Terahertz Spectroscopy in the Lab and at Telescopes

The section of the electromagnetic spectrum extending roughly from wavelengths of 3 mm to 30 μm is commonly known as the far-infrared or TeraHertz (THz) region. It contains the great majority of the photons emitted by the universe, and THz observations of molecules and dust are able penetrate deeply into molecular clouds, thus revealing the full history of star and planet formation. Accordingly, the upcoming deployments of the Herschel, ALMA, and SOFIA observatories promise to revolutionize our understanding of THz astrophysics. To fully realize this promise, however, it is essential that we achieve a quantitative experimental understanding of the dust, ice, and gas which make up the ISM. After outlining the tremendous impact that Tom Phillips has had on astronomical applications of THz radiation, this contribution will describe how emerging technologies in ultrafast lasers are enabling the development of integrated frequency- and time-domain THz facilities that can acquire high dynamic range optical constants of the major components that comprise astrophysical dust, ice and organics across the full wavelength region accessible to Herschel and other THz observatories

Chemistry in Dense Molecular Clouds: Theory and Observational Constraints

For the most part, gas phase models of the chemistry of dense molecular clouds predict the abundances of simple species rather well. However, for larger molecules and even for small systems rich in carbon these models often fail spectacularly. We present a brief review of the basic assumptions and results of large scale modeling of the chemistry in dense molecular clouds. Particular attention will be paid to the influence of the gas phase ratios of the major elements in molecular clouds, and the likely role grains play in maintaining these ratios as clouds evolve from initially diffuse objects to denser cores with associated stellar and planetary formation. Recent spectral line surveys at centimeter and millimeter wavelengths along with selected observations in the submillimeter have now produced an accurate "inventory" of the gas phase elemental budgets in different types of molecular clouds, though gaps in our knowledge clearly remain. The constraints these observations place on theoretical models of interstellar chemistry can be used to gain insights into why the models fail, and show also which neglected processes must be included in more complete analyses. Looking toward the future, truly protostellar regions are only now becoming available for both experimental and theoretical study, and some of the expected modifications of molecular cloud chemistry in these sources are therefore outlined

Carbon Chemistry in Dense Molecular Clouds: Theory and Observational Constraints

For the most part, gas phase models of the chemistry of dense molecular clouds predict the abundances of simple species rather well. However, for larger molecules and even for small systems rich in carbon these models often fail spectacularly. We present a brief review of the basic assumptions and results of large scale modeling of the carbon chemistry in dense molecular clouds. Particular attention will be paid to the influence of the gas phase C/O ratio in molecular clouds, and the likely role grains play in maintaining this ratio as clouds evolve from initially diffuse objects to denser cores with associated stellar and planetary formation. Recent spectral line surveys at centimeter and millimeter wavelengths along with selected observations in the submillimeter have now produced an accurate "inventory" of the gas phase carbon budget in several different types of molecular clouds, though gaps in our knowledge clearly remain. The constraints these observations place on theoretical models of interstellar chemistry can be used to gain insights into why the models fail, and show also which neglected processes must be included in more complete analyses. Looking toward the future, larger molecules are especially difficult to study both experimentally and theoretically in such dense, cold regions, and some new methods are therefore outlined which may ultimately push the detectability of small carbon chains and rings to much heavier species

ZEKE-PFI spectroscopy of 1:1 complexes of sodium with water and ammonia

ZEKE-PFI (zero kinetic energy pulsed field ionization) photoelectron spectra of the Na(H_2O), Na(D_2O), Na(NH_3), and Na(ND_3) complexes are reported. Spectra of all four complexes were obtained by single-photon ionization, and, for the Na(NH_3) and Na(ND_3) complexes, by two-color (1 + 1′) photoionization as well, with the Ã^2E state serving as the intermediate resonance. Improved values for the ionization energies (IE) and intermolecular vibrational frequencies of the complexes were determined. The single-photon ZEKE-PFI spectra show transitions only between states of the same vibrational symmetry, in accord with the selection rule for allowed electronic transitions. Some of the two-color ZEKE-PFI spectra, however, show strong transitions between states of different vibrational symmetry which we attribute to vibronic coupling in the intermediate state

High-Resolution 4.7 Micron Keck/NIRSPEC Spectroscopy of the CO Emission from the Disks Surrounding Herbig Ae Stars

