Coherent microwave transient spectroscopy

Arjavalingram, G., Theophilou, N., Pastol, Y Kopcsay, G.V. and Angelopoulos, M. (1990) Anisotropic conductivity in stretch oriented polymers measures with coherent microwave transient spectroscopy. J. Chem. Phys., 93, 6-9. 

Notably, measurements of the electrical conductivity in stretch-oriented polymers such as l2-doped polyacetylene, by employing the technique of coherent microwave transient spectroscopy [105] revealed a large orientation-dependence. In the case of polyacetylene, the conductivity values parallel and perpendicular to the stretch direction were cT = 34.4 and cry = 3.3 Scm [106]. The higher cT -value was believed to be due to the higher electron mobility along the oriented polymer chains, cr-values have been obtained from transmission data with the aid of Equation (1.18) ... 

Fig. 5. Pulsed-nozzle FT microwave measurements. A molecule-radiation interaction occurs when the gas pulse is between mirrors forming a Fabry-Perot cavity. If the transient molecule has a rotational transition of frequency vm falling within the narrow band of frequencies carried into the cavity by a short pulse (ca. 1 (is) of monochromatic radiation of frequency v, rotational excitation leads to a macroscopic electric polarization of the gas. This electric polarization decays only slowly (half-life T2 = 100 (is) compared with the relatively intense exciting pulse (half-life in the cavity t 0.1 (is). If detection is delayed until ca. 2 (is after the polarization, the exciting pulse has diminished in intensity by a factor of ca. 106 but the spontaneous coherent emission from the polarized gas is just beginning. This weak emission can then be detected in the absence of background radiation with high sensitivity. For technical reasons, the molecular emission at vm is mixed with some of the exciting radiation v and detected as a signal proportional to the amplitude of the oscillating electric vector at the beat frequency v - r , as a function of time, as in NMR spectroscopy Fourier transformation leads to the frequency spectrum [reproduced with permission from (31), p. 5631.

---