Molecular Methods Other than Vibrational Spectroscopy

We may conclude that these global methods are interesting for obtaining the values of various macroscopic quantities. They are not, however, sufficient to give an interpretation at the molecular level. They are consequently interesting when used in connection with some other methods that are able to provide such a molecular insight. The classification into nonfreezing and freezable water molecnles they provide is not that directly evident on this molecular scale. Consequently, the interpretations of their results on the molecular level appear tentative. 

CLASSICAL MOLECULAR METHODS OTHER THAN VIBRATIONAL SPECTROSCOPY 

These classical molecular methods encompass X-ray or neutron scattering methods, NMR spectroscopy and theoretical descriptions. Their interest for the study of H-bonds in general has been the object of Ch. 3. 

This method is complementary to X-ray scattering, as seen in Ch. 3. Neutron scattering offers, in some cases, a better efficiency than X-ray scattering in the observation of the H-atom or, 

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Classical Molecular Methods Other than Vibrational Spectroscopy... 

Infrared spectroscopy is now nearly 100 years old, Raman spectroscopy more than 60. These methods provide us with complementary images of molecular vibrations Vibrations which modulate the molecular dipole moment are visible in the infrared spectrum, while those which modulate the polarizability appear in the Raman spectrum. Other vibrations may be forbidden, silent , in both spectra. It is therefore appropriate to evaluate infrared and Raman spectra jointly. Ideally, both techniques should be available in a well-equipped analytical laboratory. However, infrared and Raman spectroscopy have developed separately. Infrared spectroscopy became the work-horse of vibrational spectroscopy in industrial analytical laboratories as well as in research institutes, whereas Raman spectroscopy up until recently was essentially restricted to academic purposes. 

There are, however, several advantages to using TDLs for measurement of gas-phase flame species. These include high resolution (typically better than lxl0 cm" ), good spatial resolution (200 to 1 mm), reasonable output power ( 1 mW), and the ability to scan over their spectral range on a millisecond or better timescale. Probably the most widely studied molecular flame species by tunable diode laser spectroscopy is CO. In addition to the reasons for study outlined above in the discussion of broadband source methods, CO possesses several fundamental (v = 0-l) and hot-band transitions (v = 1-2, V = 2-3) which occur within several line widths (approximately 0.05 cm" ) of each other. At room temperature, populations of states from which hot-band transitions occur are very low. However, at flame temperatures, populations of vibrational states other than the v = 0 state may become appreciable. When temperatures (and also species concentrations) are calculated from simultaneous measurement of a fundamental and a hot-band transition, the technique is referred to as two-line thermometry. 

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