A comparison of IR spectra of PAHs in solid Ar and frozen in solid H₂O

Polycyclic aromatic hydrocarbons (PAHs) and related aromatic materials are thought to be present in virtually all phases of the ISM (Allamandola, Hudgins, & Sandford 1999).  Interstellar PAHs are most easily seen under conditions where gas phase PAHs (both neutrals and ions) are excited by UV and visible light and cool through the emission of infrared (IR) photons, giving rise to the well-known emission spectra with prominent features at 3050, 1610, 1300, 1160 and 890 cm^-1 (3.28, 6.2, 7.6, 8.6, and 11.2 µm) (e.g., Allamandola, Tielens, & Barker 1989; Peeters et al. 2002).  However, PAHs in dense interstellar clouds will be largely screened from exciting radiation.  In addition, since dense molecular clouds are extremely cold (T < 50 K), most PAHs in these environments should be efficiently condensed onto dust grains, either as 'pure' solids or as 'guest molecules' in icy grain mantles, as is the case for most other interstellar molecules (Sandford & Allamandola 1993). The presence of PAHs in dense clouds is further suggested by the presence of aromatics in primitive meteorites and interplanetary dust particles that contain deuterium enrichments best explained by an interstellar dense cloud chemistry (Sandford 2002). Under dense cloud conditions, these PAHs will be seen in absorption

Because they fall at positions characteristic of aromatic molecules, a few weak mid-infrared absorption features have been attributed to PAHs along lines of sight to a limited number of objects embedded in dense clouds. These include bands near 3030 cm^-1 (3.3 µm) (Smith, Sellgren, & Tokunaga 1989; Sellgren et al. 1995; Brooke, Sellgren, & Geballe, 1999), and 1600 cm^-1 (6.2 µm) (Chiar et al. 2000), and 890 cm^-1 (11.2 µm) (Bregman, Hayward, & Sloan 2000).

Beyond making a general connection with aromatic materials, it has proven difficult to fully interpret these features, however. Although there is a growing database of laboratory IR absorption spectra of both neutral and ionized PAHs in inert gas matrices (e.g., Hudgins & Allamandola 1995a,b; Szczepanski, Wehlburg, & Vala 1995; Hudgins & Sandford 1998a,b,c; Mattioda et al. 2003; and references therein; see http://www.astrochem.org/pahdata/), these data have been taken to address the interpretation of the IR emission bands of interstellar gas phase PAHs. However, the PAHs in dense clouds are expected to be condensed onto dust grains, largely as neutral molecules frozen in H₂O-rich ice mantles.  Under these conditions, the PAHs will interact with each other and with other molecules.  Because these interactions will perturb their infrared spectral properties (band positions, widths, profiles, and strengths), analyses of aromatic absorption bands in the spectra of dense clouds will require laboratory absorption spectra of appropriate analog materials taken under realistic astrophysical conditions.

Lines of sight that probe dense molecular clouds show that H₂O is the dominant ice component present (Sandford 1996; Ehrenfreund & Charnley 2000; van Dishoeck 2004).  Thus, for PAHs in dense clouds, the most important molecular interaction to consider is that between PAHs and H₂O.  H₂O is a relatively polar molecule and can interact quite strongly with other molecules in the solid state (see, for example, Sandford et al. 1988; Ehrenfreund et al. 1998; Dartois et al. 1999; Gerakines et al. 1999).  A systematic study of the infrared spectral properties of the two ring PAH naphthalene (C₁₀H₈) demonstrated that interactions with H₂O resulted in relatively small changes in the positions and strengths of this molecule's vibrational bands, but significant changes in band widths (Sandford, Bernstein, & Allamandola 2004a).  However, to date, very little work has been done on the infrared spectral properties of multi-ring PAHs frozen in H₂O-rich ices.

On this web page, we present the infrared spectra of 11 additional PAHs and related species frozen in H₂O ices.  In general the positions of PAH absorptions in H₂O are not very different from those of PAHs in argon matrix.  In future papers and web pages we shall present IR spectra of PAH cations frozen in H₂O.

