224
11 Liquid Phase
11.4 Electron Spectrometrics
11.4.1 STM and STS: Strong Polarization
Figure 11.3 shows the orbital images and the dI/dV spectra of a H 2 O monomer and
a (H 2 O) 4 tetramer deposited on a NaCl(001) surface probed using STM/S at 5 K
temperature [74]. The highest occupied molecular orbital (HOMO) below the E F of
the monomer appears as a double—lobe structure with a nodal plane in between,
while the lowest unoccupied molecular orbital (LUMO) above E F appears as an
ovate lobe developing between two HOMO lobes. STS spectra at different depths
discriminate the tetramer from the monomer in the density of states (DOS) crossing
E F .
These STM/S observations [74] confirmed the occurrence of sp
3 —orbit hybridization of oxygen in H 2 O monomer occurs at 5 K or lower and the intermolecular interaction involved in (H 2 O) 4 . Therefore, the 2N number conservation of protons and
(a)
(b)
(c)
(d)
LUMO
HOMO
Fig. 11.3 STM images of a a H 2 O monomer and b a (H 2 O) 4 tetramer, and c, d the respective
dI/dV spectra obtained under conditions of V = 100 mV, I = 100 pA, and dI/dV collected at 50
pA of different heights at 5 K temperature. Grid in images denotes the Cl − lattice of the NaCl(001)
substrate. The LUMO (>E F ) and HOMO ( Reprinted with permission from [74]