INFRARED STUDY OF UV/EUV IRRADIATION
OF NAPHTHALENE IN H2O + NH3 ICE
Y.-J. CHEN1, M. NUEVO1, F.-C. YEH1, T.-S. YIH1, W.-H. SUN2, W.-H. IP2,
H.-S. FUNG3, Y.-Y. LEE3, C.-Y. R. WU4
1
Department of Physics, National Central University, Chung-Li 32054, Taiwan, R.O.C.
Institute of Astronomy, National Central University, Chung-Li, 32054, Taiwan, R.O.C.
3
National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, R.O.C.
4
Space Sciences Center and Department of Physics and Astronomy, University of
Southern California Los Angeles, CA 90089-1341, U.S.A.
2
We irradiated the smallest polycyclic aromatic hydrocarbon (PAH) naphthalene (C10H8) mixed with
water and ammonia ice at 15 K. In the present work, we focus on studying the effect of the photon
energy range on H2O+NH3+C10H8 = 1:1:1 mixtures, irradiated separately with 4–20 eV and 13–45
eV photons, i.e. in the ultraviolet (UV)/near extreme ultraviolet (EUV) and the EUV ranges,
respectively, provided by synchrotron radiation. We observed several photo-products after irradiation
with both energy ranges, namely CH4, C2H6, C3H8, CO, CO2, HNCO and OCN- , some of them
observed only after irradiation with 4–20 eV photons, some others observed in both cases, showing
that naphthalene and its photo-products are probably more efficiently destroyed by the high energy
photons (E > 20 eV). The production yields were also shown to study the correlation between the
production of these small carbonaceous molecules and the photon energy range.
1. Introduction
In the early 20th century, a series of diffuse interstellar bands (DIBs) were
recorded on photographic plates. More than 100 of such bands are observed
nowadays in the ultraviolet (UV), visible and near infrared (IR) regions of the
electromagnetic spectrum [1,2,3,4]. The identification of the carriers of these
DIBs has become one of the most active challenges in astrophysical
spectroscopy [5,6]. Polycyclic aromatic hydrocarbons (PAHs) are now thought
to be the best candidates to account for the DIBs in the interstellar medium
(ISM), or most likely their cations [7,8]. It has been suggested that PAHs could
represent ~17% of the cosmic carbon, and consequently be the most abundant
free organic molecules in the Universe [7 and references therein].
PAHs constitute a group of very stable organic molecules made up from
carbon and hydrogen only, assembled into aromatic rings of 6 sp2-carbon atoms
like benzene (C6H6) bound together. Since phenyl rings are flat, these large
compounds are in most of the cases also flat, like graphite layers. It is believed
that these molecules are formed in the outflows of dying carbon-rich stars from
which they are ejected into the ISM.
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The presence of PAHs in the ISM has been confirmed by astronomical
observations of their C-H stretching and out-of-plane bending modes, both in
emission [9,10,11] and in absorption in embedded protostars [12,13,14,15].
PAHs may condense onto refractory dust grains with other volatile species,
among which H2O is the most abundant, and are abundant and ubiquitous in
many different astrophysical environments [7,16,17]. Bernstein et al. [18,19]
and Sandford et al. [20] have been investigating the ultraviolet (UV) processing
of PAHs in H2O ices, emphasizing the possible connections between interstellar
and meteoritic PAHs, and have shown that PAHs undergo both oxidation and
reduction photoreactions in ices, resulting in the production of aromatic
hydrocarbon species similar to some of those identified in carbonaceous
chondrites and interplanetary dust particles (IDPs).
Here we present the experimental study of irradiations of naphthalene
(C10H8), the smallest PAH (only two aromatic rings), in H2O and NH3 ice
mixtures at low temperature. Photons in the 4–20 and 13–45 eV ranges, i.e. from
UV to EUV, were used to irradiate H2O+NH3+C10H8 = 1:1:1 ice mixtures at 15
K in two separated experiments. EUV photons can excite the molecules to their
ionization continua and produce neutral and ionic fragments. It has been shown
that certain molecular species can be synthesized after irradiation with EUV
photons, but not necessarily with vacuum ultraviolet (VUV) irradiation [21].
Finally, the use of two different energy ranges allowed us to study the
dependence of the photon energy with the production yields of several
photo-produced molecules.
2. Experimental protocol
A detailed description of the experimental apparatus and techniques are given
elsewhere (see e.g. Wu et al. 2002). In the present work, the UV/EUV photolysis
study of the icy samples containing naphthalene was performed in a stainless
steel vacuum chamber (P < 5 ´ 10-10 torr). The reagents used in this work and
their purities are as follows: H2O (liquid, triply distilled), NH3 (gas,
Sigma-Aldrich, 99.5% purity) and C10H8 (powder, Sigma-Aldrich, 99% purity).