Figure 1 (below): The 4000-500 cm^-1 (2.5-20 µm) infrared spectra of pyrene (C₁₆H₁₀) isolated in an argon matrix at 12 K (Ar/C₁₆H₁₀ > 1000; from Hudgins & Sandford 1998a) (top) and in an H₂O matrix (H₂O/C₁₆H₁₀~ 10) at 15 K (bottom).  The H₂O-pyrene spectrum is dominated by the H₂O features at 3320, 1630, and 750 cm^-1 (3.0, 6.1, and 13.3 µm), but also shows features associated with the fundamental vibrations of pyrene and an enhanced dangling O-H band near 3620 cm^-1 (2.76 µm).

Figure 2 (below): The 3150-2950 cm^-1 (3.17-3.39 µm) spectra of phenanthrene, coronene, and pyrene in Ar and H₂O matrices.  The H₂O-PAH spectra span a range of PAH concentrations and demonstrate how the presence of the nearby H₂O band suppresses the spectral contrast of these vibrational modes.  As with naphthalene, the C-H stretching bands of other PAHs only become weakly visible as the H₂O/PAH ratio falls below ~50, i.e., PAH concentrations in the H₂O exceed a few percent.

Figure 3 (below): The 3100-2800 cm^-1 (3.23-3.57 µm) spectra of 1,2-dihydronaphthalene and 9,10-dihydroanthracene in H₂O matrices (H₂O/1,2-dihydronaphthalene ~ 20 and H₂O/9,10-dihydroanthracene ~ 50).  The structures for these two molecules can be seen in Figures 4b and 4d.  The regions in which aromatic and aliphatic CH stretching vibrations fall are marked with horizontal bars.  While the aromatic CH stretches bands of these molecule are barely visible at these concentrations, the aliphatic CH stretching bands are readily apparent.

Figure 4 (below):   The 2000-500 cm^-1 (5.0-20.0 µm) infrared spectra of (a) H₂O/naphthalene ~ 15, (b) H₂O/1,2-dihydronaphthalene ~ 20, (c) H₂O/anthracene ~ 70, and (d) H₂O/9,10-dihydroanthracene ~ 50.  Each H₂O/PAH spectrum is compared to the spectrum of the relevant PAH isolated in an Ar matrix.  The Ar matrix data for naphthalene and anthracene were taken from Hudgins & Sandford (1998a).  The Ar matrix data for 1,2-dihydronaphthalene and 9,10-dihydroanthracene are taken from Sandford et al. (2005, in preparation).  All spectra were measured from samples at 15 K.

Figure 5 (below):   The 2000-500 cm^-1 (5.0-20.0 µm) infrared spectra of (a) H₂O/phenanthrene ~ 200, (b) H₂O/pentacene ~ 75, (c) H₂O/pyrene ~ 380, and (d) H₂O/benzo[e]pyrene ~ 35.  Each H₂O/PAH spectrum is compared to the spectrum of the relevant PAH isolated in an Ar matrix.  The Ar matrix data for phenanthrene and pyrene are taken from Hudgins & Sandford (1998a); those for pentacene and benzo[e]pyrene are taken from Hudgins & Sandford (1998b).  All spectra were measured from samples at 15 K.

Figure 6 (below):   The 2000-500 cm^-1 (5.0-20.0 µm) infrared spectra of (a) H₂O/perylene ~ 40, (b) H₂O/benzo[ghi]perylene ~ 130, (c) H₂O/coronene ~ 160, and (d) H₂O/benzo(k)fluoranthene ~ 100.  Each H₂O/PAH spectrum is compared to the spectrum of the relevant PAH isolated in an Ar matrix.  The Ar matrix data for perylene, benzo[ghi]perylene, and coronene are taken from Hudgins & Sandford (1998b); the spectrum of benzo(k)fluoranthene are taken from Hudgins & Sandford (1998c).  All spectra were measured from samples at 15 K.

Figure 7 (below): The 1350-1100 cm^-1 (7.41-9.1 µm) infrared spectra of several PAHs frozen in Ar and in H₂O-rich ices at different concentrations.  Compared to their spectra when isolated in Ar, the absorption bands of PAHs in H₂O ices generally show increased widths, but only modest shifts in position and relative band strengths.  The magnitude of these changes is largely independent of concentration over the range considered here.  All spectra were taken from samples deposited and maintained at 15 K.