Difference FTIR spectra of the CH₂CHCHO/Cl₂/para-H₂ sample illustrating the effect of the sequential 365 nm (60 min) + 2217 nm (100 min) irradiation (a), free sample standing after photolysis for 480 min (b), 365 nm (70 min) photolysis (c), 2217 nm (100 min) photolysis (d), 400 nm (20 min) photolysis (e), 330 nm (20 min) photolysis (f), 260 nm (20 min) photolysis (g), and 225 nm (20 min) photolysis (h). We note that the center-left panel is multiplied by 0.3 for a better representation. The absorption features attributed to the trans-CH₂CHCHO and cis-CH₃CH₂CHO are marked with “a” and “p”, respectively. The unassigned absorption features are labeled with question signs.