Methane (CH4)
Ozone (O3)
Carbon Dioxide (CO2)
Ammonia (NH3)
Nitrogen (N2)
Water Vapor (H2O)
MOLECULAR ABSORPTION
From Previous lectures we know how to use absorption coefficients &cross sections to calculate absorption and emission by gases in theatmosphere.
BUT:
•How do gases absorb radiation?
•Why do only certain gases absorb radiation?
• What dictates the nature of the absorption (wavelength,strength)?
Elementary Molecular Spectroscopy
E1
E2
Absorption emission
E=h
Quantum mechanics dictates that virtually all energytransitions are discrete:
•Absorption: molecule increases its energy
•Emission: molecule decreases its energy.
Elementary Molecular Spectroscopy
One of the real clues as to the natureof the absorption emission process(and for that matter the nature ofmatter itself) came from therealization that bright and dark linesoccur in the same spectral location
Bright lines of emission’
Dark lines of absorption
Wavelength or frequency
Molecules can store energy in 4 discrete ways:
1.Translational (kinetic) energy – directly associated with the theTEMPERATURE of the gas.
2.Vibrational : Most molecules are constantly vibrating! (if theirstructures allow it)
3.Rotational: Molecules can rotate on top of vibrating.
4.Electronic: Relates to energy states of electrons inside amolecule
Energy storage potential in each type is :
A.Electronic : HIGH (associated with visible/UV)
B.Vibrational: MEDIUM-LOW (associated with IR/Microwave)
C.Rotational: quite low – tacked on as a modified to vibrations,leading to “vibrational-rotational” absorption features.
IMPORTANT NOTES:
Because molecules are constantly collidingwhen in Local Thermodynamic Equilibrium (LTE),the energies are constantly redistributedamongst kinetic, electronic, vibrational, androtational modes of energy storage.
TRANSLATIONAL ENERGY is not quantized, butplays an important role in energy redistribution.Molecules that are excited byelectronic/vibrational transitions will redistributesome of this extra energy to translationalenergy (ie HEATING)
EEL > EVIB > ETRANS > EROT
The types of interactions that occur
in matter depend on the rate of
oscillations that must be induced
(i.e the wavelength of the incident
radiation).
On the whole, shorter wavelength,
radiation interacts with lighter and
smaller parts of matter whereas more
sluggish slower oscillating radiation
affects the larger parts of matter.
We are mainly concerned with
mechanisms affecting electrons,
and atoms to more bulky molecules
- mostly vibrational and rotational
spectra
ELECTRONIC TRANSITIONS ARE IMPORTANT INthe UltraViolet – It’s How Ozone protects us!
UV-A
UV-B
UV-C
•N2 generallyunimportant instratosphericchemistry
•Because of O2/O3, nophotons make it belowmid- stratospherethat can excite moreelectronic transitions.
The Electric Dipole: Separation of + and - charge
The electric dipole is a characteristic of matter important tohow E-M radiation interacts with matter.
The displacement or oscillationof charge in this the dipolecreates a time varying dipolemoment (ie. dp/dt) and in turn atime varying e-field and thus EMradiation
Dipole Moments (electric ormagnetic) ARE REQUIRED tointeract with E-M radiation
*
*
*Induced through vibrations
THE OVERALL PICTURE
MODES OF VIBRATION
Degenerate!
Analog Models: Vibration of a Diatomic Molecule
“It’s like a spring!”
Restoring Force F=-k(r-re)
harmonic oscillator predicts
the vibrational
quantum #
k=Spring constant
m’=“reduced mass”
VIBRATION INFORMATION!
•Linear diatomic molecules have a single mode ofvibration at fundamental frequency ν1.
•Triatomic (linear& nonlinear) have : ν1, ν2, ν3
•Energy of vibration E = (v+½) hν; v=0,1,2,3…
•QM rules require Δv=±1 !!!
