E. Dartois1 - O. Marco2 - G. M. Muñoz-Caro 1 - K. Brooks 3,2 - D. Deboffle 1 - L. d'Hendecourt 1
1 - Institut d'Astrophysique Spatiale, UMR-8617,
Université Paris-Sud, Bâtiment 121,
91405 Orsay, France
2 -
European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
3 -
Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
Received 13 January 2004 / Accepted 19 April 2004
Abstract
We present ESO - Very Large Telescope and ESA - Infrared Space
Observatory 3 to 4 m spectra of Seyfert 2 nuclei as compared to
our galactic center lines of sight. The diffuse interstellar medium
probed in both environments displays the characteristic 3.4
m
aliphatic CH stretch absorptions of refractory carbonaceous
material. The profile of this absorption feature is similar in all
sources, indicating the CH2/CH3 ratios of the carbon chains
present in the refractory components of the grains are the same in
Seyfert 2 inner regions. At longer wavelengths the circumstellar
contamination of most of the galactic lines of sight precludes the
identification of other absorption bands arising from the groups
constitutive of the aliphatics seen at 3.4
m. The clearer
continuum produced by the Seyfert 2 nuclei represents promising lines
of sight to constrain the existence or absence of strongly infrared
active chemical groups such as the carbonyl one, important to understand
the role of oxygen insertion in interstellar grains. The Spitzer Space
Telescope spectrometer will soon allow one to investigate the
importance of aliphatics on a much larger extragalactic sample.
Key words: ISM: dust, extinction - ISM: evolution - galaxies: active - galaxies: ISM
The exact composition of the carbon-dominated component is still under
debate. So far, the identification of the carbonaceous material seen
in absorption has proceeded via the spectroscopic analysis of the
so-called 3.4 m absorption features due to -CH2- and -CH3-
aliphatic stretching vibrations in hydrocarbon chains. This 3.4
m absorption is only clearly observed in lines of sight toward our
Galactic center (e.g., Chiar et al. 2002, and references
therein), or more recently in the foreground diffuse medium toward a
luminous Young Stellar Object (YSO) on the shoulder of a strong
circumstellar water ice mantle absorption feature
(Ishii et al. 2002). In all these lines of sight, it is clear
that, in addition to the diffuse interstellar component, high amounts
of molecular cloud material and/or circumstellar features also
contribute to the extinction, rendering the spectral analysis
more complex as one has to decipher the underlying respective
contributions of both intervening MC and DISM media. In particular,
strong ice mantle absorption features from the densest regions probed
by the infrared beam will add several contributions (e.g. 3.1
m
water ice, 3.47
m hydrate, 4.27
m CO2, 4.67
m CO,
6
m water ice, 6.8
m CH3 deformation modes, methanol,
7.7
m methane, 13
m water ice;
Dartois et al. 2002; Chiar et al. 2000). To
reduce the influence of the MC contribution to the spectra, to perform
a finer spectroscopic analysis of the intrinsic DISM absorptions, it
is essential to be able to probe DISM on larger galactic scales and not
only toward embedded individual infrared sources.
Several observations using the Infared Space Observatory (ISO)
spectrometers have allowed the identification of some extragalactic
molecular cloud components (Spoon et al. 2002) or
extragalactic diffuse components (Laureijs et al. 2000). More
recently, ground-based telescopes in specific atmospheric windows
clearly demonstrated the extragalactic sources we should focus our
attention on. Indeed, the infrared
spectra of some external galaxies such as the Seyfert 2 (Sy2) Active
Galactic Nuclei show a strong 3.4 m diffuse medium
absorption feature, with no major contribution from molecular cloud
absorption (e.g. Imanishi 2000).
Table 1: Source parameters.
We present in this paper observations of both galactic and Seyfert 2
extragalactic lines of sight from near (3.1-4 m) to mid
(5.4-7.4
m) infrared to investigate the composition of the
refractory hydrocarbons present in the interstellar grains. We first
present the observations and the observed extragalactic lines of
sights in Sects. 2 and 3, then compare in Sect. 4 the observed 3.4
m absorption profiles, in Sect. 5 the carbonaceous to silicates
ratios, and the mid-infrared spectra in Sect. 6. We discuss in Sect. 7 the implications for the nature of the DISM hydrocarbons and
conclude.
