Reflection Fourier transform infrared spectroscopy of polymer targets for CO2 laser ablation

One of the many challenges faced by laser propulsion is the long-term performance of the propellant. The chemical changes that can take place at the propellant surface during ablation can greatly modify the in-flight performance characteristics. For stable regimes for propulsion, such chemical action should be minimized. A TEA (Transverse Electrical discharge in gas at Atmospheric pressure) CO2 laser of 10.6 mu m wavelength, 300 ns pulse length, and up to 20 J pulse energy was used to ablate several types of polymer targets with a range of observable chemical changes at the surface following ablation. After 10 subsequent shots, the target samples were measured using Attenuated Total Reflectance Fourier Transform Infrared (ATR FTIR) spectroscopy then compared to unablated samples of the same polymer. An analysis of the results was made with an emphasis on laser propulsion applications, with a comparison of the propulsion performance of the targets, specifically regarding the ablated mass per spot area (Delta m(a)). Chemical reaction pathways for combustion and vaporization are discussed on the basis of the differences observed in the FTIR spectra, and the consequences for using such materials as laser propulsion propellants are explored.