Volume 43, Issue 5, 01 September 1965
Index of content:
43(1965); http://dx.doi.org/10.1063/1.1696952View Description Hide Description
The quenching of the vibrational fluorescence of CO by CH4 has been measured between 303° and 363°K. The quenching arises from the vibration—vibration exchange: CO*+CH4=CO+CH4 * where all molecules are in ground electronic states. The average number of collisions between CO* and CH4 required for a V–V transfer to occur varies from 33 000 at 303°K to 24 000 at 363°K.
Study of Collision Narrowing by Comparison of Molecular‐Beam and Gas‐Phase Nuclear Resonance Spectra43(1965); http://dx.doi.org/10.1063/1.1696953View Description Hide Description
The second moments of molecular‐beam nuclear resonance curves are compared through a simple motionnarrowing theory with linewidths from gas‐phase NMR data for CF4, SiF4, SF6, CH3F, CF3H, and CH2F2 at 300°K. From this comparison the average number of collisions necessary for these molecules to change their angular momentum is estimated to be on the order of three.
Charge‐Transfer‐Controlled Vaporization of Cadmium Sulfide Single Crystals. I. Effect of Light on the Evaporation Rate of the (0001) Face43(1965); http://dx.doi.org/10.1063/1.1696954View Description Hide Description
Light of greater‐than‐band‐gap energy was found to change markedly the vacuum evaporation rate of the (0001) face of cadmium sulfide single crystals.Evaporation temperatures of 680°—740°C and light intensities of 5.0×103−2.0×105 μW/cm2 were used. The results were interpreted assuming that charge transfer is the rate‐determining step in the sequence of vaporizationsurface reactions. An evaporation mechanism in terms of charge transfer has been proposed. Light (1) changes the free‐carrier concentrations at the vaporizing surface, and (2) under proper conditions, changes the composition of the crystals. High‐resistivity crystals showed a fivefold increase of their evaporation rate under illumination due to the increase by light of both electron and hole concentrations. In low‐resistivity crystals, in which illumination can only significantly increase the minority free‐carrier concentration above the dark equilibrium value, effects, which are due to the change of the crystal composition, dominated.
Charge‐Transfer‐Controlled Vaporization of Cadmium Sulfide Single Crystals. II. Effect of Copper Doping on the Evaporation Rate of the (0001) Face43(1965); http://dx.doi.org/10.1063/1.1696955View Description Hide Description
The evaporation rate of copper‐doped CdS single‐crystal c face was measured in the temperature range 680°—740°C. The evaporation rate was decreased markedly with respect to that of the undoped crystals. The coppersurface concentration dependence of the vaporization rate was studied by simultaneous diffusion of copper into, and vaporization of the crystal at the () and (0001) crystal faces, respectively. The presence of copper impurity in the bulk of the crystal was found to also have an effect on the evaporation rate of CdS. The results were interpreted in terms of self‐compensation in CdS via sulfur‐vacancy formation, and copper‐ and sulfur‐vacancy diffusion toward the vaporizing surface.Copperdoping rendered the crystal vaporization insensitive to light.
43(1965); http://dx.doi.org/10.1063/1.1696956View Description Hide Description
The usual use of the closure relation in the derivation of the incoherent scattering factor for electrons and x rays involves approximations and introduces errors in the final result. In this paper corrections through third order for these errors are given. The correction terms are found to be of a simple form and can be quite important not only in the limit of low energy for the incident electron or x‐ray beam but also for atoms of large atomic number. The magnitudes of the corrections for Ne and Ge are given for 40‐kV electrons. In the x‐ray case corrections are given for Ge for Cu Kα x radiation.
