Fig 2 - uploaded by Thomas J. Kulp
Content may be subject to copyright.
Source publication
Ultraviolet (UV) Raman scattering with a 244-nm laser is evaluated for standoff detection of explosive compounds. The measured Raman scattering albedo is incorporated into a performance model that focused on standoff detection of trace levels of explosives. This model shows that detection at 100 m would likely require tens of seconds, discouraging...
Contexts in source publication
Context 1
... detection of trace levels of explosive compounds has been held at the highest priority by defense and security concerns for the past decade [1,2]. As a result, investments have been made toward developing optical standoff detection approaches [3], including visible Raman scattering [4,5], laser-induced breakdown spectroscopy (LIBS) [6,7], photofragment (PF)-based approaches [8], coherent anti-Stokes Raman scattering (CARS) [9], and both passive and active infrared detection [10]. Thus far not one of these approaches has demonstrated the potential to achieve standoff detection of explosives traces with a field-deployable system on a variety of surfaces in realistic conditions. However, very recently the Swedish Defense Research Agency reported the use of ultraviolet (UV) Raman scattering for field-detection of vapor-phase nitromethane at 13 m [11]. Compared to the aforementioned alternatives, we note four differentiating features of UV Raman scattering: (1) Raman scattering is non-destructive, allowing repeatable optical sampling and signal averaging; (2) Raman scattering provides a unique chemical fingerprint of the molecule for improved specificity; (3) Raman scattering is a linear process, so the laser beam does not have to be focused at the sample to enhance the signal; (4) UV Raman scattering potentially allows the use of eye-safe laser fluences at UV wavelengths; (5) the use of UV lasers corresponds to Raman shifts of ~10 nm, less than the Stokes fluorescence shift from commo n materials, providing “fluorescence - free” measurements. While the first three advantages apply to Raman scattering in the visible wavelength range as well, the last two advantages are specific to UV probing. Also, compared to visible Raman scattering, UV Raman scattering from explosive compounds is expected to generate signals greater by several orders of magnitude due to the resonance enhancement of the Raman cross section [11-15]. In this work we evaluate the potential of standoff detection of explosives at trace levels by UV Raman scattering on typical surfaces, for use as either a stand-alone approach or as a confirmatory channel. We acquire UV Raman scattering of trace levels of the explosive TNT using a spectrally calibrated system. The measured results are implemented into a performance model to allow assessment of instrument performance. In addition to characterizing the photons resulting from the explosive compounds, the other two quantities required for detection assessment are the relative number and distinguishability of the photons generated by common backgrounds and interferences (in comparison to those generated by the explosive compounds). In other words, the ultimate performance of a detection system depends not only on capturing a sufficient number of photons from the explosive, but also on whether those photons both sufficiently exceed and are sufficiently distinguishable from those resulting from backgrounds and interferences. A Raman LIDAR consists of a co-located transmitter and receiver, which are responsible for projecting excitation light onto the target and collecting Raman scattering from it. A simple schematic diagram of a Raman LIDAR is shown in Fig. 1. The transmitter is a laser that is used in conjunction with formatting and steering optics (not shown) so that its beam intersects the field-of- view of the receiver at the point to be measured on the target. The receiver collects the scatter from the target using a large optic, such as a telescope, imaging the radiating target area onto the entrance aperture of some type of spectral-dispersion device, such as the Czerny-Turner spectrograph illustrated in Fig. 1. A rejection filter is placed in the path between the receiver optic and the spectrograph, which rejects the strong backscatter at the laser frequency (i.e., photons not frequency- shifted by Raman scattering). The spectrograph disperses the light across an array detector, which records it and converts it to digital data. The radiometric properties of a LIDAR instrument are sufficiently well understood that one can accurately predicts its performance from appropriate laboratory measurements. This is the approach taken in this project. The following steps were taken to generate a confident performance estimate: (1) assembly of a lab-scale Raman transmitter and receiver; (2) calibration of the instrument in radiometric units; (3) comparison to published Raman cross-sections for selected explosives; and (4) extrapolation to field instrumentation and conditions. The results in each of these areas are described below. The laboratory Raman system developed for this project is shown in Fig. 2. The primary differences between it and a fieldable device lie in the type of transmitter and receiver optics used. Also, as shown in Fig. 2, the illumination and collection axes are orthogonal rather than nearly parallel, for reasons of mechanical convenience. Otherwise, the laser, spectrograph, and detector array all exhibit performance comparable to similar components that would be integrated into a fieldable device. The extrapolation