Aluminum Monoxide Emission Measurements Following Laser-Induced Breakdown for Plasma Characterization

In this work, spectroscopic emissions from laser ablated aluminum samples are used to characterize the time dependent decay of laser-induced plasma. The plasma is created by tightly focusing nanosecond pulsed laser radiation. Time resolved measurements of the plasma are made with a gated, intensified linear diode array coupled to an optical multichannel analyzer and/or an intensified charged coupled device. Time resolution is achieved by synchronizing the laser with the measurement rate of the array detector.

Computed diatomic molecular aluminum monoxide emissions were used to infer the temperature of the plasma as a function of time. This was completed by comparing experimentally collected spectra to theoretical calculations with a Nelder-Mead algorithm. The theoretical spectra were calculated from accurate line strengths for selected aluminum monoxide bands. The temperature of the plasma was found to decrease from typically 5100 Kelvin to 3600 Kelvin from 10 to 90 microseconds after optical breakdown. The temperature appears to plateau to a temperature of 3800 Kelvin after 90 microseconds. Error analysis in the inferred temperature is accomplished with the fitting algorithm and the precision was found to be between 45 and 75 Kelvin. Gated camera measurements were performed to infer the temperature along the height of the plasma and found the temperature profile of the plasma to increase above the plasma edge. Superposition of hydrogen Balmer series beta emissions with aluminum monoxide spectra allow one to infer electron number densities from the plasma at time delays preceding 20 microseconds.