Light is most likely to be absorbed by a molecule when its frequency almost exactly matches the frequency of an internal motion within the molecule. For frequencies in the infrared region, the relevant internal motions are the vibrations of the molecule’s atoms relative to each other.
The simplest vibrational motion in a molecule is the oscillatory motion of two bonded atoms X and Y relative to each other. In this motion, called a bond-stretching vibration, the X-to-Y distance increases beyond its average value R, then returns to R, then contracts to a lesser value, and finally returns to R.
Such oscillatory motion occurs in all bonds of all molecules under all temperature conditions, even at absolute zero.
A huge number (about 1013) of such vibrational cycle occur each second. The exact frequency of the oscillatory motion depends primarily on the type of the two atoms involved.
For many bond types, e.g., the C-H bond in methane and the O-H bond in water, the stretching frequency does not fall within the thermal infrared region. The stretching frequency of carbon-fluorine bonds does, however, occur within the thermal infrared range; thus any molecules in the atmosphere with C-F bonds will absorb outgoing thermal IR light and enhance greenhouse effect.
Light is most likely to be absorbed by a molecule when its frequency almost exactly matches the frequency of an internal motion within the molecule. For frequencies in the infrared region, the relevant internal motions are the vibrations of the molecule’s atoms relative to each other.
The simplest vibrational motion in a molecule is the oscillatory motion of two bonded atoms X and Y relative to each other. In this motion, called a bond-stretching vibration, the X-to-Y distance increases beyond its average value R, then returns to R, then contracts to a lesser value, and finally returns to R.
Such oscillatory motion occurs in all bonds of all molecules under all temperature conditions, even at absolute zero.
A huge number (about 1013) of such vibrational cycle occur each second. The exact frequency of the oscillatory motion depends primarily on the type of the two atoms involved.
For many bond types, e.g., the C-H bond in methane and the O-H bond in water, the stretching frequency does not fall within the thermal infrared region. The stretching frequency of carbon-fluorine bonds does, however, occur within the thermal infrared range; thus any molecules in the atmosphere with C-F bonds will absorb outgoing thermal IR light and enhance greenhouse effect.
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