Thermal wave decay lengths

The infrared absorption coefficient and thermal wave decay coefficients, a(v) and flj, respectively, determine the magnitude of the photoacoustic signal. The term ot( exp —[a( +fls]x in the expression for temperature oscillation leads to a linear PA signal dependence on infrared absorption when a( sampling depth, penetration depth, or thermal diflusion depth. The sample layer extending a distance L beneath the surface contributes... 

Table 20.1. Approximale Thermal Wave Decay Lengths (pm) for Various Thermal DinDsivities"...

No matter what modulation frequency is used for PA/FT-IR spectrometry, the bands from the upper layers of the sample always dominate the spectrum. In addition, the fact that the thermal wave decay length varies as when the spectra are measured with a rapid-scanning interferometer has always led to suboptimal results. The variation of L with wavenumber may be circumvented through the use of a phase-modulated step-scan interferometer, and most contemporary PA/FT-IR spectra are now measured with this type of instrument. 

Formal 1 1 copolymers incorporating PTL ladder repeat units can be prepared in excellent yield by interfacial polymerization of ladder oligomer bis-acylhalides with neopentyldiamine. The resulting polymers are soluble, processible and are thermally stable to ca. 400 C. These new polymers exhibit values exceeding 10" esu at 532 nm, and the response of these materials at other wave lengths and the mechanism of DFWM decay is currently under investigations. 

The first criterion is most easily examined. In pure benzene vapor excited at 2536 A or longer wave lengths, both the quantum yield and the observed singlet state lifetime show only a very shallow dependence on benzene pressure above 10 torr. Addition of foreign gases such as hydrocarbons to these experiments also has little (but sometimes finite) effect on fluorescence yields " or lifetimes. Even in the extreme collisional limit of condensed phase at 77°K, the fluorescence yield of 0.2 matches that of the vapor. - (Condensed phase yields drop to about 0.05 at 300°K, but this is probably a special thermal effect somewhat apart from a collision-induced electronic decay. See Section IVC.)... 

Having established that radiative relaxation is essentially from a thermal vibrational distribution above 10 torr, it is now necessary to discover whether all nonradiative relaxation occurs from this distribution. The most sensitive test comes from examination of high-pressure fluorescence yields as a function of exciting wave length. If nonradiative electronic decay from all levels reached in absorption and during vibrational relaxation is truly slower than the vibrational equilibration, the quantum yields will be independent of exciting wave length. 

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