The H2O+C10H8+NH3 samples were vapor-deposited onto a KBr substrate at 15
K, injecting a mixture of H2O+NH3 gas and the C10H8 vapor through two
separated stainless steel thin tubes (2-mm inner diameter). The relative
proportions of the final H2O+NH3+C10H8 = 1:1:1 mixtures were controlled from
the partial pressures inside the stainless steel bottles where they were mixed. The
typical thickness of the ice films was approximately 1~3 μm, measured by
monitoring the variation of interference fringes of a He-Ne laser light reflected
by the KBr substrate.
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A Fourier-transform infrared spectrometer (FTIR) (Perkin-Elmer
FTIR-1600) was used to record infrared (IR) spectra between 4000 and 500 cm-1
with a 4 cm-1 resolution (Fig. 1). The IR and EUV beams form an angle of 90°
on the substrate, so that the IR spectrum could be recorded in situ during the
whole experiment, i.e. before, during and after irradiation. Once the gas
deposition was completed, the vacuum chamber was left idling for a few hours
so all gases could condense on the substrate. The irradiation experiments were
performed after the pressure in the chamber reaches ~5 ´ 10-10 torr.
0.20
H2O+NH3+C10H8 = 1:1:1 at 15 K, before irradiation
Absorbance
0.15
0.10
0.05
0.00
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Figure 1. Infrared spectrum of one of the H2O+C10H8+NH3 = 1:1:1 ice mixture in the 4000-500 cm-1
range, before UV/EUV irradiation.
The UV/EUV photon beams were provided by the high-flux beamline of
the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu,
Taiwan. The incident photon flux was constantly monitored by an in-line gold
mesh. The incident photon energy used was the 0th order of the white light in the
4-20 and 13-45 eV ranges of 450 and 1600 l mm-1 gratings, respectively [22].
We could therefore study the compounds photo-produced after irradiation with
4-20 and 13-45 eV photons, and compare the results for both ranges. The
irradiations were performed until a typical total integrated incident photon dose
of about 1.5 ´ 1020 photons was reached for each photolysis experiment.
3. Results and discussion
3.1. Photo-dissociation of naphthalene
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Figure 2 shows the differences of absorbance for the IR spectra of the
H2O+NH3+C10H8 = 1:1:1 mixtures recorded after UV/EUV irradiations. We
could identify several IR features, most of them being present in both 4-20 and
13-45 eV irradiated ices. The identified features and their career species are
listed in Table 1. The two IR spectra are very similar, showing that the
photo-products are almost the same after EUV irradiation of the
H2O+NH3+C10H8 ice mixtures with the two photon energy ranges. However, the
features of some of the photo-products are not present after irradiation with the
in 13-45 eV photons (Fig. 2, lower trace), maybe because these compounds are
easily destroyed by high energy (E > 20 eV) photons or because they recombine
with other products and/or radicals. In the following, we will discuss how the
photo-chemical mechanisms depend on the photon energy.
Difference of absorbances
0.06
C3H8
C2H6
CO2
C2H6
4 - 20 eV
13 - 45 eV
-
OCN
CO
+
NH4
*
0.04
HNCO
*
CH4
*
0.02
0.00
-0.02
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Figure 2. Difference of absorbances of IR spectra of H2O+C10H8+NH3 = 1:1:1 ice mixtures at 15 K
after irradiation with 4–20 (upper trace) and 13–45 eV (lower trace) photons. The features marked
with * correspond to the tentatively identified compounds in Table 1.
The photo-products formed after irradiation with 4-20 eV photons include
(see Table 1): CH4 (1305 cm-1), C2H6 (2873 and 2931 cm-1), C3H8 (2959 cm-1),
CO (2134 cm-1), CO2 (2338 cm-1), HNCO (2255 cm-1), OCN- (2159 cm-1) and
NH4+ (1453 cm-1). Three other absorption features, marked with an asterisk in
Fig. 2, were only tentatively identified. They could be assigned to CH2N
(methylene amidogen) (2722 cm-1), the C2O- anion (1898 cm-1) and the benzyl
radical (863 cm-1).
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In our H2O+NH3+C10H8 mixtures, naphthalene was the only source of
carbon. Thus, all products containing carbon atoms could only be formed from
the photolysis of C10H8. This means that the phenyl rings are dissociated by
UV/EUV photons into smaller fragments to produce aliphatic chains (carbon
chains containing only C and H atoms). By comparison, breaking aromatic rings
such as in PAHs is not an efficient process when using photons in the
photochemistry range (5–10 eV), with which only substitution reactions of H
atoms and/or addition of heteroatoms are efficient [18].
