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Lecture11

# Lecture11 - Algorithm Theoretical Basis Document ATBD 1...

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1 1 Algorithm Theoretical Basis Document ATBD 2 + - + - + + + + - + + + - + - + - - - + - - - - + - - - - - - - + - - - + - + - + + + - + + + + - + + + - + - + + + + + + + + What happens when “light” encounters matter? Recall qualitative picture of EM radiation: = EM wave can be thought of as potential to move a positive test charge. If a real charge is placed near the wave, the charge will be moved. The EM wave does work (F x d), so it must lose energy. The charge – or system of charges (molecule) – must gain energy: Absorption The movement of the charges is called a Transition . e.g., atomic electronic transition: Δ E = | E k -E j | = h ν E j E k E k E j Absorption Emission Chapter 9: Absorption by Atmospheric Gases

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2 3 Total energy of a molecule: E = E rot + E vib + E el + E tr E rot is the Rotational Energy , and refers to rotation of the molecule as a whole unit around a center of mass. E vib is the Vibrational Energy , and refers to vibrations of atoms about their equilibrium positions in a molecule. E el is the Electronic Energy , and refers to the potential energy of the electrons distributed about a nucleus. E tr is the Translational Energy , and refers to the position and velocity of the center of mass. Since E tr refers to kinetic energy, changes in it correspond to changes in temperature. Collisions redistribute energy among these types until local thermodynamic equilibrium is reached (steady state of populations) LTE is appropriate for troposphere and stratosphere Absorption or emission of photon results in change in energy state of molecule: Quantized! 4 Radiation transmitted through the atmosphere interacts with gases, aerosols and clouds, and is transformed via emission, absorption, and scattering. Nuclear magnetic resonance 0.3-300m (1000-1 MHz) Radio Wave Electron spin resonance 10-30 cm (1 – 3 GHz) Microwave Molecular rotation 30 – 10 3 μ m (10 – 300 cm -1 ) 1 – 10 mm (1 – 10 cm -1 ) (30 – 300 GHz) Far IR Microwave Molecular vibrations 1 – 30 μ m (1/ λ ~ 300 – 10 4 cm -1 ) IR Outer electron transitions 100 – 400 nm 400 – 700 nm 700 – 1000 nm UV Vis Near IR Inner electron transitions 0.01 – 100 nm X-ray, Far UV Molecular Motion Wavelength Spectral Region All these processes must be included when interpreting remote sensing data. Spectroscopy is defined as the study of the interaction of EM radiation with matter. The type of interaction depends on the EM energy Different energies correspond to different molecular motions
3 5 Not all transitions between electronic, rotational, and vibrational states are allowed – the probability of a transition between any two states is governed by selection rules . Hypothetical molecule with 3 energy levels (a) Possible transitions between levels (b) Example spectrum (line strengths are arbitrary) Petty Figure 9.1 Radiative transition from energy state E 0 (E 1 ) to energy state E 1 (E 0 ) requires absorption (emission) of a photon with energy Δ E 01 = E 1 – E 0 h ν 01 .

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