Laser diagnostics for ro-vibronic excitations in plasmas at atmospheric pressures

Pulsed plasmas at atmospheric pressure have recently attracted increasing interest for a variety of applications. Although the detailed physics and chemistry can vary considerably between different types of plasmas, a common fundamental aspect is the generation of ro-vibrationally excited states in molecules due to electron collisions. Often this primary process is augmented by collisional excitation transfer between molecules. In either case, these excited states can significantly alter the dissociation rate of molecular species in the bulk and/or transfer internal energy to surfaces. Therefore, the ability to non-invasively study the ro-vibrationally excited states with high temporal (ns) resolution is of paramount importance. Depending on the symmetry properties of the relevant states, these allow either electromagnetic dipole or Raman transitions. For more complicated molecules, such as CO2, coupling between these modes is also possible. The first mode allows absorption measurements, the second Raman scattering. In this talk, tunable diode laser absorption spectroscopy (TDLAS) with quantum cascade lasers and broadband coherent anti-Stokes Raman scattering (CARS) will be presented. In addition to the general aspects of these complementary techniques, applications in CO2 and N2 ns-pulsed plasmas are shown [1-2].  

[1] J. Kuhfeld et al., J. Phys.D: Appl. Phys. 54, 305204 and 54305205 (2021).

[2] Yanjun Du et al., J. Phys. D: Appl. Phys. 54, 34LT02 and 365201 (2021).