11.4 Electron Spectrometrics
231
is proportional to the binding energy of bond in the first coordination neighbors,
according to the tight-binding approximation [104]. A contraction of the neighboring
bonds shifts the energy levels further—deepens the energy levels further—quantum
entrapment occurs [87]. The amount of energy shift varies from level to level because
of the screening of the potential by electrons in the outer orbitals. Therefore, the
energy of the valence band composed of electrons in the outermost orbital shifts
more than an inner energy level. One has thus, E vb > E 1s, and E edge = E 1s −
E vb < 0, negative shift due to bond contraction.
On the other hand, electrostatic polarization changes the situation contrastingly.
Charge polarization screens and splits the local potential and then shifts a proportion
of electrons up in energy bands causing their negative shift, then, −E vb >> −E 1s,
and thus E edge = E 1s − E vb > 0, positive shift takes place due to the polarization. In fact, bond contraction by molecular undercoordination or salt solvation
is associated with polarization. Therefore, H–O bond thermal contraction without
polarization results in the pre-edge energy negative shift. For the salt solution, ionic
polarization becomes dominance, which overweight the effect of H–O bond contraction on the binding energy shift. The polarization shifts the energy levels up and
thus, E edge = E 1s − E vb > 0. Therefore, the XAS pre-edge shift is very sensitive to the local energetic environment, being capable of discriminating the effect
of entrapment by bond contraction and charge polarization. If the entrapment and
polarization is compatible, no shift will occur to the XAS pre-edge energy peak. This
discrimination could be the advantage of the XAS that probes the effect of valence
band polarization while an XPS detects only the O 1s level shift subjecting to weak
perturbation of the polarization.
11.4.4.3 XAS Capability: Entrapment and Polarization
NEXFAS measurements [101] revealed that Li
+ , Na
+ , and K
+ cations shift the preedge component peak energy more than the Cl
− , Br
– , and I
− anions and the pre–edge
component associated with the first hydration shell of Li
+ ion is thermally more stable
than those beyond, see Fig. 11.9. At 25 °C, the cation effect on the pre–edge shift from
the value of 534.67 eV in alkali chlorides is remarkable: Li
+ (0.27 eV), Na
+ (0.09 eV),
and K
+ (0.00 eV). The energy shift of Li
+ ion in 5 M LiCl solution (0.30 eV) is close
to that in its 3 M solution. On the other hand, in sodium halides, the anion effect
is small: Cl
− (0.09 eV), Br
− (0.04 eV), and I
− (0.02 eV). The energy trend of the
pre–edge shifts is the same as the DPS ω H of the solutions and the skin of water
[102].
Contributions from the Li:O polarization or the O:H binding energy change are
negligibly small. At polarization, the H–O bond becomes shorter and stiffer; the
stiffer H–O bond is thermally more stable than those in the bulk of ordinary water,
as it does in the skin of deionized water [102]. The identical O fw –O bw and O bw –O bw
thermal expansion in both the pure water and in the 5M LiCl solution indicates the
invariance of the Li
+ hydration shell size, which does not interfere the O–O thermal
behavior between the hydrated and non–hydrated oxygen anions.
231
is proportional to the binding energy of bond in the first coordination neighbors,
according to the tight-binding approximation [104]. A contraction of the neighboring
bonds shifts the energy levels further—deepens the energy levels further—quantum
entrapment occurs [87]. The amount of energy shift varies from level to level because
of the screening of the potential by electrons in the outer orbitals. Therefore, the
energy of the valence band composed of electrons in the outermost orbital shifts
more than an inner energy level. One has thus, E vb > E 1s, and E edge = E 1s −
E vb < 0, negative shift due to bond contraction.
On the other hand, electrostatic polarization changes the situation contrastingly.
Charge polarization screens and splits the local potential and then shifts a proportion
of electrons up in energy bands causing their negative shift, then, −E vb >> −E 1s,
and thus E edge = E 1s − E vb > 0, positive shift takes place due to the polarization. In fact, bond contraction by molecular undercoordination or salt solvation
is associated with polarization. Therefore, H–O bond thermal contraction without
polarization results in the pre-edge energy negative shift. For the salt solution, ionic
polarization becomes dominance, which overweight the effect of H–O bond contraction on the binding energy shift. The polarization shifts the energy levels up and
thus, E edge = E 1s − E vb > 0. Therefore, the XAS pre-edge shift is very sensitive to the local energetic environment, being capable of discriminating the effect
of entrapment by bond contraction and charge polarization. If the entrapment and
polarization is compatible, no shift will occur to the XAS pre-edge energy peak. This
discrimination could be the advantage of the XAS that probes the effect of valence
band polarization while an XPS detects only the O 1s level shift subjecting to weak
perturbation of the polarization.
11.4.4.3 XAS Capability: Entrapment and Polarization
NEXFAS measurements [101] revealed that Li
+ , Na
+ , and K
+ cations shift the preedge component peak energy more than the Cl
− , Br
– , and I
− anions and the pre–edge
component associated with the first hydration shell of Li
+ ion is thermally more stable
than those beyond, see Fig. 11.9. At 25 °C, the cation effect on the pre–edge shift from
the value of 534.67 eV in alkali chlorides is remarkable: Li
+ (0.27 eV), Na
+ (0.09 eV),
and K
+ (0.00 eV). The energy shift of Li
+ ion in 5 M LiCl solution (0.30 eV) is close
to that in its 3 M solution. On the other hand, in sodium halides, the anion effect
is small: Cl
− (0.09 eV), Br
− (0.04 eV), and I
− (0.02 eV). The energy trend of the
pre–edge shifts is the same as the DPS ω H of the solutions and the skin of water
[102].
Contributions from the Li:O polarization or the O:H binding energy change are
negligibly small. At polarization, the H–O bond becomes shorter and stiffer; the
stiffer H–O bond is thermally more stable than those in the bulk of ordinary water,
as it does in the skin of deionized water [102]. The identical O fw –O bw and O bw –O bw
thermal expansion in both the pure water and in the 5M LiCl solution indicates the
invariance of the Li
+ hydration shell size, which does not interfere the O–O thermal
behavior between the hydrated and non–hydrated oxygen anions.