Abstract for 11th GeoRaman International Conference, June 15-19, 2014, St. Louis, Missouri, USA

A COMBINED TIME-RESOLVED LIBS-RAMAN SYSTEM FOR SURFACE CHEMICAL ANALYSIS AT STANDOFF DISTANCES. S. K. Sharma, A. K. Misra T. E. Acosta-Maeda, and D. E. Bates, University of Hawaii, Hawaii Institute of Geophysics and Planetology, Honolulu, HI-96822 USA. sksharma@soest.hawaii.edu

Introduction: Applications of active spectro-

scopic techniques for molecular detection and mineral phase identification, combined with surface chemical composition at stand-off distances during day-time and nighttime, has been a very active field of research. Standoff Raman spectroscopy has been demonstrated to detect minerals, organic, and biogenic materials at radial distances of >100 m distance [1, 2] as well as from a robotic platform [3]. A combination of Raman with other complementary techniques such as Laser Induced Breakdown Spectroscopy (LIBS) and Laser-induced Native Fluorescence (LINF) show an enor-mous potential for planetary surface analysis as well as for other terrestrial applications including mining, ex-plosive detection, and work in harsh and remote envi-ronments [4, 5]. For future sample return missions, precise standoff analysis of drill cores will be a very valuable tool for selecting the appropriate samples, simplifying the operation strategy, and allowing a faster screening of the surroundings of a rover or lan-der, in particular, approaching outcrops.

Standoff Raman could also be applied with great advantage to Astrobiological applications in the study of water and ice and for detecting salts and organics in the subsurface of ice [6, 7]. For LIBS measurements, there is a need to induce plasma on the sample surface, which restricts the range of moderate size systems to tens of meters.

In this work, we report the results of a combined LIBS-Raman system operating in the range of 457-850 nm with high spectral resolution using 532-nm pulse laser excitation. The custom F/1.8 spectrograph is based on three volume holographic gratings and uses an intensified CCD detector that allows measurements of both time-resolved Raman and LIBS spectrum dur-ing daytime and nighttime.

Experimental Methods: Figure 1 shows a sche-matic diagram of a combined Raman spectroscopy and LIBS system. The integrated remote Raman and LIBS system uses the following common components: (i) a telescope (Meade LX200R Advanced Ritchey-Chrétien, 203 mm clear aperture, f/10); (ii) a fre-quency-doubled mini Nd:YAG pulsed laser source (Quantel Laser, CFR model, 532 nm, 110 mJ/pulse, 15 Hz, pulse width 10 ns) (iii) a 10x beam expander (iv) an intensified CCD detector (Princeton Instruments, PI-MAX); and (v) a custom F/1.8 holographic grating based spectrograph operating in the range 457-850 nm. The laser photon density at the sample was adjusted by changing the laser spot diameter from 10 mm for Ra-

man experiments to 530µm for the LIBS experiments. The pure Raman and LIBS spectra of samples at ambi-ent conditions were measured with the ICCD gate of 100 ns for Raman and 2 µs for LIBS, respectively. Both Raman and LIBS spectra could be recorded with a single laser shot when operating the laser in single pulse mode or with two laser shots when operating in double pulse mode.

Figure 1. Schematics of combined remote Raman and LIBS system using 532 nm pulse laser.

For measurements of the emission lines of Si, Al,

and Mg in the UV part of the electromagnetic spec-trum, we used a F/4 SPEX 270M imaging spectrograph equipped with a UV intensified CCD detector config-ured to cover a spectral range of 200-480 nm. The spectrograph has a unique Czerny–Turner configura-tion for optical correction that provides for exceptional imaging capabilities. The spectrograph was interfaced to a second 203-mm diameter telescope without the corrector plate. The primary and the secondary mirrors of this second telescope were recoated with Al-film protected with a MgF2 coating that produced 95% re-flectivity in the deep UV.

