A&A 379, 588-591 (2001)
DOI: 10.1051/0004-6361:20011332
J. H. Novozamsky1 - W. A. Schutte1 - J. V. Keane1,2
1 - Raymond and Beverly Sackler Laboratory for
Astrophysics, Leiden
Observatory, 2300 RA Leiden, The Netherlands
2 - Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen,
The Netherlands
Received 17 April 2001 / Accepted 20 September 2001
Abstract
The
feature of the
ion near
2160
which is produced by acid-base reactions
in cryogenic HNCO/
samples shows a small matrix induced deuterium shift. A similar shift
is found for the "XCN'' band near this position produced by photolysis of
CO/
ice.
This agreement, together with the abundant evidence already in the literature,
clearly demonstrates that the XCN feature can be assigned to
.
Key words: methods: laboratory - ISM: molecules - infrared: ISM: lines and bands
Infrared observations towards many embedded protostellar sources show
an absorption band near 2161 cm-1 (4.63 m; e.g., Pendleton
et al. 1999; Whittet et al. 2001). Its position is indicative of
the stretching mode of a CN group, and so this feature became known as
the XCN feature. It was already shown in the discovery paper that UV
irradiation of cryogenic ice mixtures containing CO and NH3produces a very similar feature (Lacy et al. 1984). To
constrain its identity, Grim & Greenberg (1987) and Schutte
& Greenberg (1997) measured three isotope shifts by labelling the
ice sample with 13C, 15N or 18O. The shifts coincided
with those of the
CN stretching band of OCN- in alkali
halide matrices. Such an agreement is strong evidence for an
identification, since isotope shifts depend only weakly on the nature
of the matrix, even though the position of a feature itself may shift
considerably (Maki & Decius 1959; Gordon & Foss Smith 1974). It was
proposed that the formation of
is preceeded by
photochemical formation of isocyanic acid (HNCO), followed by proton
transfer to
(Grim et al. 1989).
A very similar feature is produced through irradiation with energetic
ions of ices containing (at least) CO and ,
or
,
CO and
(Palumbo et al. 2000). Recently it was demonstrated
that this feature gives identical 13C and 15N isotope shift
as the XCN feature produced by UV photolysis (Hudson et al. 2001), and
thus should arise from the same species. We will therefore throughout
this paper use the designation "XCN'' for the carrier of
the feature produced by photolysis as well as by irradiation.
Besides the magnitude of the three isotope shifts, other results also
pointed to .
First, the
growth of the XCN feature during photolysis of CO/
mixtures
is strongly correlated with the growth of a feature at 1500
which can be ascribed to
(Schutte & Greenberg
1997). Next, additional features are produced at 1206, 630 and 1296
cm-1 which are close to the other infrared active modes of
(Schutte & Greenberg 1997). Next, upon warm-up to 240 K, a feature
forms at 2217
,
near the position of the OCN stretching
vibration of ammoniumcyanate (
;
Grim & Greenberg 1987).
Next, photolysis of isotopically mixed 13CO/12CO/
samples produced no more than one labelled XCN feature, consistent with
the carrier having only one C atom (Bernstein et al. 2000; Hudson et al. 2001). Next, the behaviour of the XCN feature when the ice matrix
was doped by electron donor or electron acceptor molecules gave direct
evidence that the carrier is a negative ion (Demyk et al. 1998; Hudson
& Moore 2000). Next, Hudson et al. (2001) investigated the production
of the XCN band by irradiation of a large sample of ice mixtures with
a wide variety of composition. In each case, the production or
non-production of the XCN band in these mixtures was consistent with
the
assignment. This wealth of evidence appears to make
the best identified product of energetic processing of
interstellar ice analogs.
Recently, Bernstein et al. (2000) and Palumbo et al. (2000) measured a
deuterium (D) shift of 8
for the XCN feature obtained by
photolysis of unlabelled and deuterated CO/
= 1/1 ice. From
this they concluded that the carrier should contain hydrogen and that
therefore the feature cannot be solely due to
.
