Laser-induced phonon spectroscopy

A legitimate comparison of the elastic anisotropy of different PLCs can be made by considering the behavior at the same relative temperature T = 0.9 At this temperature is about 

It is seen that ultrasonic techniques have two major advantages high accuracy and the ability to determine the complete set of elastic constants for a sample of small size. As a result of these capabilities, all five independent stiffness constants have been obtained for extruded rods of PLC of high draw ratio (2 = 15) and small diameter (0.8 mm). Moreover, it has been possible to study the skin-core structure in injection molded PLC by monitoring the variation in stiffness with position. For blends of a PLC and a thermoplastic or glass fiber-reinforced PLC, successful correlation has been obtained between the modulus data and the orientation of the PLC fibrils or glass fibers. 

There have been very few studies of the acoustic absorption of PLCs, probably because of the difficulty in interpreting the data. To date, absorption measurements have been carried out mainly to reveal the relaxation peak associated with the glass transition in side chain PLCs. 

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Laser induced phonon spectroscopy has been used to study the ultrasonic properties of nematic sidechain LCEs [47, 48], It has been shown that both the ultrasonic velocity and the attenuation show a strong anisotropy and it was concluded that the coupling between density changes and reorientation-al motion of the sidechains takes place on a time scale of 10" -10" s [47]. 

Ultrasonic experiments using laser induced phonon spectroscopy have been performed in a nematic liquid single crystal elastomer [48]. The experiments reveal a dispersion step for the speed of sound and a strong anisotropy for the acoustic attenuation constant in the investigated frequency range (100 MHz -1 GHz). These results are consistent with a description of LCEs using macroscopic dynamics [54-56] and reflect a coupling between elastic effects and the nematic order parameter as analyzed in detail previously [48]. 

Acoustic waves can be generated by optical pulses. Laser induced phonon spectroscopy is a technique applied to the study of molecular processes. In our case the 10-]um grating implies an acoustic period... 

Since the development of ultrashort lasers, nudear wavepacket dynamics of various matters have attracted continuing attention [1,2]. The research targets extend from gas phase molecules [3, 4] to molecules in solution [5, 6], and solids [7]. In general, an excitation of matter by an ultrashort pulse with sufficient bandwidth leads to the creation of coherence between vibrational (or vibronic) eigenstates [1]. The induced nuclear wavepacket then starts to evolve on a certain potential energy surface and the dynamics is probed by a suitable pump-probe spectroscopy. The direct time-domain observation of the nudear motion provides us with valuable information on photochemical reaction dynamics, vibrational excitation/relaxation mechanisms, electron-vibration (phonon) coupling, and so on. 

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