Anionic hydrogen clusters as a ABSTRACT The sources behind numerous key diffuse source of diffuse interstellar interstellar bands (DIBs) remain elusive, and 1 thus evidence is presented here that a family of bands (DIBs) seven anionic hydrogen atomic clusters 1 2-5 ( , 1n7) are pertinent contributors. Lulu Huang, Chérif F. Matta*, Daniel H 2n 1 Majaess,2 Tina Harriott,2,6 Configuration interaction calculations sho hat Joseph Capitani,7 and Lou Massa*1 these charge-induced dipole clusters are stable at temperatures characteristic of the interstellar 1Hunter College and the Graduate School, City University of medium, and emerge from the most abundant New York, New York, NY 10065, USA. 2 Department of element in the Universe. The clusters’ spectra Chemistry and Physics, Mount Saint Vincent University, 3 yield 25 absorption optical lines that align with Halifax, Nova Scotia, Canada B3M2J6. Department of observed DIBs to within the computational Chemistry, Saint Mary's University, Halifax, Nova Scotia, Canada B3H3C3. 4 Dép. de chimie, Université Laval, uncertainties. The absorption bands are due to Québec, Québec, Canada G1V 0A6. 5 Department of excitations from the ground states of the Chemistry, Dalhousie University, Halifax, Nova Scotia, clusters to metastable states. Canada B3H4J3. 6 Department of Mathematics and Statistics, Mount Saint Vincent University, Halifax, Nova Scotia, Canada B3M2J6. 7 Chemistry and Biochemistry Keywords: Interstellar medium (ISM): dust, Department, Manhattan College, Riverdale, NY 10471, USA. extinction---molecules---lines and bands; diffuse * Correspondence: [email protected], interstellar bands (DIBs); anionic hydrogen [email protected] clusters; astrophysical spectroscopy. TOC GRAPHIC 1. INTRODUCTION Light from stars and galaxies may be absorbed by interstellar material along the sight-line. Absorption typically affects the entire spectrum rather than producing lines. However, for over 100 years a set of discrete absorption features, known as diffuse interstellar bands (DIBs), have been recognized in the spectra of many stars when viewed through interstellar clouds. Indeed, the signature of that effect is partly conveyed by numerous discrete absorption features known as diffuse interstellar bands (DIBs). Yet there is no consensus as to the source(s) behind the majority of DIBs, and hence the impetus behind this research. For a comprehensive discussion concerning the discovery of DIBs the reader is referred to McCall & Griffin (2013) [1]. Briefly, Hartman 1904 [2] noted that the K line of ionized calcium did not exhibit the requisite Doppler shift identified in other lines associated with the 1 A preliminary version of this work has been spectroscopic binary Orionis, and consequently presented as a poster entitled “Anionic Hydrogen it was associated with an interstellar calcium Clusters ( ) as a Chemical Source of Diffuse H2n 1 cloud along the sightline. In 1919 Heger reported Interstellar Bands and Possible Candidates for that sodium D lines displayed a similar stationary Hydrogen Storage” by Huang, L.; Matta, C. F.; Massa, behaviour relative to other lines, and later L. at the Sagamore XVII Conference on Charge, Spin identified features at 5780 Å and 5797 Å that are and Momentum Densities (CSMD), International now recognized as DIBs [1]. That work in part Union of Crystallography; Kitayuzawa, Hokkai-do, helped catalyze this key area of research, and Japan (15-20 July 2012) and at the First Changsha significant progress has since been achieved International Workshop on Theoretical and toward expanding the census of DIBs. In 2009, Computational Chemistry with Materials, Hunan Hobbs et al. [3] reported a total of 414 DIBs Normal University, Changsha, China (8, 9 June 2012). 1 between 3900 Å and 8100 Å in the spectrum of viable source of DIBs. While the H ion is the Galactic binary star HD 204827. DIBs were found in the interstellar medium CHEMREV likewise detected beyond the Galaxy, such as at and it is also found in stellar atmospheres and 5780 Å and 5797 Å in the Magellanic clouds [4], regions such as dark molecular clouds [18]. and Junkkarinen et al. [5] found DIBs associated Indeed the ions C4H , C6H , C8H are among with observations of the BL Lac object AO the reported 2018 census [19] of 204 molecules 0235+164 (z = 0.524). identified in the interstellar medium. Even The word “diffuse” in the designation of though the binding energy of H is low ( 0.75 DIBs underscores the “somewhat hazy eV), the low temperatures ( 7-20 K) and appearance of DIBs compared with the relative densities ( 100-300 molecules/cm3) of certain sharpness of atomic transitions in the interstellar molecular clouds imply that once formed H medium”, quoting McCall & Griffin (2013) [1], could remain stable for long periods, and act as which they argue is a strong indicator that the an attractor for H2 molecules to form anionic sources of DIBs are molecular (not atomic). The hydrogen atomic