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

The overall goal of the following experiment is to measure and analyze time resolved spectral signatures of atom and or molecule emissions following generation of a laser induced spark. This is achieved by focusing pulsed laser radiation at a wavelength of 1064 nanometers onto a target region for breakdown in air or sample ablation. As a second step, adjust the time delay from optical plasma generation of the detector to measure temporally, resolved atomic and or molecular spectra.

Next, wavelength and intensity calibrate the recorded data in order to prepare experimental results. For comparison with theory predictions results are obtained that show the technique can be used to determine temperature from diatomic recombination spectra, and study the evolution in time of atomic and molecular species in the plasma. The main advantage of this technique over existing methods like electrical discharge, is that laser induced optical breakdown is relatively easy to perform.

This method can help answer key questions in the combustion field, such as laser ignition of combustible liquids, solids and gases. Atomic and molecular spectra are also helpful in understanding combustion processes. Though this method can provide insight into laser induced phenomena, it can also be applied to other systems such as pulse vapor deposition.

Moreover, detailed spectral analysis can aid an understanding of astrophysical plasma. Generally speaking, individuals new to this method will struggle because of the requirements to accomplish high accuracy in the wavelength, calibration and intensity calibration to accomplish good, accurate timing and to get a good signal to noise ratio in the experimental data. Visual demonstrations of this method is critical.

As early time measurement steps are difficult to learn because of various competing plasma dynamics phenomena. The components of this experiment are set up across two optical benches. The laser is on one bench, the spectrometer is on the other.

An outline of this setup is shown in this schematic to perform the experiments begin with a neodymium YAG laser and direct its output to a beam splitter that allows 1064 nanometer light to pass through. Use a PIN hive speed photo detector to record a portion of the laser radiation reflected off the beam splitter. Connect this photo detector to an oscilloscope with a coax cable to monitor the optical pulse.

After the beam splitter, a line infrared reflective mirrors to position the beam parallel to the slit of the spectrometer. This mirror directs the beam parallel to the slit and toward a focusing element. Use a lens on a translation stage to focus the beam to generate optical breakdown plasma parallel to the spectrometer slit.

Next, align two quartz lenses to image the plasma on the slit for measurements above 380 nanometers. Position a cut on filter between the two lenses. Once a plasma is created, its spectrum is dispersed by the spectrometer.

The spectrum is recorded by an intensified linear diode array that is connected to an optical multichannel analyzer. Prepare for data acquisition by connecting a function generator, producing a triangular wave at 50 hertz to a divide by five circuit sink box. Use the 10 hertz signal from one output of the divide by five circuit as input to a digital delay generator.

Use one output of the digital delay generator to synchronize the neodymium YAG flash lamps to operate at 10 hertz. Use another output from the digital delay through the sink box to control the triggers of the diode array intensifier and the optical multichannel analyzer operating at 50 hertz. Next, relay the adjustable trigger output of the laser to an oscilloscope and a pulse generator.

Then connect the outputs of the pulse generator to the intensified linear diode array and the oscilloscope. Finally, connect the intensified linear DDE to the optical multi-channel analyzer. Take the necessary safety precautions for working with lasers.

Begin measurements by setting the function generator to output a 50 hertz triangular pulse. Prepare the laser for operation by initiating its cooling system and power supply. Then activate it on the digital pulse generator.

Set the gate width to specify how long the DIO array is exposed to plasma radiation. Also set the delay time, which determines how long after breakdown to wait for data collection. Begin optical breakdown in air image the microplasma onto the spectrometer slit and record the data.

Once plasma data is collected, turn the laser off to begin calibration of the optical system, including the spectrometer, the DIO array, and the optical multi-channel analyzer to correct for nonlinearities. In the plasma measurements place standard calibration lamps at the place where plasma was formed. Fit the collected data with known wavelengths to obtain pixel wavelength correspondence in spectral regions of interest to start intensity calibration.

Turn on a tungsten lamp and allow it to warm up. Then use an optical porometer to measure its temperature. Next, place the lamp where the plasma was created and use the apparatus to record its spectrum.

Use the measured temperature to compute a black body curve and compare it to the measured spectrum for each spec region in which the spectrometer was used. Determine the ratio of the ideal to the recorded intensities as a function of wavelength. Apply these factors to correct recorded spectra in subsequent data analysis.

Here, the apparatus was used to record the hydrogen balmer alpha line in 107 kilopascal gaseous hydrogen. The spectra of 1000 consecutive optical breakdown events are resolved along the spectrometer slit height for the time delay of 42 nanoseconds. The maximum red shift is 1.2 nanometers.

The maximum electron density is 0.32 times 10 to the 25th electrons per meter cubed. Shown here is the spectrum for the titanium oxide electronic transition from the first excited triplet PHI state to the ground state with vibrational level differences equal to zero. The measured spectrum in black was recorded using a time delay of 95 microseconds, and a gate width of two microseconds in red is a fit to the data from an elder mead fitting algorithm using a spectral resolution of 0.09 nanometers and a temperature of 3, 300 kelvin.

As another example. This is the rotational vibrational spectrum of aluminum monoxide. The transition is from the second excited doublet sigma plus states to the ground state with vibrational level differences equal to zero.

Again, the measured spectrum is in black. In this case, the time delay was 45 microseconds and the gate width five microseconds. The fitted spectrum in red was computed using a spectral resolution of 0.09 nanometers and a temperature of 3, 900 kelvin.

When performing this procedure, it is important to record key laser radiation parameters, such as the pulse width, the pulse shape at focus, and the energy prop pulse. It is also important to calibrate and recalibrate the recorded data Following this procedure. Other methods, such as laser induced fluorescence can be used to answer additional questions such as the spatial distribution of species in the plasma.

After its early development. This technique bathed its way for laser induced spectroscopy to analyze samples remotely, for example, to analyze martian rocks on Mars. After watching this video, you should have a good understanding of how to record time resolved.

Spectra Don't forget that laser radiation can be extremely hazardous and that precautions such as wearing laser protective eyewear should always be taken while the laser is on.