•SO ΔE = hν (for a given vibrational mode)
•If you could only change one mode at a time, CO2 (e.g.)could only have 3 absorption regions. In reality it has alot more!
Analog Models: Rotation of a ‘Diatomic Molecule’
Rotating Molecule
Center of mass: m1r1=m2r2
Moment of Inertia: I=m1r12+ m2r22
Energy: E=1/2 I2 =L2/2I
Angular Momentum, L= I
The more complex the molecule geometry, the more rotationaldegrees of freedom exist, and thus the more complex is therotational absorption spectrum.
Linear molecules (CO2, N2O) - only one I, simple evenly spaced
distribution of lines)
Symmetric top molecules (NH3, CF3Cl) - non linear, I1=I2,I3
Spherical symmetric top (CH4) - non linear, I1=I2=I3
Asymmetric top (H2O) - non linear and all moments of inertia are
different - complex (random) spectrum
ROTATION INFORMATION!
•In reality, most atmospheric gas molecules haveone or two nonzero moments of inertia
•Angular momentum is quantized by
•E = ½L2/I =
•QM rules require
•Usually: ΔJ=±1 only
•Degenerate: ΔJ=±1, or 0 (not J=00)
•SO ΔE =
•Leads to equally spaced lines (J=0,1,2,3 etc)
•Rotations are often a perturbation on vibrationaltransitions
FundamentalVibrationalMode
FirstHarmonic
Vibrational
Mode
ΔJ= -1
ΔJ= 0
ΔJ= +1
ROATIONAL-VIBRATIONAL Transitions
Rotation-Vibration Modes
vibrations + rotations typically occur
together - at least < 20 m
selection rules (from q-theory) establish
which transitions are permitted
Diatomic molecule v= 1, J= 1
P&R Branch
Triatomic (linear) molecule (CO2)
v= 1, J= 1
P&R Branch
v= 1, J= 0
Q Branch
IMPORTANTSOLARABSORPTIONBANDS
From Liou, Chapter 3
•Most useful inremote sensing! Canoften derivecolumn-integratedquanties of thesegases.
•Can be importantfor energy balance(H2O especially)
P-Branch
R-Branch
Q-Branch
15 μm ν2 CO2 Transition
15 μm CO2 Transitions (mainly ν2)
The thermal IR spectrum, again
Isotopologues Matter!
Summary
electric
Permanent
magnetic
dipole -
yes
Insert fig. 8.9
VERY LITTLE RHYME OR REASON
Summary in Words of Gas Transitions (1)
•3 types of quantized transitions important to us:
•Electronic (highest energy: UV-Vis)
•Vibrational (medium energy: Vis-NIR-Thermal IR)
•Rotational (Far IR & Microwave)
•Other types of absorption are not quantized:
•Photo-Ionization : Ripping electronic off to make ion
(Occurs when photon energy > ionization energy of molecule)
•Photo-Dissociation: Tearing an atom off a molecule
(E.g. O3 O2 + O* - critical for stratospheric chemistry)
(Occurs when photon energy > dissociation energy of molecule)
•Pure rotational transitions can happen ONLY if molecule has a permanentelectric dipole moment: (e.g. H2O, CO, O3).
• Symmetric linear molecules (N2, CO2, N2O) do not have a permanent dipolemoment.
Summary in Words of Gas Transitions (2)
•Rotational transitions often accompany vibrational transitions
•Rotational quantum number J changes by (-1,0, or 1) when vibrationalquantum number v changes by ± 1.
•ΔJ = -1 “P-branch”
•ΔJ = 0 “Q-branch” if it exists! Only allowed if the vibrationaltransition is “degenerate” , e.g. the ν2 transition of CO2!
•ΔJ = +1 “R-branch”
•The energy associated with ΔJ = ±1 is proportional to the starting J state
For example: J = 34 takes 3 times more energy than J = 01 !
•The energy associated with Δv = ±1 does not depend on starting v state:they all take the same energy.