Energetic Material Detection by Laser 

Photofragmentation-Fragment Detection 

(PF-FD) Spectroscopy 

by Stephen D. Roberson and Rosario C. Sausa 

ARL-TR-5244 August 2010 

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-5244 August 2010 

Energetic Material Detection by Laser 

Photofragmentation-Fragment Detection 

(PF-FD) Spectroscopy 

Rosario C. Sausa 

Weapons and Materials Research Directorate, ARL 

Stephen D. Roberson 

National Research Council 

Postdoctoral Research Associateship Programs 

Approved for public release; distribution is unlimited. 
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Energetic Material Detection by Laser Photofragmentation-Fragment Detection 

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14. ABSTRACT 

We detect thin films of 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-hexanitro-1,3,5-triazine (RDX) by one- and two-laser 

photofragmentation (PF) with subsequent fragment detection in real time at ambient temperature and pressure. In the one-laser 

technique, a laser tuned to 226 nm excites the energetic material and both generates the characteristic nitric oxide (NO) 

photofragments and facilitates their detection by resonance-enhanced multiphoton ionization (REMPI) using their A-X (0,0) 

transitions near 226 nm. In contrast, in the two-laser technique, a 454-nm laser generates the analyte molecule in the gas phase 

by matrix assisted desorption, and a second laser tuned to 226 nm both photofragments it and ionizes the resulting NO. We 

report the effects of laser energy, analyte concentration, and matrix concentration on the ion signal and determine the rotational 

temperatures of the NO photofragments from Boltzmann, rotational distribution analysis of the REMPI spectra. We achieve 

limits of detection (LOD) (S/N=3) of hundreds of ng/cm2

 for both techniques under ambient conditions with a positive signal 

identification as low as 70 pg using a single, 226-nm laser pulse of about 50 μJ. 

15. SUBJECT TERMS

energetic materials, laser photofragmentation-fragment detection 

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Rosario C. Sausa 

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iii

Contents 

List of Figures iv

Acknowledgments v

1. Introduction 1

2. Experimental Setup 4

3. Results and Discussion 6

3.1 Two-Laser Experiments (454 nm + 226 nm) ..................................................................6

3.2 Single Laser Experiments (226 nm) ..............................................................................14

4. Conclusions 15

5. References 16

List of Symbols, Abbreviations, and Acronyms 18

Distribution List 19 
iv

List of Figures 

Figure 1. Structural diagram of RDX (top) and TNT (bottom). The outlying NO2 groups are 

the signature functional groups for energetic material detection. ..............................................2

Figure 2. Diagrams of experimental setup for the two-laser (top) and one-laser (bottom) 

setup. ..........................................................................................................................................5

Figure 3. Two-laser REMPI spectra of RDX (A), TNT (B), and reference NO (C). .....................7

Figure 4. Observed and calculated REMPI spectra of NO (A) and NO from a TNT-dye 

matrix (B). ..................................................................................................................................8

Figure 5. Signal strength vs. laser energy for fixed, surface concentrations of RDX and TNT 

under ambient conditions. ........................................................................................................10

Figure 6. Photos of grooves created by the 454-nm laser MALD of RDX- and TNTcontaining thin films with a TNT and RDX surface concentration of about 3 μg/cm2

 and 

dye concentration of about 4.6 μg/cm2

 (left) and 1.2 μg/cm2 (right). ....................................11

Figure 7. Two-laser, REMPI response plots of NO from RDX (top) and TNT (bottom) at 

298K and 1 atm. .......................................................................................................................13

Figure 8. One-laser, REMPI spectra of NO from RDX (A) and TNT (B), and observed (....) 

and calculated (-) spectra of NO from TNT (C) in the region of 226.65 to 226.95 nm. .........15
v

Acknowledgments 

We thank Dr. Rose Pesce-Rodriguez for providing us with the 2,4,6-trinitrotoluene (TNT) and 

hexahydro-1,3,5-hexanitro-1,3,5-triazine (RDX) samples and Dr. Michael Grams for his 

invaluable assistance. Also, we thank Mr. Matthew Bratcher and Ms. Kristin Denault for the use 

of their profilometer, and Mr. Donovan Harris for the use of his imaging microscope. Support 

from the National Research Council Postdoctoral Research Associateship Program is greatly 

appreciated by Dr. Stephen Roberson. 
vi

INTENTIONALLY LEFT BLANK. 
1

1. Introduction 

Now more than ever, the detection of energetic materials is vital to the safety and security of the 

world. A device that is sensitive, accurate, robust, and detects hexahydro-1,3,5-hexanitro-1,3,5-

triazine (RDX), 2,4,6-trinitrotoluene (TNT), as well as other energetic materials in real time 

under ambient conditions would be invaluable in both military and civilian applications. Lasers 

are widely used in and outside the laboratory for their ability to detect accurately low 

concentrations of energetic materials in real time under ambient conditions. Laser induced 

breakdown spectroscopy (LIBS) (1−2), Raman spectroscopy (3–5), and laser 

photofragmentation-fragment detection (PF-FD) (6–19) are three laser spectroscopic methods 

employed for energetic materials detection. LIBS uses high-powered, focused laser beams to 

completely break down a complex energetic material into its constituent elements. In contrast, 

