ANOMALOUS INFRARED ABSORPTANCE OF ALUMINUM UNDER PULSED 10.6-MU-M 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.