NONRADIATIVE TRANSFER OF MOMENTUM FROM AN ELECTROMAGNETIC-WAVE TO A CHARGED-PARTICLE

A plane-wave pulse passing by a charge particle can leave the particle with momentum and energy different from that before the arrival of the pulse. The mechanism for transmitting this momentum and energy is nonradiative and involves the interference of the electromagnetic fields of the particle and wave. A focused pulse (with a finite beam waist) cannot transfer momentum and energy in this way. However, if a charged particle is mechanically restrained and then released inside a focused pulse, momentum can be transferred non radiatively to the particle through the mechanical restraining force. We use these considerations to further clarify the source of the large drift momentum that photoelectrons have been observed to acquire in recent experiments on a atomic ionization by strong low-frequency fields, we discuss the energy quantization condition, and we illuminate the distinction between the low-frequency regimes for scattering and ionization processes. We also show that atomic stability in a high-frequency field is a consequence of the difficulty of transferring momentum. Finally, we briefly examine the distribution of drift momentum in a two-color processes, illustrating the role of the ponderomotive shift and the sensitivity to the phase at which ionization occurs.