Index of content:
Volume 42, Issue 1, 01 January 1965
Magnetothermodynamic Properties of MnCl2 from 1.3° to 4.4°K at 90 kG. A Zero Entropy Reference. The Magnetomechanical Process at Absolute Zero42(1965); http://dx.doi.org/10.1063/1.1695673View Description Hide Description
The heat capacity of single‐crystal manganous chloride has been measured in a constant magnetic field, accurately stabilized at 90 000 G and parallel to the b magnetic axis (a crystallographic axis), over the range 1.4° to 4.35°K. The heat capacity approaches zero and magnetic saturation occurs near 1°K, thus providing a reference for the zero of electronic entropy which might otherwise be difficult to obtain due to possible irreversible behavior of this antiferromagnetic system below 1°K. It has been established that loss of magnetic moment from the saturated magnetic domain of the crystal occurs with increasing temperature through the loss of individual Bohr units of angular momentum. By extrapolating reversibly measured values of the magnetic moment, on isoerstedic paths to 0°K, the magnetomechanical process of magnetization at the absolute zero has been described in fairly complete detail. The magnetization curve at 0°K consists of two nearly straight lines joined by a small deviation near 10 kG and approximating saturation at about 32 kG. The value of the quantity . It has been shown that (∂E/∂T)H for MnCl2 at 90 kG is a negative quantity, as would be expected for an antiferromagnetic substance near magnetic saturation. Magnetic work is by far the dominant effect in determining the size of the quanta absorbed near magnetic saturation. The initial increments of demagnetization, with increasing temperature, are assisted by withdrawal of stored internal energy. Thus, the quanta with the smaller energies are those connected with the loss of angular momentum from individual atomic centers, rather than proportionally from the entire cooperative system of electron spins. The limiting heat content at 90 kG is given by , and the limiting magnetic moment is represented by , where Msat=27 988 and Ngβ=11 194 G·cm3/mole, with g taken as 2.004. The abuse of the word ``point'' in thermodynamics is discussed and the word ``pernt'' is defined.
Magnetic Moment and Susceptibility of MnCl2 as a Function of Field along the b Magnetic Axis to 95 kG from 1.3° to 4.2°K42(1965); http://dx.doi.org/10.1063/1.1695732View Description Hide Description
Values of magnetic moment, isothermal differential susceptibility, magnetic work and free energy change of single‐crystal MnCl2 have been obtained by means of a digital voltmeter integrating the dc potential on a sensing coil surrounding a spherical sample. This substance becomes antiferromagnetic near 2°K. The sample was moved vertically along the measuring coil axis which coincided with the field direction in an accurately stabilized field in a long solenoid. The observations cover the range from 1.3° to 4.2°K and fields to 95 kG. The measurements show no signs of irreversibility over the above region. The sample was high purity anhydrous MnCl2 with its b magnetic axis (a crystallographic axis) parallel to the field. The saturation value of MnCl2 was found to be 27 988 G·cm3/mole corresponding to a g=2.004 for a 6 S state. Problems connected with the accurate use of digital integrators for such measurements are discussed and illustrated by means of various experiments. Magnetic effects due to air, should leaks occur into a refrigerant under vacuum within a solenoid, are discussed and a method of removing solid air from the region near the magnetic sample during magnetic measurements is suggested.
42(1965); http://dx.doi.org/10.1063/1.1695678View Description Hide Description
For the helium atom, a new type of correlation factor is tested which has the form 1+ch(r 12) where h(r 12) is a cutoff correlation variable such that h(r 12) = r 12 for 0≤r 12≤D, and h(r 12) = D for D≤r 12≤∞, with c and D adjustable parameters. In a series of energy calculations using wavefunctions which contain both configuration interaction and cutoff r 12 terms, it is found that, as far as the energy is concerned: (1) The correlation cusp is unimportant, and (2) The range of the Coulomb correlation effect is of the order of the atomic diameter. For very small D, the correlation energy per unit volume is calculated to be 0.048 hartree/bohr3.
42(1965); http://dx.doi.org/10.1063/1.1695685View Description Hide Description
Portions of the iodine absorptionspectrum have been remeasured under high resolution. A new analysis of 719 lines in the 3–6, 4–7, 5–4, 7–5, 11–1, 12–2, 13–2, 25–0, and 29–0 bands, combined with previous measurements by Mecke, Loomis, and Brown, gives the following revised constants:Constants for ground state rotational levels were found to be in good agreement with Rank's latest determinations.