of the performance of this system to that of a fieldable instrument (i.e., one which would use transmitter and receiver optics suitable for measuring a remote target) is straightforward. The specific components of the lab system are described as follows. The probe laser is a frequency- doubled continuous-wave (cw) Ar-ion laser (Cambridge Lasers Laboratories Model 3500) producing 244-nm light. The laser is formatted to a diameter of about 1-mm at the sample. It is worth noting that earlier measurements were made with tighter focusing (about 0.2 mm diameter); however, that caused photodegradation of the explosive. The receiver used for these measurements is only slightly modified from a system previously fabricated for the assessment of photofragment laser-induced fluorescence detection of surface-bound analytes [16,17]. The Raman scattering signal is collected by a pair of lenses (50-mm diameter; 300- mm focal length) that image the optical signal onto the entrance slit of a grating spectrograph. The spectrograph is a Czerny-Turner type (McPherson Inc., Chelmsford, MA; Model 218) that accepts light at f/5.3. Thus, its acceptance solid angle is slightly under-filled by that of the input light (f/6). Its entrance slit is oriented parallel to the laser beam axis and is adjusted to a width (typically 50 m) to maintain acceptable spectral resolution. Once dispersed, the optical signal is imaged by the spectrograph onto the photocathode of an intensified charge-coupled device (ICCD) array detector (Andor Model DH501i-25F-04) having 1024×128 pixels. Spectroscopic measurements are conducted by on-chip binning of the array pixels in the vertical dimension to create a linear, 1024-element-long spectrum. The photocathode has a near-uniform quantum efficiency of about 13-15% over the wavelength range of 200-300 nm (encompassing all signals measured in this project). The intensifier is gated to amplify photoelectron signals at gains of up to 400X for a 90-ms gate width. The calibration of the relevant laser parameter (power) is accomplished using an ordinary laser power meter and requires no more discussion. The receiver calibration is carried out by placing a diffusely scattering material (Spectralon) at the sample location and illuminating it with a UV lamp of known radiance. This allows the measured counts on a channel of the ICCD to be related to radiance from the sample (W/sr) within the appropriate wavelength range intercepted by that channel. The calibration function can be derived as follows. The manufacturer-supplied lamp spectral irradiance, [ I ( λ )] mfg (Wcm -2 nm -1 ), has been measured using a detector placed at 300 mm ( r mfg ) distance from the lamp and oriented at normal incidence to its illumination axis. In the configuration illustrated in Fig. 3, the irradiance I’ on the Spectralon target ...
Context 2
... detection of trace levels of explosive compounds has been held at the highest priority by defense and security concerns for the past decade [1,2]. As a result, investments have been made toward developing optical standoff detection approaches [3], including visible Raman scattering [4,5], laser-induced breakdown spectroscopy (LIBS) [6,7], photofragment (PF)-based approaches [8], coherent anti-Stokes Raman scattering (CARS) [9], and both passive and active infrared detection [10]. Thus far not one of these approaches has demonstrated the potential to achieve standoff detection of explosives traces with a field-deployable system on a variety of surfaces in realistic conditions. However, very recently the Swedish Defense Research Agency reported the use of ultraviolet (UV) Raman scattering for field-detection of vapor-phase nitromethane at 13 m [11]. Compared to the aforementioned alternatives, we note four differentiating features of UV Raman scattering: (1) Raman scattering is non-destructive, allowing repeatable optical sampling and signal averaging; (2) Raman scattering provides a unique chemical fingerprint of the molecule for improved specificity; (3) Raman scattering is a linear process, so the laser beam does not have to be focused at the sample to enhance the signal; (4) UV Raman scattering potentially allows the use of eye-safe laser fluences at UV wavelengths; (5) the use of UV lasers corresponds to Raman shifts of ~10 nm, less than the Stokes fluorescence shift from commo n materials, providing “fluorescence - free” measurements. While the first three advantages apply to Raman scattering in the visible wavelength range as well, the last two advantages are specific to UV probing. Also, compared to visible Raman scattering, UV Raman scattering from explosive compounds is expected to generate signals greater by several orders of magnitude due to the resonance enhancement of the Raman cross section [11-15]. In this work we evaluate the potential of standoff detection of explosives at trace levels by UV Raman scattering on typical surfaces, for use as either a stand-alone approach or as a confirmatory channel. We acquire UV Raman scattering of trace levels of the explosive TNT using a spectrally calibrated system. The measured results are implemented into a performance model to allow assessment of instrument performance. In addition to characterizing the photons resulting from the explosive compounds, the other two quantities required for detection assessment are the relative number and distinguishability of the photons generated by common backgrounds and interferences (in comparison to those generated by the explosive compounds). In other words, the ultimate performance of a detection system