摘要
利用脉宽为25ns的脉冲Nd∶YAG1064nm的激光,在1010～1011W·cm-2的强度下,用飞行时间质谱对乙腈分子束激光电离过程进行了研究。实验中利用氦气作为载气,乙腈分子束采用扩散和脉冲两种方式进样,当扩散束进样时,乙腈分子的谱峰可用多光子电离来解释,而当脉冲分子束进样时,实验中产生了外层价电子高剥离的Nq+(q=2～5)和Cq+(q=1～4)信号谱。实验中同时发现,激光波长增大时,高价离子的谱强度也趋向于增大。激光延迟及谱峰的相关性分析表明,这些高价离子可能来源于乙腈团簇的库仑爆炸过程。提出一个多光子电离引发,逆韧致吸收加热-电子碰撞电离模型来解释高价离子的产生,对高价离子伴随的波长效应也给出了较为合理的解释。
To further investigate the relation between cluster and multi-charged ions, the photo-ionization of acetonitrite 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 · 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, muhi-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 O. 75 ms delay time. When the wavelength of laser was changed from 532 nm to 1 064 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 simuhaneously. 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
出处
《光谱学与光谱分析》
SCIE
EI
CAS
CSCD
北大核心
2008年第10期2218-2223,共6页
Spectroscopy and Spectral Analysis
基金
国家自然科学基金项目(20573111)
国家"863"课题项目(2002AA649010
2001AA640302)资助
关键词
光谱学
乙腈团簇
库仑爆炸
高价离子
波长效应
Spectroscopy
Acetonitrile cluster
Coulomb explosion
Multi-charged ion
Wavelength dependence
