The resonant frequency spectrum for flexural vibrations in thick rectangular beams has been analyzed in terms of the elastic parameters of the beam material. An apparatus for the precise measurement of such resonant frequencies up to the tenth or higher modes in the range ∼0.3 to ∼20 kHz is described together with results for sample aluminum beams. By comparing the measured frequency spectrum with that calculated from a first-order theoretical treatment it has been found possible to derive both the elongational modulus and Poisson’s ratio from such a measurement, thereby completely characterizing the elastic response of an isotropic material.

Zusammenfassung Es werden Messungen des dielektrischen Verlustes tand im Temperaturbereich von 1 bis 80 °K und bei verschiedenen Frequenzen zwischen 10 Hz und 10 kHz an einer Reihe von Polymeren mitgeteilt.

Measurements of tanδ for polyethylene for frequencies around one kHz down to temperatures of 4.2° K show an increase in the dielectric loss (Fallou andBobo) (1). We have repeated these measurements with very sensitive equipment over a range of frequencies up to several kHz and down to temperatures of 1.2° K. We find maxima in tanδ as a function of temperature and frequency. It thus seems possible for there to be an activated mechanism at these very low temperatures. The values of the activation energy are of the order of only some cal/mol instead of the more usual kcal/mol. The loss peaks are observed in “as received” material, i. e. fabricated films and pressed plates. Additives and contaminations do not seem to play any role. The loss peaks are observed in stretched, particularly stretched crystalline, material. The dielectric losses vanish for melted and then slowly cooled or quenched material. Molecular rearrangements in the interface between crystalline and non-crystalline regions of the material may play a role in causing these losses.

Summary Measurements of tand for polyethylene for frequencies around one kHz down to temperatures of 4.2° K show an increase in the dielectric loss (Fallou andBobo) (1). We have repeated these measurements with very sensitive equipment over a range of frequencies up to several kHz and down to temperatures of 1.2° K. We find maxima in tand as a function of temperature and frequency. It thus seems possible for there to be an activated mechanism at these very low temperatures. The values of the activation energy are of the order of only some cal/mol instead of the more usual kcal/mol. The loss peaks are observed in “as received” material, i. e. fabricated films and pressed plates. Additives and contaminations do not seem to play any role. The loss peaks are observed in stretched, particularly stretched crystalline, material. The dielectric losses vanish for melted and then slowly cooled or quenched material. Molecular rearrangements in the interface between crystalline and non-crystalline regions of the material may play a role in causing these losses.

Das Auftreten von dielektrischen Verlusten an einem Material zeigt an, daß es in dem vermessenen Temperaturbereich noch Möglichkeiten für Umlagerungen, für Änderungen der gegenseitigen Anordnung von molekularen Bausteinen, zeitliche Fluktuationen, gibt. Solange dielektrische Verluste gemessen werden, sind also nicht alle Bewegungsmöglichkeiten eingefroren.

Pulse-time-of-flight measurements have been used to determine the velocities of 10-MHz transverse and longitudinal sound in the hcp phase of solid helium-four at a molar volume of approximately 21 ${\mathrm{cm}}^{3}$ per mole. The measurements have been made on single crystals grown from superfluid helium. The orientations of these crystals have been determined by an optical birefringence technique. The five elastic constants for this molar volume of hcp solid helium have been calculated using the data obtained in these experiments.

Infrared absorption spectra of electron-bombarded superfluid liquid helium have established that (a) $a^{3}{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ helium molecules and probably $2^{3}S_{1}$ atoms are present in the liquid in concentrations of the order of ${10}^{13}$/${\mathrm{cm}}^{3}$ in an excitation volume \ensuremath{\sim}0.05 ${\mathrm{cm}}^{3}$, for a 1-\ensuremath{\mu}A bombarding beam of 160-keV electrons; (b) these states are metastable with life-times of the order of a millisecond at 1.7\ifmmode^\circ\else\textdegree\fi{}K; (c) the $a^{3}{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ molecules are de-excited by $a^{3}{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}\ensuremath{-}a^{3}{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ collisions in the liquid; and (d) the local configuration of the surrounding liquid helium is different for different excited states of the helium atomic and molecular systems, and the configurational interaction with the host fluid has an appreciable effect upon the electronic energies associated with these excited states.

Optical and infrared emission spectra of electron-bombarded liquid helium reveal the presence of a number of excited singlet and triplet states of the ${\mathrm{He}}_{2}$ molecule, including the $a{^{3}\ensuremath{\Sigma}_{\mathrm{u}}}^{+}$ metastable state. The observed liquid spectra show vibrational and unresolved rotational structure. There is also preliminary evidence that excited atomic states, including the metastable $2^{3}S_{1}$ state, are populated in the liquid.