The VLT spectra presented here were extracted from the ESO Science Archive Facility (http://archive.eso.org/) from the programs (69.A-0643 and 69.B-0101) and observations from the authors programs (67.B-0332 and 71.B-0404). A summary of the main parameters are given in Table 1. Data were reduced using in-house software and classical infrared extraction techniques.
The ISO spectra were extracted from the Infrared Space Observatory (ISO) database (http://isowww.estec.esa.nl/). The different AOT bands were stitched together by applying gain factors, using overlapping regions of the spectra to determine them. The corrections applied were less than 5%-10%, in agreement with the differences of the apertures and the absolute photometric reliability of the spectrometer.
The commonly-accepted AGN model involves a central engine
(black hole plus accretion disc)
surrounded by sets of dense matter clouds (the so-called broad-line and narrow-line regions,
BLR & NLR) and by a dusty/molecular disc-like and thick structure (also called a torus)
which funnels the emission of high energy photons and particles along privileged directions
(the ionizing cone, the radio jet axis). This in turn gives rise to
viewing-angle dependent effects which are
usually invoked to explain the various types of AGN discovered so far
(type 1 with broad H
directly visible or
type 2 with broad H
not directly visible).
The spatial extension of the infrared emitting region of Seyfert galaxies
is usually in the range 10-100 pc and corresponds to dust heated by the UV emission
coming from the accretion disk (sized
pc).
The typical extinction on the line of sight is high,
mag,
with a substantial column density of X-ray absorbing material (
).
ISAAC spectroscopy has been done using a slit width of 1 arcsec, centered on the
infrared L-band brightest peak of emission of the sources, corresponding to the hot dust
(
K) around the AGN central engine.
This means that the slit used with ISAAC is getting all the flux
from this region, for all sources.
NGC 1068 is the archetypal Seyfert 2 nucleus, nearby and very bright.
It harbours a hidden nucleus with broad permitted emission lines (BLR)
seen in spectropolarimetry. The optical-UV luminosity of the central
source is
.
Adaptive optics
observations (Marco & Alloin 2000) have shown that the hot
(T>900 K) dust emission seems to be confined in a region of
10 pc radius around the core. L-band images show a bright,
unresolved central peak of emission surrounded by extended emission,
with 90% of the flux contained in a region less than 100 pc away from
the central peak.
NGC 7172 is a Seyfert 2, it has an obscured (
)
active nucleus with strong infrared emission from the nucleus
(mK=10 and
).
IRAS 19254-7245, also known as The Superantennae, belongs to the
Ultra Luminous Infrared Galaxies (ULIRG). Its southern nucleus,
the dominant source at different wavelengths,
is classified as a Seyfert 2 galaxy, with the presence of a Compton-thick AGN with intrinsic luminosity
.
An image observed with ISOCAM at 6.75
m shows an unresolved central
source of an angular size
3 arcsec, with hot dust.
NGC 5506 is Seyfert 2 with hot dust obscuring the central source.
A feature of silicate absorption has been detected at 9.7 m.
The mid-infrared continuum of the nucleus of NGC 5506,
is believed to come from warm (
300 K)
dust in thermal equilibrium, which should be located within
10 pc
of the AGN in the case of the UV luminosity with
.
It has recently been reclassified as an optically-obscured Narrow Line Seyfert 1.
Large scale images of the observed extragalactic objects are presented in Fig. 1 and their main characteristics in Table 2.
The 3.4 m hydrocarbon absorption features observed in galactic
and extragalactic lines of sight are presented in Fig. 2. A
local continuum has been subtracted from all spectra, including the wing
of a water ice stretching mode absorption band in our Galactic sources
lines of sight, as shown by Chiar et al. (2002). All optical
depth spectra have been multiplied by a factor, indicated on the left,
in order to be able to compare the intrinsic profiles of the bands
with an approximately equal optical depth.
The profile observed toward SgR A* with ISO appears peculiar as
compared to the others, with more sub-structures. This can be
explained by the mixing of many infrared emitting sources filling the
large observation beam (about 10'') of ISO, leading to some
confusion in this crowded line of sight (see
Eisenhauer et al. 2003, for a recent infrared image of this area),
even if there is a dominating infrared source.
The two independent GC IRS7 VLT observations, with different
orientations of the slit in the sky, better sample the Diffuse Medium
by avoiding such a confusion. The 3.4 m profile is therefore
better defined.
