Ultraviolet Raman Spectra and Cross-Sections of the G-seriesNerve Agents

STEVEN D. CHRISTESEN,* JAY PENDELL JONES, JOSEPH M. LOCHNER, andAARON M. HYREU.S. Army Edgewood Chemical Biological Center, APG-EA, Maryland 21010-5424 (S.D.C., J.M.L., A.M.H.); and ITT Advanced Engineering

and Sciences, 1311 Continental Dr., Suite H, Abingdon, Maryland 21009 (J.P.J.)

Ultraviolet (UV) Raman spectroscopy is being applied to the detection of

chemical agent contamination of natural and man-made surfaces. In

support of these efforts, we have measured the UV Raman signatures of

the G-series nerve agents GA (tabun), GB (sarin), GD (soman), GF

(cyclosarin), and the agent simulant diisopropyl methylphosphonate

(DIMP) at 248 nm and 262 nm, as well as taking their UV Raman and

UV absorption cross-sections. Of these chemicals, only GA exhibits any

significant pre-resonance enhancement. We also show that reduction of

the excitation wavelength from 262 nm to 248 nm effectively shifts the

Raman spectrum away from a substantial sample fluorescence back-

ground, implying a significant improvement in detection capability.

Index Headings: Raman spectroscopy; Ultraviolet Raman spectroscopy;

Cross-sections; Chemical agents; Nerve agents; Contamination detection.

INTRODUCTION

The ability of normal Raman spectroscopy to differentiatebetween structurally similar chemicals makes it an attractivetechnique for chemical agent identification, especially wheretrace level analysis is not required. Its utility has already beenproven in the field identification of chemical agents and othertoxic compounds.1,2 Applications of Raman spectroscopy tohazardous material detection and identification can only beexpected to increase with the recent proliferation of commer-cially available portable and even hand-held Raman instru-ments. Because of its small scattering cross-sections, however,normal Raman spectroscopy is generally limited to theidentification of chemicals in solution at relatively highconcentrations (perhaps down to parts per thousand) or bulkmaterial visible to the naked eye.

Surface-enhanced Raman spectroscopy (SERS) provides anincreased sensitivity via the interaction of the laser excitationand Raman scattered light with the surface plasmons of a metalsubstrate, typically gold, silver, or platinum. This technique hasrecently been applied to the detection of trace concentrations ofchemical agents in water3,4 as well as detection andclassification of biological agents.5–7 Some chemicals such ascyanide are readily detectable at the lg/L (ppb) level, butagents such as VX and HD (sulfur mustard) have only beenmeasured at tens of ppm, well above the required detectionlimits. Although much progress has been made in the area ofSERS substrate reproducibility, the variability of the SERSresponse from one substrate batch to another is still an issue,especially for colloidal SERS particles. A recent book edited byK. Kneipp, M. Moskovits, and H. Kneipp documents the state-of-the-art in SERS theory and applications, with a chapter onthe specific use of SERS for chemical agent detection in water.3

Although it promises increased sensitivity over normal Ramanspectroscopy, SERS is an inherently point-sensing techniqueand is not readily applicable to the direct, non-contact detectionof chemicals on surfaces.

Unlike SERS, ultraviolet (UV) excited Raman spectroscopycan be applied to non-contact reagentless detection andidentification of chemicals on surfaces while promisingimproved sensitivity over normal Raman detection usingvisible or NIR laser excitation. UV laser excitation results inenhanced scattering efficiency via the m4 frequency dependenceof the Raman signal. There is also the possibility of resonanceor pre-resonance enhancement of select vibrational modes asthe laser excitation energy approaches coincidence with anallowed electronic transition of the molecule. An additionaladvantage in using excitation wavelengths below approximate-ly 260 nm is the shifting of the Raman spectrum into the solarblind region, allowing for full daylight detection. Excitationwavelengths below approximately 250 nm will blue-shift thespectrum away from the native fluorescence associated withbiological material, generally peaked at around 320 nm, as wellas from most other concomitant organic material. Asher andJohnson8 also demonstrated fluorescence reduction in UV-excited resonance Raman spectra of coal liquid using 255 nmexcitation. Others have shown that fluorescence-free spectra ofbacteria are obtained with excitation wavelengths belowapproximately 270 nm.9 As will be shown later, moving theexcitation wavelength from 262 nm to 248 nm also provides asubstantial reduction in the fluorescence background in theRaman spectra of the G-series nerve agents.