Total Cross Sections for Ionization and Attachment in Gases by Electron Impact. I. Positive Ionization43(1965); http://dx.doi.org/10.1063/1.1696957View Description Hide Description
The total ionization cross sections of He, Ne, Ar, Kr, Xe, H2, D2, N2, O2, CO, NO, CO2, N2O, and CH4 have been measured from threshold to 1000 eV in a total ionization tube. More limited measurements were performed in C2H4 and SF6. Great care was taken to assure complete collection of electron and ion currents, and the absence of spurious instrumental errors. A new method was devised for obtaining absolute cross sections of gases relative to H2, and a McLeod gauge was used to obtain the absolute cross section in H2. The cross sections in NO and O2 could not be obtained by this method, and an approximate correction to direct McLeod‐gauge readings was used for these gases. It is believed that the results are as accurate as is possible with the present method. It is difficult to explain the differences found between cross sections measured by various investigators. McLeod‐gauge errors appear to account for most of the difference in absolute magnitude.
Total Cross Sections for Ionization and Attachment in Gases by Electron Impact. II. Negative‐Ion Formation43(1965); http://dx.doi.org/10.1063/1.1696958View Description Hide Description
Absolute total cross sections for negative‐ion formation in several gases (CO, NO, O2, CO2, N2O, SF6, and H2) by electron impact have been measured in a total ionization tube. Both dissociative attachment and ion‐pair formation have been measured, with careful attention paid to complete collection of the negative ions. Possible errors due to scattered electrons and energy spread are discussed.
43(1965); http://dx.doi.org/10.1063/1.1696959View Description Hide Description
The chemiluminescence and chemi‐ionization resulting from the room‐temperature reaction of atomic oxygen with acetylene have been investigated. The absolute intensity of chemiluminescent radiation and the over‐all rate of chemi‐ion formation have been measured and the relative concentrations of various individual chemi‐ions have been observed with a T.O.F. mass spectrometer.
It has been shown conclusively from these measurements that chemi‐ionization does not arise primarily from the reactions:It has been shown also that if ionization arises from the reactiona mechanism that has also received wide support, then the ground state CH (X 2π) molecule must be produced by some mechanism other than that producing CH (A 2Δ).
The CH (A 2Δ—X 2π) system was found to be the most intense feature of the chemiluminescent spectrum. Weak emission from the C2 Swan bands and from the CN violet system was also observed. The intensity of the weak OH emission was found to be consistent with it being produced by reactions with molecular oxygen. The CH emission was shown to arise from a complex series of reactions involving the participation of three molecules of acetylene and two atoms of oxygen. Two possible mechanisms for its production are discussed.
Luminescence from Transition‐Metal Complexes: Tris(2,2′‐bipyridine)‐ and Tris(1,10‐phenanthroline)Ruthenium(II)43(1965); http://dx.doi.org/10.1063/1.1696960View Description Hide Description
Absorption and emission spectra and mean lifetimes (τ m ) of the luminescence of tris(2,2′‐bipyridine)‐ and tris(1,10‐phenanthroline)ruthenium(II) are reported. For the 2,2′‐bipyridine complex, two weak absorption bands occurring at 15 050 and 18 550 cm−1 are assigned, respectively, to the 1 A 1→3 T 1 and 1 A 1→1 T 1 transitions of d—d type, and the intense luminescence with origin at 17 250 cm−1 (τ m = 5.92 μsec) is assigned to 1 T 1→1 A 1fluorescence. For the 1,10‐phenanthroline complex, no intra‐d transitions were observed in absorption, but the bright luminescence with origin at 17 700 cm−1 (τ m = 9.93 μsec) was assigned to 1 T 1→1 A 1fluorescence in analogy with the first compound. The energy levels are analyzed on the basis of an octahedral model, and empirical values for the Racah B and C parameters for Ru II and the crystal‐field parameter Δ are evaluated. Possible uses of the compounds for laser applications are suggested.
Isotope Effects on Vibrational Transition Probabilities. III. Ionization of Isotopic H2, N2, O2, NO, CO, and HCl Molecules43(1965); http://dx.doi.org/10.1063/1.1696961View Description Hide Description
Franck—Condon factors are given for the normal and stable isotope‐labeled molecules for transitions to spectroscopically known ionic states. Appreciable isotope effects on Franck—Condon factors are found in several transitions, and are related both to the differences in internuclear separation associated with different levels and to differences in reduced masses. Isotopic substitution by a heavier element results in a shift in the transition probability towards higher vibrational levels. Therefore an inversion in the ratio of ionization efficiencies of light and heavy molecules is predicted.
