Introduction to Basic Properties of Hydrogen - Hydrogen Generation, Storage, and Utilization - page 3

1.2 BASICS ABOUT THE HYDROGEN MOLECULE
When two hydrogen atoms combine, they form a stable molecule, H 2 , with
a single and strong covalent bond. The equilibrium bond length is 0.74 Å.
The bond dissociation energy is 4.52 eV or 436 kJ mol −1 .
Extensive experimental and theoretical studies have been done on H 2 in
terms of its electronic structures, optical properties, magnetic properties, and
reactivity with other elements or compounds. Its small size and light mass
make it convenient for theoretical and computational studies. For example,
potential energy surfaces (PES) or curves for many electronic states of H 2
have been calculated with high accuracy [2, 3]. Figure 1.2 shows some
examples of PES of low-lying electronic states of H 2 [4, 5]. The ground
electronic state and the first few excited states are all bound with respect to
the bond distance between the two hydrogen atoms.
Because the large energy difference between the ground and first excited
electronic states of H 2 (near 12 eV), there is no absorption of visible or UV
light by H 2 , thus H 2 gas is colorless. H 2 does absorb light in the vacuum UV
(VUV) region of the spectrum. Since the three lowest excited electronic
states are all bound, they are expected to be relatively long-lived and lead to
fluorescence when excited by light in the VUV region.
Molecular hydrogen has interesting magnetic properties, mainly due to its
nuclear spin properties. There are two different spin isomers of H 2 , ortho and
FIGURE 1.2 Examples of several low-lying PES of H 2 . Source : Reproduced with permission from
Flemming et al. [4].