Excitation mechanisms in CW He-Hg lasers

The collision processes responsible for populating and de-populating upper and lower levels of the cw He-Hg ion laser are investigated using flowing afterglow techniques. It is shown that the superior performance of the laser at 6150 Å compared to that at 7945 Å is not due to a difference in excitation mechanism — both are excited by charge transfer from He⁺ — but instead reflects a greater effectiveness of second kind collisions of electrons in depopulating the 7p²P_{$\text{1}\text{2}$} upper level of the 7945 Å transition. The charge transfer, excitation rate constants for several Hg II levels are compared, and values for the rate constant for the electron collision de-excitation process are deduced.     

Application of 15N spectroscopy and dynamic NMR to the study of ureas,thioureas and their lewis acid adducts

Rotational barriers and ¹⁵N chemical shifts have been measured in a number of ureas and thioureas. As anticipated on the basis of the ¹⁵N shifts, several previously unobserved rotational barriers could be detected by using lanthanide reagents or a high field spectrometer. Nearly constant effects on both the rotational activation energy and the ¹⁵N shift are produced on going from ureas to the corresponding thioureas, and correlations are found between the ΔG? and δ¹⁵N values. The results are discussed in terms of lone pair delocalization, and anomalies with respect to the general behaviour are tentatively explained in the light of the effect of steric torsion in crowded structures on the ¹⁵N shifts and rotation barriers.     

More on quadratic convergence in interval arithmetic

Necessary and sufficient conditions are given for quadratic convergence in interval arithmetic.Partially supported by NSF grant GJ-797.     

Resonant Raman scattering from screened longitudinal optical phonons in EuTe

Forbidden resonant Raman scattering from screened longitudinal optical, LO, phonons has been observed in the back-reflection geometry from n-type EuTe at 2°K. The Raman shift increased with increasing excitation frequency but was always between the LO and TO phonon frequencies. This effect is explained in terms of a varying ‘effective’ carrier concentration as a function of laser penetration depth through the surface depletion layer in the situation of large phonon wave vector. Conduction band and lattice parameters have been calculated from infrared Reststrahlen and plasma edge measurements.     

Observation of diffraction excitation in pp → (pπ+π−)+X at the CERN-ISR and tests of limiting fragmentation

In a single-arm magnetic spectrometer experiment at the CERN Intersecting Storage Rings (ISR), a prominent diffractive enhancement is observed for inclusive production of (pπ⁺π^?) systems near x ～ 1 at √s = 53 and 35 GeV. In testing limiting fragmentation and scaling for this system, inclusive pπ⁺π^? diffractive excitation into a fixed invariant mass range is found to possess a cross-section independent of s to within (3 ± 5)%. The main component in the (pπ⁺π^?) system is Δ(1236)π, with a charge structure compatible with the break-up of an isotopic spin $\text{1}\text{2}$ system.     

Vibronic properties of the F+ centre in strontium oxide

The shape and moments of the F⁺ absorption band in SrO have been measured between 4.2K and 300K. Analysis of the moments gives the effective frequency of the vibrational modes interacting with the centre as $\text{h}\text{?}\text{ω = 236 ± 20 cm}{}^{-1}$, and the Huang-Rhys factors for cubic and non-cubic modes as S_C = 3 ± 1 and S_NC = 4 ± 1 respectively.     

A LEED study of Ge(111); a high-temperature incommensurate structure

We present a detailed LEED study of the Ge(111) surface. The low-temperature structure is c(2×8). This structure undergoes a first order transition to an incommensurate reconstruction at 300°C. The diffraction from the high-temperature phase is consistent with either of two simple models. One is a 2 × 1 reconstruction modulated by antiphase walls which are oriented primarily perpendicular to the undoubled direction of the 2 × 1 unit mesh. The second is a 2 × 2 reconstruction modulated by a honeycomb arrangement of intersecting antiphase walls produced by shearing the 2 × 2 mesh by one primitive translation vector. Fluctuations in the I(2 × 2) structure preserve the total wall length and number of wall intersections; the free energy of this structure should be dominated by the entropy. The wall separation and correlation lengths for the high-temperature phase decrease continuously with increasing temperature above the transition.