Introduction in Molecularly spectroscopy
- 1. Molecular Spectroscopy Dr. Azza Mohamed Shaker
- 2. Spectroscopy • It is the study of the spectra generated by matter when it emits or interacts with a source of electromagnetic radiation. • Several types of spectroscopic techniques are used to study the different structures of atoms as well as molecules. • The numerous wavelengths emitted by these particles helps in the investigation of their structures and electronic configurations.
- 3. Molecular spectroscopy • The study of how electromagnetic radiation interacts with matter, specifically molecules, through emission, absorption, or scattering. • The major difference between atomic and molecular spectroscopy is: - Atomic spectroscopy deals with electromagnetic radiations emitted or absorbed by atoms. - Molecular spectroscopy deals with electromagnetic radiations emitted or absorbed by molecules.
- 4. Fundamentals of molecular spectroscopy • Molecular spectroscopy deals with the interaction of molecules with electromagnetic radiations. Due to such interaction, a spectrum (absorption pattern) is generated. This spectrum helps you to access compositional as well as structural details of the material. • When molecules interact with various electromagnetic radiations, these particles move from one energy level to another. This type of motion produces the molecular spectrum.
- 5. Molecular spectroscopy The fundamentals of molecular spectroscopy involve the excitation of molecules from the ground state to the excited state These molecules either emit or absorb various wavelengths of radiation and move from one energy level to another. As a result of this movement, spectra are produced. Molecular spectroscopy utilizes a spectrophotometer device to determine the structure and composition of different substances.
- 6. Molecular spectroscopy • According to the Born-Oppenheimer approximation rule, the total energy of a molecule is equivalent to the total of all energies exhibited by the molecule and its constituent particles. • The energies of a molecule is an additive property of each individual motion in the molecule. • These individual energy levels are termed translational, vibration, rotational and electronic spectra of the molecule. • The total energy is the molecular spectra, representing the transitions of energies occurring within the molecule.
- 7. Molecular spectroscopy • The molecular spectra are represented in the form of an equation as: ET = Et + Er + Ev + Ee + ……. Where, • ET is the total molecular energy in the spectrum. • Et is the total translational energy • Er is the total rotational energy • Ev is the total vibrational energy, and • Ee is the total electronic energy
- 8. Translational Energy • Translational energy is the kinetic energy associated with the motion of the center of mass of an object. The classical expression for translational kinetic energy is: 𝐸𝑡𝑟𝑎𝑛𝑠 = 1 2 𝑚𝑣2 • where ( m ) is the mass of the object and ( v ) is its velocity. • This formula shows that translational kinetic energy depends on both the mass of the object and the square of its velocity. It’s a fundamental concept in physics, especially when analyzing the motion of objects in a straight line.
- 9. Rotational Energy • Rotational energy is the kinetic energy due to the rotation of an object around its center of mass. For a rigid body rotating about a fixed axis, the classical expression is: 𝐸𝑟𝑜𝑡 = 1 2 𝐼𝜔2 • where ( I ) is the moment of inertia The moment of inertia (I) depends on the mass distribution of the object relative to the axis of rotation. • The moment of inertia, often referred to as rotational inertia, is a measure of an object’s resistance to changes in its rotational motion. It depends on the mass distribution of the object relative to the axis of rotation I=μr2 𝜇 = 𝑚1𝑚2 𝑚1 + 𝑚2 • r interatomic distance • 𝜇 is the reduced mass • and (ω omega) is the angular velocity ( 𝜕𝜃 𝜕𝑡 ) change by changing the angle
- 10. Vibrational Energy • The vibrational energy ((E)) of a harmonic oscillator is the sum of its kinetic energy ((T)) and potential energy ((V)): • E= V+T • Vibrational energy is the energy associated with the vibrational motion of atoms within a molecule. For a simple harmonic oscillator, the classical expression is: 𝐸𝑣𝑖𝑏 = 1 2 𝑘𝑥2 + 1 2 𝑚𝑣2 X=r-re • where ( k ) is the force constant, ( x ) is the displacement from equilibrium position, ( m ) is the mass of the oscillating particle, and ( v ) is the velocity.
- 11. Total number of degrees of freedom Total number of degrees of freedom (3N) Linear molecule Translational = 3 Rotational = 2 Vibrational = 3N-5 Non-Linear molecule Translational = 3 Rotational = 3 Vibrational = 3N-6
- 12. Example For O2 molecule: The total number of degrees of freedom = 3N = 3x2= 6 Translational = 3 Rotational = 2 Vibrational = 6-5= 1 For H2O molecule: Total number of degrees of freedom = 9 Translational = 3 Rotational = 3 Vibrational = 9-6= 3