Raman Albedo and Deep-UV Resonance Raman Signatures of Explosives

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1 Raman Albedo and Deep-UV Resonance Raman Signatures of Explosives Balakishore Yellampalle and Brian Lemoff WVHTC Foundation, 1000 Technology Drive, Suite 1000, Fairmont, WV, USA ABSTRACT Deep-ultraviolet resonance Raman spectroscopy (DUVRRS) is a promising approach to stand-off detection of explosive traces due to large Raman cross-section and background free signatures. In order to design an effective sensor, one must be able to estimate the signal level of the DUVRRS signature for solid-phase explosive residues. The conventional approach to signal estimation uses scattering cross-sections and molar absorptivity, measured on solutions of explosives dissolved in an optically-transparent solvent. Only recently have researchers started to measure solid-state cross-sections. For most solid-phase explosives and explosive mixtures, neither the DUV Raman scattering cross sections nor the optical absorption coefficient are known, and they are very difficult to separately measure. Therefore, for a typical solid explosive mixture, it is difficult to accurately estimate Raman signal strength using conventional approaches. To address this issue, we have developed a technique to measure the Raman scattering strength of optically-thick (opaque) materials, or Raman Albedo, defined as the total power of Raman-scattered light per unit frequency per unit solid angle divided by the incident power of the excitation source. We have measured Raman Albedo signatures for a wide range of solid explosives at four different DUV excitation wavelengths. These results will be presented, and we will describe the use of Raman Albedo measurements in the design and current construction of a novel stand-off explosive sensor, based on dual-excitation-wavelength DUVRRS. Keywords: Raman Albedo, Deep Ultraviolet, Resonance Raman Spectroscopy, Multiple Excitation Wavelengths, Explosive Detection. 1. INTRODUCTION Standoff trace detection is a challenging problem [1,2] and has become increasingly important due to threats from terrorist bombings and improvised explosive devices. Because of several unknown background materials and the minute quantities involved, it is important to know the strengths of explosive signatures while designing a stand-off detection instrument. Deep-ultraviolet resonance Raman spectroscopy (DUVRRS) is a promising approach for stand-off detection of explosive traces due to: 1) resonant enhancement of Raman cross-section [3], and 2) λ -4 cross-section increase [4], thus providing good selectivity and sensitivity. An added advantage of DUV excitation is that Raman signatures are free from fluorescence [5] and solar background. Although visible and IR wavelength excitation show deeper sample penetration in thick explosive materials, the DUV resonance Raman enhancement allows for higher signal levels in very thin, trace explosive samples. An important calculation for the development of a DUVRRS stand-off analytical instrument is the estimation of number of Raman signal photons for a given excitation scenario. This estimation forms a very useful basis for iterative optimization and for exploration of any trade-offs during the design of a stand-off analytical instrument. An estimation of Raman signal counts can be made using the solid-state Raman cross-section, the number of molecules involved, the laser penetration depth and the self-absorption coefficient. However, almost all known explosive Raman cross-section measurements were performed by dissolving the explosive analyte in a nearly transparent reference that is in liquid phase [6], which is less useful than a more accurate solid-state cross-section. University of Pittsburgh has only recently measured the solid-state cross-section of a few explosives [7], and it is not known for a large fraction of explosives. Another problem that arises is the estimation of number of molecules involved in the Raman scattering process, which depends on penetration properties of the excitation laser. For many solid-phase explosives, the laser penetration and