Raman spectroscopy uses a laser to probe the molecule’s vibrational and rotational states from 

the inelastic scatter of photons. When using LIBS as a detection technique, the breakdown of 

energetic material into atomic constituents can hinder positive energetic material identification. 

In contrast, PF-FD breaks down the complex explosive molecule into larger fragments or 

signature molecular groups that are subsequently detected, thus indicating the presence of the 

energetic material. When using Raman spectroscopy as a detection technique, the spectroscopic 

signal from a complex, energetic molecule like RDX or TNT is weak and difficult to interpret 

without additional computing hardware. In contrast, PF-FD produces small, signature molecules, 

whose spectra are relatively easy to interpret. 

PF-FD detects energetic molecules by photofragmenting them into specific, characteristic 

molecular fragments and facilitating their detection by chemiluminecence (8), resonanceenhanced multiphoton ionization (REMPI) (7–15), or laser induced fluorescence (LIF) (16–19). 

This technique is extremely effective for nitro-containing, energetic molecules because they 

contain the characteristic nitro groups (-NO2) that are easily fragmented from the energetic 

molecule. The structural formulas of RDX and TNT are shown in figure 1. Oftentimes, when the 

energetic molecule is excited in the ultraviolet (UV), nitric oxide (NO) molecules are readily 

generated directly from the energetic molecule or indirectly from the displaced nitrogen dioxide 

(NO2) molecules. The ability to identify the signature NO photofragments allows the device to 

detect a number of energetic materials with similar molecular characteristics. 
2

Figure 1. Structural diagram of RDX (top) and TNT (bottom). 

The outlying NO2 groups are the signature functional 

groups for energetic material detection. 

In a one-laser, PF-FD technique, the laser photolyzing the energetic molecule also excites the 

resulting characteristic NO photofragment, which is detected subsequently by LIF, REMPI, or 

both. Each fragment detection method has its advantages and shortcomings. Both LIF and 

REMPI offer high sensitivity, ease of setup, and the ability for concentration and temperature 

measurements in real time. LIF is an optical technique that allows for remote detection, whereas 

REMPI requires an ion probe and, thus, is more conducive for in-situ detection. However, 

research on the remote sensing of explosives by PF-FD with NO REMPI detection is ongoing 

(20). One significant challenge when trying to use LIF to detect energetic materials under 
3

ambient conditions is collisional quenching. Molecular collisions quench electronically excited 

NO making it more difficult to detect at atmospheric pressure. REMPI mitigates this problem to 

a large extent because the ionization process is much faster than the collision quenching process. 

The ability to detect NO using LIF depends on the quantum efficiency of the optical detector, as 

well as the overall transmission efficiency of the monochromator or filter. The LIF and REMPI 

signals are both isotropic. However, a LIF detector captures only a portion of the emission, an 

amount related to the solid angle of the optical arrangement, whereas a REMPI probe captures 

nearly all of the resulting ions or electrons formed between metal plates. Thus, REMPI is, in 

general, a more sensitive technique than LIF for detecting equal amounts of NO generated from 

the energetic molecule at atmospheric pressure. 

The vapor pressures of RDX and TNT, as well as may other energetic materials, are very low 

under ambient conditions. RDX’s concentration is 6−30 parts per trillion by volume (ppt/v) and 

that of TNT is about 1 part per billion by volume (ppb/v) at 298 K and 1 atm (21−22). The low 

numbers of airborne energetic particles created naturally under ambient conditions challenge the 

sensitivity limit achievable by PF-FD and other techniques. Surface PF-FD increases 

substantially the number of airborne energetic molecules or characteristic fragments by laser 

heating, vaporization, or surface photolysis (7− 8, 16−17). Cabalo and Sausa reported the PF-FD 

detection of TNT, RDX, octohydro-1,3,5,7-octonitro-1,3,5,7-octazocine (HMX), and 

hexanitrohexazaisowurtzitane (CL20) residues under ambient conditions (7− 8). A laser 

operating at either 248, 266 or 308 nm vaporized and/or photofragmented the target molecule 

and a second laser tuned to 226 nm ionized the characteristic NO fragment by REMPI by means 

of its A-X (0,0) transitions. They found that RDX’s sensitivity is higher at 248 nm compared to 

266 or 308 nm because its absorption coefficient is greater at 248 nm than the other two 

wavelengths (7). They determined the limits of detection (LOD) (S/N=3) to be in the range of 1 

to 15 ng/cm2

 at 1 atm and 298 K, and found them to depend on the electrode orientation and 

mechanism for NO formation. The sensitivities of RDX and HMX are comparable, and they are 

higher than those of TNT and CL20 (8). 

