The Far-Infrared Spectrum of Deuterium Iodide.

We recently described an investigation of the far-infrared (FIR) spectrum of hydrogen iodide, HI, which resulted in an accurate set of rotational and I electric quadrupole parameters for the molecule in its XS (v 5 0) state (1). In this Note, we report an extension of this work to the deuterated form of the molecule, DI. The spectrum of deuterium iodide was recorded using the third-order tunable far-infrared (TuFIR) spectrometer at the National Institute of Standards and Technology. This spectrometer has been described in detail elsewhere (2). Gaseous DI was generated in a 1.1-m silica discharge tube with an outside diameter of 22 mm. A wire electrode was located 6 cm from each end of the tube. Small amounts of solid iodine were placed in the bottom of the tube. Deuterium gas was slowly flowed through the tube at a pressure of 100 Pa (0.8 Torr), and a d.c. positivecolumn electric discharge was ignited and maintained. A liquid-nitrogen trap prevented iodine vapor from reaching the vacuum pump. The apparatus required frequent reloading with iodine during operation. To record the absorption spectrum of DI, far-infrared radiation was passed down the discharge tube and onto a liquid-helium–cooled bolometer. The FIR radiation was frequency modulated and a lock-in amplifier was used to demodulate the absorption signals and generate first-derivative spectra. A typical spectrum of DI is shown in Fig. 1. The digitally recorded lineshapes were fitted by the method of least squares using an equation employing five adjustable parameters: the transition frequency and intensity, the Gaussian and Lorentzian linewidths, and the spectrometer baseline. The rotational transitions we recorded for DI cover the range J0 5 2–10 and are listed in Table 1. These transitions display hyperfine structure arising from the I (I 5 5/2; 100% natural abundance) electricquadrupole interaction. The strong DF 5 DJ 5 11 hyperfine components are severely overlapped for all but the lowest (J 5 3–2) rotational transition we measured; above this transition, we recorded only the weaker, but well-resolved DF 5 0 hyperfine components. However, the frequencies of several DF 5 61 and 0 unblended hyperfine components of the J 5 1– 0 and 2–1 rotational lines have been reported by Gordy and coworkers (3, 4), so we enlarged our data set to include these earlier measurements. We then fitted the combined set by the method of least squares with a parametrized Hamiltonian appropriate for a diatomic molecule in a S state with quadrupole structure (5). During each iteration, the Hamiltonian matrix for each value of F was generated and diagonalized.