Cavity ring-down spectroscopy has been used to measure the absorbance
of methyl radicals near 216 nm. The methyl radicals are generated by a
hot tungsten filament heated to 2300 K in a mixture of 0.5% CH4 in H-
2 slowly flowing through the reactor at 20 Torr total pressure. CH3 ab
sorbances are detected with a noise-equivalent sensitivity of two part
s in 10(5) using a narrow pencil of UV light 0.5 mm in diameter, which
allows measurement of spatial profiles of CH3 for column densities of
3 x 10(12) radicals/cm(2) (3 x 10(12) radicals/cm(3) x 1 cm absorptio
n pathlength).