The measurement of quantitative Raman spectra is critical toany application of Raman spectroscopy to the detection andidentification of chemical agents. Qualitative spectra are requiredfor chemical identification via library and algorithm develop-ment, while Raman intensities or cross-sections are needed fordetermining sensitivities. In the UV, sample absorption andpossible resonance or pre-resonance enhancement of the Ramansignal become factors that affect sensitivity as well.

The intensity of a Raman line is expressed as the differentialRaman cross-section (rR) and has units of cm2/sr/molecule.The differential Raman cross-section is that fraction of incidentlaser radiation that is scattered into a particular Raman line permolecule irradiated and per unit solid scattering angle. It is alsoa function of the incident laser frequency m0 (in wavenumbers)and the frequency of the scattered light (m0 � mj):

rR ¼ð2pÞ4b2

j gjð45a02j þ 7c02

j Þm0ðm0 � mjÞ3

45½1� expð�hcmj=kTÞ� ð1Þ

where mj is the frequency of a vibrational transition, b2j is the

zero point amplitude of the vibration, gj is the vibrational

Received 9 April 2008; accepted 1 July 2008.* Author to whom correspondence should be sent. E-mail: steven.christesen@us.army.mil.

1078 Volume 62, Number 10, 2008 APPLIED SPECTROSCOPY0003-7028/08/6210-1078$2.00/0

� 2008 Society for Applied Spectroscopy

degeneracy, and a0j and c0

j are, respectively, the trace andanisotropy of the derived polarizability tensor. For normalRaman scattering, rR increases by the factor m0(m0� mj)

3 as theexcitation frequency increases (subsequently referred to as thestandard m4 dependence). The polarizability tensor elementsalso become frequency dependent as the excitation wavelengthapproaches resonance with an allowed electronic transition. Forthis pre-resonance enhancement, the frequency dependence canbe described by the modified Albrecht A-term:10

rR ¼ K1 3 m0ðm0 � mjÞ3m2

e þ m20

ðm2e � m2

0Þ2þ K2

" #2

ð2Þ

where K1 and K2 are fitting constants and me is the frequency inwavenumbers of the relevant electronic excited state. Both K1

and K2 are independent of m0, and Eq. 2 simplifies to thestandard A-term expression when K2 ¼ 0. According toHarmon and Asher,11 the K2 term models contributions fromhigher frequency electronic states whose contribution can beconsidered to be frequency independent. Equation 2 tends tounderestimate the energy of the resonant transition, while thestandard A-term equation with K2 ¼ 0 overestimates thatenergy.12

Raman cross-sections are generally calculated by comparingscattering intensities to those of a known standard. For the UVRaman measurements reported here we used literature valuesfor the Raman cross-sections of the acetonitrile bands at 918cm�1 and 2249 cm�1 in an internal standard method. In thisprocedure, the sample is mixed with acetonitrile in concentra-tions ranging from approximately 10 to 90% by volume. TheRaman scattering cross-sections are determined by a compar-ison of the integrated line intensity (integrated area) of aRaman band of the sample with that of the acetonitrile via thefollowing equation:

rsR ¼ rr

R

Is

I r

� �Eðm0 � mr

jÞEðm0 � ms

j Þ

" #Cr

Cs

� �ð3Þ

where the superscripts r and s refer to the reference(acetonitrile) and sample, respectively. I is the integrated areaof the Raman band, E is the spectrometer efficiency at theRaman frequency, and C is the sample concentration inmolecules per unit volume.