This
conclusion however seems at odds with the overwhelming evidence in
favor of this assignment summarized above. In addition, the XCN
feature having several carriers seems unlikely, since the various
isotopically labelled experiments in each case produced only a single
feature (Grim & Greenberg 1987; Schutte & Greenberg 1997; Bernstein
et al. 2000; Palumbo et al. 2000; Hudson et al. 2001). Alternatively,
the shift could be caused by the interaction of the
ion
with the matrix, causing some coupling of its vibrational
motion to the H atoms of its neighbors. As noticed already by
Bernstein et al. and Hudson et al., matrix induced D shifts are not
unprecedented (e.g., Nelander & Nord 1982; Zong & McHale 1997).
In this paper we investigate whether a matrix induced D shift
occurs for .
We produce the ion by acid base reactions
in ice samples of isocyanic acid (HNCO) and
.
The results are compared with the D shift of the XCN band
produced by UV photolysis of CO/
ices. Furthermore we
investigate whether the D shift of the XCN band
depends on the nature of the matrix.
We will first however make a close comparison between the spectral
properties of "XCN'' and
.
Detailed descriptions of the general procedure for creation and UV
photolysis of ice samples have been published earlier (Gerakines et al. 1995, 1996). The reagents used in these experiments were
(Praxair, 99.99
purity),
(Cambridge Isotope
Laboratories, D/H = 99) and CO (99.997
purity). The gas samples
were prepared in a glass manifold at a background pressure of
mbar. Before preparation of a
containing
mixture, the glass manifold as
well as the glass bulb in which the sample is prepared was neutralized
with 30 mbar
for 12 hr. The vacuum system was
neutralized by flushing with
at
molecules s-1for 90 min. The spectrum of the deuterated samples showed, besides the
umbrella mode of
at 830
,
a weak feature at
927
due to the umbrella mode of
.
The relative intensity of these peaks, assuming equal
intrinsic strength, showed that the final amount of H in
our deuterated experiments was
2.5%.
/
and
HNCO were deposited through seperate deposition tubes (Gerakines et al. 1995). To ensure transparency for UV, the
thickness of samples in photolysis experiments was
0.1
m.
To produce isocyanic acid (HNCO) we used the technique of Linhard (1938). This involves the thermal cracking of cyanuric acid (C3H3N3O3, Aldrich, purity 98%). After its production the HNCO was stored in liquid nitrogen. Before making a deposition, the tube was placed in an octane slush to enhance the HNCO vapor pressure.
To measure the matrix induced D shift of ,
we selected
matrices strongly dominated by
.
This is essential for a
meaningful comparison of the D shifts of
in HNCO/
and
of XCN in photolysed CO/
,
because otherwise the nature of
the matrix would be very different in these two sets of experiments,
and no agreement could be expected. An additional benefit of such
matrices is that their composition remains stable during photolysis or
warm-up. As a result, features shift very little over the course of
the experiment, allowing a more accurate measurement of the D
shift. To produce
by direct acid-base reactions we used
NH3/HNCO = 100/1. For photolysis NH3/CO = 40/1 was
used. Since for photolysis the sample thickness is limited to
0.1
m, the amount of XCN that is produced becomes too low for
accurate spectroscopy if the CO fraction is smaller. To investigate
the dependency of the D shift on sample composition we
furthermore photolysed NH3/CO = 1/1.
![]() |
Figure 1:
a) NH3/HNCO (1/1) at 12 K; b) idem,
after warm-up to 80 K;
c) NH3/CO (1/1.2) photolysed at 12K, followed by warm-up
to 80 K. Dotted vertical lines indicate ![]() ![]() ![]() |
Open with DEXTER |
To investigate the spectroscopic properties of ,
we
deposited an ice mixture
/HNCO = 1/1, followed by slow
warm-up. Figure 1
follows the evolution of the spectrum. Already at 12 K features are
present at 3240, 3040, 2820, 2155, 1500, 1295, 1210 and 630
which cannot be ascribed to the original species. Upon
warm-up, these bands grow while the bands of HNCO and
diminish. This shows that proton transfer takes place. The 3240,
3040, 2820 and 1500
bands can be ascribed to the
,
,
and
modes of
(Wagner & Hornig 1950). The 2155, 1295, 1210 and 630
features are identified with the
,
,
and
bands of
by comparing to its spectrum in salt
matrices (Maki & Decius 1959).