Wynn and coworkers detected a number of energetic molecules including TNT and RDX in the 

condensed phase under ambient conditions (19). One laser both vaporized and photolyzed the 

energetic material and also facilitated the detection of the resulting NO fragment by LIF. They 

excited NO by means of its A-X (0-1) and (0-2) transitions at 236 and 247 nm, respectively, and 

monitored its A-X (0,0) band at 226 nm for fluorescence detection. They probed NO’s 

vibrational excited states to minimize any interference from ambient NO (v ̋=0) that may be 

present in the atmosphere. They determined that NO is formed predominantly in its v ̋=1 state 

compared to its v ̋=2 state with a rotational temperature, Tr ~1000 ± 500. They detected 20 ng 

of TNT with each 236-nm laser pulse and reported that its signal is comparable to that of RDX 

above 5mJ/cm2

, but less than that of RDX at lower fluences. 

Bernstein and his group studied the laser photolysis of a variety of energetic molecules with 

subsequent NO REMPI and time-of-flight mass spectrometric detection under vacuum 
4

conditions. They employed both ns and fs lasers operating in the region of 226−258 nm for PFFD (11–15). A 532-nm laser generated the target molecule in the gas phase by Matrix Assisted 

Laser Desorption (MALD) using R6G dye as the absorption medium, and a UV laser facilitated 

its detection by PF-FD. They determined that NO is the predominant product formed in a fast 

decomposition process, 180 fs, the time of their laser pulse. The nascent NO fragment is formed 

vibrationally hot (Tv ~1800K) and rotationally cold (Tr ~20K). Its signal from RDX:HMX:CL20 

is in the ratio of 3:4:6, which corresponds to the number of NO2 in each respective precursor 

molecule. 

In this report, we report the detection of thin films of RDX and TNT in real time under ambient 

conditions using both two lasers and a single laser. In the two-laser setup, a 454-nm laser 

vaporizes mostly the energetic material by MALD and a 226-nm laser both photolizes the gas 

phase molecules and ionizes the resulting NO signature photofragments. In contrast, in the onelaser setup, a 226-nm laser both generates the NO photofragments and facilitates their detection 

by REMPI at or on the substrate’s surface. We will report the effects of laser energy and analyte 

concentration on the signal strength, our NO rotational, temperature measurements and the limits 

of detection of TNT and RDX for both laser setups. In addition, we will show the effectiveness 

of MALD of TNT and RDX under ambient conditions at wavelengths where their absorption 

coefficients are very low. 

2. Experimental Setup 

Figure 2 is a schematic of the experimental apparatus for both the one- and two-laser setup. An 

optical parametric oscillator (OPO) (Continuum Sunlite1 EX) pumped by a Nd:YAG laser 

(Continuum Powerlite2

 Precision II) generates the 454-nm laser output, and the frequency 

doubled output of a dye laser (Lumonics Hyperdye and Hypertrack) pumped by a Nd:YAG laser 

(Continuum Surelite3

 II) generates the tunable UV laser output near 226 nm. The laser output is 

10 mJ/pulse at 454 nm and 20−50 μJ at 226 nm. Both lasers systems operate at a repetition rate 

of 10 pulses per second (pps) and each pulse is about 6 ns. The UV laser’s linewidth is about 

0.15 cm-1 near 226 nm. We use a Newport (935-10) attenuator to vary the energy for our study 

of laser energy on detection efficacy. 

The REMPI probe is fabricated in-house. It contains two metals plates that are 1.3 cm by 2 cm 

in size, each with about a 3-mm hole in the center that allows the pump beam to pass through and 

strike the surface. The plates are 4 mm apart from each other, and the lower plate is about 1 mm 

from the substrate’s surface. We estimate the electric field to be about 250 V/mm for a 1000-V 

 1

 Sunlite is a registered trademark of Continuum, Inc. 

2

 Powerlite is a registered trademark of Continuum, Inc. 

3

 Surelite is a registered trademark of Continuum, Inc. 
5

bias between the plates. A current amplifier (Keithley 427, gain 106-107 V/A, time constant 0.01 

ms) amplifies the signal, which we then feed to a 125-MHz oscilloscope (LeCroy 9400) for 

display. A personal computer records the signals from a boxcar averager (Stanford Research 

Systems, SR250). We use a digital delay generator (DDG) (Stanford Research Systems, DG535) 

to trigger the UV laser to fire after the visible laser, as well as to control the time delay between 

the pump and probe lasers. The time delay between the two lasers is typically 100 μs. 