Acetonitrile was chosen as an internal reference because itscross-section value is fairly well established for ultravioletwavelength excitation12,13 and it has a UV absorption cutoffwavelength of 210 nm.14 We used the values obtained byDudik et al.12 for K1 (7.65 3 10�27 for the 2249 cm�1 line and2.26 3 10�25 for the 918 cm�1 line) and me (116 000 cm�1 forthe 2249 cm�1 line and 390 000 cm�1 for the 918 cm�1 line)with K2¼0 as inputs to Eq. 2 to calculate the acetonitrile cross-sections at the appropriate excitation wavelength. Near-infrared(NIR) Raman cross-sections of the nerve agents were alsomeasured at 785 nm excitation using acetonitrile as a standard.Cross-sections for DIMP (diisopropyl methylphosphonate),GB, and GA using excitation wavelengths between 514.5 and363.8 nm were previously published by one of the authors.15

These values were recalculated including additional data withchloroform as a reference (using the value from Abe andWakayama)16 as well as more recent values for the benzenecross-section (Biswas and Umapathy).17 The recalculations forGA, GB, and DIMP resulted in negligible changes for the

visible excitation wavelengths, but the new values for 363.8nm excitation are 27% (GB and GA) and 38% (DIMP) lowerthan previously reported.

As mentioned before, the sensitivity of a Raman surfacecontamination detector is dependent on both the Ramanscattering cross-section and the molecular absorbance. Theobserved intensity of a given Raman line is proportional to theRaman scattering cross-section and the number of moleculesthat contribute to that scattering. For a direct backscatteringgeometry, the latter term is a function of the molecularabsorbance, which determines the depth of penetration by thelaser excitation and the amount of Raman scattering reabsorbedby the sample.

Raman Signal } rR 3 q 3

Z D

0

10�ða0þaRÞr dr ð4Þ

In Eq. 4, the molecular density is given by q, D is the samplethickness, and a0 and aR are the absorbances at the laser andRaman scattered wavelengths, respectively. For nonabsorbingsamples, the integral in Eq. 4 (penetration depth) isapproximately D, but as the sample absorbance increases, thepenetration depth and, therefore, the Raman signal decrease.Thus, you can have a situation where the increase in Ramanreturn due to the increase in Raman cross-section resultingfrom resonance or pre-resonance enhancement is more thanoffset by the larger absorption cross-section and reduction inpenetration depth. Equation 4 demonstrates that, in assessingthe relative sensitivities of visible or NIR excitation versus UVexcitation for detecting surface contamination, the expectedsample thickness D must be considered.

EXPERIMENTAL

We have measured the UV Raman signatures of the G-seriesnerve agents (G-agents) GA (tabun), GB (sarin), GD (soman),GF (cyclosarin), and the agent simulant diisopropyl methyl-phosphonate or DIMP (Table I) at 248 nm and 262 nm. Wehave also measured their UV absorption cross-sections and theUV Raman cross-sections of selected intense Raman lines ofeach agent. Raman spectra and cross-sections were measuredusing an EIC Laboratories echelle spectrograph equipped withan Andor thermoelectrically (TE) cooled charge-coupleddevice (CCD) detector18 (248 nm excitation only) or a SPEX270 M spectrograph with a liquid nitrogen cooled CCD. Thespectrograph resolution was approximately 4 cm�1 for theformer and 15 cm�1 for the latter. Laser excitation wasprovided by a frequency doubled argon ion laser (Coherent Inc.I300C Motofred) operating at 248.25 nm and a quadrupledNd:YLF laser (CrystaLaser) operating at 262 nm. The formeroperates in a continuous wave (CW) mode while the latter is Q-switched at approximately 4 kHz. Both lasers output

TABLE I. Chemical information.

Agent Common name Molecular formula Purity CAS #

GA Tabun C5H11N2O2P 96% 77-81-6GB Sarin C4H10FO2P 95% 107-44-8GD Soman C7H16FO2P 98% 96-64-0GF Cyclosarin C7H14FO2P 95.5% 329-99-7DIMP Diisopropyl

methylphosphonateC7H17O3P 96% 1445-75-6

APPLIED SPECTROSCOPY 1079

approximately 40 mW average power. While the focus of our

effort was the determination of UV Raman spectra and cross-

sections, we also measured quantitative NIR Raman spectra

using 785 nm laser excitation with an EIC Laboratories echelle

spectrograph Model NIR700. This instrument is described in

detail in Ref. 19.

