trans (t) and gauche (g) conformers concerning the C 1 -C 2 conformation of trimethylene-substituted anthryl. Fluorescence excitation spectra of the t conformer show well-resolved vibrational structures of the S 1 state, while those of g were considerably congested

Vibrational mode-selective effects in the picosecond time-resolved resonance Raman spectrum of singlet excited trans-stilbene

Picosecond time-resolved resonance Raman spectra of singlet excited trans-stilbene have been obtained using several photolysis (pump) and Raman excitation (probe) wavelengths. Vibrational mode-selective changes in peak wavenumbers and bandwidths are dependent on the sample temperature, the pump wavelength, and the solvent. The effects are greatest for the band at ≈ 1570 cm −1, attributed to the olefinic bond stretching mode. The peak wavenumber of this band decreases and the band broadens as the temperature is raised. Excess energy of excitation results in an initial decrease of peak wavenumber and increase in width followed by an increase in peak wavenumber and decrease in width as the delay between pump and probe increases further, with a similar time dependence for peak position as for width. Fits to the sum of two exponentials give time constants in the range 1 to 5 ps for the initial and 9 to 12 ps for the later stages of the evolution. There is an similarity between the temperature and excess energy dependence, but bandwidth is more (× 2.9) sensitive than peak wavenumber to excess energy. The mode-selective changes and dynamics are interpreted with a model involving intramolecular vibrational relaxation and reorientation of the solvent cage in response to microdielectric stabilisation forces and quasi-equilibration with bulk solvent. The role of vibrationally “'hot” low-wavenumber modes is clarified.

Texture dependence of power losses in nonoriented SiFe electrical steels

Epstein-frame loss measurements were performed on 3.2% Si-Fe nonoriented strips cut along and transverse to the rolling direction, in the frequency range 10–100 Hz and at various peak inductions between 0.3 and 1.6 T. The behavior of hysteresis loss Ph and excess loss Pe vs peak induction B and frequency f was worked out and analyzed. The B dependence exhibits a rich structure, as a consequence of the coupling between magnetic domains and the grain structure. The f dependence follows the law Pe∝f3/2, in agreement with previous experiments. These results are interpreted in the frame of a generalized Preisach model recently proposed. It is shown that the excess loss dependence on B is in direct relation with that of the permeability B/H along the virgin magnetization curve. The results of numerical calculations of model predictions are presented and discussed in comparison with experimental data.

Long-range, photoinduced charge separation in solvent-free donor—bridge—acceptor molecules

Charge separation in isolated supermolecules, consisting of an aniline or 4-methoxy aniline donor and a cyanonaphtalene acceptor connected by a semirigid bridge, is described in terms of irreversible sequential intramolecular relaxation involving electron transfer from an electronically excited molecular state and electrostatically driven folding. Sequential folding of the charge transfer state extended the energy range of this state, which is accessible by radiationless electron transfer. The analysis of the intramolecular electron transfer dynamics led to agreement between the distance scale for charge separation inferred from energetic and structural data and accounted for the excess energy dependence of the nuclear Franck—Condon vibrational overlap factors.

ChemInform Abstract: Size and Excess Vibrational Energy Dependence of Excimer Formation in Naphthalene Clusters.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The decay of triplet pyrazine and methylpyrazine in supersonic jets. Substitution effects

We have measured the decay rates of optically excited triplet states of pyrazine and methylpyrazine in a supersonic jet. The excess vibrational energy dependence of the radiationless rate constants in the energy range between the T1 and the S1 electronic origins of the isolated molecules was explored. Decay rates between 5×102 and 105 s−1 were found in the 1800 cm−1 range of excess vibrational energy above the origin of the T1 state in pyrazine. In methylpyrazine the decay rates increase from 8×102 to 3.3×104 in the first 600 cm−1 excess energy range above the T1 origin. The decay rates are free of mode specificity and rotational effects. The wide dynamic range of the T1→S0 radiationless rates of pyrazine is substantially enhanced by methyl substitution. The results support a model which suggests that certain vibrational modes, those which undergo large frequency decreases in the excited state, control the strong vibrational energy dependence of the T1→S0 intersystem crossing of pyrazine. These large frequency changes result from the interaction of the near lying 3nπ* and 3ππ* states (the ‘‘proximity effect’’). The enhancement of the excess energy dependence of the decay rates by methyl substitution indicates a further decrease in the frequency of these modes. This effect of the methyl group is consistent with the ‘‘proximity effect.’’ The frequency reduction of these modes in the S1 manifold of methylpyrazine also support this mechanism.