Figure 2. Diagrams of experimental setup for the two-laser (top) and one-laser (bottom) setup. 
6

In the two-laser setup, we focus the visible beam with a 200-mm lens, 30 mm below the quartz 

substrate containing a thin-film matrix of RDX or TNT and laser dye in order to avoid any 

plasma formation on the sample. We then direct the UV laser beam, which we focus with a 100-

mm focal length lens, between the REMPI plates. The UV beam is normal to the visible beam 

about 3 mm from the quartz surface. In contrast, in the one-laser setup, we utilize only the UV 

laser beam. We focus it approximately 1 mm above the quartz’s surface, near the bottom of the 

REMPI plate. We do not observe any plasma formation on the substrate’s surface because the 

UV laser flux is sufficiently low under our experimental conditions. 

We prepare the thin films by coating circular, quartz plates with known concentrations of 

energetic material/methanol solution and then evaporating the solvent with a hotplate. A 

personal computer that is interfaced with an XYZ stage (Anaheim Automation LS100 series) 

allows for precise programmable movement of the substrate. Typical scan speeds range from 0.1 

to 0.2 mm/s. We record excitation scans by scanning the UV laser over the region 226.5 to 

227.3 nm at a speed of 0.025 nm/s with 10-shot averaging. We average three to five scans to 

account for any variation for film thickness during each scan. We determine the technique’s 

sensitivity by recording the REMPI response of NO from TNT or RDX, neat or in a matrix with 

equimolar coumarin dye C540A (Exciton), at various surface concentrations. C540A’s strong, 

broad absorption band in the visible, which peaks at 423 nm, makes it suitable for our 454-nm 

MALD process. Research on substituting the laser dye with energetic binders in the matrix is 

ongoing. For the limit of detection and the laser energy measurements, the UV laser beam is 

tuned near 226.8 nm and collect 8000 data points for each measurement to obtain average values 

and standard deviations of the REMPI signal. 

Our colleagues at the U.S. Army Research Laboratory (ARL) provided us with the RDX and 

TNT samples. RDX is about 98% pure and TNT is military grade. Our methanol is from Alpha 

Aesar (High Performance Liquid Chromatography [HPLC] grade) and NO gas from Airgas 

(0.097% in Argon [Ar]). We use a profilometer to measure the thickness of the films and an 

imaging microscope to observe and photograph the various laser-induced features on the 

substrate’s surface. 

3. Results and Discussion 

3.1 Two-Laser Experiments (454 nm + 226 nm) 

Figure 3 shows REMPI spectra of NO from TNT or RDX generated by exciting the thin-film 

matrix at 454 nm, while scanning the 226-nm laser at 1 atm and 298K. The figure also shows a 

reference REMPI spectrum of room temperature NO gas in a flow cell. A comparison of the 

spectra reveals that the spectral features of NO from TNT and RDX are very similar to those of 

ambient NO, indicating successful formation of NO with its subsequent selective ionization. The 
7

two broad TNT and RDX peaks near 226.80 and 226.93 nm are due to NO A-X (0,0) rotational 

lines forming the P2 and Q2 bandheads and the P12 subband, respectively. The bands show 

rotational structure with sufficient spectral resolution to resolve some of the rotational lines even 

at 1 atm. We do not observe any REMPI signal from NO when the visible laser is on and the UV 

laser radiation is tuned off an NO transition. Also, we do not observe any NO REMPI signal 

from TNT or RDX without dye under our experimental conditions. Neat TNT and RDX films 

are very weak absorbers at 454 nm, and the laser fluence is sufficiently low as not to cause any 

laser desorption under our experimental conditions. Our observations reveal that only the UV 

laser beam causes ionization of NO. The visible laser beam does not generate NO from direct 

excitation of the energetic material or through any energy transfer processes from the dye to the 

energetic material in the MALD process. Our results corroborate previous results that show that 

the MALD of energetic materials can be performed successfully to generate volatile molecules 

with minimal ionization or fragmentation (11–15, 23). 

Figure 3. Two-laser REMPI spectra of RDX (A), TNT (B), and reference NO (C). 

