N2O分子C1Π态的吸收光谱及解离动力学

Absorption Spectrum and Dissociation Dynamics of the C^1Π State of N_2O

通过四波混频差频的方法产生高分辨的真空紫外激光,用以测量143.6至146.9 nm范围内的射流冷却N2O分子吸收光谱,对应于C1Π←X1Σ+的吸收跃迁.谱图显示出三个分立的振动谱峰叠加在宽吸收背景上,谱峰间隔分别是521和482 cm-1.前人的高精度量子化学计算表明C1Π态在N—O键长方向表现为无势垒的排斥态,而在N—N键伸缩及N2O弯曲振动方向则表现为束缚态,因此观测到的振动谱峰被归属为激发态的Feshbach共振.通过反Fourier变换可以得到Feshbach共振对应的非稳定周期轨道的特征周期为61 fs,相应的振动频率为546 cm-1.鉴于这一频率与弯曲振动频率非常接近,非稳定周期轨道被认为是由C1Π态的弯曲振动与解离运动相互作用而形成的,N—N伸缩振动没有参与形成非稳定周期轨道.由此,N2O分子C1Π态光激发-解离过程得以清晰地阐述.

The absorption spectrum of the N2O molecule in the wavelength range of 143.6-146.9 nm was measured under the jet-cooled condition by using the high resolution vacuum ultraviolet radiation,which was generated by resonance-enhanced difference-frequency mixing,corresponding to the C1Π←X1Σ＋ transition.Three vibrational bands were observed with frequency intervals of 521 and 482 cm-1,and they were superimposed on a wide absorption background.Previous high-level quantum chemical calculations indicate that the C1Π state of N2O is dissociative along the N—O elongation,while it is bound along the N—N bond stretching or N2O bending.Therefore,the observed vibrational progression is a Feshbach resonance of the dissociative transition state.From an anti-Fourier transformation analysis,the recurrence period of the unstable periodic orbit of the Feshbach resonance was found to be 61 fs and the corresponding vibrational frequency was 546 cm-1.Since this vibrational frequency is close to the frequency of the bending motion,the unstable periodic orbit is mostly composed of the bending motion of the C1Π state coupled with dissociation.The N—N stretching vibration does not participate in its formation.Therefore,we describe the excitation-dissociation dynamics of the C1Π state of N2O clearly.