Changes to IR spectra observed during near-UV irradiation of C₄H₂ ice films. (a) Evolution of the differential absorbance (A_t − A_t=0) of the v₄ band of C₄H₂ as function of the time of irradiation at 355 nm at JPL. (b) Evolution of the differential absorbance of the v₄ band of C₄H₂ as function of the time of irradiation at λ > 300 nm at PIIM. (c) Evolution of the differential absorbance of the v₅ band of C₄H₂ as function of the time of irradiation at 355 nm at JPL. (d) Evolution of the differential absorbance of the v₅ band of C₄H₂ as function of the time of irradiation at λ > 300 nm at PIIM.