We observed Pluto and Triton with the CSHELL echelle spectrograph on the In
frared Telescope Facility in April and July 1996, in an effort to detect th
e R(2), R(3), and R(4) rotational lines of the 2-0 vibrational transition o
f gaseous CO. As no lines were detected, we derived 3-sigma upper limits on
the average widths of these three lines of 0.040 cm(-1) for Pluto and 0.02
8 cm(-1) for Triton. The corresponding upper limits on the gaseous CO mole
fractions depend on the assumed profiles of temperature and pressure in the
atmospheres of these bodies. If Triton's atmosphere in 1996 resembles that
measured by stellar occultation in 1997, we find a 3-sigma upper limit to
the CO mole fraction of 59%. If Pluto's atmosphere resembles the tropospher
ic model of J. A. Stansberry, J. I. Lunine, W. B. Hubbard, R. V. Yelle, and
D. M. Hunten (1994), Icarus 11, 503-513, we find a 3-sigma upper limit to
the CO mole fraction of 6%. For Pluto, this limit to the gaseous mole fract
ion argues against intimate mixtures (e.g., "salt-and-pepper" mixtures, as
opposed to solid solutions) of surface CO and N-2 frost. sigma 2001 Academi
c Press.