Vibrational relaxation of biacetyl studied by a visible—infrared double resonance technique

The vibrational relaxation of biacetyl following excitation by a highly intense laser pulse at 10.6 micron is studied. The changes in vibrational population are monitored by visible absorption from the ground electronic state to the excited n-π* state. The results indicate that thermal disequilibrium persists for at least 10^?5 seconds after the laser pulse at pressures in the range of 1–40 torr.The implications of these results for the use of high-power infrared lasers in inducing selective chemical reactions are briefly discussed.     

Direct determination of biacetyl in fats by sensitized room temperature phosphorescence

A direct method for the determination of biacetyl in butter and margarine by sensitized room temperature phosphorescence (SRTP) is described. After dissolution of the sample in hexane, biacetyl is isolated by distillation, and its native phosphorescence is sensitized by a non-polar linear furocoumarin, 4′5′-dihydro-3-carbethoxypsoralen. The limit of detection is 0.05 ng ml^?1 biacetyl, with a linear response from 1 × 10^?4 to 1 μg ml^?1 (r = 0.999). The RSD is 3.5% at 100 ng ml^?1.     

Kinetics of the phosphorescent triplet states of tetracyanobenzene complexes as studied by microwave induced delayed phosphorescence

Using microwave induced delayed phosphorescence techniques we determined the populating and depopulating rate constants for the individual spin-sublevels of the phosphorescent states of tetracyanobenzene (TCNB) and its charge-transfer complexes with benzene and hexamethylbenzene (HMB) in n-hexane at 1.25 K. It was shown that the non-radiative decay process from the shortest lifetime sublevel is most responsible for the previously observed shortening in the phosphorescence lifetime of the TCNBHMB complex.     

Electron-impact-induced phosphorescence and mechanism of triplet-state formation in biacetyl

Electron-impact-induced phosphorescence of biacetyl was observed at low vapor pressures. Excitation functions and decay characteristics of the phosphorescence were measured as functions of pressure. Mechanisms of triplet formation under electron impact are discussed in terms of these results as well as previously reported electron spectroscopic data.     

反相胶束介质中敏化双乙酰室温磷光——-α-萘乙酸为敏化剂

本文报道了AOT-C6H12-H2O反相胶束介质中α-萘乙酸(α-NAA)敏化双乙酰(BIAC)的室温磷光。详细讨论了琥珀酸二(2-乙基己基)酯磺酸钠(AOT)浓度和水泡大小(W值)对敏化磷光的影响。吸收、发光性质和微粘度性质的实验表明AOT浓度对敏化磷光强度的影响由敏化磷光寿命、能量转移效率和Poisson分布决定; 一定范围内, 随着水浓度增大, 由于粘度下降和内腔半径增大作用的相互抵销, 水泡大小仅有微弱影响。当W([H2O]/[AOT])大于20后, 内腔半径增大起主要作用, 敏化磷光强度快速下降。与普通SDS胶束相比, 磷光强度约增强13倍, 检出限约下降一个数量级。建立了灵敏的测定α-萘乙酸和双乙酰的敏化室温磷光法, 检出限分别达2.0×10^-^8mol.dm^-^3(α-NAA)和8.5×10^-^9mol.dm^-^3(BIAC)。     

Quenching of the triplet state 3Au of biacetyl by halogenated compounds

The lifetime of excited triplet ³A_u biacetyl molecules was determined in the presence of various halogenated methanes and hydrogen halides. Stern-Volmer analysis of the data gave the following quenching rate constants in the gas phase at 25 °C (in units of litres per mole per second): CF₄, less than 5 × 10²; CF₃Cl, 1.9 × 10⁵; CF₃Br, 5.0 × 10⁵; CF₃I, 9.3 × 10⁶; CCl₃Br, 8.2 × 10⁵; CH₄, less than 5 × 10²; CH₃Cl, 1.0 × 10⁴; CH₃Br, 3.7 × 10⁴; CH₃I, 4.2 × 10⁵; HCl, 5.1 × 10⁵; HBr, 2.7 × 10⁷. The quenching rate constants observed for the halogenated methanes are consistent with a halogen atom abstraction process. The data for the hydrogen halides are interpreted in terms of a mechanism involving hydrogen atom abstraction with the formation of the biacetyl ketyl radical.     

Sensitized phosphorescence studies of p-xylene+biacetyl system, an optical antenna

p-Xylene sensitized biacetyl fluorescence and phosphorescence has been investigated and photosensitized phosphorescence lifetimes of biacetyl in the vapor phase has been determined. Attempts to detect the triplet of biacetyl by its absorption spectrum were unsuccessful, due to primarily, it is believed, to the low extinction coefficients of the triplet, and the low triplet concentrations produced by the optical pumping device at room temperature.     

