Chemistry Reference
In-Depth Information
fresh cleave
after 2h of VUV exposure
after 2.5h, but previously
unexposed sample spot
T = 61 K
h = 25 eV
k = k
F
-1.5
-1.0
-0.5
0.0
energy relative to E
F
(eV)
Figure 6.11. ARUPS spectra taken at
k
F
showing the effect of photon-induced
surface degradation (
61 K). Reprinted with permission from
M. Sing, U. Schwingenschlolgl, R. Claessen, P. Blaha, J. M. P. Carmelo, L. M.
Martelo, P. D. Sacramento, M. Dressel and C. S. Jacobsen,
Physical Review B
,
68
,
125111 (2003). Copyright (2003) by the American Physical Society.
ω
25 eV,
T
discussion in Chapter 1). By using a 1D Hubbard model at finite doping
N
at
(see
Fig. 6.13), a
U
/
=
.
=
.
6 eV for the TCNQ-
related features has been derived (Sing
et al.
, 2003b). However, this seems to
contradict the accepted idea that electronic correlations are weak for TCNQ (Pouget,
1988).
On the other hand, if we assume that photoemission is able to reveal both charged
and neutral states of TTF and TCNQ, in the case where the timescale of the pho-
toemission process is much shorter than that of CT, thus considering the CTSs as
dynamical systems with regard to charge, then one could speculate whether the c,
d and d
features from Fig. 6.8 may in fact arise from the HOMOs from
neutral
TTF and TCNQ, as XPS spectra seem to suggest (see Fig. 1.31). From Fig. 6.5
we observe that both HOMOs are located
c
. 0.7-0.8 eV below
E
F
, assuming that
E
F
lies halfway between the HOMO-LUMO gaps. This second approach would
render band theories valid, except for the suppression of spectral intensity at
E
F
.
Thus, even for TTF-TCNQ, a satisfactory scenario has not yet been achieved. This
is clearly an open field that will emerge in the coming years and
old
materials
W
ratio of 1.2 with
U
1
96 eV and
W
1
