Spectroscopy: Lecture 7. Anupam K. Misra HIGP, University of Hawaii, Honolulu, USA

Transcription

1 GG 711: Advanced Techniques in Geophysics and Materials Science Spectroscopy: Lecture 7 Remote Raman Spectroscopy Anupam K. Misra HIGP, University of Hawaii, Honolulu, USA

2 Remote Raman Spectroscopy Daytime rapid detection of Minerals and organics * Range: Up to 125 m * Detection time: 1 Second : some chemicals even with single laser shot. University of Hawaii

3 Raman Effect: (Discovered by C. V. Raman 1928 ) Incident Light Elastic Scattering (Mie-Rayleigh) Strong Phenomenon Energy Inelastic scattering (Raman) Weak Phenomenon Stokes Anti-Stokes Raman spectra Vibrational modes of molecules Unique spectrum for each chemical Depends on atomic mass, bond lengths, bond strength, configuration, etc

4 Commercial (traditional) Raman system (after 1962) CW Laser Beam splitter Sample Notch/edge Filter Spectrograph C. V. Raman 1928 CCD

5 Typical Raman spectra Calcite CaCO 3 Intensity (a.u.) Notch filter 801 Cyclohexane C 6 H Laser at 0 cm -1 Raman Shift (cm -1 )

6 Daytime conditions GeoPhysics (HIGP), Univ. of Hawaii

7 15000 Issues with daytime Raman spectra 1085 Calcite CaCO 3 Intensity (a.u.) Notch filter 801 CCD over saturated Cyclohexane C 6 H Laser at 0 cm Background too strong Raman Shift (cm -1 ) 2. Raman signal too weak.

8 Remote Raman Instrument What do you need for daytime measurements * Laser: 532 nm, Nd:YAG, PULSED, 20 Hz, 8 ns * Telescope = 5 or 8 (Maksutov-Cassegrain, Meade) * Kaiser F/1.8 Holospec Spectrometer * ICCD detector (gated), Princeton Instruments

9 Remote Raman System Design (2) (1) (3) (4) (5) Key components: (1) High power pulse laser (2) Beam expander (3) Telescope (4) Highly efficient Spectrograph (VPG) (5) ICCD (gated detection)

10 U. of Hawaii Raman Lab Lightening Condition

11 Calcite at 10 m, 1 s. Operating Modes: * CW (lights on) * CW (lights off) * Gated (lights on)

12 Provides High Signal to Background

13 Simplified concept 50 m Background Pulsed-Laser Telescope Laser pulse width = 10 ns (half width) ICCD Time of arrival for first Raman Photon = 100 m 3x10 8 = 0.33 µs Gate Width = 20 ns + τ (Raman) ( 20+ ) ns

14 50 m Pulsed-Laser Spectrograph Telescope Laser pulse rate = 20 Hz ICCD T = 1s means 20 measurements In 400 ns all the Raman photons were counted Background collection time = Gate width *20 ( ) ns

15 50 m Pulsed-Laser Spectrograph Telescope Laser pulse energy = 20 mj ICCD Power = Energy/time = 20 mj x 20 Hz = 0.4 W!!! Actually very large number of Photons per pulse (in 20 ns)

16 Low Background Large Signal High Signal to Background Very few cosmic ray peaks

17 Remote Raman Systems developed at UH 5 system (532 nm) (fiber optic coupled) (NASA) 5 system (532 nm) (direct coupled) (NASA) 8 system (532 nm) (ONR) 8 UV system (248 nm) (ONR) 16 UV system (248 nm) (JIEDDO) 8 Raman+LIBS system (ONR) 2 system (532 nm) (NASA)

18 5 Remote Raman System For Official Use Only

19 8 Remote Raman System Holmes Hall (128 m)

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21 Remote Raman Applications Capabilities & data As recorded data shown without any processing

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23 Single pulse Remote Raman Spectra of Naphthalene C 10 H Daytime Intensity (a.u.) pulse, 50 m m 0 1 pulse, 100 m 100 m Raman Shift (cm -1 ) 20 mj/pulse, 532 nm, 8 ns pulse width, gate width 2 µs, 8 telescope, 100 µm slit

24 Daytime Remote Raman spectra from 100 m, 1 s Intensity (a.u.) Nitromethane CH 3 NO 2 Nitrobenzene C 6 H 5 NO Methanol O X Raman Shift (cm -1 ) 20 mj/pulse, 532 nm, 8 ns pulse width, gate width 2 µs, 8 telescope, 100 µm slit

25 Daytime Remote Raman spectra from 100 m, 1 s Intensity (a.u.) O O Raman Shift (cm -1 ) NH 4 NO 3 KClO 4 N N 2 20 mj/pulse, 532 nm, 8 ns pulse width, gate width 2 µs, 8 telescope, 100 µm slit

26 Acids at 50 m, 1s Intensity (a.u.) Nitric acid Sulfuric acid Raman Shift (cm -1 )

27 Single Pulse spectra of Sulfur from 50 and 100 m Intensity (a.u.) m m Raman Shift (cm -1 )

28 Application : Geology

29 2 system using 85 mm camera lens Detection at 50 m

30 remote Raman system daytime at 50 m Detection of home made explosive chemicals Provides clear sharp peaks for chemical identification with high SNR 10 s integration Intensity (a.u.) KClO 4 KNO 3 0 NH 4 NO Raman Shift (cm -1 ) 532 nm system, 50 ns gate width, 20 Hz, 30 mj/pulse

31 2 remote Raman system daytime at 50 m Can measure through plastic and glass bottles s Water Intensity (a.u.) propanol Acetone Raman Shift (cm -1 )

32 2 remote Raman system daytime at 50 m Can easily distinguish between very similar chemicals s Intensity (a.u.) Nitrobenzene Ethyl benzene N O O Benzene Raman Shift (cm -1 )

33 2 remote Raman system at 50 m Can measure target, atmosphere, and both Target (Gypsum at 50 m, 10s) * Intensity (a.u.) O 2 N 2 H 2 O Target with atmosphere, 350 ns Atmosphere before target, 310 ns Target, 50 ns * Atmospheric Rotational bands Raman Shift (cm -1 )

34 Signal is proportional to size of collection optics Ammonium Nitrate 50 m Intensity (a.u.) System 5 System Raman Shift (cm -1 )

35 Signal is proportional to laser pulse power Single Pulse Raman Excitation, 532 nm 8% RDX on silica, 9 m Intensity (a.u.) , Double 94 mj 94 mj 35 mj Raman Shift (cm -1 )

36 Signal is proportional to number of laser pulses Cyclohexane, 50 m Intensity (a.u.) O N Second Raman Shift (cm -1 ) 1 pulse 532nm, 20 Hz laser, 1 s = 20 pulses

37 Signal is inversely proportional to distance Single pulse detection of Naphthalene C 10 H Intensity (a.u.) pulse, 50 m m 0 1 pulse, 100 m 100 m Raman Shift (cm -1 ) 20 mj/pulse, 532 nm, 8 ns pulse width, 8 telescope

38 University of Hawaii Raman Group December 2007 Fort Irwin, Mojave Desert Successful field test for chemical detection during dust storm (O 2 ) (O 2 ) Raman Shift (cm -1 )

39 University of Hawaii Raman Group December 2007 Fort Irwin, Mojave Desert Show Video (57 seconds long) of - UH remote Raman system in field during dust storm - Single pulse spectra of gypsum at 50 m (measurements through two ¼ thick glass windows)