Escaping From an Ultracold Inferno: Classical and Semiclassical Mechanics Near Threshold

ultracold chemistry;semiclassical mechanics;classical mechanics;quantum mechanics;threshold phenomena

Soley, Micheline Barbara Szabó ; Heller, Eric J.

University:Havard University

Department:Chemical Physics

关键词: ultracold chemistry;    semiclassical mechanics;    classical mechanics;    quantum mechanics;    threshold phenomena;   

Others  :  https://dash.harvard.edu/bitstream/handle/1/37365112/SOLEY-DISSERTATION-2020.pdf?sequence=1&isAllowed=y

美国|英语

来源: Digital Access to Scholarship at Harvard

【 摘 要 】

Ultracold chemistry, the science of chemical reactions below one milliKelvin, has the potential to expedite technologies from controlled reactions to quantum computation. Experimental investigation of the ultracold KRb dimer reaction (2KRb → [K2Rb2]⋆ → K2 + Rb2) has been central to the field. Despite recent experimental and theoretical work on the reaction, it remains an open question how cold products (14 K) are formed from the hot collision complex (4,000 K). The answer to this question has been obstructed by the computational intensity of quantum mechanical simulations at cold temperatures.This dissertation makes use of the analogy of an “ultracold inferno” to circumvent the computationally intensive quantum mechanical treatments typically applied near threshold. The thesis reveals that, unexpectedly, classical mechanics is expected to be exact for elements of the ultracold KRb dimer reaction and many threshold processes previously thought to be the sole province of quantum mechanics. Classical behaviors can thus be observed in certain systems at extremely low temperatures, and classical mechanics can be employed to bring computational simulation of many threshold systems within reach.To identify when classical mechanics is valid near threshold, two semiclassical criteria of quantum-classical correspondence are presented. The criteria are then put into practice to calculate the rate of product formation in a reduced-dimensional model with the essence of the ultracold KRb dimer reaction via classical analytic phase-space counting and classical numerical dynamics simulations. A hybrid semiclassical-quantum method is also proposed to bring simulation of many full-dimensional exothermic ultracold reactions within reach. Classical “post-threshold” laws are derived and shown to operate in a plethora of cold and ultracold systems. Quantum and classical threshold behavior is unified from across physics and chemistry to emphasize the wide-reaching importance of classical mechanics near threshold.This thesis concludes classical mechanics has broad applications in threshold phenomena in fields such as ultracold and cold physics and chemistry, intense laser science, plasma science, nuclear physics, positron and positronium scattering, electron detachment and attachment, and astrochemistry. Classical techniques can be used to facilitate calculations and to better understand the mechanisms behind these threshold processes. These findings illustrate the power of time-honored classical mechanics in the age of the quantum leap.