6.4 Flashcards
what types of molecules trap heat?
2+ atoms (polyatomic)
99.9% of molecules in atmosphere don’t absorb IR radiation including:
- N2 = 78%
- O2= 21%
- Ar = 0.9%
2 molecular requirements for IR absorption:
- energy of radiation matches energy of molecular transition (nuclei/electron vibrations)
- molecular transition (atmospheric molecules) must change electric field (alter molecule’s dipole moment)
why does Ar NOT absorb IR?
no nuclei vibrations
dipole moment =
(vector sum of atomic charges) * (distance from molecule’s center of mass)
why do N2 and O2 NOT absorb IR?
- they have nuclei vibrations BUT
- NO change in dipole moment (bc symmetrical dipole always = 0)
all homonuclear diatomic molecules =
infrared inactive
homonuclear diatomic molecules
molecule w/ 2 of the same atoms (H2, O2, N2, etc.)
all heteronuclear diatomic molecules =
have altered dipole moment = absorb IR
heteronuclear diatomic molecules
molecules w/ 2 diff atoms (CO, NO, HCI)
polyatomic molecules w/ vibrations that change dipole =
IR active
all gasses that contribute to GHG effect =
polyatomic
2 most important GHG molecules =
H2O and CO2
H2O:
3 vibrations = 3 Δ-dipole
CO2:
symmetric = no Δ-dipole
- BUT net Δ-dipole = altered by stretch/bending of vibrations
molecular vibrational transitions allow for absorption of …
specific energies
rates of rotation
for each vibration, IR photons change rotational levels
rotational absorptions = ? and allow for absorption of…
gain/loss of energy during collision
- broad absorption band
CO2: bending vibration
14,922 nm
CO2: main absorption band
15 µ band
1 µ =
1 µm = 1000 nm = 1 x 10^6 m
H2O: bending vibration
6269 nm (major absorption band)
H2O rotational vibrations =
wavelengths > 20,000 nm
why do CO2 and H2O absorb no significant radiation
wavelengths = short
