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Prof. Haynes opens the programme by showing how the absorption spectrum of Carbon Monoxide falls into three regions of increasing energy; the microwave, infra-red and ultra-violet regions. Dr. then shows how the lowest energy region absorptions - the microwave absorptions are only produced by molecules with a permanent dipole. A magnetic model shows how the interaction of the electric component of light and the electric dipole results in a rotation of the molecule. This absorption of energy by rotation gives the microwave spectra. The relationship of the rotational energy to the bond length allows the calculation of bond strength and bond length from microwave spectra. Dr. Ross then picks up the story and explains with the help of magnetic models of the water molecule how infrared absorptions are related to molecular vibrations caused by bond stretching and bending. Thus, from infra-red spectra, bond strengths and angles can be calculated. He finishes by explaining that just as the fine structure of the infra-red absorption gives information on rotational energies, so the ultra-violet spectra contains in its fine structure information on both rotational and vibrational energies.
Metadata describing this Open University video programme
Item code: S24-; 01
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Duration: 00:24:15
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Producer: Barrie Whatley
Contributors: Charles Harding; Len Haynes; RA Ross
Publisher: BBC Open University
Keyword(s): Absorbtion spectrum; Carbon monoxide; Microwaves
Footage description: Len Haynes with a diagram which shows the absorption spectrum of CO. He points out the bands. Charlie Harding discusses absorption in the microwave section of the spectrum. Harding performs an experiment which demonstrates the effect of a (+) charged glass rod on a stream of CS2. He then does it with a (-) charged ebonite rod. The water is attracted by both charges. This is a demonstration of dipole molecule characteristics. Harding has demonstrated that molecules with a permanent dipole can absorb low energy electromagnetic radiation and convert this into rotation. He explains why H2O has a dipole while the CS2 does not. Charlie Harding uses a model which represents the components of electromagnetic radiation in wave form to explain why some molecules can absorb this radiation. He then demonstrates with another model on which the electric force is represented by a magnetic force giving an alternating magnetic field. A model of a dipole molecule is passed along this field and is made to rotate. Harrding explains an important difference in the behaviour of the model and the actual dipole molecule. The absorption energy in molecules is quantised while that of the model is not. He explains that this characteristic allows bond length and angle to be calculated. Bob Ross examines absorption spectra in the Higher energy regions of the spectrun. He examines first the infra red (vibrational absorption) region using the diagram of the CO spectrum to aid his discussion. The diagram reveals a fine structure consisting of many lines, caused by absorption of energies equivalent to a change of vibrational energy combined with changes of rotational energy. Ross explains how the energy due to pure vibrational energy transition is found. Computer animation shows vibrations of the triatomic molecule, water (H2O) Bending, symmetry and asymmetric stretching can be seen. Ross uses a mechanical model in the studio to show the vibration motion of a tri-atomic molecule. Electric forces are represented by magnetic forces in the model. The model shows all three vibrational modes. Further shots of the computer animation showing the three vibrational modes. He next examines the vibrational modes of CS2, a tri-atomic molecule which has no permanent dipole. He uses a molecular ball model to demonstrate that under certain conditions a dipole state does exist. Ross very briefly looks at the ultra violet absorption for the CO molecule.
Production number: NINH20270
Available to public: no