Using these new constants for the B state, Franck—Condon factors for this system were computed. The vibrational numbering originally assigned by Mecke and Loomis to the B state was decreased by one unit to bring calculated intensity distribution into complete agreement with all observed fluorescence data, including new photoelectric measurements on a number of bands. No systematic variation of the electronic transition moment could be derived from these data. These revised Franck—Condon factors were also used to recalculate radiative lifetimes from absolute absorption data which are in fair agreement with the phase‐shift determination of Brewer, Berg, and Rosenblatt.
42(1965); http://dx.doi.org/10.1063/1.1695696View Description Hide Description
A measurement of the vibrational isotope displacements between 127I2 and 129I2 has confirmed the revised vibrational numbering of the B 3Π0+u state proposed by Steinfeld et al. They have observed that the traditional numbering must be decreased by one unit to bring their calculated fluorescence intensity distributions into agreement with experiment.
42(1965); http://dx.doi.org/10.1063/1.1695700View Description Hide Description
The Raman spectrum between 3500 and 100 cm−1, including polarization measurements, and the infrared spectrum between 4000 and 265 cm−1 of liquid α,α,α‐trichlorotoluene, C6H5CCl3, have been recorded. Wavenumber assignments of the phenyl vibrations have been made on the basis of C 2v symmetry, although the molecule is undoubtably of lower symmetry.
The 24‐substituent‐insensitive ring modes were assigned without much difficulty. The nine substituent modes and the six substituent‐sensitive ring modes were assigned with less certainty, but the Raman polarization data helped in the assignment of some of these. The phenyl‐substituent torsional mode was assigned to the strong, broad Raman line observed at 118 cm−1. The six phenyl vibrations whose wavenumbers are sensitive to the nature of the substituent were assigned, in our previous notation [C. V. Stephenson, W. C. Coburn, Jr., and W. S. Wilcox, Spectrochim. Acta 17, 933 (1961)], as follows: .
42(1965); http://dx.doi.org/10.1063/1.1695706View Description Hide Description
In the iron‐group cyanides the low‐lying antibonding orbitals of the cyanide ligands do not contain electrons. These π* orbitals play an important role in the covalency of the complexes. A molecular orbital analysis of their role is presented in order to correlate the orbital reduction factors derived from the ESR experiments of Baker, Bleaney, and Bowers with the isomer shifts of the Mössbauer experiments. By fitting the wavefunctions to the ESR experiment it is shown that the Fe2+ complex donates about one more electron to the π* orbitals than does the Fe3+. This shows that the effective charges of Fe2+ and Fe3+ are approximately the same and explains their similar isomer shifts.
42(1965); http://dx.doi.org/10.1063/1.1695717View Description Hide Description
Measurements of the nuclear magnetic resonancerelaxation timesT 1 and T 2 of the water protons in aqueous solutions of nucleic acids and magnetic metal ions allow one to determine details about the metal‐ion binding. When Mn2+ is bound, the rotation of its hydration sphere becomes slower, so that its effectiveness in reducing the protonT 1 from its value in pure water is enhanced. This relaxation enhancement has been measured for Mn2+ bound to DNA,RNA, synthetic polynucleotides, and E. coli ribosomes. Compared to water as unity, the enhancement factors range from 3.7 for polyuridylic acid to 16.7 for polyadenylic acid. The equilibrium constants for binding, and the concentration of binding sites have been calculated from the dependence of enhancement upon Mn2+ concentration.
42(1965); http://dx.doi.org/10.1063/1.1695723View Description Hide Description
This paper outlines a simple semiempirical theory of the zero‐field splitting parameters of aromatic nitrenes. The theory is based on a Hückel LCAO description of the aromatic system, and takes advantage of the vanishing of odd orbital mutual atom polarizabilities in odd alternant molecules. The observed parameter for the parent imine radical is used as an empirical one‐center integral. Calculated parameters agree rather well with electron spin resonance results. The relation between the zero‐field parameters and general π‐electron spin densities is also outlined.
42(1965); http://dx.doi.org/10.1063/1.1695724View Description Hide Description
Using the WKB semiclassical approximation, the dependence of the probability of vibrational de‐excittion on the assumed form of the interaction potential is discussed. The asymptotic results show that the Morse and Lennard‐Jones potentials have, respectively, two and three correction terms in the exponential part. The magnitudes of the correction terms are important compared to the leading term. The exact form of the leading term is also dependent upon the interaction potential.
