DISSOCIATION DYNAMICS OF ACETYLENE RYDBERG STATES AS A FUNCTION OF EXCITED-STATE LIFETIME

The state selective photodissociation of acetylene, C2H2/C2D2, was stu died in the wavelength range 121.2-132.2 nm by high resolution Rydberg atom time-of-flight measurements on the atomic fragment H/D. In the w avelength region studied members of all four Rydberg series and the hi ghly excited (E) over tilde valence state were state selectively excit ed using tunable vacuum-ultraviolet laser radiation. The lifetime of t he excited states which were studied varied from 58 fs to more than 2 ps. Formation of the ethynyl radical in its (X) over tilde electronic ground state and its first electronically excited (A) over tilde state is observed with practically no indication of (B) over tilde state fr agments. Two decay channels with different dissociation dynamics were also observed. In both channels the observed decay dynamics depended s trongly on the excited state of the parent molecule. Further there are major differences between these two dissociation pathways with respec t to the measured internal energy and angular distributions. In one ch annel the dissociation is dominated by dynamical effects and the C2H f ragments are formed with a high degree of vibrational excitation. In c ontrast to this in the second channel a smooth internal energy distrib ution is observed indicating that the fragment quantum state distribut ion is spread over a considerable range of the available phase space. Moreover, this second channel can be fit with a phase space model cons trained only by conservation of energy and angular momentum. This is f urther evidence for the randomization of internal energy during the di ssociation process. (C) 1998 American Institute of Physics. [S0021-960 6(98)00137-8].