Rydberg Atoms in THz Fields

Abstract

Rydberg atoms are highly excited atoms with bizarre and exotic properties such as huge orbits and exaggerated sensitivity to fields. In this study we examine these atoms using classical methods. We present the classical trajectory Monte Carlo (CTMC) method, where the microcanonical distribution is used. This is a model based on the microcanonical ensemble from statistical mechanics and can be used to initialize the desired atomic system. The systems are propagated by classical equations solved numerically. Recent experiment found ionization probabilities of excited sodium atoms under the influence of a single-cycle THz field. Energy distributions of the electrons from ionized states were obtained. We make corresponding classical calculations and compare results. For field strengths corresponding to low ionization probabilities we see a n^(-3) scaling behaviour in experimental results as well as in classical calculations. For higher field strengths the results diverge, with classical results scaling as n^(-4) and experimental results as n^(-3). Further calculations show that states require stronger fields to ionize at very high n. Ionization probabilities for states with various angular momentum l were compared and states with relatively low l were found to ionize before states with relatively high l. Energy distribution of electrons from ionized states were compared with experimental results and the underlying trend were found to be in some agreement. The current classical method was however unable to reproduce detailed properties of the distributions. Further calculations were done for energy distributions of hydrogen and sodium in various states and results suggests that core scattering plays a role in these interactions. Momenta and positions of electrons from ionized states as a function of time are for high n found to agree with analytical calculations corresponding to a free particle in a field. Investigations of single electron trajectories are made to glean insights into ionization mechanisms.