This profile results from identified absorptions of the symmetric and
antisymmetric vibrations of -CH2- (at 3.42 and 3.50
m)
and terminal -CH3 (at
3.38 and 3.48
m) groups in
aliphatics, and are indicated above optical depth spectra in Fig. 2. Additional absorption is present around 3.28
m, an absorption
generally attributed to aromatic C-H stretching mode.
The resultant optical depth line profiles as seen in NGC 1068,
NGC 7172 and NGC 5506 are similar to the ones observed toward GC IRS7
and GCS3I with the VLT, which implies that the CH2/CH3 aliphatic
component ratio of extragalactic dust is the same in the diffuse
interstellar medium encountered in these objects, i.e. about
.
In the case of the Superantennae galaxy (IRAS 19254-7245), the 3.4
m is almost saturated (
)
but
Risaliti et al. (2003) argue that the profile is similar to our
Galactic center one.
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Figure 1: Images of the observed sources, showing the inclination of the host Seyfert 2 galaxies and where dust obscuration is evident, were retrieved from the NED database (http://nedwww.ipac.caltech.edu/).From left to right, up to down: NGC 7172 (Canada-France-Hawaii Telescope, Hickson (1994)), NGC 1068 (2MASS K band, Jarrett et al. 2003), NGC 5506 (Hubble Space Telescope, 606 nm filter, Malkan et al. 1998) and IRAS 19254-7245 (Danish Telescope, Chatzichristou 2000). A vertical bar representing a 10'' scale has been drawn. |
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Table 2: Main characteristics of the extragalactic sources.
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Figure 2:
Left: spectra of the observed sources and adopted local continua to
extract the 3.4 ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Within variations of a factor of two, which could be attributed to
galactic plane and/or inner torus dust inclinations as well as
temperature gradient effects (which for example modify the optical
depth of absorption bands and the scale of the background infrared
emitting source at 3 and 10 m), the
C
/Si
ratio in the DISM, as
probed by the aliphatics and the silicates, is of the same order.
In a review presented by Pendleton & Allamandola (2002) are
summarised the spectra of most of the interstellar analog relevant
material measured in the laboratory, apart from kerogens, and
proposed for an interstellar identification of the 3.4 m
absorption features. From this set of data, which includes the long
wavelength region of these materials, it appears clearly that the
simultaneous comparison of the 3-4
m range C-H stretching modes
absorption with the 5-8
m bending and deformation modes,
"fingerprints'' of the same molecules, now acessible via satellite
observations, will put the most severe constraint on the nature of the
carbonaceous component of the DISM refractive dust.
In order to make such a comparison, and to shed light on the line of
sight confusion/contamination, in Fig. 3 we show the
near and mid-infrared spectra of SgR A*/GC IRS8, GCS3I and NGC 1068, after subtraction of a local continuum due to
the background infrared emitting region. The overall
optical depth has been normalised to the 3.4
m aliphatic region
of the spectrum, in order to directly compare the various absorptions
arising in the whole presented spectra.
The mid infrared region of the galactic center individual sources are
quite complicated, due mostly to circumstellar and dense molecular
cloud materials along the lines of sight that do not participate in
the diffuse interstellar medium component.
SgR A* is dominated in that region by a quite strong water ice
bending mode arising in dense molecular cloud environments. This band
smears out all the very faint relatively large solid state absorptions
that could appear on the continuum of the infrared source. The
decomposition of the spectrum with several absorbing components has
been reported by Chiar et al. (2000) but clearly the result is
dependent on the water ice structure and the number of components
used, which are not constrained at all by such a "Gaussian-like''
absorption band, making the identification very difficult. The fainter CH2 and CH3 deformation modes are seen at around 6.85 m and 7.25
m and correspond to what is expected for typical
aliphatic hydrocarbons.
The case of GCS3, presented below and located at only a few arcminutes
from SgR A*, is a good example of the problems encountered in
the identification of mid infrared spectra. The strong 6.2
m absorption feature observed toward this line of sight is in fact of
circumstellar nature, as is observed in several Wolf Rayet
envelopes (Chiar & Tielens 2001), which do not display
any strong 3.4
m absorption counterpart.
Table 3: Aliphatic hydrocarbons/silicates ratio.
Table 4: List of detected and/or expected infrared absorption features.
