Coherent Anti Stokes Raman Spectroscopy

ANKIT JOSHI SEMESTER 2

1. TOPIC – COHERENT ANTI-STOKES RAMAN SPECTROSCOPY (CARS) S.P.C. G O V T . C OLL E G E , A J MER SEMINAR SESSION : 2021 - 22 SUBMITTED TO : DEPARTMENT OF CHEMISTRY S U B M I T T ED B Y : ANKIT JOSHI M . S c . PREVIOUS CHEM I S T R Y SEM E S T ER – 2 nd

2. CONTENTS :- Introduction Theoretical Background Some Basics Of Rayleigh And Raman Scattering Coherent Anti-Stokes Raman Spectroscopy (CARS) :- CARS Process Advantages of CARS Limitations of CARS Applications of CARS Summary

3. Introduction A spectroscopic technique based on coherent anti-Stokes Raman scattering (CARS) was first demonstrated by Maker and Terhune in 1965 . CARS spectroscopy is a powerful technique which has been widely applied essentially in interdisciplinary research fields on the borders of biology, chemistry , physics, healthcare, defense, remote sensing, forensics , material science and so on . The recent breakthrough achievements such as detection bacterial spores, implementation of coherent Raman microscopy, gas-phase thermometry of reacting and non-reacting flows and many others have been its state-of- art successes.

4. Theoretical Background We have read earlier that the probability or relative intensity of Rayleigh lines, stokes and anti-stoke lines occurs inside solutions are in : : Anti-stock lines have the least probability of occurring in the solution because when the ground state falls bellow the original level in the anti-stokes lines, its probability decreases. And when we take its signal, it fails. Thus anti-stoke lines are so weak that’s why we cannot study it with the help of normal R aman spectroscopy, So we need coherent anti-stokes Raman spectroscopy (CARS). A simple comprehensive theory for CARS is introduced in this PPT. It begins with CARS formulation and , based on obtained solutions, a newly recognized (somewhat neglected in the past) effect such as enhancement in coherent Raman spectroscopy at positive probe delay is introduced in detail . At the end of this section, the experimental observations are elucidated with the CARS formulation .  

5. SOME BASICS OF RAYLEIGH AND RAMAN SCATTERING The source of energy in R aman spectroscopy is laser, so l aser is used in R aman spectroscopy. Scattering of light – When sunlight enters the atmosphere of the earth. The atoms and molecules of different gasses present in the air absorb the light. Then these atoms re-emit light in all directions. This process is known as scattering of light. The atoms or particles that scatter light are called scatters. Radiation – Radiation is energy or particles that comes from a source and travel through space at the speed of light.

6. TYPES OF SCATTERING Elastic scattering   2 . Inelastic scattering If the energy of the incident beam of light and the scattered beam of light are same, then it is called as ‘elastic scattering’. If the energy of the incident beam of light and the scattered beam of light are not same, then it is called as ‘inelastic scattering’.

7. scattering   R aman scattering Rayleigh scattering is the elastic scattering process in which the electromagnetic radiation is elastically deflected by particles of matter, without a change of frequency but with a phase change. Raman scattering is the inelastic scattering of photons by matter, meaning that there is both an exchange of energy and a change in the light’s direction. Raman Stokes scattering When the energy of the scattered photons is less than the incident photons, the scattering is known as R aman Stokes scattering. When the energy of the scattered photons is more than the incident photons, the scattering is known as R aman anti-Stokes scattering. Raman anti-Stokes scattering

8. h υ 1 h υ 1 h υ 1 h υ 2 h υ 2 - Δυ h υ 2 + Δυ Δ υ Incident radiation T ransmitted radiation

9. Coherent Anti-Stokes Raman Spectroscopy (CARS) :- In this technique of coherent anti-Stokes Raman spectroscopy (CARS), two sufficient laser beams of high intensity with frequency and are focussed on a sample. M ixing in the sample generates a new coherent beam of low intensity at a frequency 2 1 - 2 = 3 . If frequency is fixed and the value of frequency υ 2 is varies such that : υ 1 - υ 2 = υ R Then the scattered radiations are occurs i.e. υ 1 + υ R = υ 3   Where υ 1 = Incident radiations frequency υ 2 = Transmitted radiations frequency υ R = Raman active vibrational of molecular system υ 3 = Relative frequency in υ 1 Or Anti-stoke Raman frequency

