The radiation from the source passes through a gaseous sample and is detected by a crystal diode detector that is followed by an amplifier and display system (chart recorder).

In order to increase the sensitivity of the instrument, signal modulation by application of a high-voltage square wave across the sample is used.

Thus any observed electronic transition will consist of a large number of closely spaced members owing to the vibrational and rotational energy changes.spectrometer systems that are commonly used for molecular spectroscopy: emission, monochromatic radiation absorption, and Fourier transform.

Each of these methods involves a source of radiation, a sample, and a device for detecting and analyzing radiation. This system is used extensively for the observation of electronic spectra.

Fourier-transform spectrometers can be designed to cover all spectral regions from the radio frequency to the X-ray.

Spectrometers allow the **study** of a large variety of samples over a wide range of frequencies.

The rigid-rotor, harmonic oscillator model exhibits a combined rotational-vibrational energy level satisfying Chemical bonds are neither rigid nor perfect harmonic oscillators, however, and all molecules in a given collection do not possess identical rotational, vibrational, and electronic energies but will be distributed among the available energy states in accordance with the principle known as the and therefore will exhibit a different rotational spectrum.The second type is the Fourier-transform spectrometer, in which the radiation is confined in an evacuated cavity between a pair of spherical mirrors and the sample is introduced by a pulsed nozzle that lowers the temperature of the sample to less than 10 K.The sample is subjected to rotational energy excitation by application of a pulsed microwave signal, and the resulting emission signal is detected and Fourier-transformed to an absorption versus frequency spectrum.The nonrigidity of the chemical bond in the molecule as it goes to higher rotational states leads to centrifugal distortion; in diatomic molecules this results in the stretching of the bonds, which increases the moment of inertia.The total of these effects can be expressed in the form of an expanded energy expression for the rotational-vibrational energy of the diatomic molecule.A molecule in a given electronic state will simultaneously possess discrete amounts of rotational and vibrational energies.For a collection of molecules they will be spread out into a large number of rotational and vibrational energy states so any electronic state change (electronic transition) will be accompanied by changes in both rotational and vibrational energies in accordance with the proper selection rules.Absorption spectrometers employ as sources either broadband radiation emitters followed by a monochromator to provide a signal of very narrow frequency content or a generator that will produce a tunable single frequency.The tunable monochromatic source signal then passes through a sample contained in a suitable cell and onto a detector designed to sense the source frequency being used.The relationship between the observed microwave transition frequency and the rotational constant of a diatomic molecule can provide a value for the internuclear distance.The quantitative geometric structures of molecules can also be obtained from the measured transitions in its microwave spectrum.

Vol.7, No.3, May, 2004. Mathematical and Natural Sciences.

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