Vibrational energy relaxation of azulene studied by the transient grating method. II. Liquid solvents

The vibrational energy dissipation process of the ground-state azulene in various liquids has been studied by the transient grating spectroscopy. The acoustic signal produced by the temperature rise of the solvent due to the vibrational energy relaxation of azulene was monitored. The temperature rise-time constant of the solvent has been determined both by the fitting of the acoustic signal to a theoretical model equation and by the analysis of the acoustic peak shift. We found that the temperature rise-time constants determined by the transient grating method in various solvents are larger than the vibrational energy relaxation time constants determined by the transient absorption measurement [D. Schwarzer, J. Troe, M. Votsmeier, and M. Zerezke, J. Chem. Phys. 105, 3121 (1996)]. The difference is explained by different energy dissipation pathways from azulene to solvent; vibrational-vibrational (V-V) energy transfer and vibrational-translational (V-T) energy transfer. The contribution of the V-V energy transfer is estimated in various liquid solvents from the difference between the temperature rise time and vibrational energy relaxation time, and the solvent V-T relaxation time.     

Vibrational energy relaxation of azulene studied by the transient grating method. I. Supercritical fluids

The vibrational energy dissipation process of the ground-state azulene in supercritical xenon, carbon dioxide, and ethane has been studied by the transient grating spectroscopy. In this method, azulene in these fluids was photoexcited by two counterpropagating subpicosecond laser pulses at 570 nm, which created a sinusoidal pattern of vibrationally hot ground-state azulene inside the fluids. The photoacoustic signal produced by the temperature rise of the solvent due to the vibrational energy relaxation of azulene was monitored by the diffraction of a probe pulse. The temperature-rise time constants of the solvents were determined at 383 and 298 K from 0.7 to 2.4 in rho(r), where rho(r) is the reduced density by the critical density of the fluids, by the fitting of the acoustic signal based on a theoretical model equation. In xenon, the temperature-rise time constant was almost similar to the vibrational energy-relaxation time constant of the photoexcited solute determined by the transient absorption measurement [D. Schwarzer, J. Troe, M. Votsmeier, and M. Zerezke, J. Chem. Phys. 105, 3121 (1996)] at the same reduced density irrespective of the solvent temperature. On the other hand, the temperature-rise time constants in ethane were larger than the vibrational energy-relaxation time constants by a factor of about 2. In carbon dioxide, the difference was small. From these results, the larger time constants of the solvent temperature rise than those of the vibrational energy relaxation in ethane and carbon dioxide were interpreted in terms of the vibrational-vibrational (V-V) energy transfer between azulene and solvent molecules and the vibrational-translational (V-T) energy transfer between solvent molecules. The contribution of the V-V energy transfer process against the V-T energy transfer process has been discussed.     

Correlation between phosphorescence lifetimes and ratios of steady state phosphorescence to fluorescence intensity for biacetyl in various solvents

The phosphorescence of biacetyl in fluid solutions is quenched not only by oxygen but also by impurities resulting from the distillation procedure and from the freeze-pump-thaw degassing technique. A linear correlation between the measured phosphorescence lifetimes and the ratios of the phosphorescence to the fluorescence intensities is established; the slope affords reliable calculations of the biacetyl triplet lifetimes from steady state measurements and additionally the determination of the rate constant k_p of radiative triplet deactivation. This constant is dependent on the solvent, whereas the fluorescence constant k_F is not.     

Vibrational and rotational relaxation of NH3 studied in a molecular jet by infrared-infrared double resonance

Infrared-infrared double resonance is applied to an expanding jet of NH₃. Molecules in the a(3,3) rotational level of the vibrational ground state are excited with a CO₂-laser into the s(4,3) level of the υ₂ vibrational state, 2s(4,3). Rotational distributions in the ground and υ₂ state are probed by the infrared absorption of color-center-laser radiation producing transitions to the υ₁ and υ₁ + υ₂ states. The time delay between pumping and probing is determined by the distance between the CO₂ laser focus and the color-center-laser focus, along the jet axis. The results indicate an average relaxation time of vibration to rotation/translation of 280 (50) ns Torr* over the temperature range 50–110 K. In that temperature range the population difference between ortho and para species comes into equilibrium within 425(190) ns Torr*, and the relaxation between (3,3) and other ortho ground state levels occurs within 56(20) ns Torr* and between the 2s(4,3) and 2s(3,3) levels within 20(8) ns Torr*. The inversion relaxation time between a(3,3) and s(3,3) is determined to 7.6 (20) ns Torr*, at 50 K. (Here 1 Torr* indicates a density, 1 Torr* = 3.27 × 10²² molecules/m³).     

Electronic relaxation of biacetyl in a supersonic jet

Emission spectra have been recorded and decay times measured for biacetyl selectively excited in the 0⁰₀ band of the S₁S₀ (¹A_u¹A_g) transition. The spectrum of the “long emission” corresponds to a superposition of the fluorescence and of the “hot” phosphorescence. The results may be treated in terms of a uniform distribution of the singlet oscillator strength among the quasi-stationary levels, in the absence of vibrational redistribution on a microsecond scale.     

