The Fritz Haber Center for Molecular Dynamics and Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
J Chem Phys. 2019 Sep 21;151(11):114308. doi: 10.1063/1.5118990.
The time evolution of a vacuum ultraviolet excited N molecule is followed all the way from an ultrafast excitation to dissociation by a quantum mechanical simulation. The primary aim is to discern the role of the excitation by a pulse short compared to the vibrational period, to discern the different coupling mechanisms between different electronic states, nonadiabatic, spin orbit, and to analyze the origin of any isotopic effect. We compare the picture in the time and energy domains. The initial ultrafast excitation pumps the molecule to a coherent electronic wave packet to which several singlet bound electronic states contribute. The total nonstationary wave function is given as a coherent sum of nuclear wave packets on each electronic state times the stationary electronic wave function. When the wave packets on different electronic states overlap, they are coupled in a mass-dependent manner whether one uses an adiabatic or a diabatic electronic basis. A weak spin-orbit coupling acts as a bottleneck between the bound singlet part of phase space and the triplet manifold of states in which dissociation takes place. To describe the spin-orbit perturbation that is ongoing in time, an energy-resolved eigenstate representation appears to be more intuitive. In the eigenstate basis, the singlet-to-triplet population transfer is large only between those vibronic eigenstates that are quasiresonant in energy. The states in resonance are different for different excitation energy ranges. The resonances are mass dependent, which explains the control of the isotope effect through the profile of the pulse.
通过量子力学模拟，我们跟踪研究了真空紫外光激发的 N 分子的时间演化，从超快激发到解离的全过程。主要目的是辨别与振动周期相比很短的脉冲激发的作用，辨别不同电子态之间的不同耦合机制，包括非绝热、自旋轨道，并分析任何同位素效应的起源。我们比较了时间域和能量域的图像。初始超快激发将分子泵送到相干电子波包，几个单重束缚电子态都对其有贡献。总非定态波函数是每个电子态上的核波包的相干和与定态电子波函数的乘积。当不同电子态上的波包重叠时，它们以质量相关的方式耦合，无论使用绝热还是非绝热电子基。弱自旋轨道耦合在束缚单重部分相空间和发生离解的三重态态之间充当瓶颈。为了描述随时间进行的自旋轨道微扰，能量分辨本征态表示似乎更为直观。在本征态基中，只有那些在能量上准共振的振动态之间才会发生单重态到三重态的布居转移。对于不同的激发能量范围，共振状态不同。共振状态是质量相关的，这解释了通过脉冲形状来控制同位素效应的原理。