depends not only on capturing a sufficient number of photons from the explosive, but also on whether those photons both sufficiently exceed and are sufficiently distinguishable from those resulting from backgrounds and interferences. A Raman LIDAR consists of a co-located transmitter and receiver, which are responsible for projecting excitation light onto the target and collecting Raman scattering from it. A simple schematic diagram of a Raman LIDAR is shown in Fig. 1. The transmitter is a laser that is used in conjunction with formatting and steering optics (not shown) so that its beam intersects the field-of- view of the receiver at the point to be measured on the target. The receiver collects the scatter from the target using a large optic, such as a telescope, imaging the radiating target area onto the entrance aperture of some type of spectral-dispersion device, such as the Czerny-Turner spectrograph illustrated in Fig. 1. A rejection filter is placed in the path between the receiver optic and the spectrograph, which rejects the strong backscatter at the laser frequency (i.e., photons not frequency- shifted by Raman scattering). The spectrograph disperses the light across an array detector, which records it and converts it to digital data. The radiometric properties of a LIDAR instrument are sufficiently well understood that one can accurately predicts its performance from appropriate laboratory measurements. This is the approach taken in this project. The following steps were taken to generate a confident performance estimate: (1) assembly of a lab-scale Raman transmitter and receiver; (2) calibration of the instrument in radiometric units; (3) comparison to published Raman cross-sections for selected explosives; and (4) extrapolation to field instrumentation and conditions. The results in each of these areas are described below. The laboratory Raman system developed for this project is shown in Fig. 2. The primary differences between it and a fieldable device lie in the type of transmitter and receiver optics used. Also, as shown in Fig. 2, the illumination and collection axes are orthogonal rather than nearly parallel, for reasons of mechanical convenience. Otherwise, the laser, spectrograph, and detector array all exhibit performance comparable to similar components that would be integrated into a fieldable device. The extrapolation of the performance of this system to that of a fieldable instrument (i.e., one which would use transmitter and receiver optics suitable for measuring a remote target) is straightforward. The specific components of the lab system are described as follows. The probe laser is a frequency- doubled continuous-wave (cw) Ar-ion laser (Cambridge Lasers Laboratories Model 3500) producing 244-nm light. The laser is formatted to a diameter of about 1-mm at the sample. It is worth noting that earlier measurements were made with tighter focusing (about 0.2 mm diameter); however, that caused photodegradation of the explosive. The receiver used for these measurements is only slightly modified from a system previously fabricated for the assessment of photofragment laser-induced fluorescence detection of surface-bound analytes [16,17]. The Raman scattering signal is collected by a pair of lenses (50-mm diameter; 300- mm focal length) that image the optical signal onto the entrance slit of a grating spectrograph. The spectrograph is a Czerny-Turner type (McPherson Inc., Chelmsford, MA; Model 218) that accepts light at f/5.3. Thus, its acceptance solid angle is slightly under-filled by that of the input light (f/6). Its entrance slit is oriented parallel to the laser beam axis and is adjusted to a width (typically 50 m) to maintain acceptable spectral resolution. Once dispersed, the optical signal is imaged by the spectrograph onto the photocathode of an intensified charge-coupled device (ICCD) array detector (Andor Model DH501i-25F-04) having 1024×128 pixels. Spectroscopic measurements are conducted by on-chip binning of the array pixels in the vertical dimension to create a linear, 1024-element-long spectrum. The photocathode has a near-uniform quantum efficiency of about 13-15% over the wavelength range of 200-300 nm (encompassing all signals measured in this project). The intensifier is gated to amplify photoelectron signals at gains of up to 400X for a 90-ms gate width. The calibration of the relevant laser parameter (power) is accomplished using an ordinary laser power meter and requires no more discussion. The receiver calibration is carried out by placing a diffusely scattering material (Spectralon) at the sample location and illuminating it with a UV lamp of known radiance. This allows the measured counts on a channel of the ICCD to be related to radiance from the sample (W/sr) within the appropriate wavelength range intercepted by that channel. The calibration function can be derived as follows. The manufacturer-supplied lamp spectral irradiance, [ I ( λ )] mfg (Wcm -2 nm -1 ), has been measured using a detector placed at 300 mm ( r mfg ) distance from the lamp and oriented at normal incidence to its illumination axis. In the configuration illustrated in Fig. 3, the irradiance I’ on the Spectralon target ...
Similar publications
Various Au-Ag bimetallic alloy nanostructures were obtained as sensitive surface-enhanced Raman scattering (SERS) substrates by changing the thermal annealing sequence. The atomic force microscopy (AFM) and scanning electron microscopy (SEM) results confirm that Au/Ag bimetallic clusters and Ag-Au core-shell like structures can be designed by therm...