Figure 4 shows the observed and calculated REMPI spectra of room temperature NO (A) and 

NO from TNT-dye film (B) in the region of 226.6 to 226.95 nm. We calculate the NO spectrum 

with a multi-parameter, computer program based on a Boltzmann, rotational line analysis of NO 

(7−8). We input the rotational energies and line strengths of NO into the program and vary the 
8

temperature, concentration, baseline, laser linewidth, and laser line shape. The best fit to the 

observed TNT data yields a rotational temperature of 450 ± 50 K using a Gaussian function for 

the line shape. We obtain a comparable temperature for NO from RDX. A similar analysis of 

room temperature NO yields a temperature of 308 ± 15K. Our results suggest that NO is formed 

in the gas phase by the 226-nm photolysis of either a parent TNT or RDX molecule or some 

daughter species. If it is formed on or near the substrate’s surface by laser-induced 

photochemical reaction at 454 nm or by surface-induced reactions, then it would be rotationally 

thermalized when we interrogate it with our probe laser, 100 μs after we fire our visible laser, 

about 3 mm above the substrate’s surface at 1 atm. 

Figure 4. Observed and calculated REMPI spectra of NO (A) and NO from a TNT-dye 

matrix (B). 

Wynn and coworkers report a NO rotational temperature of 1000K ± 500K in their one-laser, 

237-nm PF-LIF of both TNT films, and bulk TNT and composition 4 (C4), an explosive material 

containing about 90% RDX (19). Their experimental uncertainty is in part due to their laser 

linewidth, 0.03-0.04 nm (5−7 cm-1), which is about 30−50 times greater than our linewidth, 0.15 

cm

-1. Thus, they are probing simultaneously many NO rotational lines with both low and high 

J-values. They claim that the NO temperature depends somewhat on morphology and laser 
9

fluence (1−60 mJ/cm2), with larger fluencies yielding hotter NO temperatures. Our fluence is 

about 23 mJ/cm2

 for comparison purposes. We derive this value using a laser energy of 50 μJ 

and a beam area of 2.2 × 10-5 m

2

, which we obtain from the laser beam’s Airy-disk diameter, 

d = 2.44λ(f/D), where λ is the laser wavelength, f is the lens’ focal length, and D the laser beam 

diameter. The NO temperatures may differ in the two experiments because each group probes 

the NO photofragment at different stages of its temperature history, from its initial formation 

with a nascent, rotational distribution to its eventual thermalization at room temperature. 

However, the observed temperatures differ probably because the photochemistry that governs the 

NO photofragnent is different in the two experiments. It is unlikely that the NO generation from 

TNT and RDX differs much at 237 nm compared to 226 nm. Thus, the main difference in the 

measurements is that we probe NO that is generated about 3mm away from the surface, 

presumably from a gas-phase, energetic parent molecule or its daughter species, whereas they 

probe NO that is generated near or at the surface, where surface-molecule reactions, laser-surface 

effects, or both, play a role in its formation. We will explore this point in the subsequent section 

of this report where we present our results on the one-laser, 226-nm excitation of neat, energetic, 

thin films on surfaces. 

Bernstein and his group report a NO rotational temperature of 20K in their one-laser, PF-REMPI 

experiments of gas-phase RDX at 226 nm using laser fluencies in the range of 0.2−0.6 mJ/pulse 

(14). Their experimental conditions are very much different than ours and, as a result, different 

RDX photolysis mechanisms are in play. Their collisonless environment allows them to 

photolyze a single RDX molecule that is prepared rotationally cold because of its supersonic 

expansion into their vacuum chamber. These conditions will certainly influence the rotational 

distribution of the resulting NO photofragment, which they claim is a direct product of RDX 

decomposition from an isolated, electronic state molecule. In contrast, we perform our 

experiments in a collisional environment at room temperature, where collisions from other RDX 

molecules, intermediate molecules, as well as ambient nitrogen (N2) and oxygen (O2), play a role 

in the photodissociation dynamics. Also, the NO photofragment may result from a RDX 

daughter species, in addition to the parent RDX molecule, under our experimental conditions. 

Figure 5 is a graph of NO signal strength vs. pump laser pulse energy for both RDX and TNT. 

The graph reveals that both signals decrease with decreasing pulse energy, as expected. It is 

interesting to note, however, that the NO REMPI signal from RDX is greater than that from TNT 

and that the ratio of the RDX to TNT signal increases as the laser energy decreases. A plot of 

signal strength vs. dye concentration for a given RDX or TNT concentration, not shown, shows 

that the signal decreases linearly with decreasing dye concentration. The effects of laser energy 

and dye concentration on REMPI signal are similar and suggest that lowering the dye 

concentrations is tantamount to decreasing the laser energy, both resulting in the dye absorbing 

less energy. 

