Magnetic field induced cross relaxation between two different spin transitions of triplet coumarin

At certain magnetic fields, when the energy separations between the τ_x — τ_y spin levels and between the τ_y — τ_z levels of triplet coumarin become equal, intensity changes are observed in the phosphorescence of coumarin doped in a single crystal of durence cooled to 1.6 K that can be attributed to cross relaxation (CR) between the τ_x — τ_y spin levels of one triplet coumarin molecule and the τ_y — τ_z spin levels of another triplet coumarin. Rate equations that describe the time behavior of the populations of the spin levels which are involved in the CR process are described and numerically solved. CR behavior is found to be strongly dependent on the total coumarin triplet concentration.     

The Fokker-Planck-Langevin model for rotational brownian motion. IV. Asymmetric top molecules

The general series expansion for the joint angular velocity-orientation conditional probability density for a fluid composed of asymmetric top molecules is derived, and expressions for the reorientational correlation functions, correlation times, spectral densities, and the correlation times relevant to magnetic relaxation via spin-rotation interactions are presented. The reorientational correlation times and spin-rotation functions computed using this Fokker-Planck-Langevin (FPL) model with isotropic friction tensor (τ_x = τ_y = τ_z) are compared with the corresponding times and functions computed with the J-diffusion limit of the extended diffusion (EDJ) model. The models are found to predict significantly different behaviour only in the regime where free rotation and precessional effects become important. The reorientation self- and cross-correlation times are shown to follow Hubbard relations in the limit of short τ_x, τ_y, τ_z. Numerical calculations of the FPL reorientational correlation functions, correlation times and spin-rotation functions show that these properties are sensitive to the anisotropies in the friction tensor in the rotational Langevin equation.     

Accurate time-resolved laser spectroscopy on sputtered metal atoms

The delayed coincidence photon counting technique, employing a picosecond tunable dye laser as an excitation source, has been used for accurate determinations of radiative lifetimes of excited states. Excitation from meta-stable states of sputtered atoms was employed. We obtained for Cu: τ(3d ¹⁰ 4p ² P _1/2 = 7.27(0.06) ns, τ(3d ¹⁰ 4p ² P _3/2) = 7.17(0.06) ns and for Fe: τ(3d ⁷ 4p y ³ F ₂)= 9.06(0.16) ns, τ(3d ⁷ 4p y ³ F ₃) = 9.06(0.13) ns and τ(3d ⁷ 4p y ³ F ₄) = 9.11 (0.16) ns. Also, the ability of this technique to record quantum beats at frequencies up to 1 GHz is demonstrated.     

Étude conformationnelle et spectres de vibration de la β alanyl ciliatine

β-Alanyl-ciliatine (β-Ala-Cilia) studied in the solid state by X-ray diffraction and IR spectrometry, is shown to exist as the monobydrated zwitterion H₃N⁺CH₂CH₂CONHCH₂CH₂PO₃H^?, H₂O. Its conformation in the crystal (P2₁/c space group, Z = 4) is defined by: τ₁ = +66.1°, ψ = ?157.4°. φ = +140.3°, τ₂ = +175.4°, τ₃ = +50.0°, τ₄ = +80.1° and is compared with those of similar molecules. A partial interpretation of the IR spectra is given. A conformational energy calculation of the isolated zwitterion shows different conformations stabilized by various types of chelation.     

Experiments on solvated electron generation and deposition in hexamethylphosphoramide

Theory is developed and compared to experiment for the cyclic voltammetric ejection and redeposition of solvated electrons e_s^? in NaClO₄ and LiCl solutions in HMPA. In current reversal chronopotentiometry, e_s^? mass transfer can be observed free from uncompensated resistance effects by cathodic generation of e_s^? for time t_f followed by anodic current redeposition leading to a reverse transition time τ_b. The ratio τ_b/t_f depends on t_f, applied current, and LiCl electrolyte concentration. Comparison of τ_b/t_f to theory detects a highly reactive scavenger at micromolar concentration levels.     

