Collision induced ultraviolet structure in nitrogen radar REMPI spectra

Kinetics Model of Femtosecond Laser Ionization in Nitrogen and Comparison to Experiment

A zero-dimensional kinetics simulation of femtosecond laser ionization in nitrogen is proposed that includes fast gas heating effects, electron scattering (elastic and inelastic) rate coefficients from BOLSIG+ and photoionization based on filamentation theory. Key rate coefficients possessing significant uncertainty are tuned (within the range of variation found in literature) to reproduce the time-varying signal acquired by a bandpass-filtered photomultiplier tube with good agreement up to several hundred nanoseconds. Separate spectral measurements calibrate the relative strength of signal components. Derived equations relate the model to experimental measurements in absolute units. Reactions contributing to the rate of change of important species are displayed in terms of absolute rate and relative fraction. In general, decreasing the gas density lengthens the duration of early reactions and delays the start of later reactions. The model agrees with data taken in a variable temperature and pressure free jet by an intensified camera. Results demonstrate that initial signal depends primarily on gas density and secondarily on gas temperature. The optimal (maximum) initial signal occurs at a gas density below atmospheric. Decreases in gas density alter the evolution of excited-state populations, postponing the peak (while reducing its value) and slowing the rate of decay. For the optimal case, populations are favorably shifted in time with respect to the gate delay (and width) to boost the signal. Reductions in gas temperature generally enhance initial signal due to elevated dissociative recombination of cluster ions (along with excited-state coupling from quenching and energy pooling).

New diagnostic methods for laser plasma- and microwave-enhanced combustion

The study of pulsed laser- and microwave-induced plasma interactions with atmospheric and higher pressure combusting gases requires rapid diagnostic methods that are capable of determining the mechanisms by which these interactions are taking place. New rapid diagnostics are presented here extending the capabilities of Rayleigh and Thomson scattering and resonance-enhanced multi-photon ionization (REMPI) detection and introducing femtosecond laser-induced velocity and temperature profile imaging. Spectrally filtered Rayleigh scattering provides a method for the planar imaging of temperature fields for constant pressure interactions and line imaging of velocity, temperature and density profiles. Depolarization of Rayleigh scattering provides a measure of the dissociation fraction, and multi-wavelength line imaging enables the separation of Thomson scattering from Rayleigh scattering. Radar REMPI takes advantage of high-frequency microwave scattering from the region of laser-selected species ionization to extend REMPI to atmospheric pressures and implement it as a stand-off detection method for atomic and molecular species in combusting environments. Femtosecond laser electronic excitation tagging (FLEET) generates highly excited molecular species and dissociation through the focal zone of the laser. The prompt fluorescence from excited molecular species yields temperature profiles, and the delayed fluorescence from recombining atomic fragments yields velocity profiles.