We explore the high-resolution (λ/Δλ = 25,000; Δv = 12 km s^(-1)) M-band (4.7-5.1 μm) spectra of several disk-dominated Herbig Ae (HAe) systems: AB Aur, MWC 758, MWC 480, HD 163296, and VV Ser. All five objects show ^(12)CO v = 1-0 emission lines up to J = 42, but there is little or no evidence of moderate-J, v = 2-1 transitions despite their similar excitation energies. AB Aur shows ^(13)CO emission as well. The line/continuum ratios and intensity profiles are well correlated with inclination, and they trace collisionally driven emission from the inner disk (R_(th) ≾ 0.5-1 AU) as well as resonance fluorescence to much larger radii (R_(hν) ≾ 50-100 AU for J ≾ 10). The temperature, density, and radiation field profiles required to fit the CO emission are in good agreement with models of HAe disks derived from their spectral energy distributions. High-resolution and high dynamic range infrared spectroscopy of CO, and future observations of less abundant species, thus provide direct access to the physicochemical properties and surface structure of disks in regions where planet formation likely occurs

Direct measurement of the HCl dimer tunneling rate and Cl isotope dependence by far-infrared laser sideband spectroscopy of planar supersonic jets

The large amplitude tunneling motion of the HCl dimer has been directly studied with a tunable far‐infrared laser sideband/two-dimensional free jet expansion spectrometer at hyperfine resolution. Rotationless tunneling rates for the three common chlorine isotopic forms are v(35–35)=463 979.2(1) MHz, v(35–37)=463 357.7(1) MHz, and v(37–37)=462 733.7(3) MHz. Both the rotational constants and hyperfine parameters indicate that the vibrationally averaged structure shows little variation within a given tunneling state, with both HCl bond angles giving an average projection on the a-axis of 47° in all states with resolved hyperfine patterns

Evidence of fast pebble growth near condensation fronts in the HL Tau protoplanetary disk

Water and simple organic molecular ices dominate the mass of solid materials available for planetesimal and planet formation beyond the water snow line. Here we analyze ALMA long baseline 2.9, 1.3 and 0.87 mm continuum images of the young star HL Tau, and suggest that the emission dips observed are due to rapid pebble growth around the condensation fronts of abundant volatile species. Specifically, we show that the prominent innermost dip at 13 AU is spatially resolved in the 0.87 mm image, and its center radius is coincident with the expected mid-plane condensation front of water ice. In addition, two other prominent dips, at distances of 32 and 63 AU, cover the mid-plane condensation fronts of pure ammonia or ammonia hydrates and clathrate hydrates (especially with CO and N$_2$) formed from amorphous water ice. The spectral index map of HL Tau between 1.3 and 0.87 mm shows that the flux ratios inside the dips are statistically larger than those of nearby regions in the disk. This variation can be explained by a model with two dust populations, where most of solid mass resides in a component that has grown into decimeter size scales inside the dips. Such growth is in accord with recent numerical simulations of volatile condensation, dust coagulation and settling.Comment: 6 pages, 3 figures, Accepted for publication in the Astrophysical Journal Letter

Microwave and terahertz spectroscopy

Spectroscopy, or the study of the interaction of light with matter, has become one of the major tools of the natural and physical sciences during this century. As the wavelength of the radiation is varied across the electromagnetic spectrum, characteristic properties of atoms, molecules, liquid and solids are probed. In the optical and ultraviolet regions (λ ~ 1 µm up to 100 nm) it is the electronic structure of the material that is investigated, while at infrared wavelengths (~ 1-30 µm) the vibrational degrees of freedom dominate

High resolution millimeter-wave to infrared spectroscopy of circumstellar disks

The role of high spectral and spatial resolution spectroscopy in understanding the evolution of the gaseous component of circumstellar accretion disks is described. Millimeter-wave emission lines from trace constituents such as CO, CN, HCO+, and HCN can be used to probe the kinematic and physicochemical properties in the near-surface regions of disks beyond 100 AU, but, thanks to extensive molecular depletion in the midplane, they are not a reliable proxy for the disk mass. Mid-infrared observations of the pure rotational transitions of molecular hydrogen would alleviate many of these concerns, and results from the ISO SWS instrument on several \transitional" disks are presented. The measurements of these weak lines used beams substantially larger than the disk angular diameter, and so must be verified or refuted by high angular resolution spectroscopy from the ground. Finally, the high resolution M-band (5 µm) spectroscopy of CO in disks is outlined. Emission lines that are likely optically pumped by hot dust in the inner disk (R ≾ 1 AU) are seen toward inclined systems, while the absorption spectra of edge-on disks clearly reveal the molecular depletion inferred at millimeter-wavelengths