Detection techniques
Thursday, June 04, 2020
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  Laboratory for cold plasma laser spectroscopy, Institute of Physics, Zagreb  

Major detection techniques:

Cavity Ring-Down Spectroscopy

crds.jpg Cavity RingDown Spectroscopy (CRDS) is based on the loss measurements within cavity formed by high-reflective mirrors. Tunable pulsed laser beam is injected into such cavity. The injected pulse oscillate back and fort hundreds of times. Fast detector behind back mirror of the cavity records exponential drop of the light intensity passing throught the cavity which gives lifetime of photons in the cavity. For empty cavity this characteristic time is due to cavity losses. By introducing into cavity plasma losses are increased due to scattering or/and absorption by plasma particles. Difference of lifetimes between empty and full cavity gives directly scattering or absorption coefficients. By tuning laser wavelength one easily obtains absorption spectum without taking care about empty cavity losess. Comparing to classical absorption methods CRDS is independent on laser intensity. This allows measurements of very small absorptions and gives very high sensitivity to the method in the range of ppb or ppt. We have adopted CRDS method for application to laser induced plasma where absorption change rapidly during the photon lifetime in the cavity. We have demonstrated observation of cold alkali dimers and radicals produced by laser ablation.

Laser Induced Breakdown Spectroscopy

libs.jpg Laser Induced Breakdown Spectroscopy (LIBS) allows qualitative and quantitative real-time determination of elemental composition in bulk, liquids or gases. Senistivities in the range of ppb e.g. pikograms are possible. General applications includes material analyses, enviormental monitoring, forensic and biomedical studies, restoration of arts etc. Method is based on focusing of high intensity pulsed laser beam on the target surface which could be at distance from few centimeters to few meters away. One laser pulse which typicaly could last for few nanoseconds is sufficient to evaporate any material and create a cloud of very hot plasma. This plasma decays fast within few microsecond after laser pulse. During cooling process atoms in plasma recombine and emits light at characteristic wavelengths. All elements emits in the wavelength range from 200 nm to 980 nm, and could be detected with this method. System for detection which we posses in our laboratory use seven miniature fiber spectrometers each equiped with linear array CCD with 2048 elements. All devices are sinhronized with laser pulse and could record full spectum within few miliseconds. Main advantages of the method are fast, almost non-destructive remote detection of the target composition. Disadvantages are, as with any optical emission study difficulty in obtaining quantitative data. We plan to overcome this by parallel use of CRDS and LIBS tehniques.

Optical Emission Spectroscopy

oes.jpg Optical Emission Spectroscopy (OES) using miniature mobile fiber CCD spectrometers allows for fast data collection in whole optical region from 200 nm to 1000 nm with high spectral resolution. New possibilities are opened for real time monitoring of various processes in plasmas and gases, in laboratory environment but also in the field or factory. Nowadays, to take spectrum is simple, but remaining problem is to identify and interpret them and understand underlying processes. Therefore new procedures should be developed to deal with thousands of spectra and accompanied with appropriate modeling of atomic and molecular processes.
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