To compare the spectroscopic properties of
to that of XCN,
Fig. 1 furthermore shows the spectrum of
/CO = 1/1,
deposited and simultaneously photolysed for two
hours at 12 K, followed by warm-up to 80 K. We choose
similar abundances in the starting mixture to optimize the XCN
concentration. It can be seen that all four
features in the
/HNCO ice have counterparts in the
photolysed mixture. Furthermore, the features of
can be clearly recognized as well. The relative
intensity of the
and
features are quite
similar in both samples. While a difference
of
15
can be seen between the position of the
and
features of
in the warm-up experiment and the corresponding
features in the
photolysed sample, similar shifts are also seen for
in
different salt matrices (Maki & Decius 1959; Gordon & Foss Smith
1974). The difference of
40
of the
band in the two experiments is consistent with the very strong
sensitivity of this feature to temperature and matrix composition
(Grim et al. 1989; Schutte et al. 2001).
![]() |
Figure 2:
Comparison of the deuterium shift of the ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
mixture |
![]() |
T | ![]() |
![]() |
![]() |
||
![]() |
HNCO | CO | m | K |
![]() |
![]() |
![]() |
H | D | ||||||
100 | 1 | 12 | 2155.0 | 2153.2 | 1.8 | ||
30 | 2154.7 | 2152.5 | 2.2 | ||||
50 | 2153.6 | 2151.2 | 2.4 | ||||
60 | 2153.5 | 2150.4 | 3.1 | ||||
12a | 2153.6 | 2150.7 | 2.9 | ||||
40 | 1 | 10 | 12 | 2152.1 | |||
30 | 12 | 2151.9 | 2149.0 | 2.9 | |||
1 | 1 | 37 | 50 | 2158.3 | 2151.6 | 6.7 | |
37 | 100 | 2156.9 | 2150.6 | 6.3 | |||
37 | 140 | 2158.7 | 2152.6 | 6.1 |
Table 1 gives the position of the
band as a function
of temperature for the unlabelled and deuterated
/HNCO = 100/1
ice mixture, while Fig. 2a compares the features. A clear D-shift is
found, which increases from 1.8 to 3.1
during
warm-up from 12 to 60 K. As mentioned in Sect. 1, the deuterium
shift should be ascribed to the interaction of the ion with the
matrix.
The increase of the shift with temperature can be understood
in terms of
the annealing of the ice matrix, which results in rearranging of the
constituents into an energetically more favorable configuration
and strengthening of their interaction. The D shift changes
only slightly upon re-cooling to 12 K, showing that this effect
is irreversible.
Table 1 gives the position of the XCN band produced by
photolysis at 12 K of unlabelled and deuterated /CO = 40/1,
while Fig. 2b compares the features.
A clear D shift is apparent of
.
In a second set of experiments we photolysed unlabelled and deuterated
/CO = 1/1, followed by warm-up. For the deuterated
experiment an accurate measurement of the XCN position can only be
obtained after warm-up to
50 K, due to the interference with
the strong CO feature at lower temperature. The results are listed in
Table 1. The D shift equals
,
depending
slightly on temperature. This is somewhat lower that the 8
shift reported by Bernstein et al. (2000) for this mixture. We
note however that for matrices with comparable abundances of
the initial ice components, the nature of the matrix changes strongly
during the photolysis (due to the high concentration of photoproducts). For
this reason, the position of the XCN band is rather unstable, i.e., in
our
/CO = 1/1 experiment the XCN band shifted 5
over the course of the photolysis. In addition, in such ice
mixtures variation in the mixing ratio of the components will cause a
significant change in the nature of the matrix. Therefore, errors in
the measurement of the D shift may be introduced by imperfect
reproducibility between the labelled and unlabelled experiment
of the UV lamp intensity, the mixing ratio,
the sample thickness, or the spectrum of the UV
lamp. The difference of up to 2
between the measurements of the 13C and 15N
isotope shifts for the
/CO = 1/1 mixture by Grim &
Greenberg (1987) and Bernstein et al. (2000) is probably due to this effect.