Species
Peak position (cm-1)
after 4–20 eV Photon irradiation
C3H8
C2H8
CH2N*
CO2
HNCO
OCNCO
C2O *
NH4+
CH4
Benzyl radical*
Peak position (cm-1)
after 13–45 eV Photon irradiation
n.d.
n.d.
n.d.
n.d.
2338
2253
2160
2135
1902
1450
1299
n.d.
2959
2931
2873
2722
2338
2255
2159
2134
1898
1453
1300
863
Table 1. New species identified after UV/EUV irradiation of H2O+C10H8+NH3 = 1:1:1 ice mixtures.
The species marked with * were tentatively identified. n.d. = not detected.
CH4 was photo-produced after irradiation of H2O+NH3+C10H8 ice mixtures
with both ranges of photon energies, whereas bigger molecules such as C2H6 and
C3H8 are only observed after irradiation with 4–20 eV photons. Parallel and
independent experiments in our laboratory where pure C10H8 and C10H8 in water
ice were irradiated at low temperature with 4–20 eV photons already showed the
presence of CH4, C2H6 and C3H8, indicating that CnH2n+2 (n ³ 2) can be direct
products of the naphthalene photo-dissociation and/or secondary products of
rapid recombinations of smaller radicals such as CH3 and C2H5. For example,
C2H6 could be a secondary product of the photo-dissociation of CH4:
CH4 + hν → CH3 + H
CH3 + CH3 → C2H6 .
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One way to check if the CH3 radical is efficiently produced, is that it may also
recombine with the OH radical (from the photo-dissociation of H2O) to form
methanol (CH3OH). Among the absorption features of CH3OH in the IR
vibrational range, the feature around 1128 cm-1 could not be observed in our
spectra since it overlaps with features showing the depletion of naphthalene (Fig.
2). However, a feature around 1030 cm-1 may be assigned to methanol and
confirm the relatively high efficiency of the production of CH3 radicals, and thus
support from an indirect way the photochemical pathway mentioned above to
form aliphatic molecules such as C2H6 and C3H8.
As mentioned above, during the 13–45 eV irradiation, the absorption
features of C2H6 and C3H8 were not observed. Moreover, the presence of a
feature at 863 cm-1, probably due to the presence of the benzyl radical
(C6H5CH2), in the IR spectrum of the sample irradiated with 4–20 eV photons
but not detected in the IR spectrum of the sample irradiated with 13–45 eV
photons gives a hint on the formation of these radicals, aliphatic compounds,
and other small molecules. Indeed, naphthalene is photo-dissociated by
progressively by UV/EUV photons into smaller and smaller fragments. High
energy (E > 20 eV) photons can thus totally dissociate naphthalene, whereas less
energetic photons only partly dissociate naphthalene, so that intermediate
compounds of the photo- dissociation such as benzyl radical and small aliphatic
chains can be observed in the 13–45 eV experiment. C2H6 and C3H8 can
therefore be directly formed from the naphthalene photo-dissociation as well as
radical recombinations, since in both experiments naphthalene aromatic rings
are broken. In conclusion, both 4–20 and 13–45 eV photons can dissociate
naphthalene, and not only lead to substitutions and heteroatom additions after
excitation of naphthalene molecules to high energy states, as was shown by
previous studies [18].
Besides aliphatic chains, other small compounds containing O and N atoms
could be identified in our samples: CO (2134 and 2135 cm-1 in the 4–20 and
13–45 eV experiments, respectively), CO2 (2338 cm-1), OCN- (2159 and 2160
cm-1), HNCO (2255 and 2253 cm-1). According to what was discussed above
about the production of small molecules, these molecules are probably formed
from the recombination of small aliphatic compounds or radicals with OH and
NH2, produced after the photo- dissociation of water and ammonia, respectively.
The formation mechanisms of these small compounds will be confirmed in the
following after the determination of their production yields (Sec. 3.2).
Finally, some small absorption features in the 1500–1400 and the 820–720
cm-1 ranges of the IR spectra could be due to the presence of nitrogen-bearing
PAHs, called polycyclic aromatic nitrogen heterocycles (PANHs), by
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comparison to literature IR spectroscopy data of quinoline (C9H7N) and
phenanthridine (C13H9N) in water ice at 15 K [23]. Quinoline is a molecule of
naphthalene where one of the carbon atoms was substituted by a nitrogen atom,
so that its presence in our samples is likely.
These important results indicate that naphthalene and other PAHs can be
photo-dissociated in cold astrophysical environments by EUV photons and
contribute to the reservoir of carbon whose photochemical evolution can lead to
the production of complex organic molecules in the ISM.
