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....

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 a....

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.