exp25_10-06 - Experiment 25 Chemistry 541/2 Physical...

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Experiment 25 Physical Chemistry Laboratory C B band spectrum of nitrogen revised 10/06 Introduction Molecular spectroscopy is the study of the interaction of electromagnetic radiation (light) with molecules. Observed spectra characterize the difference between energy levels of molecules. Photons that are emitted (or absorbed) in purely radiative transitions must be of the same energy as the difference in energy between the two states involved. Transitions observed in the ultraviolet region of the spectrum are usually due to changes of electronic configuration. In the case of molecules, rotational and vibrational degrees of Chemistry 541/2 Chemistry 541/2
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Fig. 1. Experimental apparatus. Light from the N 2 lamp is dispersed by the monochromator and a small bandwith of light passing through the monochromator is detected by the PMT. freedom are also available and will give rise to additional structure in the observed spectrum. This structure can be analyzed in principle to obtain information about the rotational, vibrational, and electronic properties of the molecule. In this experiment, you will record an emission spectrum of nitrogen and analyze it to obtain information about vibrational properties of the molecule in two electronic excited states. Rotational structure is not resolved. Before you start. .. Read Ch. 13 in McQuarie and Simon on Molceular Spectroscopy. Bring a 3.5” diskette with you to lab in order to take your data files when you are done. Experiment 39 (pp 416-424) in Chapter XIV of Shoemaker, Garland, and Nibler [1] has particularly useful materials specific to this experiment. Apparatus The apparatus for observing the C Æ B band spectrum of N 2 is shown in Fig. 1 and consists of a nitrogen discharge lamp, a compact, computer-interfaced scanning mono- chromator (Instrument SA HR320, 1 m pathlength, 1800 lines/mm), and a photomultiplier tube (PMT) detector. A diffraction grating inside the monochromator can be rotated under computer control, and is used to provide wavelength selectivity. For a given grating position, only a very narrow spectral band of light (i.e. nearly monochromatic light) can reach the detector. A spectrum is recorded by rotating the grating slowly inside the monochromator while recording the resulting signal from the photodetector. The ability to distinguish between two peaks that are closely spaced in frequency, i.e. the instrument’s resolution , is governed by the spread of frequencies from the source (pressure, doppler or uncertainty principle broadening) as well as the slit settings at the input and output of the monochromator. The width of the entrance and exit slits, the dispersive power of the grating (determined by the number of rulings per unit length), and the pathlength of the monochromator are the factors which determine instrumental resolution. The monochromator you use in this experiment has only moderate resolution (~ 1-2 Å) mostly due to its 1 m pathlength.
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Fig. 2. The electrometer which reads the tiny current outputs of the PMT and the high voltage power supply (Pacific Instruments 227) which powers the PMT.
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exp25_10-06 - Experiment 25 Chemistry 541/2 Physical...

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