In contrast to the spectra discussed above, the spectra of the nuclear
regions of Seyfert 2 Galaxies do not pertain to the same regime and
will not therefore suffer from local circumstellar
contamination. The infrared continuum is thought to be emitted
by a central dust condensation, of a few parsecs of extension
(Tomono et al. 2001; Marco & Alloin 2000). This offers
us the opportunity to probe the matter with a large infrared beam,
probing essentially the diffuse matter, and lowering the importance of
individual specific objects as they represent a small volume filling
factor along the line of sight.
The spectrum of the NGC 1068 nucleus, recorded in the mid infrared by the
ISO short wavelength spectrometer, and displayed in Fig. 3,
does not show either the 6 m water ice bending absorption,
or the circumstellar contribution at 6.2
m observed for GCS3. The spectrum of the nucleus does present a large absorption around 5.87
m with an optical depth of about half the 3.4
m one. This absorption, intrinsically very strong in the infrared, is
typical of the C=O carbonyl group and is discussed in relation to
the other signatures in the next section.
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Figure 3:
A): Near and mid infrared spectra of two galactic center lines of
sights sampling the diffuse interstellar medium, and one Seyfert 2
inner region. All spectra are scaled as described, for clarity. The
adopted local continua used to establish the optical depth in the
absorption features are shown as dashed lines. B): Resultant optical
depth spectra comparing Galactic center dust absorptions and the one
seen along the Seyfert 2 nucleus line of sight of NGC 1068. The NIR (3-4 ![]() ![]() |
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Given the CH2/CH3 integrated absorption ratio of about two deduced from the previous analysis, the mean length of the carbon aliphatic chains (sub-chains if branched aliphatics) encountered both in the Galactic and Seyfert 2 galactic nuclei is therefore of six carbon atoms if purely aliphatic, and fewer if attached as individual aliphatic groups to an aromatic structure.
The 3.4 m absorption profile is to a first order compatible with
many materials (see for example the Table 3 of Pendleton &
Allamandola 2002). The only firm constraints obtained from these
absorption bands are: (i) an estimate of the CH3 to CH2 ratio
in the aliphatic component of the ISM dust can be made; (ii) aliphatic
chains are interconnected or branched, leading to broad vibrational
profiles, as simple aliphatics such as the ones presented in the
appendix would display much sharper transitions; and (iii) the
aromatic contribution, through its C-H stretching component around
3.28
m is at most as abundant as the aliphatic one, except if
these aromatics are very large or dehydrogenated.
In addition to the
last statement, the 6.2 m aromatic C=C vibration is absent or
weak from the Galactic center spectra, whereas this mode is much
stronger than the aromatic CH stretch, which implies there is at most
a small contribution of aromatics to this DISM material.
The spectra of the insoluble phase of carbonaceous chondrites, such as
Orgueil and Murchison, are often considered as excellent candidates
analogs for the ISM carbonaceous component, based on a good fit of the
3.4 m of its aliphatic phase (Ehrenfreund et al. 1991;
Pendleton 1995). However, the Orgueil and Murchison insoluble
material possess a C/H ratio of about 17 (Gardinier et al. 2000),
which shows that this extracted material is not dominated by aliphatic
chains, but rather by aromatic material, as already observed
(Gardinier et al. 2000; de Vries et al. 1993). If aliphatic
chains were dominant, the CH2/CH3 network would imply a C/H ratio much lower, closer to 0.5. The long wavelength absorptions in
the infrared spectra of these meteorites extracts are a mixture of the
aliphatics deformation modes and of the bulk of the aromatics
present. When the sample is heated above about 600-700 K under vacuum,
the absorptions seen at 3.4
m disappear whereas most of the
strongest absorptions in the 5 to 10
m range remains at higher
temperatures (Wdowiak et al. 1988), demonstrating that they are due
to at least two different phases, the less volatile being attributed
to large polyaromatics. This explains why parts of the Orgeuil and
Murchison meteorites are able to reproduce the interstellar 3.4
m profile but fail to account for the now observed longer wavelength
part of the spectrum.
As discussed previously, the NGC 1068 mid infrared spectrum display a
carbonyl absorption at 5.87 m. The absorption peak position
observed is caracteristic of ketones (R-(C=O)-R') or carboxylic acids
(R-(C=O)-O-H). The absence of a strong carboxylic (O-H) feature in the
Seyfert 2 spectra, is even more specific to the ketones.