Size and excess vibrational energy dependence of excimer formation in naphthalene clusters

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSize and excess vibrational energy dependence of excimer formation in naphthalene clustersHiroyuki Saigusa, Sheng Sun, and Edward C. LimCite this: J. Phys. Chem. 1992, 96, 5, 2083–2088Publication Date (Print):March 1, 1992Publication History Published online1 May 2002Published inissue 1 March 1992https://doi.org/10.1021/j100184a013RIGHTS & PERMISSIONSArticle Views190Altmetric-Citations58LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (776 KB) Get e-Alerts

Nonlinear fluctuation conductivity of a layered superconductor: Crossover in strong electric fields.

The fluctuation conductivity of a clean layered superconductor in an arbitrary electric field is studied. It is shown that in the vicinity of {ital T}{sub {ital c}} an unusual crossover from three-dimensional to two-dimensional behavior in the non-Ohmic region of the electric field excess current dependence takes place. In spite of the large size of the fluctuation Cooper pairs, the strong electric field gives rise to additional decay and to an effective freezing of the number of degrees of freedom.

Detection of Andreev reflection contribution to nonlinear IVC response of HTSC-normal metal point contacts to external microwave radiation

Measurements of the I-V characteristics of S-c-N Nb-TlBaCaCuO point contacts have been performed over the temperature range 4,2 K to 300 K under applied microwave radiation at a frequency of about 12 GHz. Two types of the excess current dependence on the power P of the external radiation were found. In one case excess current oscillates with P increase that is explained by the processes of Andreev reflection in S-c-N contacts. In the other case the excess current is found to be stimulated by the radiation power. Obtained resultes indicate the presence of complicated picture of the quasi-particles dynamics in HTSC. The mechanisms of the nonlinear effects revealed are discussed.

Energy and rotational state dependence of radiationless processes in propynal

The dynamics of propynal, HCCCHO and HCCCDO, have been investigated in an excess energy range between 2900 and 4500 cm −1 of the S 1 state using quantum beat spectroscopy. A rotational state dependence of S 1-T 1 interaction has been established. The number of interacting triplet states, determined by quantum beat counting, is shown to increase with the rotational quantum number N, and with the average eigenstate lifetime. This effect is attributed to K state mixing in the triplet state which is proposed to be induced by hyperfine interaction via the dipole-dipole term. Furthermore, the nonradiative decay rates exhibit a weak excess energy dependence up to a sudden fluorescence breakdown at ≈4000 cm −1 where they increase dramatically. The break-down, located at 86.6±0.3 kcal/mol, is attributed to the onset of the predissociation due to the radical dissociation HCCCHO→HCCCO+H which proceeds on the S 0 potential energy surface in competition with the concerted reaction HCCCHO→HCCH+CO already accessible at zero S 1, vibrational excess energy.

The decay of triplet pyrazine in supersonic jets

The decay rates of optically excited triplet states of pyrazine in supersonic expansion were measured by using three different methods. The excess energy dependence of the radiationless rate constants in the energy range between the T1 and the S1 electronic origins of the isolated molecule was explored. Decay rates between 7×102 –2.5×104 s−1 were found in the 1500 cm−1 range of excess vibrational energy from the origin of the T1 state. The decay rates are free of mode specificity and rotational effects. The pure radiative lifetime in the measured range is rovibronic independent. The results support a model which suggests that certain vibrational modes, those which undergo large frequency changes in the excited state, control the strong vibrational energy dependence of the T1 →S0 intersystem crossing of pyrazine.

