Rotational state, infrared energy absorption

Rotational absorption-A low energy-level change in the rotational state of a molecule characterized by absorption in the far infrared. 

The positions of individual absorption bands recorded in the spectra depend on the energy of the absorbed radiation. Radiation from the near-infrared region gives rise to changes in the rotational and oscillation energy states in a molecule. Narrow bands that are due to small... 

A circumvention of this problem is through the use of two-color excitation, or double resonance. An infrared laser can excite a selected rotational state (with a greatly reduced Doppler width because v is much less). As discussed in a following section on overtone excitation, the absorption of the second (visible) photon leads to an excited state in a much more precisely defined energy. 

It can change its state of rotation. The levels of the rotational energies are closely spaced. The difference amounts to approximately 0.4 kJ. That means a molecule can be excited in a higher rotational state by absorption of radiation in the far infrared. The excitation of rotational states by use of conventional intensities of light cannot induce a photoreaction. 

No emission spectrum is expected for the cloud and star light microwave absorptions by the cloud are by the lowest rotational states. At higher temperatures additional high-energy lines appear because higher energy rotational states are populated. Circumstellar clouds may exhibit infrared absorptions due to vibrational excitation as well as electrcxiic transitions in the ultraviolet. Ultraviolet absorptions may indicate the photodissocation of carbon monoxide. High temperature clouds exhibit emissions. 

The electronic states in a molecule, analogs of the atomic states and characterized by total electronic angular momentum and spin, are generally separated in energy by about the same order of magnitude as for isolated atoms, usually several electron volts (eV). Thus the transitions from molecular electronic states, which also correspond to different potentials, are best observed in the ultraviolet (UV) region. Excitation depends on the presence of UV radiation, and electronic transitions are usually seen in absorption, as in the state of H2. The strength of the transition depends on the dipole (heteronuclear molecules and ions) or the quadrupole (homonuclear) moments. Vibrational states dominate the optical and infrared, and rotational states are best observed in the millimeter and centimeter portions of the spectrum. 

Emission or absorption spectra are produced when molecules undergo transitions between quantum states that correspond to two different internal energies. The energy difference AE between the states is related to the frequency of the radiation emitted or absorbed by the equation DE = hn. Infrared frequencies in the wavelength range 1-50 mm are associated with molecular vibration and vibration-rotation spectra. 

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