Research on cluster-assistant multiple ionization of acetonitrile by intense nanosecond laser

Abstract

To further investigate the relation between cluster and multi-charged ions, the photo-ionization of acetonitrile molecular beam carried by helium was studied by time-of-flight mass spectrometry using 25 ns, 532 nm and 1 064 nm Nd-YAG laser with an intensity of 10(10) W x cm(-2). When the effused beam of CH3CN at a pressure of about 11 kPa was irradiated by a 532 nm laser, only a few mono-charged ions appeared in the mass spectrum and were clearly resolved, While the pulsed molecular beam seeded in 0.15 MPa He, multi-charged species appeared between H+ and C+, and parent ions containing one or more acetonitrile molecules were also discovered. The intrinsical difference between effused beam and pulsed one was that for the latter it was easier to generate molecule clusters. Adjusting delay time between laser and pulsed valve to shine the laser on different potions of pulsed beam, the change tendency of integral intensity of different multi-charged ions was found to remain similar to that of acetonitrile cluster, and the highest intensity of different ion species appeared at the same 0.75 ms delay time. When the wavelength of laser was changed from 532 nm to 1064 nm, the type of ions and their proportion from effused beam were alike except intensity, but the result of pulsed beam altered obviously. Multi-charged ions showed peak splitting, which characters CE (Coulomb explode). As a powerful tool to understand some chemical physics processes involved, correlation analysis of the time of flight mass spectrum was used. The spectrum of 532 nm was selected owing to its multi-charged ions distinguished easily. The correlations among N(n+) (n = 2-5) and C(n+) (n = 2-4) were all above 0.25, with some even above 0.7, indicating that these ions are highly correlated and probably come from the CE events simultaneously. An electron re-scattering and re-colliding ionization model was proposed to explain the appearances of those multiple charged ions under such low laser intensity. From the theory, the longer wavelength facilitates the energy absorption rate during inverse bremsstrahlung, which leads to the resulting wavelength dependence of the multi-charged atomic ions.