Abstract

Multiphoton ionization and fragmentation of acetone are studied with a diffuse molecular beam and aXeCl excimer laser. The present experiment shows that no parent ion peak can be observed, only two ion peaks appear at m/e = 43 and 15 which are assigned to CH₃CO⁺ and ${\text{CH}}_3^{+}$, respectively. The intensities of these ions are proportional to the 3rd and 3.6th power of laser intensity, respectively, and both are proportional to the 0.9th power of the acetone pressure. On the basis of our experimental results, a novel kinetic mechanism is presented, which takes account of the fact that acetone fails to be excited by the two-photon transition of $\tilde{D}\leftarrow {\tilde{X}}^1{A}_1$ at 308 nm. Instead it undergoes single-photon dissociation into neutral fragments of methyl and acetyl, because the serious leakages from the intermediate state ${\tilde{A}}^3{A}_2$ makes it impossible for acetone to climb the second step of the multiphoton transition ladder, resulting in the absence of the parent ion at 308 nm. Ch₃CO⁺ is formed by direct two-photon resonant ionization of acetyl radical, and ${\text{CH}}_3^{+}$ is produced by simultaneous two-photon absorption of methyl radical to $\tilde{C}$ state followed by one-photon absorption into the ionization continuum.

© 1986 Optical Society of America