Longitudinal acoustic mode

The infrared spectrum of partially deuterated polyethylene has been examined as a function of temperature 323). Bands associated with trans-trans (tt) bond pairs and trans-gauche (tg) bonds have been identified so it is possible to follow the changes in these isomers as a function of temperature. As the temperature increased, the intensity of the tg band increased, while the intensity of the tt band decreased. It is hypothesised that the extinction coefficients for the two bands are essentially equal and the nearly constant total intensity of the two bands seems to confirm this hypothesis. Thus it appears that each time a tt sequence disappears a tg sequence is created. Correlations of this infrared result with the intensity of the Raman longitudinal acoustic mode, suggest that the new thermally induced tg sequences occur mainly as point dislocations in the crystal 323). 

Low-frequency Raman spectroscopy (LFRS) has been used to measure the longitudinal acoustic mode (LAM) of semicrystalline diblocks containing poly(ethylene oxide) (Mai et at. 1996 Viras et at. 1988 Yang et at. 1995), The LAM mode frequency is inversely proportional to the crystal stem length, and indicates whether chains are folded or extended. 

Fig. 5.16 Low-frequency Raman spectrum of a PP07PE075PP07 triblock at -10 °C (Viras et al. 1988). The band between 8 and 12 cm 1 is assigned to the longitudinal acoustic mode (LAM) of the copolymer.

Recently Maret et al. (JL) have observed the longitudinal acoustic mode in oriented DNA fibres and films in a Brillouin scattering experiment. They observed the largest acoustic velocity for the driest samples, smallest for wet samples and at all times the observed velocity was larger than that for water itself. We have assumed that the velocities for propagation parallel to the helix axis are characteristic of acoustic modes in the DNA double helix and have used these values along with previously refined valence force field parameters (2,3) to fit the non-bonded force constants for the double helix. 

Figure 1. Velocity of longitudinal acoustic mode of poly(dG)-poly (dC) as a function of nonbonded strength parameter f3. Various curves as listed in text.

Scherer and Snyder (1980) investigated the low-frequency Raman spectra of liquid n-alkanes. The highly temperature dependent intensities of the longitudinal acoustic modes of the n-alkanes (n = 11 14) were measured at different temperatures and... 

Raman spectral-band frequencies for poly (thietane) as a crystallized melt annealed at 62° satisfy the rules for a longitudinal acoustic-mode frequency for deducing thickness. The morphology and physical properties of isoprene thietane block copolymers have been studied. Potential energies have been calculated for helical- and glide-type conformations of poly (thietane), and the conformations of the homopolymer have been calculated. ... 

In the framework of the model, the difference Ei — E is within the acoustic phonon bandwidth, and that is why Skinner and Trommsdorff [21] reasonably assumed that the longitudinal acoustic modes could influence the tunneling proton transfer. The interaction potential (22) was chosen in the form of the deformation potential approximation (see Ref. 74)... 

The substrate (or adsorbent) was modelled in one dimension by a linear chain of five carbon atoms, (We could have chosen any other atom.) A terminal carbon atom represents a surface atom and the remaining atoms the bulk. There is only one force constant, the C-C stretching force constant (C—C of C2 12.16 mdyn A" ). The dynamics were treated by the Wilson GF method. The substrate then had four vibrations similar to longitudinal acoustic modes (LAM) ( 10.1.2) LAM1,170 cm LAM2, 330 cm LAM3, 455 cm LAM4, 535 cm. The calculated INS spectrum comprised four bands of equal, weak intensity. 

D.A. Braden, S.F. Parker, J. Tomkinson B.S. Hudson (1999). J. Chem. Phys., Ill, 429-437. Inelastic neutron scattering spectra of the longitudinal acoustic modes of the normal alkanes from pentane to pentacosane. 

Below 600 cm are the V5 in-plane C—C—C bending modes (longitudinal acoustic modes, LAMs), the V9 out-of-plane C—C—C bending modes (transverse acoustic modes, TAMs) and the 12 lattice modes. These comprise in-phase and out-of-phase pairs of librations about the three axes, giving six modes, three optic translation and three... 

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