43(1965); http://dx.doi.org/10.1063/1.1696962View Description Hide Description
A theory for thermal transpiration based on the dusty‐gas model has been proposed by Mason, Evans, and Watson. For verification the theory ideally requires both nonisothermal and isothermal measurements on a sample of porous material. Such measurements are reported and it is found that the dusty‐gas theory satisfactorily reproduces the experimental results except when the gas adsorbed appreciably on the sample.
Calculation of Matrix Elements for One‐Dimensional Quantum‐Mechanical Problems and the Application to Anharmonic Oscillators43(1965); http://dx.doi.org/10.1063/1.1696963View Description Hide Description
A simple method using the techniques of transformation theory for the generation of the matrix elements of unusual potential functions for one‐dimensional quantum‐mechanical problems is described. It is applicable both to functions which exist as a set of points, for example, a curve or table, as well as to those in explicit form. Some representative calculations have been made for anharmonic oscillators.
43(1965); http://dx.doi.org/10.1063/1.1696964View Description Hide Description
The system considered is a molecule like triptycene where triplet excitation can jump between several equivalent sites whose Hamiltonians do not commute. The line shapes are calculated from the Bloch equations for the density matrix, and the result involves inverting a certain matrix with complex elements which depend on the jumping rate. The method is similar to Sack's method for sites whose Hamiltonians do commute. The complete line‐shape function is found in zero magnetic field and when the field is along the triad axis. General perturbation formulas are derived for finding the broadenings and frequency shifts which occur in the limits of very fast and very slow jumping. The direct calculation of the line shapes for tribenzotriptycene in a glassy matrix has not been attempted, but the results obtained for the oriented triplet agree quite well with the observed ESRspectra of the glass. The method is also applied to the problem of a spin ½ which jumps between two sites with different gtensors.
43(1965); http://dx.doi.org/10.1063/1.1696965View Description Hide Description
The pressure‐induced shifts of hydrogen fluoride 1–0 and 2–0 band lines due to noble gases were measured. The shifts have the same general features as those observed with HCl, but they display certain peculiarities that have been interpreted by using a phase‐shift theory with a particular collision model.
43(1965); http://dx.doi.org/10.1063/1.1696966View Description Hide Description
Potential‐energy curves for the X 1Σ+, A 1Π, I 1Σ−, a 3Π, a′ 3Σ+, d 3Δ i , and e 3Σ− electronic states of CO and the X 2Σ+, A 2Π i , and B 2Σ+ electronic states of CO+ have been calculated using a modification of the Rydberg—Klein—Rees method. The curves for the a 3Π, d 3Δ i , and A 2Π i states, which are intermediate between Hund's Cases (a) and (b), represent fictitious rotationless states calculated using the true B v . Benesch and co‐workers have shown theoretically that this representation is correct for such intermediate cases. The numerical results for the A 2Π i state presented here confirm their conclusions.
