Dr. Lacy has constructed an echelle spectrograph which is an order of magnitude more sensitive than other available instruments for spectroscopy of moderate to high resolution at mid-infrared wavelengths. With this grant he will continue his pioneering work in this wavelength region of the spectrum. Three projects will be emphasized: 1) He will investigate the relative importance of gas- phase and grain-surface reactions on the composition of molecular clouds by observing absorption of various molecules in the gaseous and solid phases; 2) He will study the pure rotational line emission of molecular hydrogen, to learn about the conditions in shocked gas and photodissociation regions, and to determine the ratio of hydrogen to carbon monoxide; and 3) He will observe the mid-infrared emission lines of ions of neon and sulfur to study the kinematics and excitation of infrared luminous galaxies. There are two principal advantages to the mid-infrared spectral region for astronomical spectroscopy: the small interstellar extinction relative to that at shorter wavelengths, and the presence of several interesting spectral features, notably vibrational transitions of molecules and fine-structure lines of ions. Many objects which are unobservable, because of extinction, at visible or near-infrared wavelengths become accessible in the mid-infrared. Spectral lines of molecules and ions provide valuable information about the kinematics and physical conditions of the newly revealed objects. Dr. Lacy's spectrograph represents a significant advance in the quality of instrumentation for exploring objects at mid-infrared wavelengths.