clusters. diffuseness of the DIBs is generally attributed to This work investigates anionic the short lifetimes of the excited molecular hydrogen clusters as contributors to the DIBs. species that are associated with them (See Ref. Anions, in general, are believed to be [6] and references therein). The observed widths contributing to the DIBs spectrum [20]. of DIBs generally fall between 0.05 and 0.3 nm, Moreover, Sarre has argued that anion-dipole with a typical width of ~ 0.07 nm according to a bound molecular clusters, such as those review by Tielens [7]. presented here, are good candidates as a source Research continues unabated to identify of DIBs [21]. This paper is divided as follows, the numerous sources behind DIBs, as the in Section 2 the computational approach is strengths of DIBs are not necessarily correlated. outlined. In Section 3.1 the evidence for the DIB wavelengths are unrelated to known atomic stability of these clusters under interstellar transitions, and may be linked to molecules or conditions is presented along with a molecular clusters. The existence of mechanism for their formation. Finally, the spectroscopic fine-structure in DIBs supports that spectra of these clusters and the matching DIBs conclusion, namely that the source is molecular, are discussed in Section 3.2. since the fine-structure can result from molecules composed of different isotopes of certain atoms 2. CALCULATIONS such as carbon 12 and carbon 13. The ESO Diffuse Interstellar Bands Large Exploration Configuration interaction calculations with Survey (EDIBLES) highlights in excess of 500 + single excitations (CIS) were conducted for the DIBs [8], however C60 remains the sole ground and excited states. Brillouin's theorem confirmed source for a few DIBs. For example, + states that the matrix element of a singly Maier et al. [9] advocated that fullerene C60 was excited Slater determinants (SD) with the the source of DIBs at 9577 Å and 9632 Å, and + ground state SD vanishes three other C60 bands were since identified at 0ˆ 1 9348 Å, 9365 Å, and 9428 Å [10]. ( SDH SD 0 ) [22]. The singly excited A source for certain DIBs that features states, thus, cannot improve the wavefunction or hydrogen is favored by the element’s energy of the ground state. Thus, in this work abundance. Yet atomic (H) or molecular (H2) the CIS calculations are followed by Møller- hydrogen are ruled out as sources of DIBs Plesset second-order perturbation theory (MP2) because their spectra do not match known DIBs. corrections [23,24] (with frozen core) for the Moreover, H is rare in the interstellar medium 2 ground and excited states, a method referred to [11]. The H- is also not a viable candidate since as CIS-MP2. it has only one bound state, at least within the The triple-zeta quality aug-cc-pVTZ non-relativistic, fixed nucleus approximation basis set has been used in all calculations with Coulombic forces only (and no excited whereby the geometry of any given cluster is bound states) [12]. energy minimized without constraints at the As a result, it has been suggested that CIS/aug-cc-pVTZ level of theory followed by ion induced dipole clusters H 2n 1 (where n is the a harmonic frequency calculation. No number of H2 ligands) might exist in the imaginary frequencies were found for any of interstellar medium [13-17], and could be a the optimized geometries, and hence all 2 structures are (at least) local minima on their of these clusters, H , are known to exist for all respective potential energy hypersurfaces. The n values of 3 < n (odd) < 99 [30], hence it is not wavelengths of maximum absorption ( ) max unreasonable to expect the existence of anionic were calculated at the CIS-MP2/aug-cc- clusters as well [31,32]. Recently, path integral pVTZ//CIS/aug-cc-pVTZ level of theory. This molecular dynamics simulations at 1 K by Calvo theoretical methodology has been found to and Yurtsever have pushed the search for stable reproduce UV-Vis spectra accurately (even anionic clusters up to n = 54 molecular hydrogen with smaller basis sets) by Forseman, Head- ligands attached to a central hydride [33]. These Gordon, and Pople [23]. In the present work, workers found that stable icosahedral shells of the the basis set aug-cc-pVTZ was selected since it larger ions exhibit magic numbers at sizes of 12, is relatively large, correlation consistent, and – 32, and 44 ligands (confirming experimental importantly – it is augmented with diffuse results previously published by Renzler et al. [34]) functions that are necessary for an accurate [33]. description of anionic systems, especially in CCSD//MP2 calculations by Renzler et their excited electronic states. al. with quadruple zeta quality basis set of the The calculated spectral lines (vertical) electron affinities of the neutral species correspond to E = hν, the difference between (H, H3, H5, and H7) yield 0.74, 0.77, 0.81, and 0.69 the MP2 energy of the ground state and the eV, respectively.