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Some ideas about Spectroscopy
Raman Spectroscopy
Laser Raman Spectrometer
What is spectroscopy?
Light interacting with matter as an analytical tool
“when we look at the universe in a different ‘light’ i.e., at ‘non visible’ wave length, we probe different kinds of physical conditions and we can see new kinds of objects”
Electromagnetic Spectrum
Spectroscopic data is often represented by a spectrum, a plot of the intensity of radiation as a function of wavelength or frequency.
Spectrum of Benzene molecule
Introduction to Raman spectroscopy
Sir Chandrasekhara Venkata Raman . November 7, 1888 - November 21, 1970
. Won the Nobel prize in 1930 for Physics
. Discovered the “Raman effect”
. Besides discovering the Raman effect he
studied extensively in X-ray Diffractions,
Acoustics, Optics, Dielectrics and
Colloidal solutions.
When a monochromatic radiation of frequency ʋ is passed through a non absorbing medium,it is found that most of it is transmitted without any change, and some of it is scattered. Ifthe scattered energy is analyzed by means of a spectrometer, the bulk of the energy is found at the frequency of the incident beam ʋ˳ but a small portion of the scattered energy will be found at frequencies ʋ´=ʋ˳±nM. The scattering of radiation with change of frequency is called Raman scattering.
In Raman spectroscopy, by varying the frequency of the radiation, a spectrum can be produced, showing the intensity of the exiting radiation for each frequency. This spectrum will show which frequencies of radiation have been absorbed by the molecule to raise it to higher vibrational energy states.
When Light hits a sample, It is Excited, and is forced to vibrate and move. It is these vibrations which we are measuring.
Atoms are at a certain energy level at any given time.
As a laser light hits the atom, it is excited and reaches a higher level of energy, and then is brought back down.
If an atom is at a given energy level, it can be excited then fall below the original level.
Anti-stokes spectrum are mirror spectrums of Stokes Raman Spectrums
Energy Scheme for Photon Scattering
Rayleigh Scatterin
g (elastic)
The Raman effect comprises a very small fraction,about 1 in 107 of the incident photons .
Stokes Scatterin
g
Anti-Stokes Scattering
hn0
hn0
hn0 hn0-hnm
hn0+hnm
E0
E0+hnm
Raman (inelastic)
Virtual State
IR Absorptio
n
En
erg
y
Raman Spectrum
A Raman spectrum is a plot of the intensity of Raman scattered radiation as a function of its frequency difference from the incident radiation (usually in units of wavenumbers, cm-1). This difference is called the Raman shift.
Laser Raman spectrometer
A typical Raman System
Raman Instruments
A modern Raman spectrometer
Days before Laser..
Commonly used sources were 435.8nm and 253.6nm emission lines of mercury vapour
Disadvantages the source is an extended one and the brightness available per unit area is very small the relatively high frequency of mercury radiation often causes the sample to fluorescence as colored samples absorb in this high frequency region, it is not possible to record their spectra
With the discovery of lasers …
Advantages
excellent monochromaticity good beam focusing capabilities and small line widths the second order Raman spectra can be recorded the broadening due to Doppler effect can be minimized
Working
Lasers using in Raman spectroscopyLaser sources for Raman spectroscopy include laser diodes,
diode-pumped lasers and ion lasers.
The Innova 300C and 70C series of small-frame argon or krypton ion lasers are also well suited for Raman experiments in the visible region of the spectrum.
Innova 70C Spectrum is a mixed gas lasers that can generate a number of laser lines from the UV to the near IR
Laser wavelengths ranging from ultra-violet through visible to near infra-red can be used for Raman spectroscopy. Typical examples include,
Ultra-violet: 244 nm, 257 nm, 325 nm, 364 nm
Visible: 457 nm, 473 nm, 488 nm, 514 nm, 532 nm, 633 nm, 660 nm
Near infra-red: 785 nm, 830 nm, 980 nm, 1064 nm
The Invictus 785-nm NIR laser is the excitation laser of choice for the majority of Raman spectroscopy applications from pharmaceutical to polymers.
The Invictus 830-nm NIR laser has been developed for biomedical applications of Raman spectroscopy where sample absorption characteristics require longer excitation wavelengths and reduced spectral range.
The Invictus 532-nm VIS laser is used for specific classes of Raman spectroscopy including gas phase measurements.
The choice of laser wavelength has an important impact on experimental capabilities:
Sensitivity
Spatial resolution
Optimisation of resulting based on sample behaviour.
Laser filters using in Raman spectroscopy
Optical filters
Edge
Holographic notch
Typical gratings used for Raman vary from perhaps 300gr/mm (low resolution) through to 1800gr/mm (high resolution) – more specialised gratings (including 2400gr/mm and 3600gr/mm) are also available, but have certain limitations, and should not be considered general purpose.
Raman spectrometers typically use holographic gratings, which normally have much less manufacturing defects in their structure than ruled gratings. Stray light produced by holographic gratings is about an order of magnitude less intense than from the ruled gratings of the same groove density.
Gratings using in Raman spectroscopy
Charge Coupled Device (CCD) detector is the “camera” usedto detect the Raman spectrum. A CCD detector is a two dimensional array of very low noise, silicon detectors.
Typical CCD chip.
Detectors used in Raman spectroscopy
Single-Channel Detectors
Photomultiplier Tubes Photodiodes
Array Detectors
Photographic Emulsion
Photodiode Arrays
Non-Silicon Array Detectors
Applications of Raman spectroscopy
Raman spectroscopy is commonly used in chemistry, since vibrational information is specific to the chemical bonds and symmetry of molecules. Therefore, it provides a fingerprint by which the molecule can be identified.
In solid-state physics, spontaneous Raman spectroscopy is used to, characterize materials, measure temperature, and find the crystallographic orientation of a sample.
Raman spectroscopy can be used to observe other low frequency excitations of the solid, such as plasmons, magnons, and superconducting gap excitations
Spatially-offset Raman spectroscopy (SORS), which is less sensitive to surface layers than conventional Raman, can be used to discover counterfeit drugs without opening their packaging, and for non-invasive monitoring of biological tissue
Raman spectroscopy can be used to investigate the chemical composition of historical documents such as and contribute to knowledge of the social and economic conditions at the time the documents were produced.
Raman spectroscopy is being investigated as a means to detect explosives for airport security.
Raman spectroscopy can be used as a technique for identification of seafloor hydrothermal and cold seep minerals
Used to discriminate between healthy and unhealthy tissues, or to determine the degree of progress of a certain disease.
Used in medicine , aiming to the development of new therapeutic drugs and in the diagnosis of arteriosclerosis and cancer.
References
Colin N. Banwell, Elaine M.McCash, 1994. Fundamentals of Spectroscopy, Tata McGraw-Hill Publishing Company Limited, New Delhi, 308p.
B B Laud, 1991.Lasers and non linear optics, New age International(P) Limited, New Delhi,261p.
H S Randhawa, 2003. Modern Molecular Spectroscopy, Macmillan India LTD, New Delhi,584p.
Thank you..