Using a decomposition with Gaussians as shown in Fig. 3 of
both the 3.4 and 5.87
m integrated absorbance, we can estimate
the C/O ratio needed to account for this new absorption band. Using
A(CH
cm.group-1 and
A(C=0
cm.group-1 (see Appendix where
we determine experimentally the integrated absorbances of ketone
carbonyl and CH stretches; see also d'Hendecourt & Allamandola 1986)
together with the ratio of integrated absorbance deduced from the fit,
we obtain a CH3/CO ratio of about 3. It leads to a C/O of
at least 9 taking into account a CH2/CH3 ratio of about 2.
The fact we do not see large amounts of oxygen inserted in the
aliphatic chains is not so surprising given the oxygen budget in the
solid phase of the diffuse medium, where a large part is already
locked in the silicates. Indeed, if we use the integrated absorbance
values of A(Si-O
cm.molecule-1(Dartois 1998) for the stretching mode of pyroxenes,
cm-1 for the FWHM of the band,
/
or 18.5
(Rieke & Lebofsky 1985; Mathis 1990),
cm-2 mag-1(Bohlin et al. 1978), and given that there are 3 oxygen atom per Si in a pyroxene (the most common pyroxene form is (Ca, Mg, Fe)2Si2O6), we can
deduce
.
Using a value of the oxygen
atomic abundance of 450 to 550 ppm (Sofia & Meyer 2001), about 30-40% of the oxygen is locked in the silicates. A large part of
the oxygen seems also confined to the gas phase, in the atomic phase
even at moderate to high extinctions (André et al. 2003;
Baumgartner 2003).
In infrared spectroscopy, the absence of a line in a spectrum for an
infrared active group with a high integrated absorption coefficient
(such as the carbonyl) is often a very strong constraint to define the
nature of the material, in relation to the detected bands. The
appearance of a carbonyl absorption band in the Galactic center lines
of sight should not be affected by the water ice observed one, as the
former lies in the foreground. This means we can put a useful upper
limit on the oxygen content. Based on the mid infrared spectrum of SgR A* and using the same carbonyl profile as the one used for NGC 1068, we estimate the maximum O/C that would escape detection,
as shown in Fig. 3. The upper limit derived in this way
translates into an O(carbonyl)/C(aliphatic).
Absence of the ketone carbonyl toward galactic center sources, besides the strength of this infrared absorption, suggests that the synthesis of the carbonaceous ISM is formed in a very reducing environment such as the one shown by Mennella et al. (2002). This is a strong constraint as, for example, almost all the laboratory spectra presented by Pendleton & Allamandola (2002) present a carbonyl absorption, even the ones in which no oxygen containing molecules were introduced in the experiment, but rather resulted from pollution, showing the ability of oxygen to insert in the aliphatic network and the ease of measuring this particular carbonyl mode.
In summary, observations shown in this paper tend to show that the aliphatic component of Galactic dust and the one encountered toward the nucleus of NGC 1068 are globally the same, except for the possible presence of a small amount of ketones. This last finding should of course be confirmed by further observations at mid infrared wavelength of the Seyfert 2 central dust condensation.
Until now, the observations toward the dense medium do not show the
presence of any dense cloud 3.4 m absorption, whereas the column densities
are large enough to detect it at levels comparable or better than the
diffuse medium. Laboratory ice analogues are able to produce
refractory materials, stable after the evaporation of the ices, with
an efficiency of at most 10-2 of the initial total ice mass
content (e.g., Muñoz Caro & Schutte 2003, and references therein).
Considering an optimistic maximum total ice abundance ratio of a few 10-4, at most 10-6 of the total material, with respect to
the hydrogen density, is processed in this way in one cycle of a dense
cloud. The observed diffuse medium
C
/C
abundance is around 5 to 10% (Pendleton et al. 1994; Sandford et al. 1991). One
such cycle can therefore produce at most one tenth of the observed DISM aliphatics.
A grain cycling from diffuse to dense medium of at least ten times is
required to explain the observations, if we suppose the produced
residues are not destroyed and remain in the subsequent cycling
sequence. This finding is in contradiction to the fact we do not see
the 3.4 m C-H absorptions in at least some of the observed dense
clouds.
