Journal of Physical Chemistry A, Vol.123, No.36, 7758-7767, 2019
Observation of Rainbows in the Rotationally Inelastic Scattering of NO with CH4
Using a combination of velocity-map imaging and resonance-enhanced multiphoton ionization detection with crossed molecular beam scattering, the dynamics of rotational energy transfer have been examined for NO in collisions with CH4 at a mean collision energy of 700 cm(-1). The images of NO scattered into individual rotational (j(NO)')and spin-orbit (Omega) levels typically exhibit a single broad maximum that gradually shifts from the forward to the backward scattering direction with increasing rotational excitation (i.e., larger Delta j(NO)). The rotational rainbow angles calculated with a two-dimensional hard ellipse model show reasonable agreement with the observed angles corresponding to the maxima in the differential cross sections extracted from the images for higher Delta j(NO) transitions, but there are clear discrepancies for lower Delta j(NO) (in particular, final rotational levels with j(NO)' = 7.5 and 8.5). The sharply forward scattered angular distributions for these lower Delta j(NO) transitions better agree with the predictions of an L-type rainbow model. The more highly rotationally excited NO appears to coincide with low rotational excitation of the co-product CH4, indicating a degree of rotational product-pair anticorrelation in this bimolecular scattering.