Bahng Mi-Kyung, Xing Xi, Baek Sun Jong, Qian Ximei, Ng C Y
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, USA.
J Phys Chem A. 2006 Jul 13;110(27):8488-96. doi: 10.1021/jp054672t.
The synchrotron based vacuum ultraviolet-pulsed field ionization-photoelectron (VUV-PFI-PE) spectrum of ammonia (NH(3)) has been measured in the energy range 10.12-12.12 eV using a room-temperature NH(3) sample. In addition to extending the VUV-PFI-PE measurement to include the v(2)(+) = 0, 10, 11, 12, and 13 and the v(1)(+) + nv(2)(+) (n = 4-9) vibrational bands, the present study also reveals photoionization transition line strengths for higher rotational levels of NH(3), which were not examined in previous PFI-PE studies. Here, v(1)(+) and v(2)(+) represent the N-H symmetric stretching and inversion vibrational modes of the ammonia cation (NH(3)(+)), respectively. The relative PFI-PE band intensities for NH(3)(+)(v(2)(+)=0-13) are found to be in general agreement with the calculated Franck-Condon factors. However, rotational simulation indicates that rotational photoionization transitions of the P-branches, particularly those for the lower v(2)(+) PFI-PE bands, are strongly enhanced by forced rotational autoionization. For the synchrotron based VUV-PFI-PE spectrum of the origin band of NH(3)(+), rotational transition intensities of the P-branch are overwhelming compared to those of other rotational branches. Similar to that observed for the nv(2)(+) (n = 0-13) levels, the v(1)(+) + nv(2)(+) (n = 4-9) levels are found to have a positive anharmonicity constant; i.e., the vibrational spacing increases as n is increased. The VUV laser PFI-PE measurement of the origin band has also been made using a supersonically cooled NH(3) sample. The analysis of this band has allowed the direct determination of the ionization energy of NH(3) as 82158.2 +/- 1.0 cm(-1), which is in good accord with the previous PFI-PE and photoionization efficiency measurements. Using the known nd(v(2)(+)=1,1(0)<--0(0)) Rydberg series of NH(3) as an example, we have demonstrated a valuable method based on two-color infrared-VUV-photoion depletion measurements for determining the rotational character of autoionizing Rydberg states.
利用室温氨（NH₃）样品，在10.12 - 12.12电子伏特的能量范围内测量了氨（NH₃）基于同步加速器的真空紫外脉冲场电离光电子（VUV - PFI - PE）光谱。除了将VUV - PFI - PE测量扩展到包括v₂⁺ = 0、10、11、12和13以及v₁⁺ + nv₂⁺（n = 4 - 9）振动带外，本研究还揭示了NH₃较高转动能级的光电离跃迁线强度，这在以前的PFI - PE研究中未被研究。这里，v₁⁺和v₂⁺分别代表氨阳离子（NH₃⁺）的N - H对称伸缩振动和反转振动模式。发现NH₃⁺（v₂⁺ = 0 - 13）的相对PFI - PE带强度与计算的弗兰克 - 康登因子总体一致。然而，转动模拟表明，P支的转动光电离跃迁，特别是较低v₂⁺ PFI - PE带的那些跃迁，因强迫转动自电离而强烈增强。对于NH₃⁺基态带的基于同步加速器的VUV - PFI - PE光谱，P支的转动跃迁强度与其他转动支相比占主导地位。与在nv₂⁺（n = 0 - 13）能级观察到的情况类似，发现v₁⁺ + nv₂⁺（n = 4 - 9）能级具有正的非谐常数；即振动间距随n增加而增大。还使用超声冷却的NH₃样品对基态带进行了VUV激光PFI - PE测量。对该带的分析使得能够直接确定NH₃的电离能为82158.2±1.0厘米⁻¹，这与先前的PFI - PE和光电离效率测量结果非常吻合。以已知的NH₃的nd（v₂⁺ = 1,1(0)←0(0)）里德堡系列为例，我们展示了一种基于双色红外 - VUV - 光离子耗尽测量来确定自电离里德堡态转动特性的有价值方法。