Ultraviolet–visible (UV/Vis) absorption spectra were col-

lected on a Varian Cary 50 UV/Vis spectrometer using a xenon

flash lamp light source. All data were collected at a scan rate of

300 nm/min over the wavelength range of 200–400 nm with an

interval size of 0.5 nm. Strong absorptions necessitated dilution

in acetonitrile (6.81 mM for GA, 27.3 mM for GD, and 2.7 M

for DIMP) of all of the chemicals except GB. A spectrum of

acetonitrile was obtained at the start of each experimental run

and automatically subtracted from that of the solution during

data collection.

All samples were placed in UV grade quartz cuvettes

manufactured by either Starna or NSG for both UV Raman and

UV/Vis absorption measurements. Chemical Agent Analytical

Reference Material (CASARM)-grade agents were used neat

and diluted with acetonitrile (J.T. Baker). DIMP was obtained

from Avocado Research Chemicals Ltd.

RESULTS AND DISCUSSION

The NIR/Vis and UV Raman spectra of the G-agents andDIMP are shown in Figs. 1 and 2, while the cross-sections forthe strongest lines are presented in Table II. The strong Ramanbands in these spectra between approximately 700 cm�1 and750 cm�1 all result from vibrational modes that include thephosphorus atom.20–24 The published literature, however, is indisagreement about the specific assignments of these lines. Thestrongest line of GB at 724 cm�1 is assigned by DeLong22 tothe P–O–C symmetric stretch. This band is weak in the IR andneither it nor the corresponding GD band at 729 cm�1 isassigned by Hameka et al.20 In a separate study, however,Hameka21 and his coauthors attribute the intense bands atapproximately 720 cm�1 in methylphosphonates (includingDIMP) to the molecules’ O–P–O bends. In contrast to Hameka,Van der Veken and Herman23 assign the comparable band at712 cm�1 for dimethyl methylphosphonate (DMMP) to a P–Cstretching vibration and the weaker lines at 786 and 818 cm�1

to symmetric and asymmetric PO2 stretching modes. Hameka20

assigns the strong 752 cm�1 band of GD to the P–C stretchingmode, but a weaker band in DIMP at 792 cm�1 is attributed toits P–C stretch.

All of the G-agents as well as DIMP contain the P¼Omoiety, which is assigned in all of the studies to the Ramanbands found between approximately 1240 and 1280 cm�1. The

FIG. 1. Raman spectra of GB, GD, GF, and DIMP. (a) Upper traces measuredwith 248 nm excitation. The G-agents and DIMP were measured with 4 cm�1

and 15 cm�1 resolution, respectively (see Experimental section). (b) Lowertraces measured with 785 nm excitation for GD, GF, and DIMP and 457.9 nmexcitation for GB.

FIG. 2. (a) 248 nm and (b) 785 nm excited Raman spectra of GA. The smallinsets of the 2195 cm�1 peak have been reduced in intensity by a factor of 10.

TABLE II. Measured Raman cross-sections. Values from Ref. 13 are in parentheses. See text for explanation.

Raman Line (cm�1)

rR (3 10�30 cm2/sr/molecule)

GA GB GD GF DIMP

2195 724 729 þ 751 758 718

Excitation wavelength (nm) 248.25 1025 200 335 235 252262 300 130 160 155 –363.8 53 (72) 32 (44) – – 43 (70)457.9 18 (19) 9.9 (11) – – 14 (15)488.0 13 (13) 8.0 (8.2) – – 13 (13)514.5 7.5 (7.6) 5.9 (6.0) – – 7.3 (7.4)785 1.5 1.5 1.5 1.0 1.3

1080 Volume 62, Number 10, 2008

P–F stretching vibrations in GB and GD are assigned to theweak lines at approximately 837 cm�1. By analogy we canattribute this mode to the 833 cm�1 line in GF.

The relative intensities of the Raman lines in GB, GD, GF, andDIMP remain essentially unchanged when the excitationwavelength is reduced from the NIR or visible to 248 nm (Fig.1). In addition, plots of the Raman cross-sections of the intensebands around 720 cm�1 as a function of excitation frequency(excitation profiles) for these molecules reveal little pre-resonance enhancement (Figs. 3–6). It should be noted that theerror bars for the 262 and 248 nm excitation data represent thestandard deviation of multiple measurements but do not includethe uncertainties in the reference cross-sections. In some cases,the error bars are smaller than the symbols used for the data point.

