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The excitation of the umbrella mode of CH3 and CD3 formed from photodissociation of CH3I and CD3I at 248 nm

1984; Elsevier BV; Volume: 87; Issue: 3 Linguagem: Inglês

10.1016/0301-0104(84)85121-6

1873-4421

G. N. A. van Veen, T. Baller, A.E. De Vries, N.J.A. Van Veen,

Atmospheric Ozone and Climate

Molecular beams of CH3I and CD3I are photodissociated by 248 nm light. Dissociation takes place into CH3 (CD3) and II = I(2P32)] or I*[= I(2P12)]. The quantum yields for I* formation are respectively 0.71 and 0.81. The CH3 and CD3 fragments are found to be vibrationally excited in the umbrella mode. Both distributions for CH3 peak at υ = 2. The distribution in the I* channel equals that at 266 nm, found by other authors, in sharp contrast with earlier theoretical predictions by Shapiro and Bersohn. The distribution for CH3+I is broader and contains vibrations at least up to υ = 7. In the case of CD3I the vibrational distribution of the CD3 fragments peaks at υ = 3 for the I* channel and around υ = 5 in the I channel. Simple models are developed which give a fair description of the excitation of the umbrella mode of the CH3 and the CD3 radicals in both the I and I* channels. The angular distributions of the CH3 and CD3 fragments exhibit a predominantly parallel character in both channels, the I signal being slightly less anisotropic than the I* distribution. The anisotropy parameters for the CH3 fragments are β(I) = 0.72±0.02 and β(I*) = 0.76±0.02. For the CD3 fragments β(I) = 0.77±0.07 and β(I*) = 0.83±0.04. The main absorption is due to the 3Q0 ← 1A1 transition. The I signal is almost exclusively due to curve crossing between the 3Q0 and the 1Q states. Additionally, accurate values for the CI bond strength in CH3I and CD3I are obtained. These values are respectively 2.30±0.01 and 2.33±0.01 eV.