Content uploaded by Farid Salama
Author content
All content in this area was uploaded by Farid Salama
Content may be subject to copyright.
A preview of the PDF is not available
Searching for Naphthalene Cation Absorption in the Interstellar Medium
Abstract and Figures
Interstellar naphthalene cations (C10H+8) have been proposed by a study to be the carriers of a small number of diffuse interstellar bands (DIBs). Using an archive of high signal-to-noise spectra obtained at the Apache Point Observatory, we used two methods to test the hypothesis. Both methods failed to detect significant absorption at lab wavelengths of interstellar spectra with laboratory spectra. We thereby conclude that C10H+8 is not a DIB carrier in typical reddened sight lines. © 2011. The American Astronomical Society. All rights reserved.
Figures - uploaded by Farid Salama
Author content
All figure content in this area was uploaded by Farid Salama
Content may be subject to copyright.
... On the other hand, there are disputed detections in the literature. Here, we highlight the cationic systems naphthalene (Iglesias-Groth et al. 2008;Galazutdinov et al. 2010;Searles et al. 2011) and anthracene (Galazutdinov et al. 2010;Iglesias-Groth et al. 2010), as well as neutral anthracene and pyrene (Vijh, Witt & Gordon 2004;Nayfeh, Habbal & Rao 2005). Benzene, the basic building block of PAHs, was discovered in the circumstellar envelope of CRL 618 (Cernicharo et al. 2001), and the presence of fullerenes C 60 and C 70 (Cami et al. 2010(Cami et al. , 2011, as well as C + 60 (Campbell et al. 2015), has also been confirmed in interstellar and circumstellar environments. ...
Polycyclic aromatic hydrocarbons (PAHs) constitute an essential family of compounds in interstellar (ISM) and circumstellar (CSM) media. Recently, formation routes for the corresponding polycyclic aromatic phosphorus heterocycles (PAPHs) in astrophysical environments have been proposed. In order to contribute to a better understanding of the phosphorus chemistry in the ISM, infrared (IR) spectra and selected properties of PAPHs were computed at the DFT level for neutral, cationic and anionic species. Our results reveal that several protonated PAPHs do not have planar backbones, and all species have permanent dipole moments between 2 D and 4 D. Closed-shell PAPHs have similar ionization potentials compared to the parent PAHs, below the Lyman threshold limit. In addition, all PAPHs show positive electron affinities higher than those of PAHs. Protonation preferably occurs on the heteroatom but with lower proton affinities than those of the corresponding nitrogen analogs (PANHs). In general, neutral species have similar IR spectra profile with the most intense bands around 800 cm−1 (12.5 μm) related to C−H wagging. Charge and protonation affect the IR spectra mainly by decreasing the intensities of these modes and increasing the ones between 1000 cm−1 (10.0 μm) and 1800 cm−1 (5.6 μm). The P−H stretching appears in a different spectral region, between 2300 cm−1 (4.3 μm) and 2700 cm−1 (3.7 μm). Our results are discussed in the context of distinct sources where PAHs and phosphorus are detected. PAPHs, in particular the coronene derivatives, can contribute to the Unidentified Infrared Emission (UIR) band at 6.2 μm.
... The polycyclic aromatic hydrocarbon (PAH) molecules have been thought by some investigators to be promising candidates for DIB carriers (e.g., Gredel et al. 2011), although no matches between the DIBs and PAH absorptions have been found. The simplest PAH molecule, the naphthalene cation (C H 8 10 + ), was reported to be detected in the sight line of Cernis 52 (Iglesias-Groth et al. 2008;González et al. 2009), though Searles et al. (2011) and Galazutdinov et al. (2011) showed that the naphthalene lines, if present at the known lab wavelengths, would have too broad and shallow profiles and be hard to detect with echelle spectra with confidence (Hobbs et al. 2008;Sonnentrucker et al. 2018). This issue deserves further study with much better spectra than currently available. ...
