Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Keane, J.V.

Novozamsky, J.H.

Schutte, W.A.

Leiden University Scholary Publications

A&A 379, 588–591 (2001)DOI: 10.1051/0004-6361:20011332c© ESO 2001Astronomy&AstrophysicsFurther evidence for the assignment of the XCN bandin astrophysical ice analogs to OCN−Spectroscopy and deuterium shiftJ. H. Novozamsky1, W. A. Schutte1, and J. V. Keane1,21 Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, 2300 RA Leiden,The Netherlands2 Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The NetherlandsReceived 17 April 2001 / Accepted 20 September 2001Abstract. The ν3 feature of the OCN− ion near 2160 cm−1 which is produced by acid-base reactions in cryogenicHNCO/NH3 samples shows a small matrix induced deuterium shift. A similar shift is found for the “XCN” bandnear this position produced by photolysis of CO/NH3 ice. This agreement, together with the abundant evidencealready in the literature, clearly demonstrates that the XCN feature can be assigned to OCN−.Key words. methods: laboratory – ISM: molecules – infrared: ISM: lines and bands1. IntroductionInfrared observations towards many embedded protostel-lar sources show an absorption band near 2161 cm−1(4.63 µm; e.g., Pendleton et al. 1999; Whittet et al. 2001).Its position is indicative of the stretching mode of a CNgroup, and so this feature became known as the XCN fea-ture. It was already shown in the discovery paper that UVirradiation of cryogenic ice mixtures containing CO andNH3 produces a very similar feature (Lacy et al. 1984).To constrain its identity, Grim & Greenberg (1987) andSchutte & Greenberg (1997) measured three isotope shiftsby labelling the ice sample with 13C, 15N or 18O. Theshifts coincided with those of the ν3 CN stretching bandof OCN− in alkali halide matrices. Such an agreement isstrong evidence for an identification, since isotope shiftsdepend only weakly on the nature of the matrix, eventhough the position of a feature itself may shift consider-ably (Maki & Decius 1959; Gordon & Foss Smith 1974).It was proposed that the formation of OCN− is preceededby photochemical formation of isocyanic acid (HNCO),followed by proton transfer to NH3 (Grim et al. 1989).A very similar feature is produced through irradiationwith energetic ions of ices containing (at least) CO andNH3, or H2O, CO and N2 (Palumbo et al. 2000). Recentlyit was demonstrated that this feature gives identical 13Cand 15N isotope shift as the XCN feature produced byUV photolysis (Hudson et al. 2001), and thus should arisefrom the same species. We will therefore throughout thisSend offprint requests to: W. Schutte,e-mail: schutte@strw.leidenuniv.nlpaper use the designation “XCN” for the carrier of thefeature produced by photolysis as well as by irradiation.Besides the magnitude of the three isotope shifts,other results also pointed to OCN−. First, the growthof the XCN feature during photolysis of CO/NH3 mix-tures is strongly correlated with the growth of a featureat 1500 cm−1 which can be ascribed to NH+4 (Schutte &Greenberg 1997). Next, additional features are producedat 1206, 630 and 1296 cm−1 which are close to the otherinfrared active modes of OCN− (Schutte & Greenberg1997). Next, upon warm-up to 240 K, a feature forms at2217 cm−1, near the position of the OCN stretch-ing vibration of ammoniumcyanate (NH4NCO; Grim &Greenberg 1987). Next, photolysis of isotopically mixed13CO/12CO/NH3 samples produced no more than one la-belled XCN feature, consistent with the carrier havingonly one C atom (Bernstein et al. 2000; Hudson et al.2001). Next, the behaviour of the XCN feature when theice matrix was doped by electron donor or electron ac-ceptor molecules gave direct evidence that the carrier is anegative ion (Demyk et al. 1998; Hudson & Moore 2000).Next, Hudson et al. (2001) investigated the productionof the XCN band by irradiation of a large sample of icemixtures with a wide variety of composition. In each case,the production or non-production of the XCN band inthese mixtures was consistent with the OCN− assignment.This wealth of evidence appears to make OCN− the bestidentified product of energetic processing of interstellar iceanalogs.Recently, Bernstein et al. (2000) and Palumbo et al.