Journal of Research of the National Bureau of Standards. Section A, Physics and Chemistry最新文献

The shear modulus G = 5.925 × 10 ^{- 3}(fp - 0.45)T+G* (Part I), its energy component G* = 0.0684 (fp - 0.45)+ 2.70 (Part II), and the number of effective suh-chains per unit volume v_e = (G - G*)/RT are given detailed molecular consideration. G is given in Mdyn cm^-2 for rubber cross-linked by adding p parts of dicumyl peroxide per hundred of rubber, and heating until a fraction f of the peroxide is decomposed. v_e is found to be approximately twice the density of cross-links, after a correction for impurities and chain ends is made. It can not be computed as G/RT since only the entropy component of modulus is related to v_e. The sub-chains for the most highly cross-linked rubbers studied had a molecular weight of about 575 g mol^-1, corresponding to about 8 isoprene units. The modulus corresponding to no added cross-links is not zero. It is determined chiefiy by the energy component of the modulus; it does not arise from entanglements. The "front factor" is found to be unity. An extensive literature survey yields values of the quantity RTΨ(v ₂), where Ψ (v ₂) is the Flory- Rehner equation function of v ₂, the equilibrium volume fraction obtained by swelling the cross-linked rubber. RTψ (v ₂) is found to be greater than G - G* but not as large as G itself.

Four molecular fluorescence parameters describe the behaviour of a fluorescent molecule in very dilute (~ 10^-6 M) solution: the fluorescence spectrum $F_M\left(\overline{v}\right)$ ;the fluorescence polarization P_M ;the radiative transition probability k_FM ; andthe radiationless transition probability k_IM .These parameters and their temperature and solvent dependence are those of primary interest to the photophysicist and photochemist. $F_M\left(\overline{v}\right)$ and P_M can be determined directly, but k_FM and k_IM can only be found indirectly from measurements of the secondary parameters,the fluorescence lifetime τ_M , andthe fluorescence quantum efficiency q_FM ,where k_FM =q_FM/τ_M and k_IM =(1-q_FM ) τ_M. The real fluorescence parameters $F\left(\overline{v}\right)$ , τ and ϕ_F of more concentrated (c > 10^-5 M) solutions usually differ from the molecular parameters $F_M\left(\overline{v}\right)$ , τ_M and q_FM due to concentration (self) quenching, so that τ > τ_M and ϕ_F < q_FM. The concentration quenching is due to excimer formation and dissociation (rates k_DMc and k_MD , respectively) and it is often accompanied by the appearance of an excimer fluorescence spectrum $F_D\left(\overline{v}\right)$ in addition to $F_M\left(\overline{v}\right)$ , so that $F\left(\overline{v}\right)$ has two components. The excimer fluorescence parameters $F_D\left(\overline{v}\right)$ , P_D , k_FD and k_ID together with k_DM and k_MD , and their solvent and temperature dependence, are also of primary scientific interest. The observed (technical) fluorescence parameters

Calorimetric techniques offer the photophysicist and photochemist the opportunity to measure a number of parameters of excited states which may be difficult to obtain by other techniques. The calorimetric strategy seeks to measure the heating of a sample resulting from radiationless decays or chemical reactions of excited states. Heating is best measured through volume and pressure transducers, and four calorimeters based on these are described. With calorimetric instrumentation one can perform measurements on samples in the gas, liquid and solid phases over a wide temperature range. Moreover time dependent processes with time constants ranging from microseconds to seconds are amenable to study. Examples of the application of calorimetric techniques to the determination of quantum yields of fluorescence, triplet formation and photochemistry are given.

The energy levels belonging to the configurations 3d ⁷4s ² and 3d ⁸ nℓ (nℓ = 4s, 5s, 4p 5p, 4d 5d 4f, and 5g) have been calculated. The radial energy integrals were treated as parameters and adjusted to give a least-squares fit to the observed levels. Two- and three-body effective electrostatic interactions for equivalent electrons were included, as well as two-body effective interactions for inequivalent electrons. Strong configuration interaction between 3d ⁷4s ² and 3d ⁸4d was taken into account. Values of the parameters are given for all the above configurations, and the calculated levels are given for all except 3d ⁸4s and 3d ⁸4p (for which essentially equivalent results have been published). Leading eigenvector percentages are given in appropriate coupling schemes.

Oscillator strengths for 49 lines of U II recently measured by Voight can be used to calibrate the intensity scale of the U II lines in the NBS Tables of Spectral-Line Intensities and derive a larger set of oscillator strengths of lower precision but consistent with the new measurements. The standard deviation of the differences between the two sets of gf-values for the 49 lines is 29 percent. Oscillator strengths of that precision are given for 776 additional lines from the NBS Intensity Tables. The uncertainty in absolute value is 67 percent.

The vapor pressure of water at its triple point was measured with exceptionally high accuracy by realizing it with a special apparatus and measuring the pressure with the NBS precision mercury manometer. The vapor pressure apparatus had a system for circulating the liquid water. Actual triple point conditions were established with a thin sheet of freshly distilled liquid flowing down over an exposed mantle of ice frozen on a vertical well. This technique reduced non-volatile contaminants and the vapor was repeatedly pumped to remove accumulated volatile contaminants. A diaphragm pressure transducer was used to separate the water vapor from the helium used to transmit the pressure to the manometer. The value found for the vapor pressure of water at its triple point was 611.657 Pa with an uncertainty of ± 0.010 Pa from random errors, computed at 99 percent confidence limits. The systematic errors are estimated to be insignificant relative to the random errors.

The purpose of this paper is to review in vivo NMR experiments [1, 2] on a transplantable tumor in mice and to discuss the feasibility of using noninvasive NMR for cancer detection in humans.

The theory of the measurement of luminescent quantum yields using chemical actinometry is described. The sample's emission intensity is measured by nearly completely surrounding the sample with an actinometer solution, and the excitation intensity is directly measured with the same type of actinometer. The ratio of the measured sample emission intensity corrected for the fraction escaping through the excitation ports to the measured excitation intensity is the absolute luminescence yield. Equations, a suitable cell design, and computer calculated correction factors for different cell dimensions and optical densities are given. The absolute yield of the actinometer is not needed, only its relative response with wavelength. New quantum-flat actinometers which should greatly simplify the measurements are described.

In connection with the work on development of a high resolution laser source bidirectional reflectometer, a large number of papers were collected dealing with various aspects of the geometrical distribution of the radiant energy reflected from surfaces of different types. Each paper has been classified into one or more classes on the basis of its technical content. There are eight general classes, with several subclasses in some of the general classes. Because of the interest in this field, the bibliography is being published as a service to the public.

A laser-source bidirectional reflectometer that is fully automated and has angular resolution on the order of one degree has been designed and built. The direction of incidence and viewing can be independently varied over an entire hemisphere except for directions more than 77.5° from the normal, and the two directions must be at least 2.5° apart. Bidirectional reflectances for 15 samples of black and white coatings are presented.