Chemistry Reference
In-Depth Information
A
(
ν
)
A
MAX
1112
0.25
1266
0.20
CH
3
CH
3
CH
3
CH
3
CH
3
Si
O
Si
O
Si
824
0.15
CH
3
CH
3
CH
3
n
A
INT
0.10
2965
0.05
0.00
ν
1
ν
MAX
ν
2
ν
3200
2800
1600
1200
800
(a)
Wave number in cm
-1
(b)
FIGURE 7.3
IR absorption spectrum (a) of a 85nm thin film from polydimethylsiloxane
(SILFAR
R
1 000) with characteristic absorptions at 2 965 cm
−
1
(asymmetric stretching of
C-H in CH
3
-group), 1 266 cm
−
1
(symmetric deformation of Si-CH
3
), 1 112 cm
−
1
(asymmetric
stretching of Si-O-Si backbone), and 824 cm
−
1
(symmetric rocking of Si-(CH
3
)
2
). (b) The
principle of the integral absorbance for determining the concentration of absorbing species is
shown.
absorbance
A
MAX
(
of the absorption line; however the instrumental parameters
have strong influence on this maximum absorbance. Therefore, the calculation of the
integral absorbance
A
INT
over the absorption line (7.4) is more favorable. The area
below the absorption line is proportional to the concentration of absorbing species.
Figure 7.3 shows as an example the absorption spectrum of a polydimethylsiloxane
(PDMS) thin film with characteristic absorption bands (taken by FT-IRRAS, Figure
7.3a), and the definition of the integral absorbance (Figure 7.3b)
ν
MAX
)
ν
2
A
INT
=
A
(
ν
)
·
d
ν.
(7.4)
ν
1
7.1.2 P
RINCIPLE OF
F
OURIER
T
RANSFORM
I
NFRARED
S
PECTROSCOPY
In comparison with conventional broadband IR spectroscopy using dispersive opti-
cal elements for the wave number tuning, the FTIR technique [6] allows one to
measure simultaneously all wave numbers in the chosen spectral range by means
of the interferometer. The principal scheme of FTIR spectrometer with a Michelson
interferometer is shown in Figure 7.4.
The interferometer consists of two mutually perpendicular plane mirrors. One of
them (
M
1
) is fixed and the other (
M
2
) can move along the axis which is perpendicular
to its plane. The infrared radiation emitted by a broadband IR source is divided by
the beam splitter (BS) where the infrared beam is partially reflected to the mirror
M
1
and partially transmitted to the mirror
M
2
. After both space-coherent beams return to
the beam splitter, they interfere and are again partially reflected and transmitted. The
path difference of the two interfering beams is equal to 2
·
δ, where δ is the
M
2
mirror