The A-term fit for DIMP is not substantially better than thefit provided by the standard frequency to the fourth dependence

(Fig. 3), and the value obtained for me from Eq. 2 (68 000 cm�1

or 147 nm) is well below the shortest excitation wavelengthused. The A-term fit for GB (Fig. 4) shows a slightly strongerpre-resonance enhancement than DIMP, with me ffi 46 000cm�1 (217 nm). The deviation from strict m4 dependence isevident in comparing the A-term fit to the frequency to thefourth power fit of the NIR and visible cross-sections shown inFig. 4. It is also clear from these plots that neither DIMP norGB has strong absorbance below 200 nm, nor does eitherexhibit a significant pre-resonance enhancement for excitationwavelengths down to 248 nm.

Both GD and GF exhibit at best a modest pre-resonanceenhancement of their Raman signals at 248 nm as seen in Figs.5 and 6, respectively. With only three points available, a fit toEq. 2 is not possible. The Raman cross-sections of the 729/751cm�1 doublet in GD and the 752 cm�1 line in GF areapproximately a factor of 2 larger than what can be accounted

FIG. 3. Raman excitation profile (718 cm�1 line) and absorption spectrum ofDIMP. The dashed line is an A-term fit to all of the data and the dotted line isthe standard m4 fit of NIR and visible excitation data. Units for the Ramancross-section appear on the left axis and the units for the absorption coefficientappear on the right axis.

FIG. 4. Raman excitation profile (724 cm�1 line) and absorption spectrum ofGB. The dashed line is an A-term fit to all of the data and the dotted line is thestandard m4 fit of the NIR and visible excitation data. Units for the Ramancross-section appear on the left axis and the units for the absorption coefficientappear on the right axis.

FIG. 5. Raman excitation profile (729 þ 751 cm�1 lines) and absorptionspectrum of GD. The dotted line is the standard m4 extrapolation of the 785 nmdatum. Units for the Raman cross-section appear on the left axis and the unitsfor the absorption coefficient appear on the right axis.

FIG. 6. Raman excitation profile (758 cm�1 line) and absorption spectrum ofGF. The dotted line is the standard m4 extrapolation of the 785 nm datum. Unitsfor the Raman cross-section appear on the left axis and the units for theabsorption coefficient appear on the right axis.

APPLIED SPECTROSCOPY 1081

for by extrapolation of the NIR value to 248 nm assuming noenhancement. As with DIMP and GB, GD and GF are weakabsorbers at wavelengths longer than 200 nm. Our absorptionvalues at 250 nm for GB and GD compare favorably withpublished measurements by Rewick et al.25 (Table III).

Unlike the other G agents, the relative intensities of theRaman lines of Tabun are different for 248 nm and 785 nmexcitation (Fig. 2). The assignment of the C[N stretchingvibration at 2195 cm�1 is unambiguous and shows anenhancement of between 4 and 5 over a straight m4 dependence.A value for me ffi 42 100 cm�1 (237 nm) was found using Eq. 2,but, as mentioned before, this is likely an overestimate of theenergy of the allowed electronic transition responsible for theRaman enhancement. Fitting the same data using the standardA-term equation with K2¼ 0 yields a slightly worse fit and a me

ffi 84 000 cm�1 (119 nm). An absorption maximum for GA isexperimentally found around 200 nm in our study as well as byRewick. Our measured absorption cross-section at 250 nm,however, is a factor of over 100 smaller than that reported byRewick. The source of the discrepancy is unknown at this time.

