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

Summary

Time-resolved atomic and diatomic molecular species are measured using LIBS. The spectra are collected at various time delays following the generation of optical breakdown plasma with Nd:YAG laser radiation and are analyzed to infer electron density and temperature.

Abstract

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.

Introduction

Laser-induced breakdown spectroscopy (LIBS) techniques^1-5 have applications in atomic^6-12 and molecular studies of plasma^13-20 generated with laser radiation. Time-resolved spectroscopy is essential for determination of the transient characteristics of the plasma. Temperature and electron density, to name but two plasma parameters, can be measured provided a reasonable theoretical model of the plasma breakdown is available. Separation of free-electron radiation from atomic and molecular emissions allows us to accurately explore transient phenomena. By focusing on a specific temporal window, one can “freeze” plasma decay and thereby obtain accurate spectroscopic fingerprints. LIBS has a variety of applications and recently interest in LIBS-diagnostics shows a considerable increase when measured by the number of researchers publishing in the field. Pico- and femtosecond generated microplasma is of ongoing research interest, however, historically experimental LIBS arrangements utilize nanosecond laser radiation.

Figure 1 displays a typical experimental arrangement for laser-induced breakdown spectroscopy. For this protocol, the functional breakdown energy for the initial beam is on the order of 75 mJ pulse, at the infrared wavelength of 1,064 nm. This pulse energy can be adjusted as needed. The plasma is dispersed by the spectrometer and measured with an intensified linear diode array and OMA or, alternatively, imaged onto an Intensified 2-dimensional Charge Coupled Device (ICCD). Figure 2 illustrates the timing diagram for time-resolved experiments: synchronization of pulsed laser radiation with readout, laser pulse trigger, laser fire, and gate-open delay.

Successful time-resolved spectroscopy requires various calibration procedures. These procedures include wavelength calibration, background correction, and most importantly, sensitivity correction of the detector. Sensitivity corrected data are important for the comparison of measured and modeled spectra. For an increase of signal-to-noise ratio, multiple laser-induced breakdown events are recorded.

Protocol

1. Optical System Setup

1. Place a beam splitter at the exit of the laser, allowing the 1,064 nm wavelength light to pass through and to reflect all other transient laser radiation into a beam dump.
2. Place a high-speed PIN photodiode detector to record a portion of the laser radiation reflected off the beam splitter. Connect this detector to the oscilloscope with coaxial cable to monitor the optical pulse with respect to triggering by the function generator and occurrence of the Q-switching in the Nd:YAG laser device.
3. Align three IR reflective mirrors to position the beam parallel to the slit of the spectrometer.
4. Position a lens above translational stage to focus the beam in order to generate optical breakdown plasma parallel to the spectrometer slit. Align two quartz lenses for the purpose of imaging the plasma onto the slit. The two focusing lenses optimally match the spectrometer design, meaning the final lens has an aperture to accomplish a f# identical with the f# of the spectrometer's internal optics.
5. For measurements above 380 nm, position a cut-on filter between the two lenses for the purpose of blocking radiation below 380 nm. The cut-on filter suppresses possible UV contributions (due to 2nd order of grating) to the measured spectra.

2. Data Acquisition Setup

1. Connect a wave-form function generator providing a triangular wave at 50 Hz to a custom built divide-by-five circuit to obtain 10 Hz. The optical multichannel analyzer (OMA) is operated at 50 Hz and the flash lamps of the Nd:YAG laser are synchronously operated at 10 Hz. One can use an ICCD in place of the OMA, operating synchronously at the rate of the pulsed laser radiation as well.
2. Connect one of the outputs of the custom built divide-by-five circuit to a digital delay generator. Use one output to synchronize the Nd:YAG flash lamps and another output to control the triggers of the linear diode array intensifier and optical multichannel analyzer. Again, instead of the intensified linear diode array and OMA one can use an ICCD.
3. Relay the adjustable trigger output of the laser device to an oscilloscope and to a pulse generator. The oscilloscope will be used to monitor when the pulsed laser radiation will be available for optical breakdown generation or laser ablation.
4. Connect the high voltage output of the digital pulse generator to the intensified linear diode array.
5. Connect the other output of the pulse generator to the oscilloscope.
6. Connect the intensified linear diode array output to the OMA.

3. Synchronization and Measurement

1. Set the wave-form function generator to output a triangle pulse operating at 50±1 Hz. This function generator provides the master frequency. A custom built divide-by-five circuit and a digital delay generator are used for accurate synchronization.
2. Initiate water cooling system and power supply for laser device. Activate laser.
3. Determine the time for laser radiation to travel from the exit aperture of the Nd:YAG laser to the area in front of the spectrometer slit as follows: Measure the distance of the light path and compute the transit time using the speed-of-light. Account for this transit time in setting the gate delay time in next step.
4. On the digital pulse generator, set the gate width for the measurement and the delay time from optical breakdown or laser ablation pulse, and use the oscilloscope to monitor the delay time. The delay time will determine how long to wait for data collection after breakdown occurs. The gate width determines how long the diode array is exposed to plasma radiation.
5. Generate optical breakdown in air and/or place a sample on the translational stage such that ablation will occur. Image the microplasma onto the spectrometer slit.
6. Begin measurements and record data with the intensified linear diode array and optical multichannel analyzer (or with an ICCD).

