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1. Introduction

Lasers offer multiple approaches for explosives detection that are not possible with other

techniques. In general, these can be separated into two types: (1 those based on the unique

properties of lasers for long!distance propagation of intense energy and (" those that are based

on the actual molecular and atomic spectroscopy and utili#e the high wavelength specificity that

most lasers offer. $f course, laser explosives detection is somewhat young given the fact that

lasers were invented fairly recently in 1%&'. s such, it is fair to say that laser explosives

detection is still a wor) in progress, with much having been discovered in recent years, and still

more to be discovered in the near future, particularly as more exotic laser sources (e.g., femtosecond

lasers become more common, less expensive, more rugged, and generally, more readily available.

*hese are very exciting times for the use of lasers for explosives detection. In particular, great

progress has been made in solid state lasers and in the extension of laser radiation throughout the

infrared, near!infrared, visible, and near!ultraviolet regions with regards to decrease in si#e and

cost for various systems. +or example, the neodinium ytterium aluminum garnet (d:-

laser has become very mature, with improvements over many generations. s a result, one canget reliable laser radiation in the near!infrared fundamental wavelength of 1./0 m, as well as

visible and ultraviolet radiation at the second to fifth harmonics, in a fairly compact and not!too!

expensive pac)age.

$ne particular area where the laser appears to be uniquely capable is in the standoff detection of

explosives, where the laser properties of long!distance radiation propagation are providing

capabilities not possible with other techniques. 2till, although very promising, standoff

explosives detection using lasers is an emerging application area requiring time to mature. $ne

other area where lasers offer intriguing potential is in the fusion of orthogonal laser!based

techniques, such as laser!induced brea)down spectroscopy (LI32 and 4aman. number of

researchers have started to pursue this avenue because an integrated LI3254aman system can usethe same laser and spectrometer components. *he expected dramatic improvements in

probability of detection and reduction of false alarm rates suggest that laser!based explosives

detection methods may evolve into a ma6or new technology area in the next 178 years.

2. Detection of Explosives Using Laser-Based Vibrational Spectroscop

pplications of laser!based vibrational spectroscopy to explosives detection have been widely

studied. *he literature on this topic was summari#ed by 2teinfeld and 9ormhoudt ( 1 and by

enderson et al. ( 2. Instrumentation for explosives detection to "//; was summari#ed by

over the temperature range of "'/78;/ ? ( 4

,5

. *here has been some discussion in the literatureas to whether detection of concealed explosives is best accomplished by vapor detection or by

detection of particles5explosive!laden fingerprints left behind during placement. +or example, a

single &!m diameter particle of the solid explosive 4=> contains as many molecules as a liter

of air saturated with 4=> vapor at standard temperature and pressure. dditionally,

concealment in plastic containers and some formulation ingredients designed to ma)e explosive

materials @plasticA may reduce partial pressures of explosive vapors by up to 8 orders of

magnitude compared to laboratory values ( 1.

8& &

/ .B / ./ 8

;.&8// .0 / ./ "&

;=*

/ .&

"& / ./ " 8.&4=> > 1&/// .;

**/ ./ 1&8

"/ / .8=* / ./ 1

".&/ ." 4= > > 1 &/ /* *

1& / ./ /&"/ .1

/ /1.&1/ "% ; "% 0 " % ' 8 / / 8/ " 8/ ; 8 / 0

! e "p e r at u r e # $%

1

&/.&

/ /

"'/ "%/ 8// 81/ 8"/ 88/ 8;/

! e"perature #$%

+igure 1. *he vapor pressures of the neat explosives ",; =*C ",;,0 **C and 4=> over the temperature range

of "'/78;/ ?.

2olid or vapor phase vibrational spectra of (unreacted explosives are usually measured with the

bul) sample in the solid state, mainly because most explosives are solids at room temperature

(nitroglycerin being the most well!)nown exception. +or many measurements of vibrational

spectra of vapors from solid explosives, the solid sample is heated to increase the vapor pressure

(3. +or formulations of high explosives, in which the main ingredient(s are often crystalline

when pure (e.g., D!;, whose main ingredient is 4=>, samples may be powders or semimalleable

"

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(plastici#ed solids. +or low!explosive formulations and propellants, often containing ingredients

which are polymeric when pure (e.g., nitrocellulose, samples are often in the form of grains

(compressed or formed in the shape of a right circular cylinder, coarse or fine powders, slurries,

or solid solutions.

