The primary process in the photolysis of hexafluoroacetone vapour II. The fluorescence and phosphorescence

Abstract

The relative quantum yields for the total and for the fluorescent emissions were studied over a wide range of ketone pressures with excitation at 254, 265, 313 and 366 nm. A spectral area technique showed that the absolute value of the ratio of the quantum yield for phosphorescence to that for fluorescence was zero at zero ketone pressure, but increased to ca. 6.1 at infinite ketone pressure; the limiting high-pressure value did not depend on whether excitation was at 265 or 313 nm. Quinine sulphate actinometry was used to determine an absolute value of $\varphi^f_\infty$ = 0.018$_5$ for $\lambda_{excit.}$ = 265 or 313 nm; thus $\varphi^p_\infty$ was 0.11$_3$. With $\lambda_{excit.}$ = 254 nm a reasonable assumption was required to reach the same values. The quantum yield for the phosphorescence of biacetyl vapour was re-determined as 0.16$_8$ by the same technique. $\varphi^f$ did not extrapolate to zero at zero ketone pressure; hence emission must also occur from vibrationally excited molecules in the singlet n$\pi^*$ state. The detailed shapes of the plots of $\varphi^f$ and $\varphi^p$ against pressure are reported here but are discussed in part V, where the question of the mechanism of the vibrational relaxation in the excited singlet state is considered. Because the quantum yields obtained with excitation at 366 nm were independent of pressure, these data are qualitatively different from those obtained at shorter wavelengths and are discussed in the present paper.