2 Raman self-absorption, which have strong frequency dependence in DUV, are not readily known. Therefore, for a typical solid explosive analyte, it is hard to accurately predict Raman signal strength due to uncertainty in the selfabsorption, laser penetration, and/or solid-state Raman scattering cross-section information. Therefore, it is desirable to have a simpler approach for the signal estimation needed for analytical instrumentation design. In addition to signal estimation, intensity-calibrated Raman spectra measured at several excitation wavelengths in the DUV are useful as signatures for developing analytical instruments based on multiple excitation wavelengths [8] or for optimizing wavelength choice for DUVRRS instruments. In the DUV region, the Raman intensity depends on excitation wavelength in a complex way, forming unique wavelength dependent signatures for different compounds. This is because resonance Raman cross-sections and the absorption of both the excitation and scattered light, which determine Raman intensity, vary strongly with excitation wavelength. We have measured these wavelength dependent signatures, using intensity calibrated Raman spectra, for a variety of solid explosives at four DUV excitation wavelengths. 2.1 Definition 2. RAMAN ALBEDO We have developed a technique for intensity calibration of Raman spectra of various solid optically-thick explosives and show that it is possible to calibrate the spectra in a detection-system-independent manner while simultaneously accounting for all the bulk properties like self-absorption and penetration depth. This measure is named Raman Albedo, akin to linear diffuse bulk reflectivity, or Albedo, of a surface. Since this method accounts for bulk properties, the accuracy of a Raman Albedo measurement depends upon the surface preparation and other bulk properties like granularity and packing fraction of the grains of the solid explosive analyte used in the measurements. If the goal of the measurements is signal estimation for particular real world conditions, the sample preparation should be representative of those conditions. We define Raman Albedo, A(ν), as the total power of Raman-scattered light per unit frequency per unit solid angle divided by the incident power of the excitation source. Raman Albedo is expressed in units of fraction/steradian/cm -1. We also define the Raman Band Albedo, A B, as the Raman Albedo, A(ν), integrated over a Raman band centered at ν B. Raman Band Albedo has units of fraction/steradian. Note that these definitions are independent of the measurement approach and only dependent on the sample properties and the excitation wavelength. For the measurement of Raman Albedo, two independent intensity calibration approaches were used, one based on known Raman scattering intensities of acetonitrile (ACN) and a second based on measured laser scattering from a Teflon surface. The two techniques agreed closely within our experimental error. The measured Raman Albedo can also be extended for explosives that exhibit time dependent spectra (for example due to sample degradation or photolysis [9]). 2.2 Measurement setup Figure 1 shows the experimental setup for measuring intensity-calibrated Raman spectra. The setup used a frequency doubled FRED Ar ion laser that could be tuned to one of four different frequency doubled Ar + wavelengths in the deep UV. The laser was polarized parallel to the optical table and the DUV was separated from the fundamental using a Pellin-Broca prism. The UV laser beam was directed towards a sample using a mirror and without any additional focusing optics. The excitation spot on the sample was imaged on to the spectrometer slit with a demagnification of approximately three. The collection optics consisted of a single lens, located in between the illumination spot and the entrance slit of the spectrometer. A DUV polarizer, oriented parallel to the laser, was placed before the slit in order to remove any polarization effects arising from the optics in the spectrometer. A long pass edge filter was used during measurement of Raman spectra. For some excitations, the edge filter was used at a steep angle (~ 40 degrees). At this angle the filter transmission was strongly dependent on the polarization of the light. The polarizer also ensured that the calibration is not affected by this dependence. Back reflection geometry was used to measure Raman spectra because this geometry is preferred for standoff detection of explosives. Our scattered light collection system had a 9-mm depth of focus, much longer than the liquid ACN reference length (1-mm) or solid sample illumination depth. This ensured that the collection efficiency was independent of the reference or sample length. In the measurements, explosive samples were positioned at the same location as the ACN reference sample volume. Solid explosive samples were prepared by packing the explosive powder in the depression of a metal ring whose axis was oriented at 45 o to the horizontal, to prevent the solid sample powder from

3 falling out. Due to the sample tilt, the laser excitation spot illuminated a region 1.4 mm in length along the optical axis of the collection lens. This illumination depth was much smaller than the depth of focus of the collection system. Figure 1: Top view of the experimental setup for measurement of Raman scattering spectra of explosive materials. 2.3 Calibration approach using ACN reference The first calibration approach is based on using ACN as an external reference. In this approach, the total scattered power per unit solid angle per unit laser power for the 918-cm -1 reference band of ACN is first calculated for the 1-mm long reference sample: This calculation uses the known Raman scattering cross-section for the reference band [10], known molecular number density, and refractive indices of the ACN and cuvette. Note that B ref has the same units as Raman Band Albedo and is an equivalent quantity, except that it applies to Raman scattering from a thin transparent liquid reference sample rather than an optically thick surface. Next, the Raman spectrum of the 1-mm ACN reference is measured, and the total CCD counts, C ref, corresponding to the 918-cm -1 reference band is determined for an acquisition time ref, and an excitation laser power P ref. The final step of the calibration procedure is to measure the wavelength dependence of the detection system. Using a calibrated Deuterium lamp located at the sample/reference position, the relative detection efficiency, ( ), is determined by dividing the spectrum measured by the CCD by the known lamp spectrum and then normalizing to a value of 1 at = 918 cm -1. This function accounts for the wavelength dependence of the Raman filter, polarizer, and spectrometer relative to the reference frequency. Once the detection system is calibrated using this method, Raman Albedo can be measured for solid-phase explosive samples. The experimental setup is identical while the explosive sample spectrum is measured. Raman Albedo of the sample is calculated as follows: ( ) ( ) ( ) ( ) ( ) (1.1) The measured sample spectrum, ( ), is measured in CCD counts per unit frequency. The ratio of acquisition times,, and laser powers,, account for different exposure times and excitation laser powers of the sample and