Aiming for a new and more comprehensive DIB catalog between 4000 and 9000 Å, we revisited the Atlas Catalog based on the observations of HD 183143 and HD 204827. Twenty-five medium to highly reddened sight lines were selected, sampling a variety of spectral types of the background star and the interstellar environments. The median signal-to-noise ratio (S/N) of these spectra is ∼1300 around 6400 Å. Compared to the Atlas Catalog, 22 new DIBs were found, and the boundaries of 27 (sets of) DIBs were adjusted, resulting in an updated catalog containing 559 DIBs that we refer to as the Apache Point Observatory Catalog of Optical Diffuse Interstellar Bands. Measurements were then made based on this catalog. We found our survey most sensitive between 5500 and 7000 Å, due largely to the local S/N of the spectra, the relative absence of interfering stellar lines, and the weakness of telluric residuals. For our data sample, the number of DIBs detected in a given sight line is mostly dependent on E B − V and less on the spectral type of the background star. Some dependence on the molecular fraction f H2 is observed, but it is less well determined owing to the limited size of the data sample. The variations of the wavelengths of each DIB in different sight lines are generally larger than those of the known interstellar lines CH ⁺ , CH, and K i . Those variations could be due to the inherent error in the measurement, or to differences in the velocity components among sight lines.
... 25,26 However, subsequent astronomical observations have disproved this hypothesis. 27,28 It had also been suggested that a species related to the pyrenium radical cation (Py + ) may be responsible for the 4430 Å DIB, based on matrix isolation spectra. 29 However, subsequent gas-phase measurements have eliminated Py + as a potential carrier of this (or any other identified) DIB. 30 PAH radical cations have been previously observed in the gas phase through cavity ring down 24,31 and ion trap techniques. ...
The predissociation spectrum of the cold, argon-tagged, 9-methylanthracenium radical cation is reported from 8000 cm−1 to 44 500 cm−1. The reported spectrum contains bands corresponding to at least eight electronic transitions ranging from the near infrared to the ultraviolet. These electronic transitions are assigned through comparison with ab initio energies and intensities. The infrared D1←D0 transitions exhibit significant vibronic activity, which is assigned through comparison with TD-B3LYP excited state frequencies and intensities, as well as modelled vibronic interactions. Dissociation of 9-methylanthracenium is also observed at high visible-photon energies, resulting in the loss of either CH2 or CH3. The relevance of these spectra, and the spectra of other polycyclic aromatic hydrocarbon radical cations, to the largely unassigned diffuse interstellar bands, is discussed.
I reminisce on my early life in section 1; on my education, in sections 2 and 3; on the years at Princeton as a research astronomer in section 4; on the years on the faculty at Chicago in section 5; on research on Diffuse Interstellar Bands (DIBs) in section 6; on construction of the 3.5-meter telescope at Apache Point Observatory (APO) in section 7; on work on the Sloan Digital Sky Survey (SDSS) in section 8; on work in public education in Chicago in section 9; and on my travels in section 10. My main science research is of an observational nature, concerning galactic and intergalactic interstellar gas. Highlights for me included my work on the orbiting telescope Copernicus, including the discovery of interstellar deuterium; early observations of absorption associated with five-times ionized oxygen; and discoveries concerning the phases of gas in the local interstellar medium, based on previously unobservable interstellar UV spectral lines. With other instruments and collaborations, I extended interstellar UV studies to the intergalactic cool gas using QSO absorption lines redshifted to the optical part of the spectrum; provided better definition of the emission and morphological character of the source of absorption lines in QSO spectra; and pursued the identification of the unidentified diffuse interstellar bands. For several of these topics, extensive collaborations with many scientists were essential over many years. The conclusions developed slowly, as I moved from being a graduate student at Chicago, to a research scientist position at Princeton, and then to a faculty position at Chicago. At each stage of life, I was exposed to new technologies adaptable to my science and to subsequent projects. From high school days, I encountered several management opportunities which were formative. I have been extremely fortunate both in scientific mentors I had and in experimental opportunities I encountered.