Determination of radiative lifetimes in the 3snp 1 P 1 sequence of Mg I using time-resolved VUV laser spectroscopy

The radiative lifetimes of the states in the (n=5–8) 3snp ¹ P ₁ sequence of neutral magnesium were investigated using time-resolved laser-induced fluorescence. Excitation from the 3s ^{2 1} S ₀ ground state was performed using VUV radiation, which was produced by resonant sum-difference frequency mixing in a krypton gas cell. We find τ(n=5): 54(3) ns, τ(n=6): 140(10) ns, τ(n=7): 290(20) ns. τ(n=8): 475(40) ns. These results were found to be in fair agreement with a Multi Configuration Hartree-Fock (MCHF) calculation.     

Nuclear relaxation and molecular motion of 1,3,5-trifluorobenzene-d3 in liquid solutions

The ²D and ¹⁹F spin-lattice relaxation times of 1,3,5-trifluorobenzene-d₃ in 0.15 mol fraction solutions in various solvents have been measured over the temperature range 270–400 K. The relationship between the reorientational correlation times, τ_θ, and the angular momentum correlation times, τ_J, obtained from the nuclear relaxation data has been compared with the theoretical relationships predicted by the J-diffusion limit of the extended diffusion (EDJ) model and the Fokker-Planck-Langevin (FPL) model for symmetric top molecules. It was found that the rotational motion of 1,3,5-trifluorobenzene-d₃ in most solutions was better described by the FPL model with frictional anisotropy τ₆/τ_· in the range 1–2. (τ₆ and τ_· are the correlation times for the angular velocity components along the axes parallel and perpendicular to the molecular symmetry axis.) The viscosity and temperature dependence of τ_θ has been analyzed in terms of a modified Debye equation and values for the anisotropic interaction parameter, κ, in each solvent are reported. The variation in κ with solvent is attributed mainly to the variation in the dipole moment of the solvent molecules. The frictional anisotropy (τ₆/τ_·) is found to decrease as κ increases. This is interpreted in terms of the effects of molecular shape and electrostatic interactions between CF bond dipole moment and solvent dipole moments.     

Bromine Hyperfine and quadrupole fine structure in the MODR spectrum of sym-tetrabromobenzene in its lowest triplet state

Bromine hyperfine and quadrupole fine structure has been observed in the 3 cm MODR spectrum of the photoexcited ³B_1u state of symmetric tetrabromobenzene (s?TBB) in a durene host crystal at liquid helium temperature. A preliminary analysis of the observed structure yields the two in-plane values of the bromine hyperfine tensor |A_y/gβl = 10.3 and |A_z/gβ| = 16.5 (for ⁸¹Br), and a comparison of these values with those recently observed for the α-bromomaleic acid radical suggests that the correct zero-field scheme in the lowest triplet state of s-TBB is τ_N > τ_M > τ_L.     

The radiative lifetime of the A1II state of BH

Second-order polarization propagator calculations of the X¹Σ⁺ → A¹II transition moment as well as the radiative lifetime of the A¹II state of BH are reported. The calculated vibrational lifetimes are τ(v′ = 0) = 121 ns, τ(v′ = 1) = 129 ns, and τ(v′ = 2) = 137 ns. The τ(v′ = 0 lifetime agrees with the most recent experiment of τ(v′ = 0) = 125 ± 5 ns. We show that the electronic oscillator strength computed at the ground state equilibrium is rather different from the band absorption oscillator strength f₀₀, which demonstrates that theoretical electronic oscillator strengths should not be expected to agree with experimentally determined band oscillator strengths.     

Lifetime measurements of the 2p 3d 3 D 1, 2, 3 0 and 2p 3d 1 P 1 0 levels in NII by beam-foil-laser spectroscopy

Accurate lifetimes measured by means of the cascade-free method based on laser excitation of a fast ion beam preexcited in a carbon foil are reported for four 2p 3d levels in NII. The lifetime results are: τ(2p 3d ³ D ₁ ⁰ )=0.209±0.007 ns, τ(2p 3d ³ D ₂ ⁰ )=0.219±0.007 ns, τ(2p 3d ³ D ₃ ⁰ )=0.217±0.005 ns, and τ(2p 3d ¹ P ₁ ⁰ )=0.410±0.017 ns. These results are compared to theoretical and experimental lifetimes reported previously.     