4 reference measurements. Using this calibrated spectrum of the explosive sample, Raman Band Albedo of individual bands can be calculated by integrating the contribution to Raman Albedo values arising from each band. 2.4 Calibration approach using Teflon scattering The second independent calibration approach for the measurement of Raman Albedo is based on scattering measurements from a Teflon sample located at the sample/reference position. In this approach, we measure the laser power (due to diffuse reflectivity) at the spectrometer slit coming from a registered spot on a Teflon surface and use this measurement as a reference. As shown in Figure 2 (a), a Teflon substrate was placed at the sample location and its laser return,, was measured using a power meter before the spectrometer-slit. This measurement can be used to calculate the total scattered laser power, from the Teflon substrate. In addition to measuring, we also measured the corresponding laser return spectrum, ( ) using the setup shown in Figure 2(b), and used it to determine the integrated count, arising from the laser scatter. ( ) Figure 2: Experimental approach for measurement of Raman Albedo using characterized laser return from Teflon surface. Once calibrated in this way, we placed a Rayleigh rejection filter before the slit, similar to Figure 1, and measured the Raman spectrum, ( ), of an explosive sample. The laser power and exposure time corresponding to this measurement were and respectively. From these measurements and the characterized spectrometer, polarizer and filter efficiencies, the Raman Albedo of the explosive sample calibrated in units of frac/ steradian /cm -1 can be calculated in a manner similar to the ACN reference method. 3.1 Calibration and repeatability 3. RESULTS Calibration was performed on fourteen different explosives using the above two methods. The samples in our experiments are stationary, similar to a typical stand-off detection scenario. Some experimental results obtained using 238 nm excitations are shown in Figure 3. Of all the explosives measured, Ammonium Perchlorate had the highest and TNT had one of the lowest Raman Albedos. The calibrations from both ACN and Teflon approaches, are shown as blue and green lines respectively in Figure 3. The agreement between the two methods was within an experimental error of less than 20%. The ACN based calibration was very repeatable with a variation of less than 5%. The reason for this variation was difference of the sample position relative to reference or sample-to-sample variation (surface quality such as granularity and packing fraction). The repeatability was verified by preparing a number of samples of the same materials and calibrating them in the above mentioned approach. Further, to verify that the calibration is independent of the detection system, we varied the sample and reference position (which changes the overall collection efficiency of the detection system) and found a similar degree of repeatability. For the spectra measured with 238-nm excitation shown in Figure 3, the laser power was ~18 mw and the total exposure time,, varied from 30 sec (for Ammonium Perchlorate) to 450 sec (for TNT). In all our experiments, ( )

5 was obtained by summing spectra measured sequentially from multiple, identical, time exposures. For example Ammonium Perchlorate was obtained in three exposures of 10 seconds each. While the purpose for multiple exposures was to filter any cosmic ray peaks during long exposure events; it also served to identify any nonlinear dosage (defined as excitation intensity exposure time) dependence of the measured spectra. The four explosives in Figure 3 showed almost linear dose dependence, with a total variation between the spectra from identical first and the third time exposure to be less than 4%. However, several explosive spectra showed greater deviation from linear dependence on dosage. St Dev: 4.9% Figure 3: (a) Typical Raman Albedo spectra. Blue plots are based on ACN calibration approach and green are using Teflon approach. The two approaches typically match within an experimental error of 20%. (b) Repeatability of Raman Albedo measurement using ACN approach to sample positioning, sample preparation, and laser power measurement. 3.2 Explosive Raman Albedo Raman Albedo was characterized at a total of four wavelengths (229 nm, 238 nm, 244 nm, and 248 nm) for fourteen solid explosives. A list of measured explosives appears in Table 1. Ammonium Perchlorate Table 1: List of measured explosives HMX TNT Semtex A1 C4 Black Powder Watergel Dyno-AP PETN ANFO Potassium Perchlorate Urea Nitrate Ammonium Nitrate RDX Raman Albedos of the measured explosives are shown below in Figure 4