Triplet energy transfer sensitized fluorescence in 3,9-dibromoperylene

Abstract Sensitized fluorescence of 3,9-dibromoperylene (DBP) was obtained from triplet energy transfer with ketones as donors. The maximum efficiency of this process (TETSF) in acetonitrile measured with time-correlated single-photon counting was QTS = 0.33 ± 0.08. Because of the high quantum yield of fluorescence (QF = 0.75 ± 0.10) for analytical applications of TETSF DBP is superior to the acceptor molecules investigated earlier. Particular attention was paid to a detailed elucidation of the energy transfer processes involved in TETSF. The analysis of the excess energy dependence provided evidence for the participation of a higher excited triplet state (Tn) of DBP.

ChemInform Abstract: Excess Energy Dependence of Internal Conversion in the S1 State of Azulene.

The internal conversion (IC) rates for nine single vibronic levels of the S1 state of azulene, having from 0 to 2417 cm−1 of vibrational energy, have been measured by direct absorption spectroscopy in a pulsed, planar supersonic jet. The value of kIC at the vibrationless level is 9.4×1011 s−1 , with an approximately linear increase to 3.3×1012 s−1 at the highest level studied. This excess energy dependence of the rate constant is contrasted to that for S2 →S1 IC across an almost identical electronic energy gap, and an explanation involving low‐frequency modes is offered.

Excess energy dependence of internal conversion in the S1 state of azulene

The internal conversion (IC) rates for nine single vibronic levels of the S1 state of azulene, having from 0 to 2417 cm−1 of vibrational energy, have been measured by direct absorption spectroscopy in a pulsed, planar supersonic jet. The value of kIC at the vibrationless level is 9.4×1011 s−1 , with an approximately linear increase to 3.3×1012 s−1 at the highest level studied. This excess energy dependence of the rate constant is contrasted to that for S2 →S1 IC across an almost identical electronic energy gap, and an explanation involving low-frequency modes is offered.

Theory of nonradiative decay from the lowest singlet state of benzene: Excess energy dependence

A time-independent Green function formalism is developed to study the excess energy dependence on the nonradiative decay in the S1 state of benzene. Effects of purely electronic relaxation (internal conversion, IC) and intramolecular vibrational redistribution, IVR, are taken into account at the same time. Model calculations show that the drastic increase in the nonradiative decay rate at around 3 000 cm-1 excess energy is due to the same onset of both IC and IVR rates. Our theory can explain the difference in rate constant between IVR and IC observed by Moss and Parmenter.

PHARMACOLOGIC REVERSAL Of ABNORMAL GLUCOSE REGULATION, BCAA UTILIZATION AND MUSCLE CATAROLISM IN SEPSIS BY DICHLOROACETATE

Sepsis has been shown to decrease skeletal muscle glucose oxidation by inhibiting the pyruvate dehydrogenase activity (PDHa) and to increase proteolysis and use of branched-chain amino acids (BCAA). The effects of dichloroacetate (DCA), which reverses PDHa inhibition, were studied in skeletal muscle from a septic (S) rat model of intra-abdominal abscess (E. coli + B. fragilis) and compared to control (C) and sterile inflammatory abscess (I) animals. In one set of S, I, and C animals, DCA (1 mmol/kg) was injected intraperitoneally at 0, 30, and 60 min. Septic, but not I, rats had a twofold increase in skeletal muscle lactate concentrations over C, but no changes in pyruvate. After DCA, both lactate and pyruvate were reduced (p less than 0.001) to same level in S, I, and C. Skeletal muscle alanine was increased in S compared to I or C, but after DCA was reduced threefold in C, S, and I (p less than 0.001) suggesting that alanine synthesis may be impaired due to decreased pyruvate availability. Like alanine, skeletal muscle BCAA were increased in S compared to C, but not altered in I. Following DCA, BCAA levels in muscle from S were reduced (p less than 0.001) to values seen in C or I. Muscle phenylalanine content was significantly elevated in S (p less than 0.05) compared to C or I, but was reduced (p less than 0.05) after DCA in S but not in C or I. Decreased muscle phenylalanine associated with lowered BCAA suggests DCA may decrease septic muscle protein catabolism and/or enhance protein synthesis. Coupled with an increased PDHa and reduced lactate levels, this suggests that DCA may reverse the excess muscle catabolism and BCAA dependence of sepsis by increasing glucose and lactate oxidation and may be a useful therapeutic modality.

