Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA) CNRS (UMR 7583), Université Paris Est Créteil, Université Paris Diderot, Institut Pierre-Simon Laplace, Université Paris Est Créteil, 94010 Créteil Cedex, France.
J Chem Phys. 2012 May 14;136(18):184302. doi: 10.1063/1.4705264.
A quantum approach and classical molecular dynamics simulations (CMDS) are proposed for the modeling of rotational relaxation and of the nonadiabatic alignment of gaseous linear molecules by a nonresonant laser field under dissipative conditions. They are applied to pure CO(2) and compared by looking at state-to-state collisional rates and at the value of <cos(2)[θ(z)(t)]> induced by a 100 fs laser pulse linearly polarized along z[overhead arrow]. The main results are: (i) When properly requantized, the classical model leads to very satisfactory predictions of the permanent and transient alignments under non-dissipative conditions. (ii) The CMDS calculations of collisional-broadening coefficients and rotational state-to-state rates are in very good agreement with those of a quantum model based on the energy corrected sudden (ECS) approximation. (iii) Both approaches show a strong propensity of collisions, while they change the rotational energy (i.e., J), to conserve the angular momentum orientation (i.e., M/J). (iv) Under dissipative conditions, CMDS and quantum-ECS calculations lead to very consistent decays with time of the "permanent" and transient components of the laser-induced alignment. This result, expected from (i) and (ii), is obtained only if a properly J- and M-dependent ECS model is used. Indeed, rotational state-to-state rates and the decay of the "permanent" alignment demonstrate, for pure CO(2), the limits of a M-independent collisional model proposed previously. Furthermore, computations show that collisions induce a decay of the "permanent" alignment about twice slower than that of the transient revivals amplitudes, a direct consequence of (iii). (v) The analysis of the effects of reorienting and dephasing elastic collisions shows that the latter have a very small influence but that the former play a non-negligible role in the alignment dynamics. (vi) Rotation-translation collisionally induced transfers have also been studied, demonstrating that they only slightly change the alignment dissipation for the considered laser energy conditions.
提出了一种量子方法和经典分子动力学模拟（CMDS），用于在耗散条件下通过非共振激光场对旋转弛豫和气态线性分子的非绝热对准进行建模。它们应用于纯 CO(2) 并通过比较状态到状态的碰撞速率和由沿 z[overhead arrow]线性极化的 100fs 激光脉冲引起的 <cos(2)[θ(z)(t)]> 值来进行比较。主要结果是：（i）当适当地再量子化时，经典模型在非耗散条件下对永久和瞬态对准给出了非常令人满意的预测。（ii）基于能量修正的突然（ECS）近似的量子模型的碰撞展宽系数和旋转态到态速率的 CMDS 计算与那些非常吻合。（iii）两种方法都表现出强烈的碰撞倾向，而它们改变旋转能量（即 J）以保持角动量取向（即 M/J）。（iv）在耗散条件下，CMDS 和量子-ECS 计算导致激光诱导对准的“永久”和瞬态分量随时间的非常一致的衰减。这一结果来自于（i）和（ii），只有使用适当的 J 和 M 依赖的 ECS 模型才能得到。事实上，旋转态到态速率和“永久”对准的衰减证明，对于纯 CO(2)，以前提出的与 M 无关的碰撞模型的局限性。此外，计算表明，碰撞导致“永久”对准的衰减速度比瞬态复兴幅度的衰减速度慢约两倍，这是（iii）的直接结果。（v）重新定向和去相位弹性碰撞的影响分析表明，后者的影响很小，但前者在对准动力学中起着不可忽视的作用。（vi）还研究了旋转-平移碰撞诱导转移，证明它们仅略微改变考虑的激光能量条件下的对准耗散。
