Energetic eruptions leading to a peculiar hydrogen-rich explosion of a massive star.

Every supernova so far observed has been considered to be the terminal explosion of a star. Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower-moving material that was previously hidden. In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining. Here we report observations of iPTF14hls, an event that has spectra identical to a hydrogen-rich core-collapse supernova, but characteristics that differ extensively from those of known supernovae. The light curve has at least five peaks and remains bright for more than 600 days; the absorption lines show little to no decrease in velocity; and the radius of the line-forming region is more than an order of magnitude bigger than the radius of the photosphere derived from the continuum emission. These characteristics are consistent with a shell of several tens of solar masses ejected by the progenitor star at supernova-level energies a few hundred days before a terminal explosion. Another possible eruption was recorded at the same position in 1954. Multiple energetic pre-supernova eruptions are expected to occur in stars of 95 to 130 solar masses, which experience the pulsational pair instability. That model, however, does not account for the continued presence of hydrogen, or the energetics observed here. Another mechanism for the violent ejection of mass in massive stars may be required.

Cation-π and CH-π Interactions in the Coordination and Solvation of Cu(+)(acetylene)n Complexes.

Copper-acetylene cation complexes of the form Cu(C2H2)n(+) (n = 1-8) are produced by laser ablation in a supersonic expansion of acetylene/argon. The ions are mass selected and studied via infrared laser photodissociation spectroscopy in the C-H stretching region (3000-3500 cm(-1)). The structure and bonding of these complexes are investigated through the number of infrared active bands, their relative intensities and their frequency positions. Density functional theory calculations are carried out in support of the experimental data. The combined data show that cation-π complexes are formed for the n = 1-3 species, resulting in red-shifted C-H stretches on the acetylene ligands. The coordination of the copper cation is completed with three acetylene ligands, forming a "propeller" structure with D3 symmetry. Surprisingly, complexes with even greater numbers of acetylenes than this (4-6) have distinctive infrared band patterns quite different from those of the smaller complexes. Experiment combined with theory establishes that there is a fascinating pattern of second-sphere solvation involving the binding of acetylenes in bifurcated CH-π binding sites at the apex of two core ligands. This binding motif leads to three equivalent sites for second-sphere ligands, which when filled form a highly symmetrical Cu(+)(C2H2)6 complex. Solvent binding in this complex induces a structural change to planarity in the core, producing an appealing "core-shell" structure with D(3h) symmetry.

Vibrational spectroscopy and structures of Ni+(C2H2)(n) (n =1-4) complexes.

Nickel cation-acetylene complexes of the form Ni(+)(C(2)H(2))(n), Ni(+)(C(2)H(2))Ne, and Ni(+)(C(2)H(2))(n)Ar(m) (n = 1-4) are produced in a molecular beam by pulsed laser vaporization. These ions are size-selected and studied in a time-of-flight mass spectrometer by infrared laser photodissociation spectroscopy in the C-H stretch region. The fragmentation patterns indicate that the coordination number is 4 for this system. The n = 1-4 complexes with and without rare gas atoms are also investigated with density functional theory. The combined IR spectra and theory show that pi-complexes are formed for the n = 1-4 species, causing the C-H stretches in the acetylene ligands to shift to lower frequencies. Theory reveals that there are low-lying excited states nearly degenerate with the ground state for all the Ni(+)(C(2)H(2))(n) complexes. Although isomeric structures are identified for rare gas atom binding at different sites, the attachment of rare gas atoms results in only minor perturbations on the structures and spectra for all complexes. Experiment and theory agree that multiple acetylene binding takes place to form low-symmetry structures, presumably due to Jahn-Teller distortion and/or ligand steric effects. The fully coordinated Ni(+)(C(2)H(2))(4) complex has a near-tetrahedral structure.

Solvation dynamics in Ni+ (H2O)n clusters probed with infrared spectroscopy.

Infrared photodissociation spectroscopy is reported for mass-selected Ni+ (H2O)n complexes in the O-H stretching region up to cluster sizes of n = 25. These clusters fragment by the loss of one or more intact water molecules, and their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water. The first evidence for hydrogen bonding, indicated by a broad band strongly red-shifted from the free OH region, appears at the cluster size of n = 4. At larger cluster sizes, additional red-shifted structure evolves over a broader wavelength range in the hydrogen-bonding region. In the free OH region, the symmetric stretch gradually diminishes in intensity, while the asymmetric stretch develops into a closely spaced doublet near 3700 cm(-1). The data indicate that essentially all of the water molecules are in a hydrogen-bonded network by the size of n = 10. However, there is no evidence for the formation of clathrate structures seen recently via IR spectroscopy of protonated water clusters.

Spectral signatures of hydrated proton vibrations in water clusters.

The ease with which the pH of water is measured obscures the fact that there is presently no clear molecular description for the hydrated proton. The mid-infrared spectrum of bulk aqueous acid, for example, is too diffuse to establish the roles of the putative Eigen (H3O+) and Zundel (H5O2+) ion cores. To expose the local environment of the excess charge, we report how the vibrational spectrum of protonated water clusters evolves in the size range from 2 to 11 water molecules. Signature bands indicating embedded Eigen or Zundel limiting forms are observed in all of the spectra with the exception of the three- and five-membered clusters. These unique species display bands appearing at intermediate energies, reflecting asymmetric solvation of the core ion. Taken together, the data reveal the pronounced spectral impact of subtle changes in the hydration environment.

Structural trends in transition metal cation-acetylene complexes revealed through the C-H stretching fundamentals.

Metal cation-acetylene complexes (M = V, Fe, Co, Ni) are produced in molecular beams and studied with infrared photodissociation spectroscopy in the C-H stretching region. Each complex has two vibrational bands corresponding to the symmetric and asymmetric stretches of acetylene that are shifted to the red of these vibrations in the isolated acetylene molecule. Density functional theory reveals the sources of the red-shifted vibrations and their relative magnitudes. Fe+, Co+, and Ni+ form pi-complexes with acetylene, while V+(C2H2) is a metallacycle.