Electronic spectra and intramolecular dynamics of phenanthrene in a supersonic free jet

The fluorescence excitation spectra and the fluorescence decays of phenanthrene-h10 and phenanthrene-d10 in a supersonic free jet have been measured with excitation across the S 0 → S 1 and S 0 → S 2 absorption regions. The fluorescence excitation spectra of the isolated phenanthrene molecule are compared with the absorption spectra in low temperature matrices. The presence of a 2-type out-of-plane vibrational bands in the S 0 → S 2 absorption spectra indicates that a 1 B 1(πσ*) state lies near the S 2 state. The S 2 → S 1 internal conversion rates at various vibronic levels of S 2 for both compounds have been evaluated from the analysis of profile and width of the individual vibronic bands in the excitation spectra. The excess vibrational energy dependence of the S 2 → S 1 internal conversion rate is the opposite of that shown by the total decay rate in S 1. The excess vibrational energy dependence of the electronic decay rate and the profile of the vibronic absorption bands belonging to the S 0 → S 2 transition are discussed from the viewpoint of the electronic energy gap.

Temperature dependence of the triplet lifetime of pyrazine

Very large temperature dependence of the triplet lifetime of pyrazine was observed in various solid solutions and is interpreted in terms of the increased radiationless decay rate of the thermally populated higher triplet states. The excess energy dependence of the decay rate is estimated from the simulation of the observed temperature dependence. It is suggested that the excess energy dependence of the decay rate constant is not smooth but abrupt, and this is brought by the n..pi..-..pi pi.. vibronic interaction. Evidence to support this suggestion is provided by the solvent dependence of the phosphorescence excitation spectrum.

Excess energy dependence in radiationless transitions. Triplet to ground state nonradiative rate calculations in benzene

The T1→S0 intersystem crossing (ISC) rates in benzene are calculated on the assumption that all T1 vibrational states with the same amount of excess energy are equally populated owing to fast intramolecular vibrational redistribution. The results are compared with the experimentally observed tendency that the ISC rate increases rapidly with excess energy and levels off at higher excess energy. The above tendency is seen when vibrational frequencies in T1 are assumed to be the same as in S1. Whether the rapid initial increase appears or not depends on parameters involved in the radiationless process; on the other hand, the saturation of the ISC rate in the high energy range occurs regardless of the parameters. Furthermore, the saturation proves to correlate with the fact that the slope of the density of states in a logarithmic scale becomes smaller as the energy increases.

Universality of bridge functions and its relation to variational perturbation theory and additivity of equations of state

Featuring the modified hypernetted-chain (MHNC) scheme as a variational fitting procedure, we demonstrate that the accuracy of the variational perturbation theory (VPT) and of the method based on additivity of equations of state is determined by the excess entropy dependence of the bridge-function parameters [i.e., $\ensuremath{\eta}(s)$ when the Percus-Yevick hard-sphere bridge functions are employed]. It is found that $\ensuremath{\eta}(s)$ is nearly universal for all (i.e., physical) potentials while it is distinctly different for the spheres, providing a graphical display of the jump in pair-potential space (with respect to accuracy of VPT) from hard to soft behavior. The universality of $\ensuremath{\eta}(s)$ provides a local criterion for the MHNC scheme that should be useful for inverting structure-factor data in order to obtain the potential. An alternative local MHNC criterion due to Lado is rederived and extended, and it is also analyzed in light of the plot of $\ensuremath{\eta}(s)$.