Desorption Kinetics and Binding Energies of Small Hydrocarbons

Small hydrocarbons are an important organic reservoir in protostellar and protoplanetary environments. Constraints on desorption temperatures and binding energies of such hydrocarbons are needed for accurate predictions of where these molecules exist in the ice versus gas phase during the different stages of star and planet formation. Through a series of temperature programmed desorption experiments, we constrain the binding energies of 2- and 3-carbon hydrocarbons (C_2H_2—acetylene, C_2H_4—ethylene, C_2H_6—ethane, C_3H_4—propyne, C_3H_6—propene, and C_3H_8—propane) to 2200–4200 K in the case of pure amorphous ices, to 2400–4400 K on compact amorphous H_2O, and to 2800–4700 K on porous amorphous H_2O. The 3-carbon hydrocarbon binding energies are always larger than the 2-carbon hydrocarbon binding energies. Within the 2- and 3-carbon hydrocarbon families, the alkynes (i.e., least-saturated) hydrocarbons exhibit the largest binding energies, while the alkane and alkene binding energies are comparable. Binding energies are ~5%–20% higher on water ice substrates compared to pure ices, which is a small increase compared to what has been measured for other volatile molecules such as CO and N_2. Thus in the case of hydrocarbons, H_2O has a less pronounced effect on sublimation front locations (i.e., snowlines) in protoplanetary disks

Untangling the chemical evolution of Titan's atmosphere and surface–from homogeneous to heterogeneous chemistry

The arrival of the Cassini-Huygens probe at Saturn's moon Titan - the only Solar System body besides Earth and Venus with a solid surface and a thick atmosphere with a pressure of 1.4 atm at surface level - in 2004 opened up a new chapter in the history of Solar System exploration. The mission revealed Titan as a world with striking Earth-like landscapes involving hydrocarbon lakes and seas as well as sand dunes and lava-like features interspersed with craters and icy mountains of hitherto unknown chemical composition. The discovery of a dynamic atmosphere and active weather system illustrates further the similarities between Titan and Earth. The aerosol-based haze layers, which give Titan its orange-brownish color, are not only Titan's most prominent optically visible features, but also play a crucial role in determining Titan's thermal structure and chemistry. These smog-like haze layers are thought to be very similar to those that were present in Earth's atmosphere before life developed more than 3.8 billion years ago, absorbing the destructive ultraviolet radiation from the Sun, thus acting as 'prebiotic ozone' to preserve astrobiologically important molecules on Titan. Compared to Earth, Titan's low surface temperature of 94 K and the absence of liquid water preclude the evolution of biological chemistry as we know it. Exactly because of these low temperatures, Titan provides us with a unique prebiotic 'atmospheric laboratory' yielding vital clues - at the frozen stage - on the likely chemical composition of the atmosphere of the primitive Earth. However, the underlying chemical processes, which initiate the haze formation from simple molecules, have been not understood well to date

Multiple-resonance spectroscopy of high vibrational levels in water and methanol

We have measured the rovibrational levels in the electronic ground state of the water molecule at the previously inaccessible energies above 26000 cm-1. The use of laser double-resonance overtone excitation combined with laser-induced fluorescence (LIF) photofragment detection extends this limit to 34200 cm-1, which corresponds to 83% of the water dissociation energy. These data have allowed the theoretical group of Oleg Polyansky to generate a semiempirical potential energy surface on the basis of their ab initio surface [O. L. Polyansky, A. G. Csaszar, S. V. Shirin et al., Science 299 (5606), 539 (2003)] that now allows prediction of water levels with sub-cm-1 accuracy at any energy up to the new limit. A new ab initio potential energy surface is being constructed by Polyansky et al. that is intended to reproduce the rovibrational levels of water up to the dissociation threshold. We have calculated the electronic energy of ca. 2500 nuclear geometries at multireference configuration interaction level with 6Z basis set. We have performed a direct measurement of one of the most fundamental thermochemical values: the O-H bond energy in water. Using a triple-resonance laser excitation scheme, we excite the molecule through a series of vibrational overtone transitions to access directly the onset of the dissociative continuum. The dissociation energy obtained from our experiments, 41145.94 ± 0.15 cm-1, is ca. 30 times more accurate than the currently accepted value and has important implications for other thermochemical quantities linked to the bond energy of water. We have studied the conformational dependence of intramolecular vibrational redistribution in the 5v1 OH stretch overtone region of methanol. Previous state-selected spectra in the 5ν1 region [O. V. Boyarkin, T. R. Rizzo, and D. S. Perry, J. Chem. Phys. 110, 11346 (1999)] revealed a structure indicating an intramolecular vibrational redistribution on three time scales. Whereas in that work, methanol in the 5 v1 bright state was prepared close to the staggered conformation, methanol in the "partially eclipsed" conformation is prepared in our experiments by double resonance excitation through a torsionally excited intermediate state. We detect the excited molecules by infrared laser assisted photofragment spectroscopy (IRLAPS). In partially eclipsed methanol, the strong coupling of the v1 OH stretch to the v2 CH stretch becomes weaker, but the coupling responsible for the widths of the narrowest features becomes stronger