=evelopment of laser!based explosives detection methods employing vibrational spectroscopy

begins with characteri#ation of the spectral signature of the explosive to be detected. 2pectral

signatures of interest are usually those of the neat condensed!phase explosive (e.g., residue left

over from a fingerprint or the vapor emanating from the explosive material (e.g., concealed

explosives. 3ecause the vapor pressures of many pure explosive materials are exceptionally

low (see figure 1, the vapor above a solid explosive formulation may consist mainly of the most

volatile components. +igure " shows the infrared absorption spectrum of vapor above solid mil!

spec ** at 8;/ ? (measured by the authors. *he measured spectrum is actually vapor!phase

=*, which is an impurity present at several percent in most samples of ** but with a much

higher vapor pressure than ** (see figure 1 ( 6.

!&!'D&! Vapor ()*$

!&!'D&! Vapor ()*$1(+) 1++,

/.8/.8

12)111)

/."/."

/.1/.1

//

!/.1!/.1

'// 18// 1'// "8// "'// 88// 8'//

'// 18// 1'// "8// "'// 88// 8'//

/avenu"bers #c"-1%

/avenu"bers #c"-1%

+igure ". *he infrared absorption spectrum of vapor above solid mil!spec ** at 8;/ ?. *he spectrum

is identical to that measured above solid =* at the same temperature (measured by the

authors.

Eibrational spectroscopic studies of explosives may be grouped roughly into studies of unreacted

materials and products of reaction.

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vibrations ( F 1 exhibit characteristic frequencies in the mid!infrared spectral region, lying in

the wavelength region from " to 8/ m (G8// cm to &/// cm . *able 1 shows the infrared71 71

spectral regions where many solid explosives and their vapors exhibit features. In general, the

most recogni#able features of the vibrational spectrum of an explosive are associated with the

symmetric and antisymmetric vibrations of the almost ubiquitous 7$ group, between about"

1"0/ and 18B& cm and 1;&/ and 10// cm ( 7.71 71

*able 1. 9avenumber ranges and vibrational mode assignments for spectral features commonly observed

(0&/781// cm in the infrared absorption spectra of explosives. $ symmetric stretches at71"

1"0/718"/ and 18"&718B& (indicated in bold are the strongest infrared (I4 absorption features

of explosive materials in the mid!infrared region of the spectrum.

Vibrational 0ode ssign"ent Explosive /avenu"bers

#!pe% #c" %1

$ deformation and ring stretch itramine (4=>, ** 0&/7'&/"

4ing torsion itramine (4=>,** 1///71/'/

! stretch itramine (4=> 1"//71"8/

$ symmetric stretch itramine (4=> 1"0/718"/"

D bend itramine (4=>, ** 18//71;&/"

$ antisymmetric stretch itramine (4=>,** 1;&/710//"

D! stretch itramine (4=>, **, nitrocellulose "%//781//

!$ stretch H* '&/7%&/D!D stretch ** 10"/71B//

$ symmetric stretch ** 18"&718B&"

$ bend itrocellulose '//7%//"

$ symmetric stretch itrocellulose 1"//718//"

$ antisymmetric stretch itrocellulose 10//71B//"

D!$ stretch H* 1///71/;/

ote: H* F pentaerythritol tetranitrate.

significant challenge in using vibrational spectroscopy for explosive detection (especially in

the vapor phase arises because of the combination of low!vapor pressures and relatively low

cross section for absorption in the infrared and the low scattering cross section for 4aman

spectroscopy. +or example, typical pea) absorption cross sections, a, for the $ stretching"modes are near 1 J1/ cm 5mole in the infrared (for comparison, pea) ultraviolet KEM0 "

absorption cross sections for ** approach &/ J1/ cm 5mole. +or 4aman spectroscopy,0 "

scattering cross sections in the E may approach 1 J1/ cm 5mole (3, 8.7"

"

2.1 Laser Infrared bsorption Spectroscop of Explosives

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methods of detection by infrared absorption techniques are often limited by the bandwidth of the

light source (this is not necessarily the case with 4aman spectra of many neat solid explosivesC

for this reason, 4aman spectroscopy has been used extensively for analysis of solid explosives

(11 (see section ".".