Samples: The rock-forming mineral samples were from Ward’s Natural Science Establishment, Inc., Rochester, New York. These samples were used as targets without any cleaning, polishing, or cutting. Samples of chemicals used in homemade explosives (ammonium nitrate, potassium nitrate, potassium per-chlorate, sulfur, etc.) were analytical grade chemicals in powder form obtained from Fisher Scientific, USA.

Results and discussion: Figure 2 shows the LIBS spectral image and the processed spectrum of iron metal measured with a custom holographic spectrome-ter in the range 457-850 nm and a double 532 nm laser pulse. The ICCD gain was set to 25. We identify all iron emission lines in this spectral range.

5029.pdf11th International GeoRaman Conference (2014)

Abstract for 11th GeoRaman International Conference, June 15-19, 2014, St. Louis, Missouri, USA

Figure 2. Iron (Fe) metal LIBS image and spectrum meas-ured using the custom holographic grating spectrograph with double pulse at 9 m distance, 532 nm, 100 mJ/pulse, 1 double pulse, pulse separation 1 µs, gate delay 2 µs, gate 10 µs.

Figure 3. Single and double pulse LIBS spectra of gypsum and anhydrite at 9 m distance.

Figure 3 shows the standoff LIBS spectra of gyp-sum and anhydrite measured with a single and double pulse of 532-nm excitation at 9 m. The intensities of the LIBS spectra for both gypsum and anhydrite with the double pulse method are much higher by a factor of 2.5 to 4 than the intensities of corresponding lines with single pulse measurements. Similar enhancement of LIBS intensities have been observed with double pulse excitation in the 200-480 nm spectral range. Both gypsum and anhydrite LIBS spectra show the presence of trace amounts of Li, Na, K, and Ba. The intensity of the hydrogen line at 656.28 nm is higher for gypsum than the intensity of the corresponding line in anhy-

drite, as is anticipated due to presence of water mole-cules in gypsum.

Figure 4 shows the Raman spectra of sulfur and gypsum measured with a single 532-nm laser shot with intensified set to maximum (256) gain, at a distance of 120 m in the daytime. The Raman fingerprints of sul-fur and gypsum do allow for easy detection of these chemicals. Detection of the LIBS spectrum of sulfur under ambient conditions is difficult because of highly reactive sulfur species in the plasma [8]

Figure 4. Standoff Raman spectra of sulfur powder and gypsum rock from a distance of 120 m with a single 532-nm laser pulse excitation showing five measurements for each.

In conclusion, the custom spectrograph with a gated intensifier allows the measurement of both LIBS and Raman spectra with a single laser in the spectral range 457-850 nm. The LIBS spectra at a standoff dis-tance of 9 m provide detailed information about the major, minor, and trace elements in the sample. The standoff Raman spectra of samples allow measure-ments at much longer (120 m) distances, and allow identification of their molecular and mineral structure from their respective Raman fingerprints. The com-bined Raman-LIBS instrument indentifies molecules and mineral phases and gives a detailed elemental analysis of the target samples.

Acknowledgement: This work was supported in part by research grants from NASA and the Office of Naval Research.

References: [1] Sharma, S. K., et al. (2006) Appl. Spectrosc. 60, 871-876. [2] Misra, A. K., et al. (2012) Appl. Spectrosc. 66, 1279-1285. [3] Abedin, M. N, et al. (2013) Appl. Optics, 52, 3124-3126. [4] Sharma, S. K., et al. (2009) Spectrochim. Acta, A 73, 468-476. [5] Moros, J. and Laserna, J. J. (2011) Anal. Chem. 83, 6275-6285. [6] Rull, F., et al (2011) Spectrochim. Acta, A 80, 148-155. [7] Sharma, S. K., et al. (2014) LPSC, 45, 1678. [8] Sharma, S. K., et al. (2007), Spec-trochim. Acta, A, 68, 1036-1045.

5029.pdf11th International GeoRaman Conference (2014)