CO desorption signals detected by the QMS for m/z = 28 during irradiations of the mixed CO:PHP and layered CO/PHP ices, baselined and offset for clarity. Irradiation energies, E_irr, are denoted above each spectrum. For better visual comparison, the desorption trace for the PHP mode irradiation of the layered ice with higher irradiation energy has been scaled by a factor of two-thirds. Each individual desorption peak corresponds to a 5 Hz macropulse of the FEL. The zero-time for irradiation is defined as the onset of the first CO desorption peak.