2.1.1 !unable Diode Laser Spectroscop #!DLS%

*=L2 uses mid! and near!infrared semiconductor light sources and detectors (similar to thoseused in D= players and laser pointers to measure (usually minute changes in light intensity

caused when the light beam passing through a region of space containing an explosive5explosive

gas is partially absorbed. *=L2 can achieve high sensitivity by virtue of phase sensitive

detection, combined with modulation techniques that discriminate against 15f noise of the laser

source (12. Light sources are commercially available throughout the mid! and near!infrared

spectral region. 4ecent developments in the last decade of quantum cascade (ND and interband

cascade lasers offer the promise of room temperature, continuous wave operation throughout the

infrared fingerprint region (8710 m (13. n illustration of the measurement process using the

3eer!Lambert Law is shown in figure 8.

bsorption SpectroscopBeer-La"bert La34

I

I

5 I

5 I

exp # -

-

s

s

L&%

L&%

t

t

*

s

s5 absorption coefficient

5 absorption coefficient

L 5 pat6 lengt6

L 5 pat6 lengt6

& 5 nu"ber of absorbers

& 5 nu"ber of absorbers

Li"itation4 0easure"ent of s"all difference bet3een t3o large nu"bers

Vapor fro" detectordetector

source

sourceexplosive

I

II

I

*

* t

t

+igure 8. cartoon schematic of the application of the 3eer!Lambert Law to explosive vapor sensing.

pplications to explosives sensing using semiconductor light sources have been reviewed by

llen et al. (14. *raditional detection methods (3eer!Lambert law!type experiments are

somewhat limited because the broad spectral features of many neat explosive vapors ma)e phase

sensitive detection methods difficult ( 10, 12. 3ecause of this, *=L2 is often used to detect

and measure light gases (e.g., $ and $ produced by decomposition of the parent explosive."

&

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9ormhoudt et al. ( 15 used a near!infrared diode laser to measure $ produced during hot

filament pyrolysis of ** in soils. 4iris et al. ( 16 used a lead!salt *=L to detect $ produced"

following cataly#ed thermal decomposition of &71/ pg of 4=> in a heated sample cell. *=L2

is often used in tandem techniques to detect explosive fragments produced by photofragmentation

of the parent explosive. +or example, 3auer et al. (17

have used a ND mid!infrared laser to detect

$ produced by 1.&&! m laser photofragmentation of ** and 4=>.

2.1.2 7ptical 8ara"etric 7scillators #787s%

$H$s provide an alternative method of generating coherent radiation in the infrared spectral

region and may exhibit a broad tuning range. n $H$ converts an input laser wave ( 7 pumpp

frequency into two output waves of lower frequency ( 7 signal frequency and

7 idlers i

frequency by means of nonlinear (usually crystal borne optical interaction. *he sum of the

output wave frequencies is equal to the input wave frequency: s O i F p. mploying a

nonlinear optical crystal for frequency conversion, quasi!phase!matching may be accomplished

by periodically changing the nonlinear optical properties of the crystal (periodical poling. +or

example, output wavelengths from B// to &/// nm can be produced in periodically poled lithium

niobate. Dommon pump sources are d:- lasers at 1./0; or /.&8" m. ffenberger and

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at surfaces and interfaces. Dompared to bul) , the surface vibrational features are

blueshifted, and observed splittings are larger. *he technique may have application to detection

of explosive residues on surfaces.