Also preferentially enhanced in GA is the 710/727 cm�1

doublet and the lines at 1003 and 1321 cm�1. The former areassigned by Hameka20 to the P–CN stretch (710 cm�1) and theP–O–C (727 cm�1) bend, while Holmstedt and Larsson24

assign these peaks to stretching of the P–N bond. The bands at1003 cm�1 and 1321 cm�1 are associated with the aminegroup,20,24 with the latter likely due to the N–C stretch.20 Forcomparison, the lines at approximately 1100 cm�1 (probableC–C stretching mode) and 1266 cm�1 (P¼O stretch)20,24 show

no pre-resonance enhancement. The former has measuredRaman cross-sections of 5.2 3 10�31 and 5.4 3 10�29 cm2/sr/molecule with 785 nm and 248 nm excitation, respectively.The cross-sections measured for the 1266 cm�1 line are 1.1 310�30 cm2/sr/molecule for 785 nm excitation and 2.0 3 10�28

cm2/sr/molecule for 248 nm excitation.For GA, an additional peak emerges at 1596 cm�1 with 248

nm excitation that is not present in the visible and NIR excitedspectra. This peak is consistent with an amorphous carbonband26 likely produced by the laser irradiation, possibly at theliquid–quartz cell interface. It is not clear, however, why asimilar peak is not observed in the GF, which we measure tohave a similarly high absorption cross-section at 250 nm (TableII). The presence of the 1596 cm�1 peak in GA and not GFcould possibly lend credence to Rewick’s higher measuredabsorbance of GA.

As evidenced by Fig. 8, the difference in fluorescencebackground observed for 248 nm and 262 nm excitation isstriking. The former places the G-agent Raman spectra in afluorescence-free region with little or no background, while thelatter produces moderate (GB) to severe (GA) degradation ofthe Raman spectra. With agent purities in the range of 95–98%,sample impurities cannot be ruled out as the source of thisfluorescence. This fluorescence background at 262 nmexcitation limited peak area and hence cross-section measure-ments to only the strongest Raman lines of each agent.Although post-processing with automated background subtrac-tion routines such as the rolling circle filter27 can suppressbroad fluorescence, the degradation in signal-to-noise ratioproduced by this background emission is significant. It is clearthat UV Raman detection of these chemicals using excitationwavelengths above approximately 255 nm is problematic.

With resonant excitation, there is the possibility of transientspecies production or saturation effects.28,29 These effects areobserved with relatively high peak power pulsed lasers at orvery near resonance with an electronic transition. Because weused CW (248 nm) or quasi-CW (262 nm) UV laser sourceswhose wavelengths are off-resonance, depletion of the groundstate resulting in saturation of the Raman intensity is unlikely.In the case of GA, however, we have not ruled out some laser-induced sample damage as the source of the 1596 cm�1 peak.

TABLE III. Absorption cross-sections at 250 nm (cm2/molecule).

Rewick23 This study

GA 9.0 3 10�19 8.3 3 10�21

GB 1.2 3 10�21 1.2 3 10�21

GD 4.0 3 10�21 2.8 3 10�21

GF 8.1 3 10�21

DIMP ,6.0 3 10�22 2.4 3 10�22

FIG. 7. Raman excitation profile (2198 cm�1 line) and absorption spectrum ofGA. The dashed line is an A-term fit to all of the data and the dotted line is thestandard m4 fit of NIR and visible excitation data. Units for the Raman cross-section appear on the left axis and the units for the absorption coefficient appearon the right axis.

FIG. 8. UV Raman spectra of GA, GF, GD, and GB excited at 262 nm.

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CONCLUSION

With sufficient signal-to-noise ratio and spectral resolution,Raman spectra can be used to unequivocally identify neatchemical agents. Given this, the issue becomes one ofsensitivity and whether the detection limits are sufficient toprovide warning of a toxic environment. Key to making thatdetermination are measurements of the relevant chemical agentRaman scattering cross-sections and UV absorption cross-sections. To this end, we have measured the UV/Vis absorptionand Raman cross-sections of the G-series nerve agents. OnlyGA exhibits any appreciable pre-resonance enhancement at248 nm excitation. The measured Raman excitation profiles ofGA, GB, and DIMP from the NIR to the UV allow forextrapolation to any excitation wavelength between 248 and785 nm via either the modified Albrecht A-term or the standardm4 frequency fit. The limited wavelength data for GD and GFindicate that the standard m4 dependence holds for excitationwavelengths down to approximately 260 nm.

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