  Two color laser pulses are used to form an air plasma and generate broadband
infrared radiation suitable as a seed for backward Raman amplification of CO2
laser pulses. Broadband radiation in the atmospheric window from 8-14 microns
is observed. The infrared radiation is characterized using a long wavelength
grating spectrometer specially designed to accept an ionizing laser filament at
its input plane. The LWIR yield is greatly enhanced by chirping the drive
pulse. Unidirectional pulse propagation simulations suggest that this is due in
part to the dependence of the nonlinear refractive index on the pulse duration.
