邱荣涛 2009年11月26日下午4：3018教学楼405 The intensity gradients of inhomogeneous laser-light fields impose ponderomotive forces on charged particles. Such forces have been used to trap and manipulate ions, diffract electrons, and generate charge waves in plasmas. But they were thought to act only very weakly on neutral atomshaving to rely on the polarizability of an atoms charge distribution. Now, however, a group at the Max Born Institute in Berlin has reported the use of intense ultrashort laser pulses to accelerate neutral helium atoms for about 100 femtoseconds at 10 15 m/s 2 . Thats eight orders of magnitude greater than the acceleration (or deceleration) one can get with the continuous-wave techniques used in laser cooling of neutral atoms. The ponderomotive force of an inhomogeneous light field pushes a charged particle toward lower light intensity with a force proportional to the square of its chargeirrespective of signand inversely proportional to its mass. The Berlin group argues that the strong laser pulse excites an electron to the outer reaches of the helium atom where it quivers in the oscillating light field and experiences the ponderomotive force almost as a free electron would. But still bound to the atoms ionic core, it tugs the much heavier core with it away from the laser beams focus. The figure shows how the maximum velocity thus acquired by neutral atoms in the Berlin experiment increases with pulse duration. The dashed curves show the theoretical expectation for the groups model of electron excitation and the consequent ponderomotive force. Such ultrastrong acceleration of neutral atoms, they suggest, could be exploited for atomic-beam optics, atom deposition, and controlled chemical reactions. 附件中是相关文章 Acceleration of neutral atoms in strong short-puls
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