Photolysis of Formaldehyde at 1470 and 1236 Å

The photolysis of formaldehyde vapor has been examined at 1470 and 1236 Å. Mixed isotope (H₂CO+D₂CO) and radical scavenger (C₂D₄) experiments have been utilized in addition to inert gas pressurization and lamp intensity attenuation. The quantum yields for H₂ and CO formation are both equal to 2; and both are independent of pressure and lamp intensity at 1470 and 1236 Å. The primary quantum yield for molecular hydrogen elimination ${\mathrm{(\Phi }}_{\mathrm{II}})$ was determined from the scavenger results and also from the mixed isotope work. The two values obtained are in reasonably good agreement, viz., ${\Phi }_{\mathrm{II}}\text{\hspace{0.17em}=\hspace{0.17em}0.5}$ . The mechanism most consistent with the experimental observations is as follows: $${\mathrm{H}}_2\mathrm{CO}\mathrm{+h\upsilon \to }{\mathrm{H}}_2+\mathrm{CO},$$ $${\mathrm{H}}_2\mathrm{CO}\text{+hυ→2}\mathrm{H}+\mathrm{CO},$$ $$\mathrm{H}+{\mathrm{H}}_2\mathrm{CO}\to {\mathrm{H}}_2+\mathrm{HCO},$$ $$2\mathrm{HCO}\to {\mathrm{H}}_2\text{+2}\mathrm{CO},$$ where the quantum yields of primary process (II) and (III) are both equal to 0.5.