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This animation replays a series of FM spectra that show collisional
cooling of CN photofragments produced in the 248 nm photodissociation
of ICN. The displayed data measure the speeds, angular distributions,
and angular momentum polarization of one particular rotational state of
CN. The top panel is a sequence of raw FM spectra, drawn at increasing
times after the photodissociation, from 0 to 50 microseconds. The
broader, more structured spectra were observed immediately after pulsed
laser photolysis. Collisions of the initially fast and strongly
polarized CN radicals gradually slow down and randomize the motion of
the CN radicals, leading to a narrower and less structured spectrum at
later times. The lower panel shows the absorption line shape derived
from the FM data. These typical, low noise data were acquired in a
single frequency scan of the Ti:sapphire laser, using 20 shot averaging
of the excimer photolysis laser, and required less than 5 minutes to
record.
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