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

在内壳激发的多原子分子中，一个有吸引力的话题是在俄歇电子发射后，选择性键解离或“光子剪刀”切断特定的化学键。俄歇过程的典型衰变时间为10^<-15 >0 s，比分子振动和旋转的10^<-14>-10^<-11> s短。因此，由俄歇过程引发的键解离预计发生在分子的有限区域内。然而，在气相分子中，位点特异性反应并不总是被明确地观察到。在本工作中，我们测量了在C、F和/或O k壳层激发的CF_3COCH_3、CF_3CCH和CF_2CH_2中，位点依赖的碎片化与分子内能量弛豫相竞争。然后我们发现观察到的分子内能量弛豫使得位点选择性模糊。通过改变分子长度和选择性地激发一系列CH_3CO(CH_2)_nCN（N =0-4）中的N和O k壳层电子，估计分子中能量弛豫的有效距离约为1 nm （10^<-9> m）。我们观察到，1999年首次发现的CF_3SF_5是一种强温室气体，其C和F k壳激发导致了小的位点特异性光破碎。在hν=2484 eV处，S1s电子激发具有很大的选择性，这可能是由于l壳层的级联俄格电子发射所致。这一发现表明，在k壳激发的多原子分子中，元素周期表第三行中的一个原子可以有效地为特定位点的断裂工作。

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.