42(1965); http://dx.doi.org/10.1063/1.1695725View Description Hide Description
The powder spectra of γ‐irradiated salts and amides of some fluorinated acids were interpreted based on the spectral shapes of the α‐fluorine hyperfine‐coupling line. Because of the large anisotropy of α‐fluorine coupling tensor, it was possible to observe the shoulders corresponding to the maximum principal value of the coupling tensor. The separation between the wing patterns and the wing hyperfine structures made it possible to identify the radical species produced. The probable radicals were the ones formed by the removal of the atoms attached to the α carbon.
42(1965); http://dx.doi.org/10.1063/1.1695726View Description Hide Description
The ionization‐efficiency curves for the parent and fragment ions from CH3CHO and (CH3)2CO have been examined by the second differential method. The appearance potentials for CH3CO+, namely, 10.2±0.1 eV [(CH3)2CO] and 10.5±0.2 eV (CH3CHO), are much lower than previously accepted electron‐impact values. Apart from CO+(CH3CHO) no appearance potentials can be easily obtained for any of the other fragment ions. No kinetic energy was found for the light ion CH3 +, nor the heavier ions CO+, HCO+(CH3CHO), and CH3CO+[(CH3)2CO].
42(1965); http://dx.doi.org/10.1063/1.1695727View Description Hide Description
An electron spin resonancespectrum observed in irradiated synthetic quartz crystals is attributed to atomic hydrogen in interstitial sites. The quartz specimens were irradiated with 1.7 MeV electrons at ∼300°K (1018 electrons cm−2) and then with 60Co γ‐rays (∼107 R) at ∼80°K and measurements were made without warming. The hydrogen spectra had a hyperfine constant A = 521.3 Oe (1453.1 Mc/sec) when the applied field was along the crystal threefold axis. An anisotropy of the hyperfineinteraction of the order of 0.75 Oe was detected. The g value was 2.0021±0.0005 and evidence for a small variation in this value with rotation was inferred. There are three sites for the atomic hydrogen in the unit cell which are related by the three fold symmetry operator of the crystal and which are otherwise equivalent. Additional hyperfineinteractions with 29Si nuclei were also observed enabling a tentative model of the sites to be proposed. The width of the line was 0.1 Oe and was attributed mainly to field inhomogeneity. Another spectrum was observed with a slightly smaller hyperfineinteraction and was also attributed to atomic hydrogen in a different interstitial site. Detailed measurements were not made due to the low intensity of the spectrum.
42(1965); http://dx.doi.org/10.1063/1.1695728View Description Hide Description
The proton spin—lattice relaxation timeT 1 has been measured by a pulse technique as a function of temperature in a series of tantalum‐hydrogen samples with hydrogen content from H/Ta=0.1 to 0.66. The spin—lattice relaxation is determined by the rapid translational diffusion of the hydrogen atoms in the tantalum lattice. Discontinuities in the T 1 curves indicate two phase transitions in the temperature range investigated, 200° to 400°K. The transition temperatures depend on the composition of the sample in a way which is in conflict with the present view of the phase relations in the Ta/H system. The activation energy in the low‐temperature phase (β1 phase) is found to decrease linearly with increasing hydrogen content from 3.7±0.5 kcal/mole in TaH0.10 to 2.6±0.3 kcal/mole in TaH0.66. In the high‐temperature phase (α phase) the activation energy is 1.6±0.4 kcal/mole, and in the intermediate‐temperature phase (β2 phase) the activation energy is 6.3±0.6 kcal/mole. Neither of the latter values changes significantly with the composition of the sample.
42(1965); http://dx.doi.org/10.1063/1.1695729View Description Hide Description
The effects of solvents on the coupling constants of the compoundshave been studied. The coupling constants are found to vary with the solvents. This implies that intermolecular interactions affect coupling constants to a degree that is not negligible. The variations in the coupling constants are to some extent correlated with the size, electronic character, and dipolar nature of the solvent molecules. The temperature dependence of fluorine coupling constants that has been reported previously may be due to this temperature‐dependent molecular interaction as well as the population of excited vibrational states.
