Measurements of the ethyl radical -CH2 out-of-plane rocking vibrational fun
damental by transient diode laser absorption spectroscopy are reported. The
new data provide information on the structure of, and the barrier to inter
nal rotation in, the radical, and how these quantities change on vibrationa
l excitation. We find that the effective barrier decreases from approximate
ly 17 cm(-1) in the zero point level to 10 cm(-1) in the excited vibrationa
l level. The assigned data set now contains approximately 450 rotation-tors
ional transitions and has been fit to a model effective Hamiltonian. The de
rived molecular parameters generally reproduce the measured line positions
to better than 0.01 cm(-1), but this does not approach the estimated measur
ement accuracy of 0.001-0.002 cm(-1). (C) 1999 American Institute of Physic
s. [S0021-9606(99)00144-0].