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MICHAEL L. HAUSE, TREVOR J. SEARS AND GREGORY E. HALL, Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000.
We examine the collisional kinetics of the CN radical using transient hole-burning and
saturation recovery. Narrow velocity groups of individual hyperfine levels in CN are depleted
(X2
+) and excited (A2
) with a saturation laser, and probed by a
counterpropagating, frequency modulated probe beam. Recovery of the unsaturated absorption is
recorded following abrupt termination of an electro optically switched pulse of saturation
light. Pressure-dependent recovery kinetics are measured for precursors, ethane dinitrile,
NCCN, and pyruvonitrile, CH3COCN, and buffer gases, helium, argon and nitrogen with rate
coefficients ranging from 0.7-2.0 x 10-9 cm3 s-1 molec-1. In the case of
NCCN, recovery kinetics are for two-level saturation resonances, where the signal observed is
a combination of X- and A-state kinetics. Similar rates occur for three-level crossover
resonances, which can be chosen to probe selectively the hole-filling in the X state or the
decay of velocity-selected A state radicals. However in the case of CH3COCN, which has a
dipole moment of 3.45 D, the X-state kinetics are faster than the A-state due to an
efficient dipole-dipole rotational energy transfer mechanism as the X-state dipole moment
is 1.5 D and the A-state dipole moment is 0.06 D. The observed recovery rates are 2-3 times
faster than the estimated rotationally inelastic contribution and are a combination of
inelastic and velocity-changing elastic collisions.
Acknowledgement: This work was carried out under Contract No. DE-AC02-98CH10886 with the U.S.
Department of Energy.