Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C₂, CN, and TiO Spectra Following Laser-induced Optical Breakdown

LIBS utilizes pulsed laser radiation to sufficiently ionize a sample to form plasma. Laser-induced breakdown of gaseous substances will create plasma that is centered about the focal region of the excitation beam, while laser ablation on solid surfaces will produce plasma above the sample's surface. The plasma is generated by focusing the optical radiation on the order of 100 GW/cm² for nanosecond breakdown pulses. To produce laser ablation plasma, typically 1 GW/cm² is more than sufficient. The induced plasma radiation is dispersed by a spectrometer. Only for specific delay times from optical breakdown, atomic and diatomic spectra are well developed. Figures 3-7 illustrate representative results.

Early in the plasma decay, atomic lines evolve as the free electron background radiation diminishes. Subsequently, primarily due to recombination, molecular spectra can be recorded. In the experiments, the intensifier of the linear diode array must be accurately triggered. Alternatively, an ICCD must be accurately triggered as well. This triggering (or timing) must occur consistently, so that acquired data are composed from contributions at an accurate delay time from the breakdown event. Subsequently, the OMA digitizes the signal from the intensified linear diode array and stores data. For an ICCD, timing is equally important and data is stored similarly. In the experiments with the intensified linear diode array, the OMA records only the first of five scans, the other four remaining scans are redundant but consecutive scans are ignored. Figure 2 illustrates how the representative intensified linear diode array results are obtained.

For investigations regarding atomic species, atomic lines are studied with respect to the spectral lines' shape and width. The measured atomic line shapes and widths are compared to corresponding theoretical models. This comparison allows one to determine the electron density in the observed region of the plasma. In investigations of diatomic molecular spectra, usually many spectral lines contribute to several molecular bands. For each measurement, the spectra are analyzed with respect to the shapes, widths, and relative intensities of the individual lines contained in the molecular bands. Comparison of the measured and computed spectra utilizes line strengths derived from diatomic quantum theory together with high-resolution spectral data that are available in the literature. This comparison allows one to infer an excitation temperature of the diatomic molecules in the observed region of the plasma. The fitting is accomplished with a Nelder-Mead algorithm, allowing one to perform a multiparameter, nonlinear fit of computed spectra to the measured spectra. In the following, results are presented for selected experiments that address diatomic molecular and atomic line spectra.

Figure 3 illustrates measured and fitted TiO spectra. When imaging certain areas of the TiO plume, the fitted temperature versus time shows a local minimum. This minimum is possibly due to combustion.

The primary analysis objective for the hydrogen spectra is to determine the electron density and temperature from the time-resolved spectra for the hydrogen-alpha and hydrogen-beta lines. Figure 4 shows hydrogen alpha profiles recorded along the slit height. Laboratory air species such as nitrogen and oxygen cause the hydrogen alpha lines to emerge after the initial optical breakdown event. The evolution of the hydrogen-alpha line is investigated for time delays of 0.4 µsec up to 30 µsec after the optical breakdown. The hydrogen alpha figure also indicates a higher electron density in the center of the plasma, displayed in the figure near the middle of the indicated slit axis.

The C₂ molecule shows that its lowest rotational levels are sensitive to temperature variation, and higher rotational levels are sensitive to the surrounding gas density and radiation field. Figure 5 displays a typical measured and fitted result following laser ablation in air. Analysis of the carbon Swan spectra includes studying the formation of C₂ due to recombination in an effort to understand the formation of C₂ and CN following laser ablation of graphene. Figure 6 shows recorded and fitted CN spectra, and the figure also shows excellent agreement with theory. The experimental spectra match well with the computed spectra, and convincing agreement can be seen in the trailing ends and band heads. Like the C₂ spectra, the rotation/vibration spectra of aluminum monoxide show well developed molecular bands for the elevated temperature in the decaying laser-induced microplasma. Figure 7 shows measured and fitted AlO spectra. Since AlO is an early product in the combustion of aluminum, spectroscopic studies AlO has applications in aluminum combustion studies, primarily for temperature determination.

Figure 1. The schematic arrangement shows the following experimental components: (1) Beam splitter to deflect a small portion of 1,064 nm laser radiation; (2) Infrared reflective mirrors to vertically align the beam in front of the spectrometer; (3) lens to focus the beam to a tight spot for optical breakdown; (4) the star symbolizes optical breakdown or laser ablation plasma; and (5) two lenses to match the focusing optics of the spectrometer. The sync box, wave, and delay generator accomplish electronic synchronization of the laser and intensified linear diode array and OMA (or ICCD) operating frequencies. The photodiode, pulse generator, and oscilloscope allow one to accurately control and monitor the measurement window for LIBS. Click here to view larger image.

Figure 2. Timing diagram for time-resolved LIBS measurements. OMA sync and laser flash lamp sync are provided by a wave and delay generator and custom-built circuitry. Laser pulse trigger and fire pulse is provided by the Nd:YAG laser device together with Q-switch delay control. An extra pretrigger, electronic pulse (not shown) is also provided by the Nd:YAG laser manufacturer. This pretrigger allows us to accurately and efficiently control the detector gate delay of the OMA/ICCD system. The OMA readout occurs subsequent to exposure of the diode array, including 4 subsequent but ignored readout scans. For single pulse experiments, individual OMA readouts are stored separately in memory. For 100-scan average measurements, the OMA readout scans are added and then stored. Click here to view larger image.

Figure 3. The measured and synthetic emission spectra show the TiO A³φ→X³Δ Δv = 0 transition. The measured data was collected with the intensified linear diode array at delay time of t_delay= 95 µsec with a gate width of 2 µsec. The measured spectra is fit with computed spectra corresponding to a spectral resolution of FWHM = 0.09 nm and an inferred temperature T = 3,300 K. Click here to view larger image.

Figure 4. Hydrogen Balmer alpha line recorded in 1.07 x 10⁵ Pa gaseous hydrogen. The emissions of 1,000 consecutive optical breakdown events were dispersed with the spectrometer and collected with an ICCD, 0.042 µsec from optical breakdown and spatially resolved along the spectrometer slit-height. For this time delay, the maximum electron number density is 0.32 x 10²⁵ electrons/m³, and the maximum Hα red shift amounts to 1.2 nm. Individual spectra of hydrogen alpha and beta lines are illustrated for example in Parigger⁶. Recording of hydrogen alpha and beta lines following breakdown in air is part of the documentary video of this JoVE publication. Click here to view larger image.

Figure 5. The C₂ Swan spectra were collected with the intensified linear diode array at a time delay t_delay = 20 µsec using a gate width of 1 µsec.  The measured spectrum was fit with computed spectra for a FWHM spectral resolution of 0.07 nm and a temperature of T = 5,600 K is inferred. Click here to view larger image.

Figure 6. The CN B²ε+→X²ε+ violet transition was recorded with the intensified linear diode array at time delay of t_delay = 70 µsec with a gate width of 4 µsec. The measured spectrum is fit with computed spectra using a FWHM spectral resolution of 0.09 nm. The inferred temperature is T = 6,600 K. Click here to view larger image.

Figure 7. The AlO B ²ε+→X²ε+ diatomic spectra for the Δ = 0 transitions were collected with the intensified linear diode array at a time delay of t_delayn= 45 µsec using a gate width of 5 µsec. The fitted spectra are computed using a FWHM spectral resolution of 0.09 nm. The inferred temperature amounts to T = 3,900 K. Click here to view larger image.