Molecular-beam scattering Moment

Figure 1 An overview of high resolution electron energy loss spectroscopy. Top left the incident electron beam is shown as a narrow, intense peak on the intensity vs energy loss axes. The specularly reflected beam is shown with loss peaks due to adsorbed molecules, with modes tuo. Centre The scattering mechanism is illustrated with the three diatomic molecules adsorbed on the sur ce with perpendicular and parallel orientation relative to the sur ce. Mode has a dynamic dipole moment pa which is perpendicular to the sur ce, and induces a second image dipole in the same direction, so that the electron scatters from a combined dipole moment of 2p . This is the dipole scattering process. The mode o>2 is parallel to the surface, and the induced image dipole cancels the molecular dynamic dipole moment. The mode is screened and is not present in the spectrum if there is no impact contribution to the scattering. Mode (03 is shown with the dynamic dipole moment equal to zero (the orientation is not relevant). The mode will be observed as an impact mode.

Thus light of a particular frequency can simultaneously induce a dipole moment in a molecule and then couple with the dipole components to result in light absorption Raman spectra are observed within the spectram of light scattered from an intense source. Induced vibrational transitions are observed with a dispersive device (monochrometer) and some sort of electronic detection (in the visible range) at 9(f from the light source (laser) beam. Remarkably, C. V. Raman first observed this effect with a handheld spectroscope in 1928 for which he received the Nobel Prize in 1930. Thus we can examine the symmetry properties of second-order combinations of the Cartesian coordinates (in column 2 ) and use them to indicate a yes/no answer as to whether a given molecular vibration will occur in Raman spectroscopy. 

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