Matrix isolation spectroscopy with para-hydrogen (p-H2) has previously been employed to record IR absorption spectra of hydrogenated and protonated polycyclic aromatic hydrocarbons (PAHs), prospective carriers of unidentified infrared and diffuse interstellar bands. Despite the promising prospects of p-H2 as matrix host, especially the rather weak interaction with the guest molecules and the resulting small matrix shifts, p-H2 matrix isolation spectroscopy has rarely been applied to study electronic transitions of guest molecules. Here, we present the dispersed fluorescence and fluorescence excitation spectrum of the 1-hydronaphthyl radical (1-C10H9) isolated in solid p-H2. We observed a strong 000 band associated with the electronic transition to the first excited electronic state at 18881 cm-1, red-shifted by ∼68 cm-1 relative to a value reported for jet-cooled 1-C10H9. From a comparison of our experimental results to simulated vibrationally resolved electronic absorption and emission spectra computed on the basis of (TD-)DFT geometry optimizations and scaled harmonic vibration calculations using the FCclasses code, we derived assignments for observed vibronic transitions. The dispersed fluorescence spectrum of 1-C10H9 is new; it complements the infrared spectrum and identified many vibrational modes unidentifiable with infrared. The excitation spectrum covers a much wider spectral range than previous reports. We compare the excitation spectrum in solid p-H2 to the reported electronic absorption spectrum of jet-cooled gaseous 1-C10H9 and that of 1-C10H9 isolated in solid Ne to assess the influence of p-H2 as a matrix host on the electronic transition of 1-C10H9 and discuss a potential contribution of 1-C10H9 to the diffuse interstellar bands.
Context. This study reports the index of aromaticity calculated by numerical integration of the magnetically-induced current density for cyclic hydrocarbon molecules both known to exist in astrophysical media as well as those proposed to exist.
Aims. This study promotes the ring current strength (RCS) value for quantifying aromaticity as a means of predicting astrophysical detectability.
Methods. Density functional theory (DFT) calculations at the B3LYP/aug-cc-pVTZ level provide optimized structures and the wave-functions needed to provide the RCS values for the molecules analyzed.
Results. The known interstellar molecules examined c -C 3 H 2 , c -(O)C 3 H 2 , c -C 3 HC 2 H, o -benzyne, benzonitrile, 1-cyano and 2-cyanonaphthalene all have RCS values of 9.9 nA T ⁻¹ (nanoampere per Tesla) or above. The known antiaromatic species have RCS values of less than 0.0 nA T ⁻¹ as expected. Several proposed interstellar molecules likely will not persist if they form due to low RCS values including c -(C)C 3 H 2 . Other species such as p -benzyne and c -HCNN ⁺ have high RCS values of 19.9 nAT ⁻¹ and 14.4nAT ⁻¹ , respectively.
Conclusions. Cyclic hydrocarbons previously observed in astrophysical media have high RCS values. Those with low or negative RCS values have yet to be observed implying that such a metric can indicate astrophysical significance.
The noncovalent intermolecular interaction of Helium, Neon and Argon with naphthalene is investigated for selected dimer configurations by means of electronic structure calculation at the MP2C level of theory and exploiting complete basis set extrapolation techniques. An analytical formulation of the global potentials, based on simple and efficient atom-atom expressions, is also provided by optimizing few and physically meaningful parameters on the calculated benchmark interaction energies. The proper representation of the interaction so obtained has been exploited to predict and analyze energies and structures of the low-lying minima in naphthalene-RgN (N=1-8) clusters by using Basin-Hopping and Diffusion Monte Carlo approaches.
Unrestricted DFT (UDFT), time‐dependent DFT (TDDFT) and spin‐flip TDDFT (SF‐TDDFT) were used to investigate the potential energy surfaces of the ground and first two electronic excited states of the naphthalene, pyrene and perylene radical cations. In particular, conical intersections (which play a central role in the photophysics of these cations) were located with these DFT‐based methods. The results are consistent with accurate multiconfigurational wavefunction‐based ab initio methods. These show that naphthalene and pyrene cations can quickly relax nonradiatively from their excited states back down to the original ground state species through easily accessible conical intersections, but the perylene cation cannot do so, due to the absence of any accessible funnels between the lowest excited state and the ground state, leaving radiative decay as the most probable photophysical pathway. This study paves the way for using computationally efficient density functional theory (DFT)‐based methods in future investigations of the photophysics of much larger polycyclic aromatic hydrocarbons, for which multiconfigurational wavefunction‐based methods become prohibitively expensive.