(2000) measured a deuterium (D) shift of 8 cm−1 for theXCN feature obtained by photolysis of unlabelled andJ. H. Novozamsky et al.: Assignment of the XCN band to ν3 OCN− 589deuterated CO/NH3 = 1/1 ice. From this they concludedthat the carrier should contain hydrogen and that there-fore the feature cannot be solely due to OCN−. This con-clusion however seems at odds with the overwhelming ev-idence in favor of this assignment summarized above. Inaddition, the XCN feature having several carriers seemsunlikely, since the various isotopically labelled experi-ments in each case produced only a single feature (Grim& Greenberg 1987; Schutte & Greenberg 1997; Bernsteinet al. 2000; Palumbo et al. 2000; Hudson et al. 2001).Alternatively, the shift could be caused by the interac-tion of the OCN− ion with the matrix, causing some cou-pling of its vibrational motion to the H atoms of its neigh-bors. As noticed already by Bernstein et al. and Hudsonet al., matrix induced D shifts are not unprecedented (e.g.,Nelander & Nord 1982; Zong & McHale 1997).In this paper we investigate whether a matrix inducedD shift occurs for OCN−. We produce the ion by acidbase reactions in ice samples of isocyanic acid (HNCO)and NH3. The results are compared with the D shift ofthe XCN band produced by UV photolysis of CO/NH3ices. Furthermore we investigate whether the D shift ofthe XCN band depends on the nature of the matrix. Wewill first however make a close comparison between thespectral properties of “XCN” and OCN−.2. ExperimentalDetailed descriptions of the general procedure for cre-ation and UV photolysis of ice samples have been pub-lished earlier (Gerakines et al. 1995, 1996). The reagentsused in these experiments were NH3 (Praxair, 99.99% pu-rity), ND3 (Cambridge Isotope Laboratories, D/H = 99)and CO (99.997% purity). The gas samples were pre-pared in a glass manifold at a background pressure of∼1 × 10−4 mbar. Before preparation of a ND3 contain-ing mixture, the glass manifold as well as the glass bulbin which the sample is prepared was neutralized with30 mbar ND3 for 12 hr. The vacuum system was neutral-ized by flushing with ND3 at 5 × 1015 molecules s−1 for90 min. The spectrum of the deuterated samples showed,besides the umbrella mode of ND3 at 830 cm−1, a weakfeature at 927 cm−1 due to the umbrella mode of ND2H.The relative intensity of these peaks, assuming equal in-trinsic strength, showed that the final amount of H inour deuterated experiments was ∼2.5%. NH3/ND3 andHNCO 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 µm.To produce isocyanic acid (HNCO) we used the tech-nique of Linhard (1938). This involves the thermal crack-ing of cyanuric acid (C3H3N3O3, Aldrich, purity 98%).After its production the HNCO was stored in liquid nitro-gen. Before making a deposition, the tube was placed inan octane slush to enhance the HNCO vapor pressure.To measure the matrix induced D shift of OCN−,we selected matrices strongly dominated by NH3. Thisis essential for a meaningful comparison of the D shiftsof OCN− in HNCO/NH3 and of XCN in photolysedCO/NH3, because otherwise the nature of the matrixwould be very different in these two sets of experiments,and no agreement could be expected. An additional ben-efit of such matrices is that their composition remainsstable during photolysis or warm-up. As a result, fea-tures shift very little over the course of the experiment,allowing a more accurate measurement of the D shift.To produce OCN− by direct acid-base reactions we usedNH3/HNCO = 100/1. For photolysis NH3/CO = 40/1was used. Since for photolysis the sample thickness is lim-ited to ≤0.1 µm, the amount of XCN that is producedbecomes too low for accurate spectroscopy if the CO frac-tion is smaller. To investigate the dependency of the Dshift on sample composition we furthermore photolysedNH3/CO = 1/1.3. Results3.1. Spectroscopic properties of OCN− and XCNTo investigate the spectroscopic properties of OCN−, wedeposited an ice mixture NH3/HNCO = 1/1, followed byslow warm-up. Figure 1 follows the evolution of the spec-trum. Already at 12 K features are present at 3240, 3040,2820, 2155, 1500, 1295, 1210 and 630 cm−1 which can-not be ascribed to the original species. Upon warm-up,these bands grow while the bands of HNCO and NH3 di-minish. This shows that proton transfer takes place. The3240, 3040, 2820 and 1500 cm−1 bands can be ascribed tothe ν1 +ν5, ν2 +ν4, 2ν4 and ν4 modes of NH+4 (Wagner &Hornig 1950). The 2155, 1295, 1210 and 630 cm−1 featuresare identified with the ν3, 2ν2, ν1 and ν2 bands of OCN−by comparing to its spectrum in salt matrices (Maki &Decius 1959).3500 3000 2500 2000 1500 1000 5000.000.050.100.150.200.250.300.35cbaAbsorbanceν (cm-1)Fig. 1. a) NH3/HNCO (1/1) at 12 K; b) idem, after warm-upto 80 K; c) NH3/CO (1/1.2) photolysed at 12K, followed bywarm-up to 80 