It seems plausible that the 1.6
difference between our result and that of Bernstein et al. derives
from this effect as well.
The shift of the
feature produced by proton transfer
between HNCO and
in our unlabelled and deuterated
/HNCO = 100/1 mixtures clearly shows that
can produce
a D shift even though it does not contain hydrogen. As already
explained in Sect. 1, such a behaviour is not unprecedented and can be
ascribed to the intermolecular bonding of the ion with species in the
matrix. After warm-up to 60 K and recooling the shift equals 2.9
.
This is in excellent agreement with the D shift of 2.9
for the XCN band produced by photolysis of
/CO = 40/1,
giving strong support to the
assignment.
The /HNCO = 100/1 experiments were not fully
deuterated, because of the hydrogen in the HNCO. If the hydrogen would
stay in close contact with the
ion, e.g., when a
.
complex is formed, we would expect the D
shift for the
/HNCO experiment to be somewhat less than for
the photolysis experiments. The good agreement therefore suggests that
the H atom is not in the direct neighbourhood of the
.
However, this conclusion depends on the extend of annealing
caused by the photolysis, since below 60 K the D shift of
in the
/HNCO sample is smaller.
The D shift of the XCN band in the photolysed
CO/ = 1/1 and 1/40 samples differ considerably, i.e.,
6.4
and 2.9
,
respectively.
This demonstrates that the shift depends on the
matrix. This seems inconsistent with the shift being caused
by a covalently bonded H atom, since in that case the influence of
the matrix should be small (e.g., the C, O, and N
isotopic shifts of the
mode of
remain constant
within 2
in different salt matrices, even though the
band position itself shifts by up to 80
;
Maki & Decius 1959; Gordon & Foss Smith 1974). However, if
the shift originates from the interaction of the XCN carrier
with the matrix, a clear matrix dependency is plausible.
Besides the good agreement in the D shift,
the coincidence of four spectral features produced in the photolysed
ices with the four
features is
"hard evidence'' in favor of the
assignment. This
agreement was earlier noticed by Schutte & Greenberg (1997), however, at
the time the comparison was only done with literature data of
in
salt matrices.
The evidence presented in this paper, together with the evidence already
published earlier (Sect. 1) clearly demonstrates that the
XCN band near 2160
produced by processing of ices containing
CO and
is due to the
feature of
.
The identification of the laboratory XCN feature with OCN-makes this species a prime candidate for the carrier of the
interstellar XCN band. While it has been thought that organic nitriles or
iso-nitriles could also be candidates, a thorough investigation of a
large variety of such species showed that none satisfied the
spectroscopic constraints (Bernstein et al. 1997). Another argument in
favor of
is the ease with which this species could be
produced under interstellar conditions. Besides through photolysis or
irradiation of ices with CO and
(see also Hudson et al.
2001), isocyanic acid (HNCO) may be produced directly by grain surface
chemistry (Hasegawa & Herbst 1993). In the presence of
,
proton transfer would be straightforward if a modest amount of energy
is supplied to the ice matrix, e.g., through some warm-up (Fig. 1).
Therefore it appears that the identification of the interstellar 4.62
m "XCN'' feature with
is secure. We therefore
propose to hereafter designate it accordingly.
Acknowledgements
Advice by Helen Fraser on the physical/chemical aspects of this work was a great asset. The help of Rian van de Nieuwendijk of the department of chemistry in the HNCO synthesis is gratefully acknowledged. The manuscript benefitted greatly from the comments made by the referee, Max Bernstein. The help by Almudena Arcones with the experiments in the final stages of this research was a major support.