The recent discovery of benzonitrile (C6H5CN), one of the simplest nitrogen-bearing polar aromatic molecules, in the interstellar medium motivates structural characterization of the benzonitrile-containing molecular ions as potential precursors for nitrogen-containing complex organics in space. Herein, we present mass-selected ion mobility measurements combined with density functional theory (DFT) calculations to reveal, for the first time, the structures of the benzonitrile dimer radical cation, the protonated dimer, and the protonated hydrated small clusters in the gas phase. The measured collision cross sections of the investigated ions in helium are in excellent agreement with the calculated values of the lowest energy DFT structures. Unlike the dimer radical cations of nonpolar aromatic molecules which adopt parallel sandwich configurations, the (C6H5CN)2·+ displays a symmetrically planar geometry with a double hydrogen bond formed between the nitrogen and hydrogen atoms. The protonated dimer has the structure of a proton-bound dimer (C6H5CNH⁺NCC6H5) where the bridging proton connects the nitrogen atoms in the two benzonitrile molecules resulting in a calculated collision cross section of 101.1 Ų in excellent agreement with the measured value of 103.3 Ų. The structure of the hydrated protonated trimer consists of a hydronium ion core solvated by three benzonitrile molecules. By locating the proton on the lower proton affinity water molecule, the resulting hydronium ion can be fully solvated by forming three ionic hydrogen bonds with the benzonitrile molecules. These unique structural motifs could be useful for the molecular design and recognition involving charged aromatic systems and also for the search of nitrogen-containing complex organics in space.
1-methylpyrene radical cations undergo the loss of a hydrogen atom at internal energies above the first dissociation threshold. Imaging photoelectron photoion coincidence spectroscopy was employed in combination with RRKM modelling to determine a 0-K activation energy of 2.78 ± 0.25 eV and an entropy of activation of 6 ± 19 J K-1 mol-1 for this H-loss reaction. The ionization energy of 1-methylpyrene was measured by mass-selected threshold photoelectron spectroscopy to be 7.27 ± 0.01 eV. These values were found to be consistent with calculations at the CCSD/6-31G(d)//B3-LYP/6-31G(d) level of theory showing that the formation of the 1-methylenepyrene cation (resulting from H loss from the methyl group) is kinetically more favorable than the formation of a tropylium-containing product ion that is structurally analogous to the formation of the tropylium cation in H loss from ionized toluene. The shift away from a tropylium-containing structure was found to be due to the increased ring-strain imposed on the C7 moiety when it is bound to three fused benzene rings. The RRKM results allow for the derivation of the ΔfH0o (1-methylenepyrene cation) of 945 ± 31 kJ mol-1.
We have investigated the correlations between the equivalent widths of 21 selected diffuse interstellar bands (DIBs) and the corresponding interstellar column densities N(C2), N(CN), and N(CH), toward 53 stars with color excesses 0.11 ≤ E(B-V) ≤ 1.99. The observational data were derived primarily from echelle spectra acquired at R = 38,000 as part of our extensive, continuing survey of the bands. All but six of the 53 final spectra show signal-to-noise ratios ≥800 at 5780 Å. The principal result presented here is that seven of the 21 bands prove to be examples of "the C2 DIBs," a class of weak, narrow bands whose normalized equivalent widths Wλ(X)/Wλ (λ6196) are well correlated specifically with N(C2)/E(B-V) via power laws. In contrast, the similarly normalized equivalent widths of the 14 other, well-known DIBs analyzed here are uncorrelated, or weakly anticorrelated, with N(C2)/E(B-V), to within the observational uncertainties. Thus, the polyatomic molecule(s) presumed to cause these seven C2 DIBs may bear a direct chemical relation to C2 that is not shared by the polyatomic molecules putatively responsible for the other 14 bands. The C2 DIBs also show positive correlations with N(CN)/E(B-V) and N(CH)/E(B-V) in our particular sample of light paths, although generally with shallower slopes in the case of N(CN) and with greater scatter in the case of N(CH). Eleven additional C2 DIBs are also identified but are not analyzed here. Among the 18 C2 DIBs identified, four apparently have not been previously detected. The λ4963 band is generally the strongest of the 18 C2 DIBs, while the λ4734 band shows the most sensitive correlation with N(C2).