K. Dotted vertical lines indicate OCN− fea-tures, solid lines NH+4 features, the arrow indicates the HNCOfeature, dots the NH3 features.590 J. H. Novozamsky et al.: Assignment of the XCN band to ν3 OCN−To compare the spectroscopic properties of OCN− tothat of XCN, Fig. 1 furthermore shows the spectrum ofNH3/CO = 1/1, deposited and simultaneously photolysedfor two hours at 12 K, followed by warm-up to 80 K. Wechoose similar abundances in the starting mixture to op-timize the XCN concentration. It can be seen that all fourOCN− features in the NH3/HNCO ice have counterpartsin the photolysed mixture. Furthermore, the features ofNH+4 can be clearly recognized as well. The relative inten-sity of the NH+4 and OCN− features are quite similar inboth samples. While a difference of ∼15 cm−1 can be seenbetween the position of the 2ν2 and ν1 features of OCN−in the warm-up experiment and the corresponding fea-tures in the photolysed sample, similar shifts are also seenfor OCN− in different salt matrices (Maki & Decius 1959;Gordon & Foss Smith 1974). The difference of ∼40 cm−1of the ν4 NH+4 band in the two experiments is consistentwith the very strong sensitivity of this feature to temper-ature and matrix composition (Grim et al. 1989; Schutteet al. 2001).3.2. The D shift of OCN− and XCN3.2.1. Warm-up of NH3 or ND3 with HNCOTable 1 gives the position of the ν3 OCN− band as afunction of temperature for the unlabelled and deuteratedNH3/HNCO = 100/1 ice mixture, while Fig. 2a comparesthe features. A clear D-shift is found, which increases from1.8 to 3.1 cm−1 during warm-up from 12 to 60 K. As men-tioned in Sect. 1, the deuterium shift should be ascribedto the interaction of the ion with the matrix. The increaseof the shift with temperature can be understood in termsof the annealing of the ice matrix, which results in re-arranging of the constituents into an energetically morefavorable configuration and strengthening of their inter-action. The D shift changes only slightly upon re-coolingto 12 K, showing that this effect is irreversible.Fig. 2. Comparison of the deuterium shift of the ν3 featureof OCN− with the XCN band: a) OCN− in NH3/HNCO =100/1 (solid line), and ND3/HNCO = 100/1 (dashed line),after warm-up to 60 K, and recooling to 12 K; b) XCN inNH3/CO = 40/1, photolysed for 30 min (solid line), and inND3/CO = 40/1, photolysed for 30 min (dashed line). Thepeak at 2138 cm−1 is due to CO.Table 1. Position of the XCN/OCN− feature in various unla-belled and deuterated experiments.mixture tirr T ν ν ∆νNH3 HNCO CO m K cm−1 cm−1 cm−1H D100 1 12 2155.0 2153.2 1.830 2154.7 2152.5 2.250 2153.6 2151.2 2.460 2153.5 2150.4 3.112a 2153.6 2150.7 2.940 1 10 12 2152.130 12 2151.9 2149.0 2.91 1 37 50 2158.3 2151.6 6.737 100 2156.9 2150.6 6.337 140 2158.7 2152.6 6.1a Recooled.3.2.2. UV photolysis of NH3 or ND3 with COTable 1 gives the position of the XCN band producedby photolysis at 12 K of unlabelled and deuteratedNH3/CO = 40/1, while Fig. 2b compares the features.A clear D shift is apparent of 2.9± 0.1 cm−1.In a second set of experiments we photolysed unla-belled and deuterated NH3/CO = 1/1, followed by warm-up. For the deuterated experiment an accurate measure-ment of the XCN position can only be obtained afterwarm-up to ≥50 K, due to the interference with the strongCO feature at lower temperature. The results are listed inTable 1. The D shift equals 6.4 ± 0.3 cm−1, dependingslightly on temperature. This is somewhat lower that the8 cm−1 shift reported by Bernstein et al. (2000) for thismixture. We note however that for matrices with compa-rable abundances of the initial ice components, the natureof the matrix changes strongly during the photolysis (dueto the high concentration of photoproducts). For this rea-son, the position of the XCN band is rather unstable, i.e.,in our NH3/CO = 1/1 experiment the XCN band shifted5 cm−1 over the course of the photolysis. In addition, insuch ice mixtures variation in the mixing ratio of the com-ponents will cause a significant change in the nature ofthe matrix. Therefore, errors in the measurement of the Dshift may be introduced by imperfect reproducibility be-tween the labelled and unlabelled experiment of the UVlamp intensity, the mixing ratio, the sample thickness, orthe spectrum of the UV lamp. The difference of up to2 cm−1 between the measurements of the 13C and 15Nisotope shifts for the NH3/CO = 1/1 mixture by Grim &Greenberg (1987) and Bernstein et al. (2000) is probablydue to this effect. It seems plausible that the 1.6 cm−1difference between