An alternative interesting scenario would be that in the diffuse medium, due to the high atomic hydrogen content in the gas phase, there exists a mechanism that can re-hydrogenate the aliphatic carbon backbone of the otherwise hydrogen UV abstracted material (Mennella et al. 2001, 2002; Muñoz Caro et al. 2001). When entering the dense clouds, the atomic hydrogenation is stopped, due to the passage of a reducing environment dominated by atomic hydrogen to a rather unreactive molecular hydrogen one, and giving rise to a chemistry leading to the formation of the ice mantles. The CH stretching modes would then disappear. To validate this scenario, one must therefore search for the mid infrared signature, if strong enough, of the much more stable carbon backbone still present, in deeply embedded objects.
In our Galaxy, the 3.4 m absorption feature is sensitive to the
environment, as it is not observed in dark clouds. Seyfert 2 and
Galactic center lines of sights should probe different mediums, but
the solid absorption features in the type 2 Active Galactic Nuclei
lines of sight and our Galactic diffuse medium share common properties
as underlined by the similarities of their carbonaceous infrared
absorption bands.
AGNs possess a powerful central source (Black hole + accretion disk)
that produces the ionizing radiation that heats the dust we see. The
temperature of the dust is shown to decrease very fast with the
distance to the central source, to reach 100-200 K at a few hundred
parsecs. At the location where the bulk of the absorption takes place,
the temperature might not differ too much from the one encountered in
the diffuse medium. The material seen via these features must be very
UV-resistant and in the solid state as it must stand the strong UV field of the Diffuse Medium. Therefore, only if the temperature of
these carbonaceous grains could reach above about 300 K would we see
these lines in emission. The case of the 3.3 m, the so called PAH feature, is different as the excitation mechanism is molecular and
transient after absorption of single UV photons by relatively small
molecular systems.
An important clue toward the understanding of the 3.4 m feature
composition lies in the dust cycling timescales of the inner dust
torus of Seyfert 2 versus our Galactic diffuse medium. They should be
quite different. It seems to favor an interpretation of in-situ
formation of this 3.4
m component, without invoking many dust
cycles, or the hydrogenation of an almost pure carbon dominated dust,
as explained above.
Acknowledgements
Part of this work was funded and performed during a visiting scientist program of one month at ESO Chile Headquarters. The authors wish to thanks the anonymous referee as well as G. Matrajt for fruitful comments that improved the submitted paper.
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Figure 4:
Transmittance spectra of a film of hexane ice ( left) and
2-nonanone ice ( right) deposited on a thick CsI window, at 10 K. Note the
fringes due to the plane parallel ice film acting as an infrared
Fabry-Perot. The meaning and use of the local maxima
![]() ![]() |
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The transmittance T of a plane-parallel film deposited on a non-absorbing thick substrate is a complex function of the wavenumber ,
the substrate refractive index
,
the film refractive index
,
the imaginary part of the film index
,
and the film thickness
(Mini 1982). In the regions of the spectrum where absorption is very weak (
), this equation takes the simpler form:
This transmittance shows a fringe pattern whose maximum and minimum are given, in the case of
,
by:
The thickness can now be evaluated using the wavenumber difference
between maxima, as presented in Fig. 4, using the classical
interfringe relation
], where
is the wavenumber
difference between two adjacent maxima (minima or maxima) of the
fringes. The film presented in Fig. 4 has therefore a
thickness of 1.86
m.
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Figure 5:
Optical depth spectra of hexane, octane and decane at 10 K in the CH stretch infrared region of the spectrum, after fringes local baseline
correction. All spectra have been normalized to the antisymmetrical CH2 stretching mode around 3.42 ![]() |
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Figure 6:
Optical depth spectra of heptanone, nonanone and undecanone where the
carbonyl is either positionned at the end or the middle of the carbon
skeleton, after fringes local baseline correction, recorded at 10 K in
the CH stretch infrared region of the spectrum. All spectra have been
normalized to the antisymmetrical CH2 stretching mode around
3.42 ![]() |
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Figure 7:
Optical depth spectra of heptanone, nonanone and undecanone, where the
carbonyl is either positionned at the end or the middle of the carbon
skeleton, recorded at 10 K in the C=O stretch infrared region of the
spectrum. All spectra have been normalized to the antisymmetrical CH2 stretching mode around 3.42 ![]() |
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