Anomalous infrared absorptance of aluminum under pulsed 10.6‐μm laser irradiation in vacuum

Above a distinct threshold laser flux of 70‐MW/cm2 peak (for a pulse consisting of a 150‐ns spike and a 1.8‐μs tail), the calorimetrically measured effective absorptance of unpolished 3003 alloy aluminum increases abruptly to roughly 14%. While too high to be ascribed to ordinary metal infrared absorptance at any reasonable temperature, this value is typical of total‐energy thermal coupling in air with air plasma ignition. We propose that this absorptance is in fact due to the ignition of a plasma in metal vapor produced by thermally isolated metal defects, rather than to a metal‐dielectric transition.