our result and that of Bernstein et al.derives from this effect as well.4. DiscussionThe shift of the OCN− feature produced by protontransfer between HNCO and NH3 in our unlabelled andJ. H. Novozamsky et al.: Assignment of the XCN band to ν3 OCN− 591deuterated NH3/HNCO = 100/1 mixtures clearly showsthat OCN− can produce a D shift even though it does notcontain hydrogen. As already explained in Sect. 1, sucha behaviour is not unprecedented and can be ascribed tothe intermolecular bonding of the ion with species in thematrix. After warm-up to 60 K and recooling the shiftequals 2.9 cm−1. This is in excellent agreement with theD shift of 2.9 cm−1 for the XCN band produced by pho-tolysis of NH3/CO = 40/1, giving strong support to theOCN− assignment.The NH3/HNCO = 100/1 experiments were not fullydeuterated, because of the hydrogen in the HNCO. If thehydrogen would stay in close contact with the OCN− ion,e.g., when a NHD+3 .OCN− complex is formed, we wouldexpect the D shift for the NH3/HNCO experiment to besomewhat less than for the photolysis experiments. Thegood agreement therefore suggests that the H atom is notin the direct neighbourhood of the OCN−. However, thisconclusion depends on the extend of annealing caused bythe photolysis, since below 60 K the D shift of OCN− inthe NH3/HNCO sample is smaller.The D shift of the XCN band in the photolysedCO/NH3 = 1/1 and 1/40 samples differ considerably, i.e.,6.4 cm−1 and 2.9 cm−1, respectively. This demonstratesthat the shift depends on the matrix. This seems incon-sistent with the shift being caused by a covalently bondedH atom, since in that case the influence of the matrixshould be small (e.g., the C, O, and N isotopic shifts ofthe ν3 mode of OCN− remain constant within 2 cm−1 indifferent salt matrices, even though the band position it-self shifts by up to 80 cm−1; Maki & Decius 1959; Gordon& Foss Smith 1974). However, if the shift originates fromthe interaction of the XCN carrier with the matrix, a clearmatrix dependency is plausible.Besides the good agreement in the D shift, the coinci-dence of four spectral features produced in the photolysedices with the four OCN− features is “hard evidence” in fa-vor of the OCN− assignment. This agreement was earliernoticed by Schutte & Greenberg (1997), however, at thetime the comparison was only done with literature dataof OCN− in salt matrices.The evidence presented in this paper, together with theevidence already published earlier (Sect. 1) clearly demon-strates that the XCN band near 2160 cm−1 produced byprocessing of ices containing CO and NH3 is due to the ν3feature of OCN−.5. Astrophysical implicationsThe identification of the laboratory XCN feature withOCN− makes this species a prime candidate for the carrierof the interstellar XCN band. While it has been thoughtthat organic nitriles or iso-nitriles could also be candi-dates, a thorough investigation of a large variety of suchspecies showed that none satisfied the spectroscopic con-straints (Bernstein et al. 1997). Another argument in fa-vor of OCN− is the ease with which this species could beproduced under interstellar conditions. Besides throughphotolysis or irradiation of ices with CO and NH3 (seealso Hudson et al. 2001), isocyanic acid (HNCO) may beproduced directly by grain surface chemistry (Hasegawa& Herbst 1993). In the presence of NH3, proton transferwould be straightforward if a modest amount of energyis supplied to the ice matrix, e.g., through some warm-up(Fig. 1). Therefore it appears that the identification of theinterstellar 4.62 µm “XCN” feature with OCN− is secure.We therefore propose to hereafter designate it accordingly.Acknowledgements. Advice by Helen Fraser on the physi-cal/chemical aspects of this work was a great asset. The help ofRian van de Nieuwendijk of the department of chemistry in theHNCO synthesis is gratefully acknowledged. The manuscriptbenefitted greatly from the comments made by the referee, MaxBernstein. The help by Almudena Arcones with the experi-ments in the final stages of this research was a major support.ReferencesBernstein, M. P., Sandford, S. A., & Allamandola, L. 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Further evidence for the assignment of the XCN band in astrophysical ice analogs to OCN^-. Spectroscopy and deuterium shift

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Further evidence for the assignment of the XCN band in astrophysical ice analogs to OCN^-. Spectroscopy and deuterium shift

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