Accelerating neutral atoms

Accelerating neutral atoms. The intensity gradients of inhomogeneous laser-light fields impose ponderomotive forces on charged particles. Such forces—proportional to the square of the particle’s charge and inversely proportional to its mass—push the particle toward lower light intensity and 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 atoms—having to rely on the polarizability of an atom’s 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 10¹⁵ m/s². That’s 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 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 atom’s ionic core, it tugs the much heavier core with it away from the laser beam’s 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 group’s 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. (U. Eichmann et al., Nature 461 , 1261, 2009 http://dx.doi.org/10.1038/nature08481 .)