Chemistry Reference
In-Depth Information
R
0
−
R
4
·
sin
2
ϕ
4π
·
k
2
4
·
sin
2
ϕ
R
=
=
cos ϕ
·
·
d
=
cos ϕ
α
·
d
.
(7.7)
R
0
n
2
·
λ
n
2
·
R
0
and
R
are the reflectivity without and with an absorbing thin film, respectively,
and
k
2
is the imaginary part of the complex refractive index of the thin film.
Further, in the case of a weakly absorbing thin film (α
·
d
1), it follows
=
(
I
0
−
)/
∼
α
·
from the Beer-Lambert law the approximation
d
and the
ratio of absorption of a thin film comparing IRRAS and transmission experiment is
expressed by
I
I
I
0
·
R
4
sin
2
ϕ
I
=
cos ϕ
.
(7.8)
n
2
·
For example, using the arrangement with a thin polymer film (
n
2
=
1.5) of thickness
d
, ambient vacuum (
n
1
=
75
◦
the sensitivity in the
IRRAS experiment is increased by the factor of about four compared with the trans-
mission experiment. By use of proper experimental condition the received IRRAS
spectrum corresponds practically to the transmission spectrum [14]. The advantage
of this technique is that thin films can be prepared on the metallic substrate by plasma
deposition processes or spin/dip coating procedure in order to study the interaction
of the nonthermal plasma with polymer surfaces and thin organic films in situ. For
example, it has been investigated the plasma modification of polymer surfaces [15],
as well as the plasma solidification of liquid polydimethylsiloxane films [16].
1), and an angle of incidence ϕ
=
7.1.3.2 Evanescent Wave Spectroscopy
The evanescent wave spectroscopy (EWS) in the IR spectral range is well known as
the
attenuated total reflection (ATR) spectroscopy
[17].
When radiation passes into a medium 1 with higher refractive index
n
1
than
the surrounding medium 2 with
n
2
, the radiation will be trapped inside if the angle
of incidence ϕ at the reflecting surface in the medium 1 exceeds the critical angle
ϕ
c
=
. A standing wave is established in front of the reflecting interface.
The radiation is totally reflected at proper conditions and propagates through the
medium 1. In the medium 2 with lower refractive index an evanescent wave is
observed for the electric field strength
E
sin
−
1
(
n
2
/
n
1
)
(
z
)
(7.9) with characteristic penetration
depth
d
p
(7.10), see Figure 7.6.
exp
,
z
d
p
E
(
z
)
=
E
0
·
−
(7.9)
λ
d
p
=
n
1
·
√
sin
2
ϕ
(7.10)
2π
·
−
(
n
2
/
n
1
)
2
with λ the vacuum wavelength.
Without absorption in medium 2 the Pointing vector in
z
-direction disappears
averaged over the time. In the interesting case of an absorbing medium 2, the
reflected intensity is reduced. ATR spectroscopy is used for chemical analysis
of the interface of bulk material (semi-infinite sample
d
d
p
), or for thin films
