物理化学学报 >> 1987, Vol. 3 >> Issue (03): 229-236.doi: 10.3866/PKU.WHXB19870303

研究论文 上一篇    下一篇

VA族元素氢化物的紫外光解和多光子电离
Ⅰ. NH3分子紫外激光光解

倪图强; 俞书勤; 马兴孝; 孔繁敖   

  1. 中国科学技术大学近代化学系, 合肥
  • 收稿日期:1985-11-04 修回日期:1986-04-05 发布日期:1987-06-15
  • 通讯作者: 孔繁敖

THE UV PHOTOLYSIS AND THE MULTIPHOTON IONIZATION OF HYDRIDES OF GROUP VA ELEMENTS
I. UV LASER PHOTOLYSIS OF NH3

Ni Tuqiang; Yu Shuqin; Ma Xingxiao; Kong Fanao   

  1. Department of Modern Chemistry; University of Science & Technology of China; Hefei; Anhui
  • Received:1985-11-04 Revised:1986-04-05 Published:1987-06-15
  • Contact: Kong Fanao

摘要: 在光强为10~6 W cm~(-2)的ArF激光作用下, NH_3分子被光解为NH(A~(3Π))先后吸收两个193 nm光予, 经过NH_3(Ã ~1A_2″)和NH_2( Ã ~2A_1)两个中间物的过程。NH(A~(3Π)→X~(3∑~-))的发射光谱表明, 该产物具有7700 K的转动高温。此系NH_2在激发过程中剧烈的变角效应所致。
我们利用高功率的紫外激光器及时间分辩的测量仪器, 系统地研究了这些分子的光谱能级, 反应、光解机理和电离过程等, 显然有重要的意义。

Abstract: The NH(A ~3Π)→X ~3∑~-)) fluorescence spectrum, of 331-343 nm, wavelength region was recorded when NH_3 was irradiated by the 193 nm ArF laser beam(Fig.1). The ratio of the intensity I_f of Q(0, 0) to that of Q(1, 1) suggests that 5% of population of NH(A ~3Π) is in the first vibrational state (v′=1). For the ro- tational lines of the P branch, a diagram of In I_f/(2N′+1) vs. Be N′(N′+1) is plotted in Fig.2, in which I_f corresponds to the relative intensities of the ro- tational lines of the P branch and N′ is the rotational quantum number of NH(A ~3Π, v′=0). NH(A ~3Π) is highly rotationally excited in the firt 12 levels with a Boltzmann temperature of T_R=7700 K.
A single-exponential decay of the NH(A ~3Π→X ~3Σ~-) emission was found and a lifetime of (470±40) ns was obtained. The Sterm-Volmer plots are shown in Fig.3. The quenching rate coefficients of NH(A ~3Π) by various gases were measured from the slopes as: k_(H_2)=(4.8±0.5)×10~(-11) cm~3 molec~(-1) s~(-1), k_(NO)=(7.3±1.0)×10~(-11) cm~3 molec~(-1) s~(-1), k_(CH_4)=(6.6±1.5)×10~(-11) cm~3 molec~(-1) s~(-1), and k_(O_2)=(2.9±0.2)×10~(-11) cm~3 molec~(-1) s~(-1). The quenching rate coeffecient of Ar gas is of order of 10~(-13) cm~3 molec~(-1) s~(-1).
The first 193 nm photon is absorbed by NH_3(X ~1A_1′) and makes it to NH_3(Ã). The predissociation of NH_3(Ã ~1A_2″) is very rapid. The nascent NH_2(Ã ~1A_2″) can further absorb a second 193 nm photon to the NH_3(B ~2B_2) state. The bond angle changes tremendously from 141.4° to 50° for a above process. The fragment NH(A ~3Π) species then possesses a high rotational excitation population which agrees the experimental temperature of T_R=7700 K.