转动分辨的NH3X→C′→Ã跃迁的离子凹陷光谱

NH_3Ã(~1A_2~″)是一个快速预解离态, 以往的光谱研究从未得到它的转动分辨光谱。我们用两个脉冲染料激光器研究了NH_3C′→X的3+1多光子电离光谱和NH_3X→C′→Ã的折叠光学双共振多光子电离光谱, 称为“离子凹陷光谱”。对前者观察到了较高分辨的转动光谱, 得出C′v_2~′=O的高精度转动常数。通过谱线宽度, 得到了C′v_2~′=0,1,2能级的预解离寿命。从离子凹陷光谱上, 首次观察到了C′→Ã跃迁的0-0, 1-1, 2-2等带的转动分辨光谱。由双共振选择定则简化的光谱使漫散至～50 cm~(-1)线宽的转动线得以分开, 由此得到Ãv_2~″=0,1,2能级的转动常数和各谱带的带原点。在消除功率加宽的情况下, 测出Ãv_2~′=1能级的寿命。这种方法对研究分子的快速预解离态有一定的普遍意义。

ROTATIONALLY RESOLVED ION DIP SPECTROSCOPY OF NH3X→C'→Ã TRANSITIONS

Ion dip spectroscopy, a folded OODR process with ion detection, has hitherto only been applied to the study of high-lying vibrational levels of ground state polyatomics. We have succeeded in employing this technique to obtain rotationally resolved spectra of NH_3Ã_1A_2″, a fast predissociating species. Conceivably, the method can be generalized to deal with a large number of other predissociating species. We first obtain high resolution REMPI spectra of C′←X transition via a 3+1 photon scheme by scanning a pulsed dye laser in the 430—480 nm. A sample spectrum of the 0-0 band is shown in Fig.1. Rotational constants B_0=10.80±0.01 cm~(-1) and C_0=5.23±0.01 cm~(-1) are obtained for the C′~A_1′v_2=0 level predissociation lifetimes are inferred from the rotational linewidth for the C′v_2′=0,1,2 levels. We then induce the C′→Ã stimulated emission by scanning a second dye laser from 560—580 nm while holding the first laser to a specific C′←X rovibronic transition. The competition between C′→A and the ionization process from C′ state results in the ion dip spectra shown in Fig.2. The rotational lines of C′→A 0-0, 1-1 and 2-2 bands are listed in Table 3. While selection rules allow Δv_2=0,±2,±4... transition, Franck-Condon factor favors Δv_2=0 only.....