Figure 6 are images from the 454-nm laser removal of RDX and TNT from the substrate. The 

concentration of both RDX and TNT is about 3 μg/cm2 and that of the dye is about 4.6 and 1.2 
10

μg/cm2 for the images on the left and right, respectively. The edges’ location is somewhat 

arbitrary because it is not well defined. Nevertheless, we note that the width of the grooves 

created by the 454-nm laser decreases as the dye concentration decreases. The grooves created 

by the laser for both RDX and TNT further show the importance of efficient energy absorption 

of the thin-film matrix. The images show that the edges of TNT’s grooves, and to some extent 

those of RDX, appear rough and not as uniform as the neat, non-irradiated film. These 

observations suggest that the energetic material or possibly the dye could be decomposing, 

resolidifying, or both, particularly at the laser beam’s outskirt. Therefore, it is plausible that the 

laser beam’s center, which contains the bulk of the energy, results in energetic material 

vaporization, whereas its outskirt, which contains less energy, causes melting, resolidification, or 

partial decomposition of the energetic material or dye. Scanning electron microscopy and 

chemical analyses of the samples, particularly the feature’s edges, are ongoing. 

Figure 5. Signal strength vs. laser energy for fixed, surface concentrations of RDX and 

TNT under ambient conditions. 
11

Figure 6. Photos of grooves created by the 454-nm laser MALD of RDX- and TNT-containing thin 

films with a TNT and RDX surface concentration of about 3 μg/cm2

 and dye concentration of 

about 4.6 μg/cm2

 (left) and 1.2 μg/cm2 (right). 

Figure 7 shows plots of NO signal response vs. RDX and TNT surface concentration. We 

determine the technique’s analytical sensitivity by calculating the LODs, defined as 

CL  3 R , where CL is the concentration, R is the signal response, and σ is the standard 

deviation of the background signal. The best fits of the signal response vs. concentration data, 

along with the noise measurements, yield RDX and TNT LODs of 400 and 543 ng/cm2, 

respectively. The sensitivity values arise from a convolution of laser-induced photochemical, 

photothermal, and photophysical mechanisms leading ultimately to the ionization of NO. These 

complex mechanisms must include the condensed phase, MALD process where the dye absorbs 

the laser radiation and transfers it to the energetic material, causing it to vaporize, and must 

account for any dye and/or energetic material melting, sublimation, or decomposition. In the gas 

phase, the absorption coefficients of RDX and TNT are important at 226 nm, as is their 

photochemistry and the role of collisions from other parent molecules, photolytic intermediates, 

ambient O2 and N2, or combination thereof, on their photodissociation and NO generation. Our 

results show that RDX exhibits a higher sensitivity than TNT. These results are consistent with 

those obtained previously by our group and other research groups under a variety of 

experimental conditions (6−7, 9, 19). However, our sensitivity is much lower than that achieved 

in our laboratory using a 248-nm laser to excite neat RDX or TNT, and a 226-nm laser for 

probing the resulting NO fragments. If we account for the differences in the probe’s electrode 

field, rotational line probed, and laser energy at 226 nm, the major difference in the sensitivities 

stems from using a different laser-excitation wavelength and exciting the energetic materials in a 

matrix. The different experimental conditions suggest two distinct mechanisms for NO 
12

generation. The 248-nm laser excites the neat energetic material and produces both 

photofragments and molecular ions (7, 24). The NO photofragments have a rotational 

temperature of 325K, which indicates that they are rotationally equilibrated by the time they are 

probed by the 226-nm laser (7). This suggests that the NO photofragments are generated from 

the 248-nm laser radiation on or near the substrate’s surface and that the 226-nm laser radiation 

acts as a probe, instead of photolizing any resulting gas-phase parent or daughter species that the 

248-nm laser generates. In contrast, the 454-nm laser excites the dye, which in turn transfers its 

absorbed energy to the energetic material and causes it to vaporize mostly under our present 

measurements. Subsequently, the 226-nm laser both photolyzes the energetic molecule and 

excites the resulting NO fragments. The NO photofragments have a rotational temperature of 

450K suggesting that they are thermalized partially under our experimental conditions. Neither 

the energetic material nor dye is ionized at 454 nm because we do not observe any ions with our 

electrodes, but they may decompose in part during the MALD process. However, if they do 

decompose in the MALD process, NO is not the main decomposition product. The NO 

fragments that we observe are generated mostly from the gas-phase photolysis of the parent 

molecule or daughter species that is a precursor to NO, as our spectral results indicate. Although 

we do not understand fully the mechanisms’ intricacies for NO formation, the two-laser technique 

with NO REMPI detection is a viable method for detecting TNT and RDX residues in situ and in 

real time under ambient conditions. 
13

Figure 7. Two-laser, REMPI response plots of NO from RDX (top) and TNT 

(bottom) at 298K and 1 atm.
14

3.2 Single Laser Experiments (226 nm) 

A one-laser setup is less complex than a two-laser setup for energetic material detection because 

it requires only one laser to both generate and ionize the resulting NO photofragments. In 

addition, aligning one laser beam spatially for signal optimization is simpler than aligning two 

laser beams both spatially and temporally for signal optimization. Figure 8 shows REMPI 

spectra of NO from neat, RDX (A) and TNT (B) films in the region of 226.65 to 226.95 nm 

using about 50 μJ of laser energy. The spectra are similar to those obtained with our two-laser 

setup and their spectral features result mainly from NO A-X (0,0) transitions of the P2, Q2, and 

