The photodissociation of azomethane following absorption of a single 3
51 nm photon was studied using the method of molecular beam photofragm
ent translational spectroscopy. The dissociation was observed to proce
ed via cleavage of both C-N bonds to yield N2 and two methyl radicals.
The measured time-of-flight spectra show evidence that the two methyl
radicals possess unequal velocities in the azomethane center of mass
suggesting that the dissociation is not symmetric. The angles between
the asymptotic center-of-mass velocities for all three fragments are s
trongly correlated, implying that the methyldiazenyl radical (CH3N2) i
ntermediate decomposes within a fraction of its rotational period. We
conclude, therefore, that the dissociation is concerted, not stepwise
as was inferred from recent time-resolved experiments. The overall tra
nslational energy distributions for all the photofragments in the azom
ethane center of mass reveal that an average of 60% of the total avail
able energy appears as translation. A possible mechanism, consistent w
ith the experimental findings, will be proposed and discussed.