An alternative interpretation of the multiple subharmonics seen in electron emission from ultracold plasmas irradiated by monochromatic radiowaves. Instead of Tonks-Dattner resonances (standing Langmuir waves) -- which suffer from ill-defined boundary conditions, wrong temperature dependence, and insensitivity to cloud shape -- the authors attribute the peaks to quasi-classical multiphoton ionization of secondary Rydberg atoms formed in the expanding, cooling cloud. Numerical simulations show the ionization efficiency has well-defined peaks and is independent of the cloud's boundary conditions and global shape, making it a viable replacement for the resonance picture.
Ultracold plasmas irradiated by monochromatic radiowaves show multiple subharmonics in their electron emission. The long-standing interpretation as Tonks-Dattner resonances (standing Langmuir waves) …