Visible aluminum monoxide emission during long pulse mid-infrared ablation of aluminum in air

Abstract

The long pulse ablation of aluminum wafers in ambient air using an Er:YAG laser at 2.94 μm wavelength is presented. Visible emission spectra collected during ablation are assigned to the B2Σ+→X2Σ+ molecular electronic transition of aluminum monoxide (AlO). A rovibronic model including self-absorption within the plume is developed to determine the molecular temperature. For a 60.2 μs pulse at a fluence of 249.92±40 J/cm2, a temperature of 2843±32 K and 3013±30 K was found with the linear and nonlinear models, respectively. A greybody background with an emissivity of approximately 1.3×10−6 was observed, implying a low-volume mixing fraction of particulates in the plume. A linear fit of the Δv=−1 sequence was developed to rapidly analyze hundreds of spectra taken as background pressure was varied from 400 to 1000 mbar. The AlO temperature is approximately independent of background pressure. Finally, comparisons to other laser ablation studies at shorter wavelengths and shorter pulse durations are made wherever possible.