Using a method based on the Magnus expansion, the survival of Rydberg atoms in intense laser fields is studied. Usually, for atoms in intense laser fields, effects beyond the electric dipole approximation need to be considered. In the past few years, researchers have primarily paid attention to the nondipole effects induced by the electric quadrupole term of the optical field. The quadrupole term plays a more important role than the electric dipole term in the calculation of the survival probability of Rydberg atoms in intense laser fields even though the magnitude of the former is smaller than that of the latter in terms of the multipole expansion of the optical vector potential. Meanwhile, the effects of higher-order corrections beyond the quadrupole term remain ambiguous. In this paper, we confirm that corrections containing higher-order terms, except for the quadrupole term, can be neglected in the calculation of the transitions of Rydberg atoms in external fields. We then use the second-order time evolution operator containing the quadrupole term to study the scaling law of the transition probability of Rydberg He in laser fields with different parameters.
Currently, the Magnus expansion method is widely used in studying Rydberg atoms in intense laser fields. In these studies, pulse lengths are usually set as an integer multiple of wavelength, at the same time, the initial phase of the laser pulse are usually set as zero, i.e. , without considering the effect of the initial phase in the calculation. However, this will result in some items multiplied by zero. Then, contributions from these items will be lost. Herein, we investigated the effects of the non-integer-cycle pulse and laser initial phase in the calculation of the transition probability of Rydberg atoms in intense laser fields. Our calculation result shows that these effects are trivial when the nondipole approximation is adopted. In dipole approximation i.e. , when the wavelength of the laser field exceeds the characteristic size of atomic or molecular orbitals, these effects are trivial only in the case of a long pulse duration and a large principal quantum number, however in the case of ultrashort laser pulses, these effects are obvious.
The photoionization of the
4s Rydberg state in hydrogen by an intense femtosecond $400\text{\ensuremath{-}}\mathrm{nm}$ light pulse is studied beyond the dipole approximation. We show that a recently predicted nondipole, high-frequency ionization mechanism [M. F\o{}rre et al., Phys. Rev. Lett. 97, 043601 (2006)] has a low-frequency counterpart in excited atomic systems. This opens the possibility that the effect could be measured in the near future.
We treat the interaction of an atom with a short intense few-cycle laser pulse by the use of the Magnus expansion of the time-evolution operator. Terms of the Magnus expansion up to the third order in the pulse duration are evaluated explicitly, and expressions for the transition probability between atomic eigenstates are given. The transition operators up to this order are particularly simple, involving only momentum and position shifts arising from the laser field alone. The connection to other existing approaches is established. For a variety of generic short-pulse forms the results of the Magnus approximation are in excellent agreement with time-dependent transition probabilities obtained from accurate ab initio numerical calculations. However, the limitation of the Magnus expansion for pulses having both vanishing momentum and position shifts is demonstrated also.
Synopsis We develop a time-dependent perturbation theory beyond the dipole approximation to analyze the two-photon ionization dynamics of atoms exposed to short laser pulses with an arbitrary polarization. Subse-quent applications of the PT method in the nonresonant regime allow us to analyze one of the most important nondipole effects, i.e., the linear-momentum-transfer law in the two-photon ionization process and to unveil the important role of the interferences between the dipole and nondipole transition pathways to the photoelectron momentum shift along the laser-propagation direction.
Rydberg-state excitation of atoms in strong infrared laser fields provides a new complementary aspect of the perspective of atom--strong field interactions. In this article, we perform an experimental and theoretical study on the corresponding process of diatomic molecules,
N2 and
O2. We show that neutral molecules can also survive strong 800-nm laser fields in high Rydberg states, while their behavior is remarkably different in comparison with their companion atoms, Ar and Xe. The Rydberg excitation of
N2 generally behaves similarly to Ar, while that of
O2 is more significantly suppressed than the ionization compared to Xe in a high intensity region, which can be understood in the frame of a semiclassical picture, together with their different structures of molecular orbitals. However, distinct quantum features in the Rydberg excitation processes that are apparently beyond the semiclassical picture have been identified, i.e., the less suppressed probability of
O2 at low intensity and the oscillation behavior of the ratio between
N2 and Ar, indicating that our understanding of the relevant physics is still far from complete.