Echelle spectra of the double-lined spectroscopic binary HD 204827 were obtained on five nights, at a resolving power R = 38,000 and with a S/N = 750 near 6000 Å in the final, combined spectrum. The stars show E(B − V) = 1.11 and spectral types near O9.5 V and B0.5 III. A catalog is presented of 380 diffuse interstellar bands (DIBs) measured between 3900 and 8100 Å in the stars' spectrum. The central wavelengths, the widths (FWHM), and the equivalent widths of nearly all of the bands are tabulated, along with the minimum uncertainties in the latter. The reliable removal of very weak stellar lines from the catalog, and of some stellar lines from the less severe blends with DIBs, is made generally easy by the highly variable radial velocities of both stars. The principal result of this investigation is that the great majority of the bands in the catalog are very weak and relatively narrow. Typical equivalent widths amount to a few mÅ, and the bandwidths (FWHM) are most often near 0.55 Å. Therefore, most of these DIBs can be detected only in spectra obtained at a resolving power and a S/N at least comparable to those used here. In addition, the anomalous interstellar reddening and the very high value of the ratio N(C2)/E(B − V) seen toward HD 204827 indicate that the physical conditions in one or more of the several interstellar clouds seen in this direction differ significantly from those found toward the prototypical DIB target HD 183143, for example. Probably primarily for these reasons, 113 of the 380 bands (30%) were not detected in four previous modern surveys of the DIBs seen in the spectra of stars other than HD 204827. No preferred wavenumber spacings among the 380 bands are reliably identified which could provide clues to the identities of the large molecules thought to cause the DIBs.
This paper presents high-quality CCD spectra (obtained at spectral resolving power λ/Δλ ≈ 2000) of the region surrounding the diffuse interstellar band (DIB) centered near 4428 Å, in some 35 O and early B stars located in the Cyg OB2 association. The reddenings of the stars range from E(B-V) ≈ 1.0 to 2.5 mag, allowing us to study the properties of the feature at high extinction, as compared with previous studies of DIBs in diffuse clouds. To our knowledge this is the first significant survey of the 4428 Å DIB profile to be based on CCD-quality spectra, allowing us to probe the properties of this feature with high accuracy. Among our conclusions are (1) the profile of λ4428 is symmetric and invariant within the uncertainties; (2) there is no clear-cut dependence of band properties on stellar spectral type; (3) the equivalent width of the 4428 Å DIB as a function of increasing extinction appears to level off at around E(B-V) ≈ 1.0; (4) there is no evidence for emission wings associated with λ4428; and (5) within the uncertainties the profile of the feature is fitted perfectly by a Lorentzian function, strongly suggesting that the band is broadened by natural damping. We conclude that the feature is created by rapid internal conversion in a molecular carrier.
We establish correlations between equivalent widths of eight diffuse interstellar bands (DIBs), and examine their correlations with atomic hydrogen, molecular hydrogen, and E
B–V
. The DIBs are centered at λλ 5780.5, 6204.5, 6283.8, 6196.0, 6613.6, 5705.1, 5797.1, and 5487.7, in decreasing order of Pearson's correlation coefficient with N(H) (here defined as the column density of neutral hydrogen), ranging from 0.96 to 0.82. We find the equivalent width (EW) of λ5780.5 is better correlated with column densities of H than with E
B–V
or H2, confirming earlier results based on smaller data sets. We show that the same is true for six of the seven other DIBs presented here. Despite this similarity, the eight strong DIBs chosen are not correlated well enough with each other to suggest they come from the same carrier. We further conclude that these eight DIBs are more likely to be associated with H than with H2, and hence are not preferentially located in the densest, most UV shielded parts of interstellar clouds. We suggest that they arise from different molecules found in diffuse H regions with very little H2 (molecular fraction f < 0.01). Of the 133 stars with available data in our study, there are three with significantly weaker λ5780.5 than our mean H-λ5780.5 relationship, all of which are in regions of high radiation fields, as previously noted by Herbig. The correlations will be useful in deriving interstellar parameters when direct methods are not available. For instance, with care, the value of N(H) can be derived from W
λ(5780.5).
The interstellar medium constitutes a physically and chemically complex component of galaxies and is important in the cycle of matter and the evolution of stars. From various spectroscopic clues we now know that the interstellar medium is rich in organic compounds. However, identifying the exact nature of all these components remains a challenge. In particular the identification of the so-called diffuse band carriers has been alluding astronomers for almost a century.
In recent decades, observational, experimental and theoretical advances have rapidly lead to renewed interest in the diffuse interstellar bands (DIBs). This has been instigated partly by their perceived relation to the infrared aromatic emission bands, the UV extinction bump and far-UV rise, and the growing number of (small) organic molecules identified in space.
This chapter gives an overview of the observational properties and behaviour of the DIBs, and their presence throughout the Universe. I will highlight recent progress in identifying their carriers and discuss their potential as tracers and probes of (extra)-Galactic ISM conditions.
