TRIPLET-SINGLET EMISSION SPECTRA IN CRYSTALLINE TOULENE AT $4^\circ K$ AND $77^\circ K$ AND IN EPA SOLUTION AT $77^\circ K.^{*}$
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Date
1957
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Ohio State University
Abstract
“A study of the triplet-singlet emission spectra of toluene crystals at $4^\circ K$ and $77^\circ K$ showed that the spectra are different at the two temperatures. At $4^\circ K$ a spectrum of sharp bands appears with a 0.0 transition at $28 785 cm^{-1}$. This spectrum has been analyzed. There is also a diffuse spectrum underlying the sharp bands. The diffuse spectrum looks like the toluene spectrum obtained in rigid EPA solution, except for a small shift of the latter to shorter wave lengths. Our explanation is that the sharp spectrum belongs to the crystal and the diffuse spectrum to the amorphous state or to a conglomerate of extremely small crystals. The crystal spectrum at $77^\circ K$ looks entirely different. It consists of seven rather broad diffuse bands going from $25290 cm^{-1}$ toward longer wavelengths. By slowly raising the temperature above $4^\circ K$ while keeping exposure times and sector rotation constant, we obtained between 20.0 and $22.7^\circ K$ the diffuse “short $\lambda$” spectrum and only a few of the sharp bands. From $24.5^\circ K$ on the spectrum has changed. It begins with a line-like band on the short wave length side of the first diffuse band which is also the first band of the $77^\circ K$ crystal spectrum, while the “short $\lambda$” spectrum is entirely gone. The life times of the two sets of spectra at $4^\circ K$ and above $25^\circ K$ are also different: the short $\lambda$ spectrum has a life time of 8 10 sec., and the long $\lambda$ spectrum has a life time of 0.2-1 sec. The sharp bands of the second set disappear when the temperature is raised still further, and the diffuse bands become identical with those of the crystal spectrum at $77^\circ K$. Since the $77^\circ K$ crystal spectrum has no analog in EPA solution, it is not characteristic of the toluene crystal. The question arises then whether it is due to an impurity or whether its carrier could have been produced photochemically. Since great care was taken in purifying the samples, the possibility of photochemical reactions under the influence of light illumination was examined. If produced photochemically, the carrier of the “long $\lambda$” spectrum must be formed at the cost of the carrier of the “short $\lambda$” spectrum. Experiments were carried out in which toluene crystals were subjected to a long illumination at $77^\circ K$ during which a few exposures were taken. Immediately after the illumination the crystal was transferred to the liquid helium container. The result was that the “short $\lambda$” spectrum-sharp and diffuse bands-appeared again, but the “long $\lambda$” spectrum was absent. As will be discussed, these results indicate that a higher temperature is needed for the sensitized excitation of an impurity which is photochemically formed.”
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$^{*}$Supported by the National Science Foundatiol
Author Institution: Department of physics, Duke University
Author Institution: Department of physics, Duke University