3.2. Production yields
We calculated and compared the production yields of CO, CO2 and OCN-,
formed during the irradiation of H2O+NH3+C10H8 = 1:1:1 mixtures with 4–20
and 13–45 eV photons at 15 K The column densities of these three compounds,
plotted in Figs. 3 (CO), 4 (CO2) and 5 (OCN-) as a function of the photon dose,
were calculated using the following absorption strengths: A(CO, 2134 cm-1) =
1.1 ´ 10-17 cm molec-1 [24], A(CO2, 2340 cm-1) = 7.6 ´ 10-17 cm molec-1 [24] and
A(OCN-, 2160 cm-1) = 4 ´ 10-17 cm molec-1 [25].
Figures 3–5 show clearly that the production yields for these three
compounds are significantly higher during the irradiation with 13–45 eV
photons (triangle symbols): after an integrated photon dose of approximately 1 ´
1020 photons cm-2, the yields for CO, CO2 and OCN- are respectively about 3.6,
2.2 and 1.9 times higher during an irradiation with 13–45 eV photons than
during an irradiation with 4–20 eV photons. This result confirms that small
C-bearing molecules are formed from the direct photo-dissociation of
naphthalene, only source of carbon in our mixtures, and that the
photo-dissociation efficiency is higher when the photon energy is higher.
Figures 3 and 4 also show that CO is produced with a higher efficiency than
CO2, since the column densities for CO were calculated to be 3.5 and 5.8 higher
than those of CO2 during the 4–20 and 13–45 eV irradiations, respectively, for
an integrated photon dose of 1 ´ 1020 photons cm-2. This most probably means
that CO2 is a secondary product of the photolysis of CO. A possible mechanism
to produce CO2 from CO would be the direct photo-dissociation of CO into CO2
[26]:
CO + hν → CO* (A1П)
CO* + CO → CO2 + C
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4 - 20 eV
13 - 45 eV
30
25
15
-2
CO column density (x 10 molec cm )
35
20
15
10
5
0
0
5
10
15
20
19
25
30
-2
Photon dose (x 10 photons cm )
Figure 3. Column density of CO plotted as a function of the 4–20 and 13–45 eV photon doses.
4 - 20 eV
13 - 45 eV
5
4
15
-2
CO2 column density (x 10 molec cm )
6
3
2
1
0
0
5
10
15
20
19
25
30
-2
Photon dose (x 10 photons cm )
Figure 4. Column density of CO2 plotted as a function of the 4–20 and 13–45 eV photon doses.
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4 - 20 eV
13 - 45 eV
-
15
-2
OCN column density (x 10 molec cm )
6
5
4
3
2
1
0
0
5
10
15
20
19
25
30
-2
Photon dose (x 10 photons cm )
Figure 5. Column density of OCN- plotted as a function of the 4–20 and 13–45 eV photon doses.
The carbon atom released in this mechanism does not remain free in the medium
but recombines immediately with a molecule of CO to form C2O. This
mechanism is supported by the presence of the 1898 and 1902 cm-1 features in
the IR spectra of the samples irradiated with 4–20 and 13–45 eV photons,
respectively (Fig. 2), assigned to the C2O- anion.
CO can also react with the OH radical (produced from the
photo-dissociation of H2O) to form CO2 [27] as following:
H2O + hν → OH + H
OH + CO → COOH → CO2 + H .
The photolysis of CO to form CO2 and COOH radicals constitute the first steps
for the formation of more complex organic molecules. In particular, the
recombination of CO2 and/or COOH with aliphatic chains and NH3 and/or its
photo-produced radical NH2 at low temperature could lead to the production of
molecules as complex as amino acids in astrophysical environments.
4. Conclusion
In this work we studied the effect of the irradiation of H2O+NH3+C10H8 = 1:1:1
ice mixtures with the two broad energy 4–20 and 13–45 eV ranges. We
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identified several photo-products in the IR spectra, among which CH4, C2H6,
C3H8, CO, CO2, HNCO and OCN-. Methyl amidogen (CH2N), the C2O anion
and the benzyl radical were also tentatively identified. The possible presence of
some photo-products such as C2H6, C3H8, CH2N and the benzyl radical in the
sample irradiated with 4–20 eV photons but not in the sample irradiated with
13–45 eV photons, as well as the study of the production yields of CO, CO2 and
OCN-, significantly higher in the 13–45 eV irradiation case, indicate that EUV
photons can efficiently photo-dissociate naphthalene and probably other PAHs at
low temperature. This result has important consequences on the photochemical
evolution of PAHs in astrophysical environments, where their carbon reservoir
could contribute significantly to the production of complex organic molecules.
Acknowledgments–We are grateful for the support of the staff of the National
Synchrotron Radiation Research Center, Hsinchu, Taiwan. This research is based
on the work supported by the NSC grant #NSC-95-2112-M008-028, the NSF
Planetary Astronomy Program under Grant AST-0604455 (C.-Y. R. W.), and the
NASA Planetary Atmospheres Program under Grant NAG5-11960 (C.-Y. R. W.).
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