  
|
High resolution spectroscopy resolves individual rotational transitions in a
vibrational band. Reflection spectra detect broad troughs are produced when solar
photons pass through the atmosphere and are absorbed before being scattered back by
clouds. Symmetrical molecules have quadrupole moments, but no dipole moments, so they
exhibit only small absorption. Reflection spectra are taken in the visible to near
infrared portion of the spectrum. At long wavelengths, reflected sunlight is negligible. Thermal
emission spectra of solid planets detect sharp emission lines are produced at the
wavelengths of vibrational transitions when the upper atmosphere is warmer than the
surface. Troughs will be present where the atmospheric temperature is cooler than the
surface. For gaseous planets, thermal emission spectra detect sharp emission lines are
produced at the wavelengths of vibrational transitions when the upper atmosphere is warmer
than the lower atmosphere. Troughs will be present where the atmospheric temperature is
higher at lower levels. Thermal spectra are taken in the infrared at wavelengths of  10  m.
Molecular spectroscopy is the study of absorption of light by molecules. In the gas phase
at low pressures, molecules exhibit absorption in narrow lines which are very
characteristic of the molecule as well as the temperature and pressure of its environment.
In the microwave and long-wavelength infrared regions of the spectrum, these lines are due
to quantized rotational motion of the molecule. At shorter wavelengths similar lines are
due to quantized vibration and electronic motion as well as rotational motion. The precise
frequencies of these lines can be fit to quantum mechanical models which can be used both
to determine the structure of the molecule and to predict the frequencies and intensities
of other lines. Because this absorption is so characteristic, it is very valuable for
detecting molecules in the Earth's stratosphere, planetary atmospheres, and even the
interstellar medium.
Molecular spectra entails the absorption of electromagnetic radiation by molecules. This
includes vibrational and rotational transitions as well as electronic transitions. The
transitions of molecules are more complex than those of atoms. This is because molecules
have interactions from vibrations within the molecule associated with the stretching or
bending of bonds between atoms, or the rotation of the molecule about its center of
gravity. Molecules may have some kinetic energy associated with its straight-line motion
in a certain direction. The energy levels involved with these various transitions differ
between one another. The energy with the movement of an electron from one orbital to
another, electronic, is about 10- 9 joules. The energy involved in vibration is
about 10- 19 joules and that of rotational is around10-21 joules.
The energy of transitions change is much smaller than those at about 10-35
joules. Each electronic state of a molecule has several possible vibrational states and
each of those has several rotational states. Example follows:
The energy absorbed by a molecule has specific wavelengths in ultraviolet, visible and
infrared regions.
|
