Relaxation Dynamics of Polyatomic Molecules Studied by Site-Selective Excitation of K-Shell Electrons

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One of the attractive topics in inner-shell excited polyatomic molecules has been a site-selective bond dissociation or "photon-scissors" to cut a specific chemical bond following Auger electron emission. Typical decay time of the Auger process is in the order of 10^<-15> s is shorter than the periods of molecular vibrations and rotations in 10^<-14>-10^<-11> s. Thus, the bond dissociation initiated by the Auger process is expected to occur in a limited area of the molecule. However, the site-specific reaction has not been always observed decidedly in gas phase molecules. In the present work, we measured that the site-dependent fragmentation competes with intramolecularenergy relaxation in the C, F and/or O K-shell excited CF_3COCH_3,CF_3CCH and CF_2CH_2. Then we showed that the intramolecular energy relaxation observed makes the site-selectivity vague. The effective distance in a molecule for the energy relaxation was estimated to be about 1 nm (10^<-9> m) by changing molecular lengths and by selectively exciting the N and O K-shell electrons in a series of CH_3CO(CH_2)_nCN(n=0-4).We observed that the C and F K-shell excitation in CF_3SF_5, which has been found for the first time in 1999 to be a strong greenhouse gas, results in a small site-specific photofragmentation. The S1s electron excitation at hν=2484 eV gives a huge site-selectivity, probably being due to a cascade Auger electron emission in L-shells. This finding suggests that an atom in the third row of the periodical table would effectively work for the site-specific fragmentation in the K-shell excited polyatomic molecule.