2.1.+ 9avit =ingdo3n Spectroscop #9=DS%

D4=2 is a technique used to enhance measured absorption of light by a chemical species by

greatly increasing the light path through the sample (see figures 8 and ;. *his is achieved byplacing the sample within an optical cavity that uses two highly reflective mirrors to create a

stable optical resonator, such that the alignment of the reflective mirrors on each end of the

cavity serves to @trapA light within the cavity. 9hen a pulse of light enters the cavity, it can

ma)e thousands of round trips before its intensity dies off, resulting in effective path lengths of

)ilometers. *he decrease in intensity with time, called @ringdown time,A is measured by

allowing a small amount of light to lea) through one of the mirrors to impinge on a fast

photodetector. Dontributions to the ringdown time by species absorption of light may be readily

separated from other causes of loss of intensity (scattering, mirror imperfection, etc.. scan of

ringdown time vs. wavelength can yield the absorption spectrum of a species present in

extremely low concentrations. *his is shown schematically in figure ;.

=ingdo3n 9avit

Detector

Laser

Laser

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of explosives and energetic materials. *odd et al. ( 19 have used a mid!infrared $H$ to measure

vapor!phase mid!I4 spectra of common explosives (**, triacetone triperoxide K**HM, 4=>,

H*, and *etryl using the D4=2 technique. Harts!per!billion concentration levels wereQ

detected with no sample preconcentration. collection5flash!heating sequence was

implemented to enhance detection limits for ambient air sampling. =etection limits were not

determined but were expected to approach B& ppt for **, with similar concentration levels for

the other explosives.

2.2 Laser =a"an Spectroscop

nalytical techniques based upon 4aman spectroscopy have been widely used for explosive

detection and characteri#ation. +ollowing theoretical prediction of inelastic light scattering in

transparent media ( 28, the effect was experimentally verified in liquids by 4aman in 1%"'C this

phenomenon is )nown as the @4aman effect.A *he first demonstration of the 4aman effect in

gases was demonstrated by 4. 9. 9ood (Dl gas and +. 4asetti (D$ and D$ . *he"

frequencies observed in 4aman scattering correspond to the frequency of the incident light

shifted by some characteristic frequency of the scattering molecule. *he difference in energy

between the incident and scattered photons (the 4aman spectrum of the molecule is typically a

function of the vibrational energy levels within a molecule.

Hrior to the invention of the laser, 4aman spectroscopy relied on arc lamps to provide incident

light, and long periods of exposure were necessary to record a spectrum. *he advent of high

intensity, monochromatic laser radiation generated renewed interest in 4aman spectroscopy in

the 1%0/s. *he implementation of +* spectrometers and lasers with output wavelengths that

minimi#ed sample fluorescence increased the utility of the technique and fostered application to

explosive analysis.

Dhemical species that exhibit a change in polari#ability with vibration (including all )nown neat

molecular explosives exhibit 4aman spectra that are uniquely determined by their vibrational

mode structure. Laser 4aman spectroscopy has been shown to be a valuable technique for the

characteri#ation of many explosives and explosive formulations, especially those containingmolecular crystals (e.g., 4=> ( 29, 30. 4easonably good spectra of neat polymeric samples

(e.g., nitrocellulose KDM may also be obtained. owever, 4aman spectroscopy may be limited

for bul) analysis of many colored formulations of polymeric energetic materials (D containing

formulations are often colored, e.g., R",

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narrower than those measured using absorption!based measurements, it is possible to see slight

impurities in samples and in mixtures of many explosive materials. Laser 4aman spectra of the

explosive formulation D!;, its main ingredient 4=>, and samples of 4=> of different origin are

shown in figure & ( 11.

Ingredient IdentificationIngredient Identification

8oint of 7rigin Infor"ation - =D>8oint of 7rigin Infor"ation - =D>

"// /

"// /

4=>

4=>

?0> i"purit

?0> i"purit 2

2

1// /

1// / ;///;///

/

/8///

8///

!1///

!1///

D;

D;

DI

DI

"///

"///

!"///

!"///

1///

1///

!8///

!8///

&// 1/// 1&/ /

&// 1/// 1&/ /

B// '// %// 1/// 11//

B// '// %// 1/// 11//

=a"an S6ift #c" %- 1

+igure &. Laser +*!4aman spectra of the explosive formulation D!;, its main ingredient 4=>, and samples of 4=>

of different origin. *he exciting laser wavelength was 1/0; nm ( 11.

