仪器分析英文课件Chapter 2-Introduction to Spectrometric Methods

Three types
Bands continuum
Fig. 6-15 Emission spectrum of a brine obtained with an oxyhydrogen flame(p.132)
Line spectra
Band spectra
The single outer electron in the ground sห้องสมุดไป่ตู้ate energy level E0
Each molecular outer electronic energy level contain many vibrational and rotational energy level.
Fig 6-17 Energy level diagrams (p.133)
Continuum Spectra Very broad band spectra in emission from solids Produced by blackbody radiation - thermal excitation and relaxation of many vibrational (and rotational) levels.
many small collisional relaxations tiny temperature rise of surrounding species
UV-VIS Absorption (Emission, Fluorescence)
10-180 nm
180-780 nm
1×106 -5×104
5×104 1.3×104 1.3×104 3.3×102
Bonding Electron
Bonding Electron Molecule Vibration (Rotation) Molecule Rotation Electron Spin in Magnetic field
When a species changes its state, it absorbs or emits an amount of energy exactly equal to the energy difference between the states. DE = E1 - E0
2. When atoms, ions or molecules absorb or emit radiation in making the transition from one energy state to a second , the wavelength or frequency of radiation is related to the energy difference between the states by the equation
Wave parameters Amplitude A- length of the electric vector at a maximum in the wave wavelength λ- linear distance between any two equivalent points on successive maxima (or minima) Period p – time for 1 l to pass fixed point Frequency - number of l passing per second Wavenumber - number of l per cm Velocity v - distance point on wave travels per second v = ×l In a vacuum: vvacuum = c = 2.99792×108 m/s
Relaxation Processes:
Lifetime of excited state is short (fsms) - relaxational processes return excited species to ground state
a. Nonradiative relaxation
Emission of radiation
Characterized by
Emission spectrum
Plot of emission intensity wavelength (frequency) Lines
EMR is produced when excited particles relax at lower energy levels by giving their excess energy as photons.
Fig 6-18 Blackbody radiation curves (p.134)
Absorption of radiation
Characterized by
Absorption spectrum
For absorption of radiation to occur, the energy of the exciting photos must exactly match the energy difference between the ground state and one of the excited states of the absorbing species. Sharp peaks
Infrared Adsorption (Raman 0.78-300 µm Scattering) Microwave Absorption Electron Spin Resonance Nuclear Magnet Spin
0.75-3.75 mm 13-27 3 cm 0.6-10 m 0.33
§2 Wave properties of electromagnetic radiation
EMR is conveniently represented as electric and magnetic fields that undergo in-phase, sinusoidal oscillations.
Chapter 2. An Introduction to Spectrometric Methods
Spectroscopy (spectrometry): The study of the interaction between electromagnetic radiation (EMR) and matter.
1×10-3-1.7×10- Nuclear Spin in Magnetic 2 field
EMR phenomena
(1) Absorption
However
(2) Emission
(3) Scattering
(4) Refraction (5) Reflection (6) Interference (7) Diffraction (8) Polarization When EMR is emitted or absorbed, a permanent transfer of energy from the emitting object or to the absorbing medium occurs. Thus, it is necessary to treat EMR as a stream of discrete “particles” called photons (or quanta) Could be explained by wave descriptions of EMR
§3 Particle (quantum-mechanical) properties of EMR
Photoelectric effect
1. First observation: In 1887, Heinrich Hertz
2. Theoretical explanation:
In 1905, Einstein: E=h (h= Planck’s Constant = 6.626 x 10-34 Joule-sec)
§1 General properties of electromagnetic radiation (EMR) •What is EMR?
- an oscillating (sinusoidal) electric and magnetic field which travels through space (Wave) - a discrete series of “particles” that possess a specific energy but have no mass (Particle) BOTH!
For example:
UV and Visible EMR has enough energy for valence (bonding) excitations
X-ray region has enough energy for core (inner) excitations
Molecular absorption The energy E, associated with the bands of a molecule, is made of three components, namely electronic, vibrational and rotational energy levels
For each electronic state - many vibrational states For each vibrational state - many rotational states many features Absorption spectra affected by (1) number of atoms in molecule more features (2) solvent molecules blurred features