Oscillator strengths for resonance transitions in neutral selenium and tellurium derived from time-resolved laser spectroscopy

We have measured radiative lifetimes for selenium and tellurium resonance levels by using short pulse laser excitation in differentially heated resonance cells. We obtained τ(³ S ₁ Se I)=2.9(5) ns and τ(³ S ₁ Te I)=3.1(5) ns, which together with branching ratio values given by Ubelis and Berzinsh yield the absorption oscillator strengthf _abs(Se I, 196.0 nm)=0.074(16) andf _abs(Te I, 214.3 nm)=0.098(17).     

Positron annihilation in amino acids

Positron lifetime measurements were performed on eight different amino acids, τ₁, τ₂ and I₂ values were observed at around 0.35 nsec, 0.7–1.2 nsec and 20–30%, respectively. The long-lived component seems to be produced by positronium states trapped in the crystalline lattice or by positronium atoms bound to molecules.     

Depolarized Rayleigh scattering studies of molecular motion of 1,3,4-[D2] thiadiazole

Depolarized Rayleigh scattering spectra have been measured using a Fabry-Pérot interferometer for 1,3,4-[D₂] thiadiazole at temperatures between 20 and 92.5°C, covering a large portion of the liquid and supercooled liquid regions. From the spectral halfwidth measurements, the Rayleigh relaxation times, τ_Ray, have been obtained. A semilog plot of τ_Ray versus T^?1 shows that the data fit well to an Arrhenius equation, with activation energy equal to 2.4 kcal/mole. The relation of τ_Ray to pair orientation correlations is discussed.     

Reorientation of hydrogen-bonded solvents around a large dipole

Using picosecond spectroscopy, we find rise times τ_R ? 20 ps for p-dimethylaminobenzonitrile S₂ → S₀ fluorescence in the weakly hydrogen-bound alcohols 2,2,2-trifluoroethanol and 2-fluoroethanol. Since we previously obtained τ_R ≈ 40 ps in methanol and ethanol, this provides evidence that hydrogen-bonding-retarded solvent reorientation largely governs the fluorescence in these solvents.     

Slowing down of oxygen migration in the processes of radical oxidation and of phenanthrene phosphorescence quenching in methanol glasses at 90 K

The hydroxymethyl radical oxidation kinetics follows the second-order equation with a time-dependent rate constant, K(t). The annealing effect is described by way of dividing K(t) into two factors, one of them depending on the preliminary annealing time (τ): K(t) = K₁(t + τ)K₂(t). The time dependence of both factors is fairly well approximated by the power functions: K₁(t + τ) ≈ (t + τ)^−0.18 and K₂(t ≈ t^−0.26. The oxygen quenching of phenanthrene phosphorescence follows an exchange mechanism, with the static conditions setting in at 77 K. At 90 K oxygen diffusion adds to the quenching efficiency. The time of oxygen jumps (τ_j) and its time dependence under the matrix annealing at 90 K are determined by comparing the theoretical 1/τ_j dependence of the quenching volume with experiment. The 1/τ_j(τ) is well described by the power function τ^{−0.18 ± 0.02)}. The annealing time functions of the oxidation rate constant and of the inverse jumping time are similar. The oxidation rate constant and the diffusion constant coincide in the order of magnitude. Consequently, the slowing down of oxygen migration contributes essentially to the time dependence of the rate constant.     

Fluorescence properties of tetraphenylporphyrin-polypeptide complexes as model photosynthetic systems

The steady-state and time-resolved fluorescence spectra of complexes formed from α,β,γ,δ-tetrakis(4-carboxyphenyl) porphin and the synthetic sequential polytripeptides poly(L-lysyl-L-leucyl-L-alanine)_n (I) (MW 7900) and poly(L-lysyl-L-alanyl-L-alanine) _n (II) (MW 6900) are studied in phosphate buffer (pH=7.2). The peptide matrices were found to bond 4-5 pigment molecules per one polypeptide. The complexed pigment fluorescent decay kinetics is two-exponential with components τ₁ =240 ps and τ₂=2400 (I) and 3500 ps (II). The relative contributions of τ₁ correlate with the degree of α-helicity of the polypeptide matrix.     

