R—O—C≡N Species Produced by Ion Irradiation of Ice Mixtures: Comparison with Astronomical Observations

Abstract

We have investigated the effects induced by ion bombardment of mixtures containing nitrogen-bearing compounds at low temperatures. The results show the formation of a band at 2080 cm-1 in binary mixtures, NH3:CH4 and N2:CH4, which we attribute to HCN embedded in the organic residue formed by ion irradiation. In addition to this band, ternary mixtures containing an oxygen-bearing species (i.e., H2O) form a compound with a prominent absorption band at about 2165 cm-1 (4.62 microns). We ascribe this band to a nitrile compound containing O that is bonded to the organic residue. A detailed comparison of the laboratory results with astronomical data of the 4.62 microns absorption band in protostellar spectra shows good agreement in peak position and profile. Our experimental studies show that N2, which is a more likely interstellar ice component than NH3, can be the molecular progenitor of the carrier of the interstellar band. This is an alternative to the pathway by which UV photolysis of NH3-containing ices produces the 4.62 microns band and implies that ion bombardment may well play an important role in the evolution of interstellar ices. Here, we discuss the implications of our studies for the chemical route by which the carrier of the 4.62 microns band is formed in these laboratory experiments.

Hydrogen Isotopic Substitution Studies of the 2165 Wavenumber (4.62 Micron) “XCN” Feature Produced by Ion Bombardment

“…In many cases, an absorption band results which is remarkably similar to that seen in the interstellar medium in both proÐle and peak position (Moore et al 1983 ;Lacy et al 1984 ;Grim & Greenberg 1987 ;Tegler et al 1993 ;Schutte & Greenberg 1997 ;Demyk et al 1998 ;Palumbo et al 2000 ;Hudson & Moore 2000). Here we report irradiation experiments using ion bombardment of and ices.…”

“…Irradiation of the mixture also produces a CH 3 OH : N 2 feature at 2168 cm~1 whose proÐle (shape, width, and peak position) is nearly identical to that presented by Palumbo et al (2000) after ion irradiation of other ice mixtures which span a variety of C, H, O, and N sources. When deuterium is substituted for hydrogen, the feature shifts to 2160 cm~1, thereby implicating hydrogen in the carrier of the band.…”

“…542 cm~1) band (Schutte & Greenberg 1997 ;Demyk et al 1998 ;Palumbo et al 2000 ;Bernstein et al 2000). The results reported here and in the companion paper by Bernstein et al (2000) indicate that hydrogen is also contained in the carrier molecule.…”

“…A detailed description of the experimental apparatus used to obtain in situ IR spectra of irradiated and unirradiated frozen gases can be found elsewhere (Baratta & Palumbo 1998 ;Palumbo et al 2000). In a stainless steel vacuum chamber, frosts were accreted onto a cooled (10È300 K) substrate.…”

“…We have previously studied the proÐle of the feature at 4.62 km (2165 cm~1) formed after ion irradiation of a variety of icy mixtures containing H, C, N, and O (e.g., Palumbo et al 2000). Here we present the results relative to the mixtures and Figure 1 shows CH 3 OH : N 2 CD 3 OD : N 2 .…”