Knowledge Bank

The absorption and emission spectrum of formaldehyde was studied in xenon, krypton and sulfur hexafluoride, and without solvent, at temperatures between $20\circ K$ and $200\circ K$ in the spectral region of $2600\backslash AA$ to $6000\backslash AA$. The $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A\leftarrow 1A$ absorption is broad in all cases, and the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}A\leftarrow 1A$ system is too weak to be studied in detail. Strong emission in the $3800\backslash AA$ to $5400\backslash AA$ region is noted. In pure formaldehyde the spectrum is due to fluorescence of $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A\to 1A$; in matrices, to simultaneous and overlapping fluorescence and phosphorescence. Phosphorescence is caused by intersystem crossing. The emission spectrum is independent of the exciting frequency between $2700\backslash AA$ and $3500\backslash AA$. The phosphorescence yield increases with solvent polarizability and decreases with increasing temperature. In $SF6$ at $20\circ K$ only 5% of the total emission is due to induced phosphorescence. The fluorescence resembles the gas phase spectrum and is analyzed accordingly. Krypton spectra, due to partial diffusion of formaldehyde during deposition, exhibit fluorescence of pure formaldehyde, and fluorescence and phosphorescence of matrix isolated molecules. The lifetime of the phosphorescence is 0.23 seconds in xenon, 1.1 seconds in krypton, and 0.63 seconds in $SF6$. The intensity of the long-lived lifetime component and the reversible temperature dependent component of the emission intensity were used to synthesize the phosphorescence spectrum.