*he first review of laser 4aman spectroscopy of explosives appeared in the late 1%0/s ( 31.

patent application for laser 4aman applied to the remote identification of ha#ardous gases from

explosives decomposition was filed a few years later ( 32.

3ecause many explosive samples fluoresce when exposed to visible laser radiation, the use of

4aman spectroscopy for explosives analysis was accelerated by the development of near!I4 laser

sources and +* 4aman techniques. 3eginning in the late 1%B/s, +*!4aman spectroscopy began

to be used for analysis of propellants and energetics characteri#ation ( 33. lso around this time,coherent anti!2to)es 4aman spectroscopy (D42 began to be developed for explosives analysis

(34. In D42, two laser frequenciesWa pump frequency ( and a tunable frequency ( W1 "

are mixed and focused onto the target species, producing a third coherent frequency ( , where8

F " 7 . 9hen 7 is equal to the frequency of a 4aman transition in the molecule,8 1 " 1 "

the D42 signal intensity increases. s a result, a D42 spectrum can be produced by scanning

and recording the resulting D42 intensity. *he use of laser 4aman spectroscopy for the"

trace identification of energetic materials was first reported by Darver and 2inclair, with limits of

detection of 1 ng or less for 4=>, H*, and ** ( 35. *rott and 4enlund (36 reported single

pulse 4aman studies of the solid explosive triaminotrinitro ben#ene (**3.

%

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s laser +*!4aman spectroscopy found wider use, 4aman spectroscopy, in general, found wider

application to the study of energetic materials. 2urvey +*!4aman spectra of most common

explosives and propellants were reported in the early 1%%/s ( 29, 30, 37. D42 for real!time

diagnostics of explosions was first reported in 1%%1 ( 38. are et al. used picosecond 4aman

spectroscopy to study energy transfer in shoc)!initiated explosives ( 39. upta used fast!time

resolved 4aman spectroscopy to study the flow of vibrational energy behind a shoc) wave in

nitromethane (40

. lso around this time, the use of the 4enishaw 4aman microscope forexplosive detection and analysis was first reported ( 41. Limits of detection for most explosives

studied were in the picogram range.

=uring this time, there was an increasing effort to employ 4aman spectroscopy as a tool in a

field deployable explosives detector ( 42. =emonstration of enhancement of the 4aman signal as

the wavelength of the incident light approaches the wavelength of an allowed transition

(resonance 4aman spectroscopy and the elimination of fluorescence when using incident

radiation near ";; nm were first reported for explosives in 1%%B71%%' ( 43, 44. 4aman

measurements on dilute ** and =* solutions in acetonitrile with ";'!nm laser excitation

have shown that for the !$ stretching 4aman modes, there is significant resonant enhancement"

(8. +or **, the 4aman cross section of the 18&1.0 cm !$ stretching mode is &.1' ( X/.;71"

1/ cm 5molecule, and for ",;!=*, the 4aman cross section of the 18&1.8 cm !$7"0 " 71"

stretching mode is B."& ( X1.1 1/ cm 5molecule. In "//0, 3lanco et al. ( 45 have described7"0 "

the use of resonance 4aman spectroscopy for measurement of trace amounts of =* and **

dispersed in sands and soils. Hattern recognition algorithms and the use of neural networ)s and

principal component analysis for classifying 4aman spectra of explosives appeared in the late

1%%/s (;07;'.

*he use of surface enhanced 4aman spectroscopy (242 for trace explosive detection was first

investigated during the late 1%%/s ( 49. In 242, an enhancement in 4aman intensity is

observed by placing the sample in close contact with a metallic surface, typically gold or silver

nanoparticles. *his effect is caused by surface plasmon resonance. 2oon thereafter ( 50, 242

detection of ",;!=* vapor to G1 ppb was demonstrated. 9ithin a year ( 51, a field portableunit had demonstrated a limit of detection of &!ppb vapor =* and the ability to locate buried

land mines.

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