We have measured and assigned more than 800 new far-infrared absorption lines and 12 new microwave absorption lines of the ammonia dimer. Our data are analyzed in combination with all previously measured far-infrared and microwave spectra for this cluster. The vibration-rotation-tunneling (VRT) states of the ammonia dimer connected by electricdipole-allowed transitions are separated into three groups that correspond to different combinations of monomer rotational states: A +A states (states formed from the combination of two ammonia monomers in A states), A +E states, and E+ E states. We present complete experimentally determined energy-level diagrams for the K,=O and K,= 1 levels of each group in the ground vibrational state of this complex. From these, we deduce that the appropriate molecular symmetry group for the ammonia dimer is G,# This, in turn, implies that three kinds of tunneling motions are feasible for the ammonia dimer: interchange of the "donor" and "acceptor" roles of the monomers, internal rotation of the monomers about their C, symmetry axes, and quite unexpectedly, "umbrella" inversion tunneling. In the K,= 0 A + E and E+ E states, the measured umbrella inversion tunneling splittings range from 1.1 to 3.3 GHz. In K,= 1, these inversion splittings between two sets of E+E states are 48 and 9 MHz, while all others are completely quenched. Another surprise, in light of previous analyses of tunneling in the ammonia dimer, is our discovery that the interchange tunneling splittings are large. In the A+A and E+E states, they are 16.1 and 19.3 cm-', respectively. In the A + E states, the measured 20.5 cm-' splitting can result from a difference in "donor" and "acceptor" internal rotation frequencies that is increased by interchange tunneling. We rule out the possibility that the upper state of the observed far-infrared subbands is the very-low-frequency out-of-plane intermolecular vibration predicted in several theoretical studies [C. E. Dykstra and L. Andrews, J. Chem. Phys. 92, 6043 (1990) Chem. Phys. 97, 4750 (1992>], make it unlikely that the structure proposed by Nelson et al. for the ammonia dimer is the equilibrium structure.

A detailed description is presented for a tunable far infrared laser spectrometer based on frequency mixing of an optically pumped molecular gas laser with tunable microwave radiation in a Schottky point contact diode. The system has been operated on over 30 laser lines in the range 10-100 cm-i and exhibits a maximum absorption sensitivity near one part in 106. Each laser line can be tuned by f 110 GHz with first-order sidebands. Applications of this instrument are detailed in the preceding paper. THE BERKELEY TUNABLE FAR-INFRARED LASER SPECTROMETERA. General description Tunable far-infrared (FIR) lasers have become powerful tools for investigating the structures of ions, radicals, and clusters, and for probing intermolecular forces through measurement of FIR spectra of van der Waals complexes. In the preceding paper we have described the rapid evolution of FIR laser spectroscopy and some recent applications. In this article we present a detailed description of the tunable FIR laser spectrometers currently used at Berkeley. We begin with a relatively general overview of the design, and then proceed to the details of construction and operation. It is our hope that this article will serve as a useful guide to those who seek to construct similar systems.The design of the tunable FIR laser systems used at Berkeley is similar to that of Farhoomand et al. ' In the following, we first present a general description of this design, in sufficient detail to afford all readers a reasonable understanding of the underlying principles and function. We then proceed to describe each component of the system in sufficient detail to effectively guide those actually seeking to construct a similar apparatus.The overall experimental design is diagrammed in Fig. 1. The complete spectrometer is built on a 5 ft. x 12 ft. vibration isolation honeycomb table. A COz. laser provides an intense mid-infrared beam (maximum power > 150 W) that is used to pump a molecular gas FIR laser. The CO2 laser is line tunable over some 100 different vibrationrotation transitions between 9.1 and 11 .O pm using a precision grating in first-order autocollimation. The output frequency is fine-tuned over the 65 MHz free spectral range of the cavity (limited by its 2.3 m length) using a piezoelectric transducer (PZT), and the zeroth-order beam reflected from the grating is focused into a CO2 spectrum analyzer to identify the laser line.The FIR laser is pumped coaxially by the COz laser beam, which circulates between the FIR laser end mirrors after expanding through a 4 mm hole in the input coupler. The 2.5 m cavity of the FIR laser is of the dielectric waveguide design, with planar gold-coated copper end mirrors. FIR power is coupled out through a lo-mm-diam hole in the end mirror, which is backed by a hybrid quartz/ dielectric mirror to reflect the pump beam, while transmitting the FIR output. The output beam of the FIR laser then enters a Martin-Puplett polarizing diplexer, which couples the laser radiation onto the (Schottky diode) comer cube mixer, while s...

The state of the art in far infrared (FIR) spectroscopy is reviewed. The development of tunable, coherent FIR radiation sources is discussed. Applications of tunable FIR laser spectrometers for measurement of rotational spectra and dipole moments of molecular ions and free radicals, vibration-rotation-tunneling (VRT) spectra of weakly bound complexes, and vibration-rotation spectra of linear carbon clusters are presented. A detailed description of the Berkeley tunable FIR laser spectrometers is presented in the following article.

We search for an infrared signature of the transiting extrasolar planet HD 209458b during secondary eclipse. Our method, which we call 'occultation spectroscopy,' searches for the disappearance and reappearance of weak spectral features due to the exoplanet as it passes behind the star and later reappears. We argue that at the longest infrared wavelengths, this technique becomes preferable to conventional 'transit spectroscopy'. We observed the system in the wing -2of the strong ν 3 band of methane near 3.6 µm during two secondary eclipses, using the VLT/ISAAC spectrometer at a spectral resolution of 3300. Our analysis, which utilizes a model template spectrum, achieves sufficient precision to expect detection of the spectral structure predicted by an irradiated, low-opacity (cloudless), low-albedo, thermochemical equilibrium model for the exoplanet atmosphere. However, our observations show no evidence for the presence of this spectrum from the exoplanet, with the statistical significance of the non-detection depending on the timing of the secondary eclipse, which depends on the assumed value for the orbital eccentricity. Our results reject certain specific models of the atmosphere of HD 209458b as inconsistent with our observations at the 3σ level, given assumptions about the stellar and planetary parameters.