Undoped and Li-doped ZnO films were fabricated by screen printing approach on1 sapphire substrate. The effect of Li doping and annealing temperature on the luminescent, optical, electrical and structural properties of the films has been investigated by the photoluminescence (PL), Raman scattering, conductivity, Atomic Force microscopy and X-ray dif...
Raman spectroscopy has been used to identify bacterial strains, bacterial components, such as protein and DNA bases and the ratio of live to dead bacteria before and after exposure to ultraviolet (UV) radiation. New vibrational bands and changes in their intensity as a function of UV irradiation time have been recorded by high resolution Raman spec...
In this Letter, we investigate the energy-scaling rules of hollow-core fiber (HCF)-based nonlinear pulse propagation and compression merged with high-energy Yb-laser technology, in a regime where the effects such as plasma disturbance, optical damages, and setup size become important limiting parameters. As a demonstration, 70 mJ 230 fs pulses from...
The main objective of this thesis is to deepen the knowledge of fundamental properties of kesterite materials (Cu2ZnSnS4 and Cu2ZnSnSe4) and their solid solutions Cu2ZnSn(S,Se)4, which are used as absorber layers in solar cells. This principally included full characterization of structural and vibrational properties of these materials mainly using...
Citations
... However, development of these instruments has been ongoing for at least 20 years, driven by both military and planetary exploration missions. [416][417][418] These instruments may not be handheld yet, but have been vehicle-portable and humanportable. ...
Until very recently, handheld spectrometers were the domain of major analytical and security instrument companies, with turnkey analyzers using spectroscopic techniques from X-ray fluorescence (XRF) for elemental analysis (metals), to Raman, mid-infrared, and near-infrared (NIR) for molecular analysis (mostly organics). However, the past few years have seen rapid changes in this landscape with the introduction of handheld laser-induced breakdown spectroscopy (LIBS), smartphone spectroscopy focusing on medical diagnostics for low-resource areas, commercial engines that a variety of companies can build up into products, hyphenated or dual technology instruments, low-cost visible-shortwave NIR instruments selling directly to the public, and, most recently, portable hyperspectral imaging instruments. Successful handheld instruments are designed to give answers to non-scientist operators; therefore, their developers have put extensive resources into reliable identification algorithms, spectroscopic libraries or databases, and qualitative and quantitative calibrations. As spectroscopic instruments become smaller and lower cost, “engines” have emerged, leading to the possibility of being incorporated in consumer devices and smart appliances, part of the Internet of Things (IOT). This review outlines the technologies used in portable spectroscopy, discusses their applications, both qualitative and quantitative, and how instrument developers and vendors have approached giving actionable answers to non-scientists. It outlines concerns on crowdsourced data, especially for heterogeneous samples, and finally looks towards the future in areas like IOT, emerging technologies for instruments, and portable hyphenated and hyperspectral instruments.
... UVRR spectroscopy shows great promise as a sensitive technique for trace explosive detection. 1,5,8,[11][12][13][14][15]20 Most explosives show deep UV absorption bands below 260 nm. 1,5 Excitation of resonance Raman spectra also result in the absorption of numerous UV photons since the absorption cross sections exceed resonance Raman cross sections by ∼10 8 -fold. ...
Ultraviolet resonance Raman spectroscopy (UVRR) is being developed for standoff trace explosives detection. To accomplish this it is important to develop a deep understanding of the accompanying UV excited photochemistry of explosives, as well as the impact of reactions on the resulting photoproducts. In the work here we used 229 nm excited UVRR spectroscopy to monitor the photochemistry of pentaerythritol tetranitrate (PETN) in acetonitrile. We find that solutions of PETN in CD3CN photodegrade with a quantum yield of 0.08 ± 0.02, as measured by high performance liquid chromatography (HPLC). The initial step in the 229 nm UV photolysis of PETN in CD3CN is cleavage of an O-NO2 bond to form NO2. The accompanying photoproduct is pentaerythritol trinitrate (PETriN), (CH2ONO2)3CCH2OH formed by photolysis of a single O-NO2. The resulting UVRR spectra show a dominant photoproduct band at ~1308 cm⁻¹, which derives from the symmetric stretch of dissolved NO2. This photoproduct NO2 is hydrolyzed by trace amounts of water, which downshifts this 1308 cm⁻¹ NO2 Raman band due to the formation of molecular HNO3. The dissociation of HNO3 to NO3⁻ in the presence of additional water results in an intense NO3⁻ symmetric stretching UVRR band at 1044 cm⁻¹.
