Franck Condon Principle

3 de Feb de 2023

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Franck Condon Principle

  1. Samrat Prithviraj Chauhan Government College Ajmer 2020-2021 FRANCK CONDON PRINCIPLE Submitted By Bhuvneshwari Saini M.Sc. Chemistry Semester 2nd Department of chemistry
  2. Table of Content  Introduction  Energy Levels  Vibrational Transition  Frank Condon Principle  Explanation of FCP  Reference
  3. Introduction  Franck Condon Principle was proposed by German physicists James Franck and U.S. Physicists Edward U. Condon in 1926.  Before the development of the Schrödinger equation Franck put forward qualitative arrangements to explain the various type of intensity distribution found in vibronic transition in a molecule take place much more rapidly than a vibrational transition. James Franck Edward U. Condon
  4. Energy levels  A quantum mechanical system or particle that is bond- that is confined spatially can only take on certain discrete values of energy called energy levels.  Types of Energy levels : 1. Translation Energy: It is generated by the translation motion of the molecule. 2. Rotational Energy: It is generated by the rotational motion of the molecules. It means it is generated due to the rotational form of centre of gravity of the molecule. Minimum rotational energy-0.005eV. Selection Rule ∆J=+1,-1. Example – Rigid Rotor
  5. 3. Vibrational Energy: It is generated due to the vibration of the atom of the molecule with respected to each other. Vibrational Energy= 0.1eV. Selection Rule ∆J=+1,-1 and ∆V=+1,-1 Example- Harmonic Oscillator
  6. 4. Electronic Energy :The electronic energy levels are quantized and discrete and the energy gap between the highest occupied and lowest unoccupied orbital determines the large degree of their stability, reactivity, and electronic properties.
  7. Molecular Spectra
  8. Vibrational Transition  Vibrational Transition of a molecule refers to the moment of the molecule from one vibrational level to another.  We can also name it as vibronic transition  This type of transition occurs in between different vibrational levels of the same electronic state.
  9. Franck Condon Principle  It states that an electronic transition take place so rapidly that a vibrating molecule does not change its internuclear distance appreciably during the transition.  This principle is, to a first approximation, true since the electrons move so much fast than the nuclei that during the electronic transition than nuclei do not change their position. Internuclear Distance Energy
  10. The most likely place to find and oscillating object is at its turning point. The nuclei do not have a chance to move. The transition represented by “VERTICAL ARROW” on the diagram. In the ground state the molecule is most likely in v=0
  11. Explanation of FCP  CASE 1: Here the excited electronic state has a slightly smaller internuclear separation than the ground state. A vertical transition form the v"=0 level will be more likely to occur into the upper vibrational state v'=2, transition to lower and higher state being less likely; in general the upper state most probably reached will depend on the difference between the equilibrium separations in the lower and upper states. In case of CO molecule this type of band system is observed. Internuclear distance Energy Intensity
  12.  CASE 2:When upper electronic state having the same equilibrium internuclear distance as the lower. Now the FCP suggests that a transition occurs vertically on this diagram since the internuclear distance does not change. Thus the strongest spectral line of the v" =0 progression will be the (0,0) . This observed in O₂ molecule. Internuclear distance Energy Intensity
  13.  CASE 3: When the upper state separation is drawn as considerably greater than that of the lower state. We see that. The vibrational level to which a vertical transition take place has a high v' value, here transitions can occur to a state where the excited molecule has energy in excess of its own dissociation energy. From such states the molecule will dissociate without any vibrations, since the atoms which are formed may take any value of kinetic energy the transition are not quantised and a continuum is observed. This type of band system is observed in I₂ Internuclear distance Energy Intensity
  14. Reference  Atomic and Molecular Spectra: Laser by Raj Kumar  Fundamentals of Molecular Spectroscopy by C.N. Banwell  Modern spectroscopy by J.M. Hollas (Wiley)
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