  L-band spectroscopy is a powerful probe of cool low-gravity atmospheres: The
P, Q, and R branch fundamental transitions of methane near 3.3 $\mu$m provide a
sensitive probe of carbon chemistry; cloud thickness modifies the spectral
slope across the band; and H$_{3}^{+}$ opacity can be used to detect aurorae.
Many directly imaged gas-giant companions to nearby young stars exhibit L-band
fluxes distinct from the field population of brown dwarfs at the same effective
temperature. Here we describe commissioning the L-band spectroscopic mode of
Clio2, the 1-5 $\mu$m instrument behind the Magellan adaptive-optics system. We
use this system to measure L-band spectra of directly imaged companions. Our
spectra are generally consistent with the parameters derived from previous
near-infrared spectra for these late M to early L type objects. Therefore,
deviations from the field sequence are constrained to occur below 1500 K. This
range includes the L-T transition for field objects and suggests that observed
discrepancies are due to differences in cloud structure and CO/CH$_{4}$
chemistry.

Solar and Stellar Astrophysics