... Several groups have begun exploring the feasibility of standoff UV Raman for explosive detection. [141][142][143][144][145][146] [141][142][143] Waterbury et al. (Alakai Defense Systems) have recently constructed a truck mounted UV Raman system for standoff detection. [144] The Swedish Defense Research Agency has developed deep UV Raman instruments for standoff detection. ...
... Reichardt et al. constructed an instrument that used 244 nm excitation for the detection of TNT that employed a conventional Czerny-Turner spectrograph with an ICCD detector. [146] The authors utilized their measured spectral S/N ratios for standoff measurements to estimate the accumulation times necessary to detect TNT. They concluded that detection of >6 μg/cm 2 amounts of TNT at~100 m standoff distances would require >10 s accumulation times. ...
Due to terrorist use of explosive devices, intense interest has been directed towards the development of techniques and instrumentation to detect explosives. A number of analytical techniques are currently under investigation for trace explosive detection. Several methods have been successfully commercialized including colorimetric kits, fluorescence quenching sensors, and ion mobility spectrometers that see widespread field use. A key limitation of all currently utilized field techniques is that they require close proximity or physical interaction with the object being analyzed. The ideal trace explosive detection technique would be able to detect trace amounts of explosives from a distance to ensure personnel safety. Laser-based spectroscopies are the only currently viable techniques that can be utilized to sense trace amounts of explosives at standoff distances. This review compares currently used explosive detection techniques with recently developed standoff methods. We emphasize the development of UV resonance Raman spectroscopy for standoff explosive detection. This work has recently been enabled by advances in laser and spectrometer design. Trace explosive detection has also been advanced by the recently developed understanding of deep UV photochemistry for several explosive molecules. Copyright
... More recently, inkjet printing, using "inks" containing explosives or narcotics dissolved in an appropriate solvent, has been gaining increased attention as a method to prepare samples for validating and optimizing the response of various trace chemical sensors to a known mass of contraband material [15][16][17][18][19][20][21][22][23][24]. The current study builds on earlier efforts to develop inkjet printing for quantitative solution deposition [17,[24][25][26] while adding critical control and characterization of additional factors in the printed deposits. ...
A method has been developed to fabricate patterned arrays of micrometer-sized monodisperse solid particles of ammonium nitrate on hydrophobic silicon surfaces using inkjet printing. The method relies on dispensing one or more microdrops of a concentrated aqueous ammonium nitrate solution from a drop-on-demand (DOD) inkjet printer at specific locations on a silicon substrate rendered hydrophobic by a perfluorodecytrichlorosilane monolayer coating. The deposited liquid droplets form into the shape of a spherical shaped cap; during the evaporation process, a deposited liquid droplet maintains this geometry until it forms a solid micrometer sized particle. Arrays of solid particles are obtained by sequential translation of the printer stage. The use of DOD inkjet printing for fabrication of discrete particle arrays allows for precise control of particle characteristics (mass, diameter and height), as well as the particle number and spatial distribution on the substrate. The final mass of an individual particle is precisely determined by using gravimetric measurement of the average mass of solution ejected per microdrop. The primary application of this method is fabrication of test materials for the evaluation of spatially-resolved optical and mass spectrometry based sensors used for detecting particle residues of contraband materials, such as explosives or narcotics.
Portable Raman spectrometers have become smaller over the last 20 years, while their performance has increased. This has been made possible by closer coupling of all the components, use of transmission gratings rather than reflection gratings, and general advances in electronics, displays, and battery technologies. An obvious question to ask is whether this trend can continue. This paper describes the technologies and evolution of these instruments, existing limitations, the current landscape of miniature Raman spectrometers, and the state of the art. Finally, the paper also looks at what emerging technologies could be applied in this area, and how those could lead to new applications
The excellent chemical identification ability of Raman based
spectroscopy provides a versatile and widely applicable method to
identify hazards within a complex chemical environment. However, the
small spontaneous Raman scattering cross section can limit standoff
detection of trace quantities. Coherent anti-Stokes Raman scattering
provides the same chemical specificity, but with the potential for much
greater signal due to coherent addition in the non-linear spectroscopy.
Utilizing CARS, we have demonstrated μg/cm2 level
detection of an explosive simulant using a single laser producing less
than 8mW of laser power in the near IR. This detection level was
achieved on a simulant present as only a small part within a polymer
mixture. In addition, we present standoff chemical images of trace
compounds within a complex chemical environment, which effectively
demonstrate the unique capabilities of the method. Further, the temporal
pulse shape can be tailored to excite specific Raman transitions, adding
versatility to this method.
