Michael J. Witte

Measurement and analysis of atomic hydrogen and diatomic molecular AlO, C2, CN, and TiO spectra following laser-induced optical breakdown.

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement is used. Here we operate a Nd:YAG laser at a frequency of 10 Hz at the fundamental wavelength of 1,064 nm. The 14 nsec pulses with anenergy of 190 mJ/pulse are focused to a 50 µm spot size to generate a plasma from optical breakdown or laser ablation in air. The microplasma is imaged onto the entrance slit of a 0.6 m spectrometer, and spectra are recorded using an 1,800 grooves/mm grating an intensified linear diode array and optical multichannel analyzer (OMA) or an ICCD. Of interest are Stark-broadened atomic lines of the hydrogen Balmer series to infer electron density. We also elaborate on temperature measurements from diatomic emission spectra of aluminum monoxide (AlO), carbon (C2), cyanogen (CN), and titanium monoxide (TiO). The experimental procedures include wavelength and sensitivity calibrations. Analysis of the recorded molecular spectra is accomplished by the fitting of data with tabulated line strengths. Furthermore, Monte-Carlo type simulations are performed to estimate the error margins. Time-resolved measurements are essential for the transient plasma commonly encountered in LIBS.

Hydrogen Alpha Self-Absorption Effects in Laser-Induced Air Plasma

Time-resolved spectroscopy measurements of the hydrogen alpha Balmer series line following laser-induced optical breakdown in laboratory air are designed to investigate in detail the determination of electron density from Stark-broadened spectral line shapes. Comparisons of results obtained from Hβ and Hγ lines indicate higher electron density inferred from Hα early in the plasma decay, suggesting self-absorption occurs. However, detailed comparisons for time delays of 300 and 400 ns after optical breakdown reveal the minute extent of self-absorption in air breakdown experiments from (i) differences of electron density determined from the N+ lines and the Hα line, and/or from (ii) differences in recorded data sets with/without the mirror for the various time delays in the experiments.

Hydrogen Balmer Series Measurements in Laser-Induced Air Plasma

Time-resolved spectroscopy is employed to analyze micro plasma generated in laboratory air. Stark-broadened emission profiles for hydrogen alpha and beta allow us to determine plasma characteristics for specific time delays after plasma generation. Stark shift, asymmetry, and full width half maximum measurements are used to infer electron density. The measurements of hydrogen alpha and beta Balmer series line shapes are analyzed using various theory results. Our laser-induced breakdown spectroscopy arrangement uses a Q- switched Nd:YAG laser operating at the fundamental wavelength of 1064 nm that is focused for plasma generation. The hydrogen alpha and beta lines emerge from the free electron background radiation for time delays larger than 0.3 ps and 1.4 ps, respectively. Neutral and ionized nitrogen emission lines allow us to infer electron density for time delays from 0.1 to 10 μs. The electron density values are compared with results obtained from hydrogen Balmer series line shapes.

Carbon Swan Spectra Measurements Following Breakdown of Nitro Compound Explosive Simulants

Our measurements of micro-plasma following laser-induced optical breakdown of nitro compound explosive simulants, here 3-nitrobenzoic acid, show well-developed molecular spectra during the first several hundreds of nanoseconds. Analysis of recorded carbon spectra is accomplished using accurate line strengths for the diatomic molecular Swan system. Presence of hydrogen-beta allows us to infer electron density in the plasma evolution. Computational challenges include accounting for background variation and appropriate modeling of hydrogen embedded in molecular spectra. Recorded and computed spectra agree nicely for time delays on the order of 1.6 μs from optical breakdown when using a single temperature for local thermodynamic equilibrium plasma.

Laser-Induced Spectroscopy of Graphene Ablation in Air

Carbon Swan spectra are observed following laser ablation of graphene in laboratory air. Previous experiments showed temperatures that ranged from 4500 to 7500 K for the Δv = 0 transition and 4200 to 4500 K for the Δv = −1 transition for time delays on the order of 1.6 μs to 70 μs. This experiment explored in greater detail time delays > 10 μs for both molecular bands. Temperatures were found to be similar, ranging from 4500 to 6700 K for the Δv = 1 transition and 3200 to 5500 K for the Δv = -1 transition. Investigation is also made into spatially resolving the plasma emissions along the slit height. In addition, efforts are made to investigate the applicability of the local thermodynamic equilibrium (LTE) assumption. Comparisons are discussed in view of previous work that utilized Stark broadening of the Hβ line, confirming LTE for delays < 10 μs, yet further research needed for later delays.

Emission spectroscopy of nitric oxide in laser-induced plasma

In this paper, we investigate the laser-induced breakdown spectra of nitric oxide. Nitric oxide spectra are studied from laser-induced plasma emissions from plasma initiated both in laboratory air. Temperatures are inferred from the spectroscopic emissions using two methods. Spectra are fit to theoretical calculations of the diatomic spectra using the method of diatomic line strengths. For a time delay of 25 μs the temperature is found to be 6800 Kelvin. Comparisons are also provided to previously determined temperatures using a non-equilibrium air radiation fitting (NEQAIR) program.