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8 Figure 4: Raman Albedo measurements of solid optically thick explosives at 4 excitation wavelengths It is important to note that the observed dependence on excitation wavelength of the measured Raman Band Albedo of pure nitrates is not easily explained using known solution-phase Raman cross-section measurements and molar absorptivity. For example, from the known solution-phase Raman cross-sections and molar absorptivities of ammonium nitrate, the Raman Albedo is expected to increase with decreasing wavelength in the range between 229 nm and 248 nm. However, our Raman Albedo measurements show an opposite trend. This suggests that time-dependent phenomena (e.g. sample degradation or photolysis) or more complex solid-state phenomena could be responsible. 3.3 Sample degradation and Raman Albedo Several explosives strongly absorb DUV radiation, resulting in photo-degradation, which results in decreasing Raman signal with time. In addition, certain explosives, like nitrates, exhibit chemical changes due to light absorption [9]. In our experiments, we measured three successive Raman spectra with equal exposures times and laser excitation intensities. From the three measurements, we calculated fractional variation of Raman Band Albedo (maximum minimum band albedo divided by mean band albedo). As an example at 244 nm, most explosives with the exception of ANFO (95%), AN (22%), Black Powder (33%), SEMTEX (52%) and Watergel (38%) showed a variation of less than 15% for their most prominent Raman band. Exact variations were dependent on the exposure dose, Despite such intensity variations, the measured Raman spectra closely represent spectra in a typical detection scenario. Hence, they are useful as real signatures and in calculation of signal strength from a sensor operating in a similar detection scenario. As a first order approximation, the dose dependence was approximated to ( ) ( ), allowing us to extrapolate the Raman Band Albedo of a fresh sample,. Here, is the dose constant, which was determined using a linear fit to the time-dependent data. The approximation is valid in the limit of small dose. 3.4 Estimation of Raman Signal Photons The Raman band albedo presented in Table 2 can be used for signal estimation during the design of an analytical instrument. Using the measured band Albedo ( ) of explosives, detected power in a given band can be estimated for a given laser power ( ) and overall detection efficiency ( ) as ( ). Here and are the collection lens diameter and standoff distance to the target, respectively. As an example, assume a 20-mw laser power illuminating a spot covered with an optically-thick explosive film at a stand-off distance of 50 feet. A sensor with a detection efficiency of 17% and a collection lens aperture of 2 inches, results in 380 counts/sec for PETN and 171 counts/sec for TNT at 244 nm excitation wavelength.

9 Table 2: Raman Band Albedo ( ) of prominent bands of explosives. Substances marked with * showed less than 10% deviation from linear dose dependence in our experiment ( mj/ m 2 ). Material Band shift cm nm 238 nm 244 nm 248 nm frac/sr ANFO AN AmPerchlorate* BlackPowder C4* DynoAP* PETN PotPerchlorate* SEMTEX TNT* UN* Watergel DISCUSSION We have introduced the concept of Raman Albedo for solid explosives and presented two calibration methods. The first method uses the known Raman cross-section of an ACN reference and the second method measures the laser light diffusely reflected from a Teflon target. Raman Albedo measurements based on both the methods agree well within our experimental error. We have measured Raman albedos of fourteen, optically thick solid explosives at four different deep-uv wavelengths. The results show a rich variation of Raman Band Albedo with wavelength. This suggests that a DUVRRS technique that uses multiple excitation wavelengths has a rich set of signatures that may offer higher selectivity than a single-excitation-wavelength technique. These Raman Albedo measurements are useful in estimating the signal strength when designing a potential stand-off explosive sensor based on deep-ultraviolet resonance Raman spectroscopy. 5. ACKNOWLEDGEMENT This work was funded by the Department of Homeland Security Science and Technology Directorate under contract number HSHQDC-09-C and the Office of Naval Research under contract number N C The authors wish to acknowledge the technical contributions of William McCormick, Mikhail Sluch, Robert Ice, and Robert Martin, and many helpful discussions with Prof. Sanford Asher. REFERENCES [1] Moore, D. Sensing and Imaging: An International Journal 2007, 8, [2] Moore, and D. Scharff, R. Analytical and Bioanalytical Chemistry 2009, 393, [3] Asher, S. Analytical Chemistry 1993, 65, [4] Asher, S. In Ultraviolet raman spectrometry, Chalmers, J., and Griffiths, P., Eds., John Wiley & Sons, 2002, Vol. 1, pp [5] Asher, S., Johnson, C. Science 1984, 225, 311. [6] Tuschel, D.; Mikhonin, A.; Lemoff, B.; Asher, S. "Deep UV Resonance Raman Excitation Enables Explosives Detection", Applied Spectroscopy 2010, 64, [7] L. Wang and S.A. Asher, Refractive-Index Matching Avoids Local Field Corrections and Scattering Bias in Solid- State Na2SO4 Ultraviolet Raman Cross-Section Measurements, Applied Spectroscopy, 2012, 66, [8] Yellampelle, B; Sluch, M; Asher, S; Lemoff B; Multiple-excitation-wavelength resonance-raman explosives detection, SPIE Defense, Security, and Sensing, [9] S.A. Asher, D.D. Tuschel, T.A. Vargson, L. Wang and S. J. Geib, Solid State and Solution Nitrate Photochemistry: Photochemical Evolution of the Solid State Lattice, J. Phys. Chem. A, 2011, 115, [10] J. M. Dudik, C. R. Johnson, and S. A. Asher, "Wavelength Dependence of the Preresonance Raman Cross Sections of CH3CN, SO42-, ClO4- and NO3-", J. Chem. Phys. 1985, 82,