10. The relative radiation frequency υ 3 of υ 1 is the anti-stokes Raman radiation. Which is very intense and paired. This is called coherent anti-stokes Raman spectroscopy (CARS). The intensity of a Raman spectrum can be increased by CARS. This technique is based on the fact that if two laser radiation whose frequency are υ 1 and υ 2 , is passed through a sample. So they are paired in such a way that the coherent frequencies of the different values can be obtained. One of these frequencies can be written as :- υ ’ = 2 υ 1 - ( υ 1 - Δυ ) υ ’ = υ 1 + Δυ This frequencies is the corresponding frequency of the anti-stokes line. In this way, coherent radiation of high intensity will be obtained which are called coherent anti-stokes Raman spectroscopy ( CARS).

11. CARS Process The CARS process consists of two coherent laser radiation beams υ 1 and υ 2 , which are almost collinear in the molecular medium. If the frequency υ 2 of the other beam is varied by keeping the frequency υ 1 constant of one beam. Then the frequency difference between the two beams come. Which is equal to Raman shift υ R . Due to which a third beam υ 3 is produced which is called coherent anti-stokes Raman spectroscopy ( CARS).

12. This is almost parallel to the incident beam ( υ 1 ) and It is also paired with them. υ 3 = υ 1 + ( υ 1 - υ 2 ) = 2 υ 1 - υ 2 or υ 3 = υ 1 + υ R The υ 3 beam can be separated from the incident beam ( υ 1 ) by filtering. The CARS process depends on the square of the normal Raman scattering and the square of the number of molecules.

13. Advantages of CARS CARS signals are stronger by 8-10 orders of magnitude than normal R aman experiment. CARS signal can be easily visualized compared to fluorescence. Scattering intensity is increase enormously. ( It is non linear technique. It reveals high resolution which is determined by line width of lasers. Even microquantities ( - ) can be detected by resonance CARS. High Raman conversion efficiencies are obtained. Excellence collection efficiency is possible since CARS is generated as a beam. Narrow spectra are obtained without the need for a monochromator.  

14. Limitations of CARS Requires complicated set up, difficult adjustment costly equipment. Spectra evaluation is non-trivial. Band shapes are often distorted. Samples may decompose by high power laser beam focusing. Signal fluctuation are caused by frequent instabilities of the lasers. No quantitative conclusion can be drawn from signal intensity.

15. Applications of CARS Molecular structure determination is important tool. To study the rotational spectrum of gases. Even difference between υ rot. & υ vib. a nd derivation from Boltzmann distribution can be analysed. Gas phase combustion process are mostly studied by CARS. (CO2, CO, O2, N2, CH4, H2O, H2 etc. in flames can be studied). Excellence technique for studying biological samples in aqueous solution which often produce strong fluorescence in normal Raman scattering. Enhancement of CARS signals occurs of radiation approach the electronic absorption frequency of the system. Medicinal samples like ferrocytochrome, cyanocobolamin have been studied in dilute ( solution. Numerous application of CARS in molecular beam studies energy transfer and plasma diagnostics are excepted.  

16. Summary This ppt presented a brief overview to the basics of coherent anti-Stokes Raman spectroscopy. First we introduced the CARS technique and its strengths and barriers. Namely , we presented the integral formulae for coherent anti-Stokes and Stokes Raman scattering , and discussed the closed-form solutions, its complex error function, and the formula for maximum enhancement of the inferred pure coherent Raman spectra. The time-resolved coherent Stokes Raman scattering experimental observations were also quantitatively elucidated as an example . Moreover , various experimental realizations of narrowband probe pulses were illustratively explained. Finally , several experimental data were presented and discussed based on all Gaussian approach presented in this review. Understanding the essentials of coherent Raman spectroscopy promotes importance of a number of experiments including the ones utilizing a broadband excitation with a narrowband delayed probing for successful background suppression emphasized in this work.

17. References : FUNDAMENTALS OF MOLECULAR SPECTROSCOPY BY P. S. SINDHU

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