P12 bands. Our results indicate successful generation of NO from the energetic material with 

subsequent selective ionization of NO. We do not observe any signal from NO when we detune 

the laser from the NO resonant transitions. Our results corroborate those reported by both our 

group and others (10, 11–15, 19, 25). Figure 8C shows both our observed and calculated spectra 

for TNT. A Boltzmann population distribution analysis of the data yields a rotational 

temperature of 350 +/- 50 K. We obtain a similar temperature for RDX. Our results suggest that 

the NO fragments are generated near or at the substrate’s surface and that collisonal energy 

transfer from other species equilibrates them to a near, room-temperature, rotational distribution 

about 1 mm above the surface. Wynn and coworkers report a NO temperature of 1000 K ± 500K 

from the one-laser, PF-FD of condensed-phase TNT and RDX, which suggests that they probe 

NO photofragments that are more nascent in the photodissociation process than ours. 

We determine the technique’s analytical sensitivity from our signal strength vs. concentration 

and noise measurements. Our analysis yields similar LODs (S/N=3) for RDX and TNT, in the 

range of 600−700 ng/cm2

 The laser fluence is about 10 mJ/cm2 for a 226-nm beam with energy 

of about 50 μJ and a diameter of 2.6 x 10-5 m at 1 mm from the surface. We obtain the beam 

diameter from the beam’s, Airy radius and estimating its variation along the z-axis by equation R 

(z) = Ro [(1+ (Z (z)/Zr)

 2] 1/2, where Ro is its radius at the z-axis origin, Zr, defined as πRo2

/λ, 

where λ is the laser wavelength. Our RDX and TNT REMPI results are consistent with those 

from previous one-laser, PF-LIF studies (19). Wynn and coworkers find that the NO LIF signal 

from RDX and TNT depends on the laser fluence: RDX’s sensitivity is greater than that of TNT 

for fluencies less than 5 mJ/cm2

, but it becomes fairly similar to that of TNT above 5mJ/cm2, 

where some saturation occurs in TNT’s NO LIF. 

We are able to detect the energetic materials with a single, 226-nm laser shot. Five laser shots 

completely remove films that are 10 to 15 μ thick and contain 0.25 mg of RDX or TNT over an 

area of about 20 cm2. The energetic material’s removal volume ranges from 8 × 10-9 to 2.4 × 

10-8 cm3

 for impact craters of 70 to 100 μ and a removal rate of 2 to 3 μ/pulse. This means that 

our one-laser REMPI system is able to detect NO fragments from 70 to 300 pg of energetic 

material using a single laser pulse with an energy of about 50 μJ. These values are comparable 

to the 200-pg value obtained for RDX by Raman microscopy (5), and RDX and TNT by laser 

desorption-ion mobility spectrometry (23), but lower than the 20-ng value obtained for TNT by 

PF-LIF (19). 
15

Figure 8. One-laser, REMPI spectra of NO from RDX (A) and TNT (B), and observed (....) 

and calculated (-) spectra of NO from TNT (C) in the region of 226.65 to 226.95 nm. 

4. Conclusions 

We have demonstrated the detection of TNT and RDX residues by one- and two-laser PF-FD 

employing REMPI of the signature, NO photofragment in situ and in real time under ambient 

conditions. The technique’s analytical sensitivity depends on the photochemical and 

photothermal processes producing NO. Our NO rotational temperature measurements reveal that 

in the two-laser configuration, the 454-nm radiation vaporizes mostly the energetic material by 

MALD and the 226-nm laser radiation both generates and ionizes the resulting NO 

photofragments. In contrast, in the one-laser configuration, the 226-nm radiation excites the 

neat, energetic material and both facilitates its decomposition and the detection of the resulting 

NO photofragments on or near the substrate’s surface. The limits of detection are 400 and 543 

ng/cm2 for RDX and TNT, respectively, for the two-laser configuration, and in the range of 600 

to 700 ng/cm2

 range for both RDX and TNT for the one-laser configuration. We can detect as 

low as 70 pg of energetic material with a single pulse of low power, 226-nm radiation, and we 

anticipate that the technique’s effectiveness can be increased with improvements in laser 

hardware. The technique is not restricted to TNT and RDX, and its extension to other energetic 

materials is ongoing. 
16

5. References 

1. Dikmelik, Y.; McEnnis, C.; Spicer, J. B. Opt. Express 2008, 16, 5332, and reference therein. 

2. Munson, C. A.; Gottfried, J. L.; De Lucia, J. Frank C.; McNesby, K. L.; Miziolek, A. W. 

Counterterrorist Detection Techniques of Explosives, Yinon, J., Ed.; Elsevier B. V.: The 

Netherlands, 2007; p. 279, and references therein. 