Vibrational relaxation by reversible intersystem crossing

The time- and energy-resolved fluorescence spectra of CO obtained under selective excitation of the A ¹Π (υ′ = 1) level have been studied. The apparent vibrational relaxation in the singlet manifold is shown to be mainly due to the three-step process: the singlet → triplet crossing from υ′ = 1 level, vibrational relaxation in the triplet manifold and reverse crossing to the A ¹Π (υ′ = 0) level. The decay form may be fitted by assuming the relaxation rate constants in triplet (and singlet) manifolds of the order of 10⁵ s^?1 Torr^?1 i.e. smaller by one order of magnitude than previously proposed.     

The electronic relaxation of biacetyl in the vapor phase

The emissions of biacetyl after pulsed dye-laser excitation were studied at pressures down to 0.05 mtorr. At all energies the time-resolved fluorescence was composed of a nanosecond and a microsecond component. At “zero” pressure the long lived phosphorescence was absent while the “hot” phosphorescence has the same time characteristics as the slow fluorescence. By increasing the pressure the slow fluorescence was quenched while the milisecond phosphorescence was induced. We determined the low-pressure emission characteristics and the pressure effects as a function of excitation energy.From our data we obtained the parameters describing the intermediate type singlet-triplet coupling, the radiative and non-radiative relaxation rates from the singlet and triplet levels and the cross sections for the slow fluorescence quenching, all as a function of energy. Strong evidence is obtained for the participation of rotational states in the intra-molecular relaxation. The important difference between the situation where the singlet levels are isolated (low energy) and where the singlet level widths overlap (at higher energies) is demonstrated. In the former situation very large fluorescence quenching cross sectios were found. It is further shown that for high energies at least two effective collisions are needed to obtain a thermalized triplet; the mean energy removed per effective collision is 2200 cm^?1.     

Reversible intersystem crossing,fluorescence lifetime lengthening and collisionally induced phosphorescence in biacetyl

The emissions of biacetyl excited at 4200 Å were studied at pressures down to 10^?3 torr. Apart from the well-known nanosecond fluorescence, a new emission of the same spectral composition was found with a non-exponential decay in the microsecond range. Furthermore the phosphorescence, as defined by its spectral composition, was found to be collisionally induced.The results imply that after excitation, the molecule rapidly transfers (rate constant k_S→T) to the triplet state, giving rise to the nanosecond decay time; and can then transfer back to the singlet state (rate constant k_T→S), giving rise to the microsecond emission. At the same time internal conversion can occur (k_S→S0). From an analysis of the data we find for k_S→S0 = 2.4 × 10⁷ sec^?1, k_S→T = 7.6 × 10⁷ sec^?1, k_T→S = 1.9 × 10⁵ sec^?1. The kinetic treatment can be transformed to a quantum mechanical one, yielding values for the triplet level density (?_T), the coupling element V_ST and the number of triplet states (N) coupled to the singlet excited. At 4200 Å we find ?_T = 6.3 × 10⁵cm, V_ST = 1.0 × 10^?5 cm^?1, N = 400.Phosphorescence occurs only when the molecule is deactivated by collisions to a vibronic triplet state below the vibrationless excited singlet state. The efficiency of biacetyl collisions is 0.54.     

Vibrational relaxation by collision in polyatomic molecules

A model of a harmonic oscillator with friction is used to discuss the conversion of translational energy to vibrational by collision of an atom with a polyatomic molecule. It is shown that adiabatic conversion implies that E ² may substantially exceed the value calculated, neglecting the interaction of the bond with the rest of the molecule.     

Temperature effects on the quenching of the triplet state 3Au of biacetyl

Rate data for the quenching of the triplet state ³A_u of biacetyl were determined for a number of collision partners in the gas phase as a function of temperature. Quenching by oxygen and nitric oxide resulted in negative temperature coefficients and the results were interpreted on the basis of the formation of collision complexes. The Arrhenius parameters determined for the reaction of triplet biacetyl molecules with acetaldehyde are an indication that the n,π* configuration of biacetyl behaves like a biradical in the gas phase. The results are consistent with a hydrogen atom abstraction reaction channel.     

Vibrational relaxation carbon tetrachloride

By use of spontaneous Brillouin scattering, the vibrational relaxation of carbon tetrachloride was determined as a function of temperature. Comparison of experimental results with theoretical predictions leads to the conclusion that there is a simultaneous relaxation of three internal modes.     

Vibrational relaxation of ozone by para and normal hydrogen

The relaxation of O₃(001) by p- and n-H₂ was measured over the temperature range 167–424 K. n-H₂ quenched ozone more efficiently, with the difference between rate constants increasing with temperature. These observations are explained by a near-resonant V—R process, in agreement with the Sharma—Brau theory.     

Vibrational dephasing and energy relaxation of admolecules by phonons

An investigation is made of the vibrational dephasing of a diatomic molecule adsorbed on a surface. Explicit analytic forms for the rate of dephasing by phonons are derived. For comparison, an expression for energy relaxation is given which is appropriate for OH on SiO₂. It is found that the dephasing rate is considerably faster for this system than the energy relaxation rate. These conclusions are compared with the results of a recent experiment.