4. Wavelength Calibration

1. Record spectra from standard calibration lamps: neon, mercury, and hydrogen lamps. Use the experimental arrangement with lamps put at the place where plasma was generated.
2. Using the known wavelengths from lamps, perform a linear or cubic fit to obtain the pixel-wavelength correspondence. The purpose of an accurate calibration is to correct for nonlinearities that are usually associated with measurement of spectra.
3. Repeat calibrations for H, C₂, CN, and TiO spectral regions of interest.

5. Intensity Calibration

1. Turn on a tungsten calibration lamp and wait for it to warm up.
2. Use an optical pyrometer to measure the temperature of the active lamp.
3. Use the experimental arrangement to record the spectrum of the active lamp.
4. Compute a blackbody curve from Plank's radiation law using the measured temperature as an input parameter.
5. Fit the computed curve to the spectrum of the active lamp. Determine the factors by which the recorded intensities from the computed curve. Apply those factors to correct recorded spectra for wavelength-dependent sensitivity of the detector.
6. Repeat this for each region the spectrometer was used.

6. Data Transfer

1. Prepare medium for file transfers.
2. For each data measurement, record it onto the medium.
3. Take this medium and upload the files on it to a work computer.

7. File Preparation

1. For each file, parse it into sections, one containing recorded data, and the others specifying starting wavelength and average wavelength shift per data point.
2. Use these sections to create a new file to match wavelengths with recorded data.

8. Diatomic Molecular Analysis

1. Select the wavelength file and corresponding line strength file.
2. Select the baseline offset.
   1. Set whether the offset is constant, linear, or quadratic.
   2. Set the corresponding coefficients to either fixed or variable values.
3. Set the resolution and temperature, both of which can be either fixed or varied.
4. Set the tolerance of fit for the synthetic spectra to be fit to the measured spectra.
5. Fit the computed to the experimental spectra using a Nelder-Mead algorithm.
6. Using the best fitting parameters of the computed spectra for each measurement, infer the microplasma parameters observed at the various time delays and gate widths used.

Results

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 ind...

Discussion

The time resolved measurement protocol and representative results are further discussed here. It is important to synchronize the laser pulses, generated at a rate of 10 Hz, with the 50 Hz operating frequency of the intensified linear diode array and OMA (or ICCD). Furthermore, accurate timing of laser pulses and opening of the gate of the intensified linear diode array (or alternatively ICCD) is essential. The wave generator, indicated in the experimental schematic, is used to synchronize the laser pulses and intens...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Custom Box	UTSI	None	Signal divider and conditioner. An oscilloscope can be used in place of this.
Four Channel Digital Delay/Pulse Generator	Stanford Research Systems, Inc.	Model DG535	Companies: Tequipment, diyAudio
Four Channel Color Digital Phosphor Oscilloscope	Tektronix	TDS 3054	500 MHz - 5 GS/sec, Companies: Amazon, Tektronix, Fluke, Agilent Technologies, Pico Technology
Wavetek FG3C Function Generator	Wavetek	FG3C	Companies: Tequipment, Stanford Research Systems, BK Precision
Nd:YAG Laser	Quanta-Ray	DCR-2A(10) PS	Laser radiation, Class IV.  Companies: Lambda Photometrics, Continuum, Ellipse, Newport
Si Biased Detector	Thorlabs	DET10A/M	200-1,100 nm, with ND10A reflective filter. Companies: Canberra, Edmund Optics
Nd:YAG Laser Line Mirror, 1,064 nm	Thorlabs	NB1-K13	Companies: Edmund Optics, Newport
1 in Fused Silica Bi-Convex Lens, uncoated	Newport	SBX031	Companies: Edmund Optics, Thorlabs
2 in Fused Silica Plano-Convex lens, uncoated	Newport	SPX049	Convex lens, f/4.  Companies: Edmund Optics, Thorlabs
Spectrograph	Instruments S.A. division Jobin-Yvon	HR 640	Companies: Andor, Newport, Horiba
Manual and electronic controller for Spectrograph	Instruments S.A. division Jobin-Yvon	Model 980028	Manual and electronic controller for Spectrograph
Mega 4000	Mega	Model 129709	Computer interface for Spectrograph
Gateway 2000 Crystal Scan 1024 monitor	Gateway	PMV14AC	Monitor for computer interface
20 MHz Oscilloscope	BK Precision	Model 2125	Companies: Amazon, Tektronix, Fluke, Agilent Technologies, Pico Technology
6040 Universal Pulse Generator	Berkeley Nucleonics Corporation	Model 6040	Companies: Agilent Technologies, Tektronix, Quantum Composers
Separate component to 6040 Universal Pulse Generator	Berkeley Nucleonics Corporation	Model 202 H	Separate component to 6040 Universal Pulse Generator
ICCD Camera	EG&G Parc	Model 46113	Companies: Andor, Standford Computer Optics, LaVision, Hamamatsu
OMA III	EG&G Parc	Model 1460	Spectral data acquisition and analysis. Unit discontinued, replaced by software installed on computers.