43(1965); http://dx.doi.org/10.1063/1.1696967View Description Hide Description
A long‐range coupling across the alkoxycarbonyl group of about 1.4–1.5 G has been observed in the EPR spectra of radicals formed in the hydroxyl‐radical‐initiated homopolymerization of methyl acrylate, ethyl acrylate, and methyl methacrylate within an aqueous flow system at 25°±1°C. This coupling has been further investigated by way of a comprehensive EPR study of a series of simpler, related radicals prepared under similar conditions from methyl formate, ethyl formate, methyl acetate, ethyl acetate, methyl propionate, and methyl isobutyrate by hydrogen‐atom abstraction with hydroxyl radicals. For ester radicals other than those produced from the formates there was a long‐range coupling of 1.4–1.5 G with protons δ to the unsaturated carbon atom when this unsaturated carbon atom was adjacent to the oxycarbonyl group, i.e., for radicals with the following functional structures, Ċ–CO–O–C and C–CO–O–Ċ. The two formates yielded only the radicals HCOOĊH2 and HCOOĊHCH3. These displayed a γ coupling with the formyl proton of 2.4–2.5 G. All these results are shown to be compatible with a simple model based on a delocalized π‐bond system. Comparison of the observed proton coupling constants with the available data for similar unsubstituted aliphatic radicals suggests that the value of Q α in the McConnell equation a α=Q αρ is significantly lower in a conjugated radical than in a similar nonconjugated radical. In support of the data from this study of hydroxyl‐radical attack on esters, the substrates formic, acetic, propionic, n‐butyric, and isobutyric acids and methanol, ethanol, and acetone have been examined in a like fashion. Many of the esters and acids investigated underwent hydrogen‐atom abstraction at more than one site. For those substrates, it was possible to estimate qualitatively the selectivity of hydroxyl‐radical attack from the relative intensities of the EPR absorptions of the different reaction products. The results so obtained are reckoned to generally support the expected electrophilic character of the hydroxyl radical in these reactions.
Intensity Relations for Determining Gas‐Phase OH, Cl, Br, I, and Free‐Electron Concentrations by Quantitative ESR43(1965); http://dx.doi.org/10.1063/1.1696968View Description Hide Description
The theoretical intensities for the absorption lines of gas phase OH and NO as observed with an ESR spectrometer are discussed. Since both species have transitions of the electric‐dipole type, it is shown how the absolute concentration of OH may be determined by comparing its integrated intensity with that of the stable NO used as a reference gas. A similar procedure for determining the concentration of free electrons from their electric‐dipole cyclotron resonance absorption signal is also outlined. Finally, the intensity relations useful for determining all of the halogen atoms (except fluorine) using O2 as the reference gas are derived.
43(1965); http://dx.doi.org/10.1063/1.1696969View Description Hide Description
An application of the intensity relations derived previously for determining absolute concentrations of H atoms and OH radicals in the gas phase by ESR spectroscopy is described. Magnetic‐dipole transitions in O2 and electric‐dipole transitions in NO are used as calibration standards for determining H and OH, respectively. The construction of a cylindrical TE011 cavity suitable for observing both types of transitions in a fast flow system is described. Results of measurements of H and OH concentrations during the ``titration'' reaction H+NO2→NO+OH are presented. The internal consistency of the theoretical OH intensity relations is demonstrated by experimental measurements on and OH lines. At reaction times of a few milliseconds, the H titration curve is shown to have an initial linear portion which extrapolates to an NO2/H ratio of unity. It then becomes curved as expected by the generation of an additional H fromThe OH titration curves yield k 2=1.2±0.3×1012 cm3 mole−1·sec−1 at 300°K in good agreement with the only other reported value. Finally, some qualitative observations on a striking generation of free electrons associated with the main H–NO2reaction are noted.
43(1965); http://dx.doi.org/10.1063/1.1696970View Description Hide Description
The melting curves of the rubidium halides have been determined to 30–40 kbar. The present curve for RbCl is in good agreement with Clark's curve to 12 kbar. The pressures of the triple points between the low‐ and high‐pressure solid phases and liquid for RbCl, RbBr, and RbI agree well with the values to be expected by extrapolation of earlier work at lower temperatures. The triple point on the melting curve of RbF is located near 32 kbar.
43(1965); http://dx.doi.org/10.1063/1.1696971View Description Hide Description
The light‐scattering spectrum for the bead‐and‐spring model of flexible polymers is computed. The spectrum is shown to consist of a sum of Lorentzians centered about the incident frequency each with a half‐width proportional to the relaxation time of a given normal mode. The contribution of each normal mode to the spectrum is proportional to the ratio of the equilibrium mean‐squared length of a mode to the square of the incident‐light wavelength.