42(1965); http://dx.doi.org/10.1063/1.1695730View Description Hide Description
The equilibrium polymerization theory of Tobolsky and Eisenberg is extended to the case of initiation by polyfunctional species, under the assumption that only a single monomer is involved and no ring or loop formation occurs. A general indexing scheme is given which permits derivation of the relation between degree of polymerization and equilibrium constants for initiation and propagation steps for any functionality of initiator. An explicit derivation for the case of trifunctional initiator is given, and the result for general functionality is provided.
42(1965); http://dx.doi.org/10.1063/1.1695731View Description Hide Description
The theory of phase separation from a single phase fluid by a spinodal mechanism is given. The predicted structure may be described in terms of a superpositioning of sinusoidal composition modulations of a fixed wavelength, but random in amplitude, orientation, and phase. Sections through a calculated structure are shown. These show that the structure has many of the geometrical features found in phase separable glasses, in particular the high degree of connectivity among particles of each phase.
Observation of the Products of Ionic Collision Processes and Ion Decomposition in a Linear, Pulsed Time‐of‐Flight Mass Spectrometer42(1965); http://dx.doi.org/10.1063/1.1695653View Description Hide Description
A retarding potential technique has been investigated which permits study of secondary processes occurring after ion acceleration in a time‐of‐flight mass spectrometer. By application of a retarding field near the end of the flight path, peaks due to neutral species and fragment ions formed after acceleration are separated from their parent ion peaks and displayed in the same panoramic mass spectrum.Charge exchange, collision‐induced dissociation, and spontaneous (metastable) ion decomposition are easily observed. The identities of initial ions and fragment ions involved in secondary processes can be determined unambiguously without complications of peak overlap.
42(1965); http://dx.doi.org/10.1063/1.1695654View Description Hide Description
The phases and phase transformations in the argon—nitrogen system have been determined by x‐ray diffraction with results very different from previous published phase diagrams. The solid phase in equilibrium with the liquid between 1% and 100% N2 is close‐packed hexagonal in spite of the fact that pure argon is face‐centered cubic. The argon‐rich solid solutions with 1% to 55% N2 undergo a strain‐induced martensitic transformation from cph (stable at high temperatures) to fcc at low temperatures. The highest temperatures at which this transition becomes possible are those at which the combination of thermal expansion plus solid solutionexpansion gives a lattice constant for the fcc phase of a 0=5.486 Å, which corresponds to that obtained by extrapolating the curve of a 0 for pure argon to a point 5° above the melting point. The c/a value for the cph argon‐rich phase is 1.633 within experimental uncertainty at all compositions and temperatures.
Stacking faults are infrequent in the as‐frozen samples but are very numerous in the deformed samples. At temperatures and compositions where fcc and cph have nearly equal free energies it is possible to destroy the crystalline diffraction pattern by cold work, leaving only an amorphous pattern.
The α‐N2solid solution forms spontaneously and isothermally from the β‐N2solid solution at N2 contents above 77.5% N2. The transformation appears to be that known in metallic systems as a ``massive'' type, and presumably involves only short‐range diffusion in the phase interface. The curve of distance between nearest molecular centers in the β phase is not an extension of the curve for the argon‐rich phase. The α phase, which is cubic, is not continuous with the cubic argon‐rich solid solution; samples with 55%—75% N2 do not transform from the hexagonal form even when cold worked at 4.2°K.
Influence of Paramagnetic Resonance on the Static Susceptibility. Spin—Lattice Relaxation Time of Cupric Sulfate Pentahydrate42(1965); http://dx.doi.org/10.1063/1.1695655View Description Hide Description
The static direct‐current susceptibility of cupric sulfate pentahydrate was measured as a function of microwave power absorbed at electron spin resonance. This technique was used to study the spin relaxation process of cupric sulfate pentahydrate as a function of temperature, microwave power level, and crystal size. Up to less than one‐half saturation, the relaxation time τ1 was found to be independent of the degree of saturation and inversely proportional to the temperature τ1 T=0.2 sec·°K, showing that the predominant mechanism at liquid‐helium temperatures is the direct spin—lattice process. At high saturation levels there is evidence of crystal heating. These results are compared with those obtained by the cw saturation method on cupric sulfate pentahydrate, and the discrepancy in the magnitude of the relaxation time and its temperature and power dependencies are explained. This new technique is shown to be especially useful for studying power‐transfer mechanisms in concentrated spin systems.