The relationship between interstellar extinction and distance in the direction of dark clouds in the areas around the open cluster IC 348 and the association Per OB2 is determined using the results of photoelectric photometry of 189 stars in the Vilnius photometric system. Two absorbing layers are found. The nearest layer, covering the whole area around IC 348, shows the mean extinction A(V) of about 0.7 mag. It begins at the distance of 160 pc and probably is an extension of the Taurus dark clouds to the northwest. The second absorbing layer has the form of a chain of dark condensations named L1468, L1470, and L1471 and is at 260 pc distance. This layer has a higher density, its mean extinction being about 2.0 mag. The cluster IC 348 is at about the same distance and is physically related to the dark cloud L1470. The distance of the Per OB2 association is found to be 340 pc and the mean extinction of its members is 0.95 mag. A model of the spatial distribution of the Perseus and Taurus dark clouds based on photometric distance determinations in this and previous papers is proposed. Six stars in the IC 348 area are suspected to have emission in the H-alpha line.
A painstaking comparison of laboratory gas-phase spectra of the naphthalene cation with an extensive set of astrophysical spectra was carried out. In the vicinity of both naphthalene features, found in the laboratory experiment, we detected relevant interstellar features. The strongest of them is situated at 6710.5Å, which is reasonably close to the laboratory feature at 6706.5Å and which is also of comparable width, i.e. about 20Å. The second feature was found at 6493Å - also close to the 6488.4-Å laboratory band of a similar width. The structures can be observed only in spectra of O stars because of the growing stellar line contamination in colder objects. The possible third feature is intertwined with a strong stellar helium line, but the unusual strength of the latter suggests that this naphthalene cation band is present as well as the two former ones. We estimated the column density of the carrier, for the reddening value E(B-V)=1.0, as roughly 4×1015cm-2. The lack of precise wavelength coincidence between laboratory and observed features, however, makes the identification uncertain and further laboratory and observational works are both highly desirable.
Twenty-two new diffuse interstellar bands (DIBs) have been discovered on
high signal-to-noise Reticon scans of reddened O- and B-type stars in
the 5840-8650 A region, with special attention being given to HD 183143
(B7 Ia). Most of the new DIBs occur in regions masked by atmospheric O2
and H2O. Attempts to find DIBs at positions expected for a transition in
the (hypothetical) spectrum of interstellar H(-), and at wavelengths of
lines in the laboratory spectrum of Cr(3+):MgO, were inconclusive. A
systematic search was made in the wavenumbers of the 105 DIBs now known
for vibrational sequences of the type 0 to v-prime; none of those found
are very convincing. The large number of DIBs now known, far more than
would be expected in the spectrum of a single species at interstellar
temperatures, must mean that a substantial number of different carriers
are responsible for the DIB spectrum.
The intensities of 10 of the yellow/red and the 4430 A diffuse
interstellar bands (DIBs) and the broad 2200 A feature of the high
Galactic latitude star Zeta Persei are compared with those for three
other stars using high SNR spectra of either 1.1 A or 0.15 A resolution.
The DIBs in Zeta Persei appear to fall into three well-defined groups:
(1) 4430 and 6180 A are absent; (2) 5780, 6196, 6203, 6269, and 6284 A
are about one-third the strength expected for the color excess; and (3)
the 2200 A feature and the 5797, 5850, 6376, 6379, and 6614 A are of
approximately normal strength for the color excess. This suggests that
the DIBs may be caused by at least three agents where proportions vary
from cloud to cloud. It is noted that other stars with markedly
anomalous DIB intensities also tend to lie at high Galactic latitudes.
The diffuse interstellar bands are absorption features observed in the spectra of stars seen through significant column densities of interstellar material. Of the 127 confirmed DIBs in the optical region between 0.4 and about 1.3 /Lm, only 2 have (tentatively) been identified with a specific carrier. Because DIB strengths increase roughly in proportion to color excess, they were originally suspected of being produced on or in the interstellar grains, but current evidence favors some species of free polyatomic molecules, either neutral or ionized. DIBs are conspicuously broader than the atomic interstellar lines, having widths at half-depth ranging from 0.8 to about 30 Å. These widths are presumably due to unresolved rotational structure, possibly compounded by lifetime broadening of the upper state. Recent proposals that C60, some members of the PAH family, or polycarbon chains are responsible for the DIB spectrum are either not supported by observation or await better data.