Grant for Exploratory Research: Electric Field Induced Orientation of Rotationally Cooled Molecules

Dudley Herschbach教授获得了实验物理化学项目探索性研究小额资助，通过应用强电场研究冷分子束中的分子取向。这些测量为解决有关反应分子间相对取向的重要性以及分子取向对化学反应性的影响等问题提供了一种新的方法。具有三倍或更高对称轴的极性分子在某些旋转状态下会在电场中进动而不是翻滚。因此，偶极矩在这种状态下并不是平均的，所以电场可以施加一个产生实质性取向的扭矩。对于不对称的分子，电场定向被认为是不切实际的。双原子、线性或不对称顶端分子的旋转运动使一阶偶极矩平均，因此与电场的相互作用要弱得多。赫施巴赫博士进行了可行性计算，结果表明，即使在这些情况下，当分子束中的线性分子（如ICN）冷却到2开尔文时，也只需要相对适度的6万伏特/厘米的斯塔克场来定位其最低旋转状态。赫施巴赫希望通过实验验证这一预测，它将对整个反应动力学领域产生巨大的影响。这将为研究碰撞分子束的取向效应提供一种重要的新方法，这种方法在容易获得的实验条件下是可以实现的。

Professor Dudley Herschbach is supported by a Small Grant for Exploratory Research from the Experimental Physical Chemistry Program to study molecular orientation in cold molecular beams by the application of strong electric fields. These measurements provide a new method to address questions concerning the importance of relative orientation between reacting molecules and the influence which molecular orientation has on chemical reactivity. Polar molecules with a three-fold or higher axis of symmetry in certain rotational states will precess in an electric field rather than tumble. The dipole moment therefore does not average out for such states, so an electric field can exert a torque which produces substantial orientation. Orientation by an electric field has been considered quite impractical for less symmetrical molecules. The rotational motion of diatomic, linear, or asymmetric top molecules averages out the dipole moment in first-order, so the interaction with an electric field is much weaker. Dr. Herschbach has performed feasibility calculations which show, however, that even in these cases only relatively modest Stark fields of 60,000 volts/cm are required to orient the lowest rotational states of linear molecules such as ICN when they are cooled to 2 degrees Kelvin in a molecular beam. This prediction, which Herschbach hopes to verify experimentally, would have an enormous impact on the whole field of reaction dynamics. It would provide an important new capability to study orientation effects in colliding molecular beams which would be achievable at easily attainable experimental conditions.