Wells Kym L, Perriam Gareth, Stavros Vasilios G
Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, United Kingdom.
J Chem Phys. 2009 Feb 21;130(7):074308. doi: 10.1063/1.3072763.
Following excitation of the A state nu(2) (')=4 mode in ammonia, we show how the time scale to dissociation of the N-H bond depends on the internal energy imparted to the NH(2) photofragment. Using a combination of femtosecond pump/probe spectroscopy and velocity map ion imaging techniques, the time and energy resolved H-atom elimination can be directly related to the nonadiabatic nature of the photodissociation for high kinetic energy H atoms with evidence for adiabatic dynamics to dissociation giving the lowest energy H atoms. Extrapolation of the time scales for dissociation versus internal energy of the NH(2) photofragment implies that dissociation to the vibrationless ground state of NH(2) occurs in <50 fs, in very good agreement with frequency resolved measurements. The anisotropy of the H fragments with the highest kinetic energies seems to also suggest that the NH(2) partner fragment comes off with very low rotational excitation.
在氨分子中A态ν(2)(')=4模式被激发后，我们展示了N - H键解离的时间尺度如何取决于赋予NH(2)光碎片的内能。通过结合飞秒泵浦/探测光谱学和速度映射离子成像技术，对于具有高动能的H原子，时间和能量分辨的H原子消除过程可直接与光解离的非绝热性质相关，同时有证据表明绝热动力学导致解离产生能量最低的H原子。NH(2)光碎片解离时间尺度与内能关系的外推表明，解离至NH(2)的无振动基态发生在<50飞秒内，这与频率分辨测量结果非常吻合。具有最高动能的H碎片的各向异性似乎也表明，NH(2)伴生碎片以非常低的转动激发脱离。
