Visible & Infrared Laser Induced Breakdown Spectroscopy of Potassium based Energetic

Materials Deposited on SubstratesKeith Tukes

Hampton UniversityDepartment of Physics

Research Advisors:Uwe Hommerich

EiEi BrownEric Kumi-Barimah

Outline• Introduction

– Basic Idea of Laser Induced Breakdown Spectroscopy– Structure and Spectroscopy of Atoms– Structure and Spectroscopy of Molecules

• Research Objective• Experimental Details

– Sample Preparation: bulk pellets, films on substrates– Un-gated visible LIBS using a Miniature Spectrometer System– Infrared LIBS setup

• Results and Discussion– Pictures of prepared samples– Visible LIBS spectra in air and element identification– MIR LIBS spectra of bulk and thin film samples– LWIR LIBS spectra of bulk and thin film samples– Identification of IR atomic and molecular LIBS emissions

• Conclusions

What is LIBS?

Method of identifying the elemental composition of a sample by examining the spectrum of light created by striking a sample with a highly-focused laser pulse. The spark that is created contains blackbody, atomic, and molecular emissions.• Blackbody (Thermal radiation)

–> Broad-band emission (UV-VIS-IR)

• Atomic Emissions–> Narrow line emission (UV-VIS & IR)

• Molecular Emissions–> Broad-band emission (mainly IR)

Schematics of LIBS setup[1]

Research ApplicationsLIBS is an especially valuable technique for material identification but at the present, only information about the UV-Visible region has been collected. Current applications of LIBS include:• De-Mining- Detection and

identification of landmines and other hazardous objects.

• Robotic LIBS- Using rovers to identify materials on deep-space landings.

• Stand-off Detection of Explosive materials

• Underwater LIBS- Specifically its use in identifying archeological materials.

Advantages of LIBSLIBS is used in many different ways simply because of the versatility of the technique, but it also boasts additional advantages such as:• Minimal Sample Preparation• Spectra may be obtained from long

distances• Ability to analyze solids, liquids, &

gases• Minimal amount of material is required

for analysis• Simple and rapid collection of data and

analysis

[3]

[4]

[2]

Blackbody Radiation• Blackbody Radiation

– Thermal Radiation– Any object based on its temperature

emits blackbody radiation– Appears as broad-band spectra with

low resolution– The peak amount of radiation occurs

at different wavelengths depending on temperature

– Planck’s Equation gives the spectral radiance distribution as a function of the temperature T

– Blackbody emission is overlapping LIBS atomic emissions (-> gated spectroscopy needed)

1

1*

2)(

/5

2

kThce

hcI

T=300K T~5000K

Atomic Transitions• Bohr’s Model: The atom looks like the solar

system.• There exist discrete atomic energy levels

and states (orbits)• Atomic Emission

– Occurs due to electrons being excited into higher energy states (absorption)

– Following excitation, electrons return to the ground state by emitting photons (Emission)

Figure 2: Bohr’s Atomic ModelSpectroscopic notation

2

2( 13.6 )n

ZE eV

n

Z=1 for hydrogen

Molecular Transitions• Molecular Emission

– Slowest emissions– Broad bands (Infrared region)

• Molecular Transitions:– Create the smallest changes in the total

energy of a molecule– Total energy:

• Nuclear energy levels are quantized• Vibrational Energy

• Rotational Energy

• Schrodinger’s Equation is used to identify the energy of electron in atoms– Quantum numbers can be derived

from Schrodinger’s Equation

• Quantum Numbers describe the characteristics of electrons and their orbitals

• Principal Quantum Number (n)– Describes the amount of energy

the electron will have.

• Spectroscopic Notation– Notation that uses quantum

numbers to describe an electron• L: Total orbital angular

momentum• S: Total Spin• J: Total Angular Momentum

Quantum Numbers and the Quantum Mechanical Model

2

2( 13.6 )n

ZE eV

n hydrogen

Research Objectives

• Prepare bulk pellets and thin-films of potassium based compounds used as energetic materials (e.g. KClO3, KClO4). Use an air-spray technique to deposit films on substrate including aluminum, cement, asphalt, and glass.

• Determine amount of deposited material necessary to obtain sufficient emission signatures in LIBS.

• Use visible LIBS to identify atomic emissions from potassium based compounds as bulk materials and then as thin films. Use the NIST database to identify background emissions from substrate materials.

• Extend visible LIBS to the infrared region and use the NIST database to identify atomic and molecular emission wavelengths from bulk and thin-film materials. Identify background emissions from substrates. Use IR emission signatures to assist material identification.

Sample Preparation• Bulk Pellets

– Compressed pellets of the energetic material (i.e. KClO3)

Uncoated Substrates

•Substrate Films– Substrates coated with

energetic materials

Sample Press(7 metric tons)

Bulk Pellet(KClO3)

Energetic Material(KClO3)Weighed 5.2 g for each sample

Aqueous Solution(2 g KClO3)

Spray GunConnected to Nitrogen gas tank(25 psi)

Coated Substrates

Aluminum Substrate

Cement Substrate

Asphalt Substrate

ND:YAG Laser

Boxcar AveragerOscilloscope Computer

Sample Stage

Infrared Spectrometer

CF

Infrared Detector

Visible LIBS Spectrometer

Visible LIBS mount and detector

CF

• Pulsed Nd:YAG laser beam is directed and focused onto sample surface• Sample is struck by laser which results in ablation of material from surface• Emitted light is collected using collimating lenses and focused onto fiber or spectrometer• Visible LIBS:

• CaF2 Lenses, Optical fiber, miniature CCD array-spectrometer system, Ungated• Infrared LIBS:

• ZnSe lenses, grating spectrometer, IR detector (InSb)• Boxcar and Oscilloscope are use for gated LIBS detection

Experimental Details: UV-VIS & IR LIBS

Experimental Details: UV-VIS & IR LIBS

CaF2 Lenses

ZnSe LensesSample Stage

UV-Vis-NIR Fiber Optic Detector

IR SpectrometerLiquid Nitrogen Cooled IR Detector

Mounted Sample

ND:YAG Laser

Data Collecting Interface

Boxcar Averager

Spectrometer Controller

Sample Stage Controller

Visible-NIR LIBS spectrum of background air

Observed obs(nm)

Literature* lit (nm)

Element

399.5 399.5 N-II403.9 403.9 Ar-II444.9 444.9 Ar-II500.6 500.7 N-II567.7 567.9 N-II795.1 795.1 O-I868.4 868.3 N-I

300 400 500 600 700 800 900 1000 1100 1200

0

10000

20000

30000

40000

50000

60000

70000

Inte

nsity

(A

U)

Wavelength (nm)

N-II 399.5 nmAr-II 403.9 nm

Ar-II 444.9 nmN-II 500.6 nm O-I 795.1 nm N-I 868.4 nm

laser scatter

- Air spark shows a rich UV-VIS LIBS emission spectrum. - Several atomic emissions were identified including those of Nitrogen, Argon and Oxygen.

*NIST atomic database

AIR

Visible-NIR LIBS spectrum of bulk KClO3Pressed pellets:

800

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Inte

nsity

WaveLength (nm)

Aluminum Sample2 KClO3 (0.0374g) Sample3 KClO3 (0.0474g) Sample1 KClO3 (0.0593)

766.490nm 769.896 nm

Dominant Potassium Emission Lines:

• Bulk KClO3 sample shows dominant potassium emission lines at 766.490nm and 769.896 nm (consistent with NIST database)• Thin film of KClO3 also reveals potassium emission lines. In addition, strong emission lines at ~395nm from aluminum substrate was observed

300 400 500 600 700 800 900 1000 1100 1200

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Inte

nsi

ty

WaveLength (nm)

(K) 766.5 nm

Air

Aluminum

KClO3

KClO3 on Aluminum

MIR (2-4 µm) LIBS spectrum of KClO3: bulk sample and films on substrates

Pressed pellets

Film on Al-substrate

• MIR LIBS shows no emission features from background air.• MIR LIBS shows distinct potassium MIR atomic emission lines from bulk sample and from a thin film of KClO3 on aluminum substrate. NIST database shows K-emissions at: 2.7µm, 3.2µm, 4.03µm

Al-substrate

2000 2500 3000 3500 4000 4500

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Inte

nsity

Wavelength (nm)

Pure KClO3

Air

KClO3 on Aluminum

Aluminum

~2.7µm~4.0µm

~3.2µm

LWIR (4-12 µm) LIBS spectrum of KClO3: bulk sample and films on substrates

4000 6000 8000 10000 12000

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

Inte

nsity

(a.

u.)

Wavelength (nm)

Bulk KClO3

KClO3 film on Aluminum

Aluminum

FTIR spectrum

• Bulk pellet of KClO3 shows atomic and molecular LIBS emission features.• The molecular emission at ~10500nm matches IR absorption of chlorate anion (ClO3

-). • Molecular emission of KClO3 films is only weak from film, due to small sample amount.

~6.2µmatomic ~10.5µm

molecular

Conclusions• Bulk pellets and thin-film samples of KClO3 were prepared for UV-VIS and IR

LIBS studies. The thin-film samples were deposited on aluminum substrates using an air-spray system.

• A combined UV-VIS and IR LIBS experimental setup was carefully aligned for LIBS studies in air.

• Background air revealed strong UV-VIS atomic emission signatures from oxygen, nitrogen, and argon. Observed UV-VIS atomic emissions were identified using the NIST database. Background air did not reveal any emission in the MIR region from 2000-4000nm.

• KClO3 pellets and thin films exhibited characteristic potassium emission lines in the UV-VIS and IR region useful for sample identification.

• Initial LWIR LIBS were performed and showed indications of a molecular LIBS emission at ~11000nm due to the ClO3- ion.

• Additional studies using different substrate materials and different sample concentrations need to be carried out in the future.

Acknowledgment

• Dr. Hommerich• Dr. Brown• Dr. Kumi-Barimah• Hampton University Physics Department• Army Research Office• National Science Foundation

Results and Discussion

– Visible LIBS spectrum of background air– Visible LIBS spectra of KClO3 (bulk and film on

substrate)– Infrared LIBS spectrum of air– Infrared LIBS spectrum of KCLO3 (bulk and film on

substrate– Identification of atomic and molecular emissions

Laser-Induced Breakdown Spectroscopy (LIBS) is a valuable tool for the chemical analysis of materials. In this study we used LIBS to identify atomic and molecular emissions of potassium-based energetic materials in the ultraviolet-visible (UV-VIS) and mid-infrared (MIR) regions. LIBS has become useful because of its ability to rapidly obtain information from a sample in solid, liquid or gaseous form. LIBS has been applied to element identification in planetary exploration, stand-off detection of dangerous chemicals and explosives, and deep-sea geochemical studies. In this research, the samples were prepared using an air-spray technique to deposit thin-films of potassium-based energetic materials onto substrates. During analysis, the substrate emissions were identified and subtracted from sample emissions leaving only the relevant atomic and molecular LIBS features from the energetic materials to be used for sample identification.

Abstract