Fluorescence and Induced Phosphorescence of Formaldehyde in Solid Low-Temperature Solutions

The absorption and emission spectrum of formaldehyde was studied in xenon, krypton, and sulfur hexafluoride and without matrix between 20° and 200°K in the region of 2600–6000 Å. Matrix isolation of formaldehyde, even at $\mathrm{M/R}$ 1000, is dependent on solvent. The SF₆ yields more than 99% isolation, xenon 90%, and krypton only 50%. The $S_1{\mathrm{\leftarrow S}}_0$ absorption is broad in all cases, and the system $T_1{\mathrm{\leftarrow S}}_0$ is too weak to be studied in detail. Strong emission in the 3800–5400‐Å region is observed. The emission spectrum is independent of the exciting frequency between 2500 and 3500 Å. In pure formaldehyde it is due to fluorescence of $S_1{\mathrm{\to S}}_0$ only, and in matrices it is due to simultaneous and overlapping fluorescence and phosphorescence. The fluorescence resembles the gas‐phase spectrum and is analyzed accordingly. Phosphorescence is caused by intersystem crossing. The phosphorescence yield increases with solvent polarizability and decreases reversibly with increasing temperature until diffusion occurs. Phosphorescence yields are always smaller than 5% of fluorescence. The lifetime of phosphorescence is 0.23 sec in xenon, 1.1 sec in krypton, and 0.63 sec in SF₆. The phosphorescence spectrum is deduced from the intensity of the long lifetime component, and also from the reversible temperature‐dependent component of the emission intensity.