Introduction to spectroscopy basics.pptx

spectrsocopy basics

1. “Seeing the unseeable”

2. Electromagnetic radiation

3. Maxwell's Theory of Electromagnetic radiation Maxewell found that electromagnetic radiation is made up of two mutually perpendicular electric and magnetic fields in planes at right angles to each other as a sign wave.

4. one second λ 1 λ 3 λ 2 ν 1 = 4 cycles/second ν 2 = 8 cycles/second ν 3 = 16 cycles/second amplitude peak trough node Electromagnetic radiation

5. Electromagnetic radiation can travel in vacuum. This velocity of light in vacuum has a value c = 2.997925 x 10 8 m s -1 The distance between two peaks or two troughs is called the wavelength and represented by λ (lambda). The product of the wavelength and the frequency (number of cycles per second) is found to be c or c = λν (1) where ν is the frequency of the radiation. ν and λ are characteristic quantity of the radiation for it is generally wavelength which is measured and always frequency is more significant for the interpretation of spectra. Electromagnetic radiation

6. Thus, spectral region may be presented in terms of

7. Wavelength ( λ ) - Distance for a wave to go through a complete cycle (distance between two consecutive peaks or troughs in a wave) Frequency ( ν ) - The number of waves (cycles) passing a given point in space per second Cycle - Crest-to-crest or trough-to-trough Speed (c) - All waves travel at the speed of light in vacuum (3.00 x 10 8 m/s) Electromagnetic radiation

8. Plane Polarized Light - Light wave propagating along only one axis (confined to one plane) Monochromatic Light - Light of only one wavelength Polychromatic Light - Consists of more than one wavelength (white light) Visible light - The small portion of electromagnetic radiation to which the human eye responds Electromagnetic radiation

9. Allow the monochromatic beam to fall on a detector , which will measure its intensity Take a white source of radiation Collimate the beam of radiation Place your sample in the path of radiation Disperse the transmitted beam of radiation Use a slit to isolate a monochromatic beam of radiation Scan through the range of frequencies to obtain data at a reasonable number of frequencies Collimated light  is  light  whose  rays  are parallel

10. By passing through a prism By reflection on a diffraction grating Place the source at the focus of A convex lens (glass or quartz for visible or uv radiation) A front silvered concave mirror for infrared radiation

11. I o I t Sample Absorbance = log (I o /I t ) Transmittance (%) = (I t /I o )x100 To measure I o, replace the sample with a reference, or use a dual beam configuration Spectrum is presented as a plot of Absorbance or Transmittance vs frequency or wavelength

12. The Beer-Lambert Law, also known as Beer’s Law, relates the absorption of light to the properties of the material through which the light is traveling. This law is crucial in fields like chemistry and physics for understanding the absorption characteristics of solutions. Let's derive the Beer-Lambert Law step by step. Assumptions : Monochromatic Light : The light used has a single wavelength. Homogeneous Medium : The absorbing medium is uniform in concentration. Straight Path : Light travels in a straight line through the medium. No Scattering : Only absorption is considered; scattering is negligible. Beer- Lambart Law x dx

13. Basic Concept When a beam of light of intensity 𝐼 ​ enters an absorbing medium, its intensity decreases exponentially with the distance traveled through the medium. The decrease in intensity 𝑑𝐼 over a small distance 𝑑𝑥 is proportional to the intensity of light 𝐼 and the concentration of the absorbing species 𝑐 . Mathematical Formulation Differential Form : here, x is the proportionality constant that depends on the nature of the absorbing species and the wavelength of light. Separation of Variables : Rearrange the differential equation:

14. Integration : Integrate both sides of the equation. On the left side, integrate with respect to 𝐼 from 𝐼 ​ (initial intensity) to 𝐼 (intensity after traveling distance 𝑥). On the right side, integrate with respect to 𝑥 from 0 to 𝑥:

15. Spectral Line Broadening: Line broadening is classified as either homogeneous, when all the atoms or molecules experience the same effect, or inhomogeneous, in which each atom or molecule is affected differently. In the former class comes natural line broadening, while among the latter is Doppler broadening.

16. Pressure or collisional broadening When collisions occur between gas phase molecules, their charge distribution is disturbed, causing an induced dipole that can subsequently absorb or emit radiation. This leads effectively to a broadening of energy levels. If τ is the mean time between collisions, and each collision collision results in a transition between two states, there is a line broadening Δ v of the transition, where Δν = (2πτ coll ) according to the Uncertainty Principle. Like natural line broadening, this broadening is homogeneous, and usually produces a Lorentzian line-shape because of the similarity in the decay functions. However, for transitions at low frequencies, the line-shape is unsymmetrical. This kind of broadening increases with the pressure. The time between collisions is related to the attraction between molecules; therefore, line-width investigations are a common technique used to investigate intermolecular forces.

17. Translational motion of a molecule is the motion of the molecule along arbitrarily defined x, y and z axes. The translational energy of a molecule is only the kinetic energy. Translational motion generally cannot be used by any spectroscopic techniques, but it is important as part of the overall motion of a molecule. Also, the kinetic energy - thus translational energy - a molecule has can be lost by emission of a photon, giving rise to emission spectra.

18. electromagnetic relationships:   λυ = c λ µ 1/υ  E = hυ E µ υ   E = hc/λ E µ 1/λ λ = wave length υ = frequency c = speed of light E = kinetic energy h = Planck’s constant λ c

19. Spectroscopy “seeing the unseeable” Using electromagnetic radiation as a probe to obtain information about atoms and molecules that are too small to see. Electromagnetic radiation is propagated at the speed of light through a vacuum as an oscillating wave.  

20.   Two oscillators will strongly interact when their energies are equal.   E 1 = E 2 λ 1 = λ 2 υ 1 = υ 2   If the energies are different, they will not strongly interact! We can use electromagnetic radiation to probe atoms and molecules to find what energies they contain.

21. some electromagnetic radiation ranges Approx. freq. range Approx. wavelengths Hz (cycle/sec) meters Radio waves 10 4 - 10 12 3x10 4 - 3x10 -4 Infrared (heat) 10 11 - 3.8x10 14 3x10 -3 - 8x10 -7 Visible light 3.8x10 14 - 7.5x10 14 8x10 -7 - 4x10 -7 Ultraviolet 7.5x10 14 - 3x10 17 4x10 -7 - 10 -9 X rays 3x10 17 - 3x10 19 10 -9 - 10 -11 Gamma rays > 3x10 19 < 10 -11