TEYs near the O K-edge measured for a pure HCOOH ice at 90 K (top left panel) and 15 K (top right panel), a ¹³CO:HCOOH ice−1:1 ice at 15 K (bottom left panel), and a H₂O:HCOOH−1:1 ice at 15 K (bottom right panel). The displayed photon fluence represents the fluence received by the ice prior to measurement, except for cases in which it is indicated as <1 × 10¹⁵ photons cm⁻². In these instances, the TEY was measured immediately after deposition and the value of 1 × 10¹⁵ photons cm⁻² corresponds to the fluence received by the ice at the end of the TEY scan. The ice thickness is equivalent to ~100 ML. The top left panel also shows the X-ray absorption spectrum of gas phase HCOOH taken from Prince et al. (2003).