3. Comanescu, G.; Manka, C. K.; Grun, J.; Nikitin, S.; Zabetakis, D. Appl. Spectrosc. 2008, 62, 

833, and references therein. 

4. Carter, J. C.; Angel, S. M.; Lawrence-Snyder, M.; Scaffidi, J.; Whipple, R. E.; Reynolds, J. 

G. Appl. Spectrosc. 2005, 59, 769. 

5. Cheng, C.; Kirkbride, T.; Batchelder, D.; Lacey, R.; Sheldon, T. J. Forensic Sci. 1995, 40, 

31.

6. Monterola, M.; Smith, B.; Omenetto, N.; Winefordner, J. Anal Bioanal Chem 2008, 391, 

2617. 

7. Cabalo, J.; Sausa, R. Appl. Opt. 2005, 44, 1084. 

8. J. Cabalo and R. Sausa, Applied Spectroscopy 2003, 57 (9), 1196. 

9. Lemire, G. W.; Simeonsson, J. B.; Sausa, R. C. Anal. Chem. 1993, 65, 529. 

10. Simeonsson, J. B.; Sausa, R. C. TrAC Trends in Analytical Chemistry 1998, 17, 542. 

11. Guo, Y.; Bhattacharya, A.; Bernstein, E. J. Phys. Chem. A 2009, 113, 85. 

12. Im, H.; Bernstein, E. R. J. Chem. Phys. 2000, 113, 7911. 

13. Guo, Y. Q.; Greenfield, M.; Bhattacharya, A.; Bernstein, E. R. J. Chem. Phys. 2007, 127, 

154301. 

14. Guo, Y.; Greenfield, M.; Bernstein, E. Journal of Chemical Physics 2005, 122, 244310. 

15. Greenfield, M.; Guo, Y.; Bernstein, E. Chem Phys. Letters 2006, 430, 277. 

16. Bourdreax, G.M.; Miller, T.S.; Kunefke, A. J.; Singh, J. P.; Yueh, F.; Monts, D. Applied 

Optics 1999, 38, 1411. 

17. Heflinger, D.; Arusi-Parpar, T.; Ron, Y.; Lavi, R. Opt. Commun. 2002, 204, 327. 

18. Arusi-Parpar, Heflinger, D.; Lavi, R. Applied Optics. 2001, 40, 6677. 
17

19. Wynn, C. M.; Palmacci, S.; Kunz, R. R.; Clow, K.; Rothschild, M. Appl. Opt. 2008, 47, 

5767. 

20. Angel, S. M. Department of Chemistry and Biochemistry, University of South Carolina, 

Columbia, SC 29208. Private communication. 

21. Dionne, B. C.; Rounbehler, D. P.; Achter, E. K.; Hobbs, J. R.; Fine, D. H. Journal of 

Energetic Materials 1986, 4, 447, and references therein. 

22. Eiceman, A.; Preston, D.; Tiano, G.; Rodriguez, J.; Parmeter, J.E. Talanta 1997, 45 (57). 

23. Duo, S.; Kolaitis, L.; Lubman, D. Applied Spectroscopy 1987, 41 (8), 1771. 

24. Dikinson, J.; Jensen, L.; Doering, D.; Lee, R. J. Applied Physics 1990, 67, 3641. 

25. Tang, T.; Chaudhri, M.; Rees, C.; Mullock, S. J. Material Science 1987, 22, 1037. 
18

List of Symbols, Abbreviations, and Acronyms 

Ar Argon 

ARL U.S. Army Research Laboratory 

C4 composition 4 

CL20 hexanitrohexazaisowurtzitane 

DDG digital delay generator 

HMX octohydro-1,3,5,7-octonitro-1,3,5,7-octazocine 

HPLC High Performance Liquid Chromatography 

LIBS laser induced breakdown spectroscopy 

LIF laser induced flourescence 

LOD limits of detection 

MALD Matrix-Assisted Laser Desorption 

N2 nitrogen 

NO nitric oxide 

-NO2 nitro group 

NO2 nitrogen dioxide 

O2 oxygen 

OPO optical parametric oscillator 

PF photofragmentation 

PF-FD photofragmentation-fragment detection 

ppb/v parts per billion per volume 

ppt/v parts per trillion per volume 

pps pulses per second 

RDX hexahydro-1,3,5-hexanitro-1,3,5-triazine 

REMPI resonance-enhanced multiphoton ionization 

TNT 2,4,6-trinitrotoluene 

UV ultraviolet 
NO. OF 

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