Determination of radiative lifetimes in neutral nitrogen using short laser pulses from a distributed feedback dye laser

Radiative lifetimes were determined for two short-lived states in neutral nitrogen. Following photo-dissociation and two-photon excitation employing the same UV source, excitation to higher states was performed with a distributed feedback dye laser (DFDL). The lifetimes were found to be τ(2p ²4d ⁴ D _7/2)=17(3) ns and τ(2p ²5s ⁴ P _5/2)=22(3) ns.     

The direct resist-free deposition of a lithographic mask from vapor initiated by an electron beam

A dry resist-free process of electron-beam vapor deposition of an undecane precursor (C₁₁H₂₄) mask on SiO₂-on-silicon and copper-on-silicon substrates was studied. The band section was trapezoidal, and the band width at the base was much larger than the diameter of the electron beam and depended on the substrate (it was three or four times larger on copper than on SiO₂). In mask deposition on copper, the mask thickening rate v was found to depend strongly on the scan time τ_sc when scanning was performed along the band. When τ_sc changed from 20 ms to 13 s, v decreased by a factor of 7.4 (beam current 1.0 nA). This was probably caused by significant diffusion delay of precursor transport to the reaction zone during pixel time when τ_sc was 13 s. The ion-beam etching of the substrates through the deposited masks was performed (the SiO₂ substrate was etched with SF₆ ions, and the copper substrate, with Ar ions).     

Calculating Permittivity and Dielectric Loss Frequency Spectra for Aqueous Electrolyte Solutions

Analytic expressions for dielectric permittivity factor ε₁(ω) and dielectric dissipation factor ε₂(ω) of electrolyte solutions are obtained, based on the ratio between complex factors of dielectric permittivity and specific conductivity. The range of frequency dispersion of dynamic factors ε₁(ω) and ε₂(ω) for aqueous solutions of LiCl, NaCl, KCl, and CsCl is considered. Numerical calculations are performed for friction coefficients β _a and β _b ; relaxation times τ _a , τ _b , and τ _ab ; and factors ε₁(ω) and ε₂(ω) in a wide range of variation for ρ; concentration c; temperature T; and frequencies ω. The resulting theoretically calculated ε₁(ω) and ε₂(ω) values and the Cole–Cole diagram are in quantitative agreement with experimental data.     

Vibrational relaxation of hydrogen-bonded species in solution. Il. Analysis of vs(XH) absorption bands

The v_s(XH) absorption bands of the following complexes have been recorded in digitised form: phenol (OH)- and phenol (OD)- dioxan in CCl₄; phenol (OH)- and phenol (OD)-acetonitrile in CCl₄; and phenol (OH)-acetonitrile in CDCl₃. All these bands are broad and nearly featureless, and have widths at half height ranging from 75 to 148 cm^?1. With one possible exception they appear to be free from Fermi resonances. By least-squares fitting of the theoretical bandshape function derived in part I to the experimental spectra we have been able to estimate the parameters Δ, ω₂, and τ_C which characterise the theoretical autocorrelation function, and also to determine the v_s(XH) stretching frequency very accurately. In the case of phenol (OD)-dioxan in CCl₄ the discrepancies between the observed and fitted spectra can be entirely attributed to errors of observation. It is found that the Kubo parametér τ_C Δ is close to unity in all cases, showing that the v_s(XH) bands are subject to partial (but not extreme) motional narrowing. The v_σ(XH?Y) vibration is always heavily damped and has a broad spectral density. The changes in the spectral profiles produced by deuteration and change of solvent are easily understood in terms of the theory. Deuteration affects the amplitude of modulation, while changing the solvent affects the value of τ_C. In either case τ_CΔ changes, and the speed of modulation is altered. Since the system is far from the gaussian (slow modulation) limit, the bandshape changes markedly in consequence.