ATMOS 5140 Lecture 11 Chapter 9

Transcription

1 ATMS 5140 Lecture 11 hapter 9 Absorption by Atmospheric Gases Rotational Vibrational Applications

2 Basis for Molecular Absorption/Emission hanges in the translational kinetic energy of molecules (i.e. temperature) hanges in the rotational kinetic energy of polyatomic molecules hanges in the vibrational energy of polyatomic molecules hanges in the distribution of electric charges within a molecule, possibly including the complete separation (or reunification) of two components previously bound by electrostatic forces

3 Basis for Molecular Absorption/Emission hanges in the translational kinetic energy of molecules (i.e. temperature) hanges in the rotational kinetic energy of polyatomic molecules hanges in the vibrational energy of polyatomic molecules hanges in the distribution of electric charges within a molecule, possibly including the complete separation (or reunification) of two components previously bound by electrostatic forces

4 Basis for Molecular Absorption/Emission Molecule of smaller Immediate consequence hanges in the translational kinetic energy of molecules (i.e. temperature) hanges in the rotational kinetic energy of polyatomic molecules hanges in the vibrational energy of polyatomic molecules hanges in the distribution of electric charges within a molecule, possibly including the complete separation (or reunification) of two components previously bound by electrostatic forces

5

6 Relationship between energy level transitions and absorption/emission line spectrum λ = c ν = hc ΔE Electrons are FREE ΔE 1 For example, if ΔE 01 Then ΔE,- = ΔE -. ν,- = ν -. DEGENERATE lines collapsed onto 1 Thus lines are stronger.. Absorption ross Section ν 01 ν 1 ν

7 Relationship between energy level transitions and absorption/emission line spectrum λ = c ν = hc ΔE Electrons are FREE ΔE 1 ΔE 01 Relative Strengths of the lines: 1. Fraction of molecules in particular state required for transition. Likelihood photon has right energy and will encounter molecule in required energy state Absorption ross Section ν 01 ν 1 ν

8 MST IMPRTANT SLIDE Energy Wavelength Band Dominant Transition Lower Energy > 0 um Far IR, Microwave Rotation 1um 0 um Near IR, thermal IR Vibration igher Energy <1um Visible, UV Electronic

9 RTATINAL Molecule xygen Structure Permanent Electirc Dipole Moment? (has magnetic dipole moment) Nitrogen N N Moment of Inertia arbon Monoxide I = 0 r - δm arbon Dioxide Nitrous xide N N Water asymmetric top zone asymmetric top Methane spherical top

10 RTATINAL Ar Simple Spectra Most omplex Spectra Description Moments of Inertia Examples Monoatomic I 1 = I = I 3 = 0 Ar Linear I 1 = 0; I = I 3 > 0 N,,, N Spherical Top I 1 = I = I 3 > 0 4 Symmetric Top I 1 0; I = I 3 > 0 N 3, 3 l, F 3 l Asymmetric Top I 1 I I 3, 3 N

11 RTATINAL Molecule xygen Structure Permanent Electirc Dipole Moment? (has magnetic dipole moment) Nitrogen N N Moment of Inertia arbon Monoxide I = 0 r - δm arbon Dioxide Angular Momentum of a rigid molecule is restricted to discreet values. Nitrous xide Water N N asymmetric top v=0,1,,. Rotational Quantum number zone asymmetric top Methane spherical top

12 J' Need to consider combination of rotational state and electronic state Energy { v' { v" P Q R Branch J" ν ν 0

13 RTATINAL Molecule xygen Structure Permanent Electirc Dipole Moment? (has magnetic dipole moment) Nitrogen N N Moment of Inertia I = 0 r - δm arbon Monoxide arbon Dioxide Nitrous xide N N Either an applied magnetic or electric field has the capacity to exert a torque on the molecule Water asymmetric top zone asymmetric top Methane spherical top

14 RTATINAL Molecule xygen Structure Permanent Electirc Dipole Moment? (has magnetic dipole moment) Nitrogen N N rotation Moment of Inertia arbon Monoxide I = 0 r - δm arbon Dioxide rotation Without Vibration changing geometry Nitrous xide N N Water asymmetric top zone asymmetric top Methane spherical top

15 RTATINAL Molecule xygen Structure Permanent Electirc Dipole Moment? (has magnetic dipole moment) Nitrogen N N rotation Moment of Inertia arbon Monoxide I = 0 r - δm arbon Dioxide rotation Without Vibration changing geometry All other major molecules found in the atmosphere exhibit permanent electric dipole moment Nitrous xide Water N N asymmetric top reating major rotational absorption bands zone Methane asymmetric top spherical top

16 VIBRATINAL Diatomic (N,, ) Molecule behaves like a spring Vibrational transitions associated with larger energy than rotational transitions (shorter wavelengths) Vibrational and rotational transitions often occur simultaneously Linear triatomic (, N ) Symmetric stretch Bending Asymmetric stretch ν 1 ν ν 3 n Triatomic (, 3 ) Symmetric stretch Bending Asymmetric stretch ν 1 ν ν 3

17 v 1 Energy 1st Excited State 0 Ground State Electronic + Vibration + Rotation v 1 0 J

18 v λ = c ν = hc ΔE 1 ΔE Energy 1st Excited State 0 Ground State Electronic + Vibration + Rotation Shortest λ v 1 0 Longest λ UV, Visible Near IR, Far IR Thermal IR, Microwave J

19 Absorption (a) without broadening N Flux Atmospheric Radiation Irrelevant Wavelength (b) with broadening LINE SAPE Absorption Wavelength

20 Describe a line shape σ v = Sf ν ν o σ F = absorption cross section per molecule S = line strength f ν ν N = line shape function α, P - = half width at half maximum (WM)

21 Frequency shift (temperature important) Doppler Pressure important Lorentz (ν ν 0 )/α

22 Absorption oeff a) at 100 mb pressure b) at 1000 mb pressure 3 Absorption oeff Freq. (Gz) Freq. (Gz)

23 Transmission in air over 1 m path a) p = 1000 mb b) p = 1000 mb Problem 9.6 You need to use figure 6.4 c) p = 100 mb