Effect of antisymmetric C-H stretching excitation on the dynamics of O(1D) + CH4 → OH + CH3.

The effect of antisymmetric C-H stretching excitation of CH₄ on the dynamics and reactivity of the O(¹D) + CH₄ → OH + CD₃ reaction at the collision energy of 6.10 kcal/mol has been investigated using the crossed-beam and time-sliced velocity map imaging techniques. The antisymmetric C-H stretching mode excited CH₄ molecule was prepared by direct infrared excitation. From the measured images of the CH₃ products with the infrared laser on and off, the product translational energy and angular distributions were derived for both the ground and vibrationally excited reactions. Experimental results show that the vibrational energy of the antisymmetric stretching excited CH₄ reagent is channeled exclusively into the vibrational energy of the OH co-products and, hence, the OH products from the excited-state reaction are about one vibrational quantum hotter than those from the ground-state reaction, and the product angular distributions are barely affected by the vibrational excitation of the CH₄ reagent. The reactivity was found to be suppressed by the antisymmetric stretching excitation of CH₄ for all observed CH₃ vibrational states. The degree of suppression is different for different CH₃ vibrational states: the suppression is about 40%-60% for the ground state and the umbrella mode excited CH₃ products, while for the CH₃ products with one quantum symmetric stretching mode excitation, the suppression is much less pronounced. In consequence, the vibrational state distribution of the CH₃ product from the excited-state reaction is considerably different from that of the ground-state reaction. [ABSTRACT FROM AUTHOR]