Hydrogen molecules, compression energies

FIGURE 3.13 The three normal vibrational modes of water. For the top mode (the symmetric stretch) both O-H bonds are extended or compressed at the same time. For the middle mode (the antisymmetric stretch) one O-H bond is extended when the other is compressed. The bottom mode is called the bend. In every case the hydrogen atoms move more than the oxygen, because the center of mass has to stay in the same position (otherwise the molecule would be translating). For a classical molecule (built out of balls and perfect springs) these three modes are independent. Thus, for example, energy in the symmetric stretch will never leak into the antisymmetric stretch or bend modes. 

Figure 1.1 Morse curve characterizing the energy of the molecule as a function of the distance R that separates the atoms of a diatomic molecule such as hydrogen. At a distance equal to Re, which corresponds to point 0, the molecule is in its most stable position, and so its energy is called the molecular equilibrium energy and expressed as Ee. Stretching or compressing the bond yields an increase in energy. The number of bound levels is finite. Dq is the dissociation energy and De the dissociation minimum energy. The horizontal lines correspond to the vibrational levels.

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