We investigate nonresonant contributions to resonant Rayleigh scattering cross sections of atoms. The problematic nonresonant contributions set a limit to the accuracy to which atomic spectra determine energy levels. We discuss the off-resonance effects in one-photon transitions. We also show that off-resonance contributions for the 1S-2S two-photon transition in atomic hydrogen are negligible at current and projected levels of experimental accuracy. The possibility of a differential measurement for the detection of off-resonance effects in one-photon transitions in atomic hydrogen is discussed.
The nonresonant (NR) corrections are estimated for the most accurately measured two-photon transition 1s-2s in the hydrogen atom. These corrections depend on the measurement process and set a limit for the accuracy of atomic frequency measurements. With the measurement process adopted in the modern experiments the NR contribution for 1s-2s transition energy can reach 10^{-3} Hz while the experimental inaccuracy is quoted to be +/- 46 Hz.
In the present paper, we study nonresonant corrections for experimental measurements of the transition frequencies in the helium atom. Having attracted more attention, such effects can make a significant contribution to experiments based on one- and two-photon atomic spectroscopy. The quantum interference effects in the measurements of $2^3S_1-n^3D_1$ ($ n=3,\,4,\,5 $) transition frequencies based on Doppler-free two-photon spectroscopy, are considered as a possible source of current discrepancy between the experimental and theoretical data. We demonstrate that line profile asymmetry caused by…
Photoelectron angular distributions of the two-photon ionization of neutral atoms are theoretically investigated. Numerical calculations of two-photon ionization cross sections and asymmetry parameters are carried out within the independent-particle approximation and relativistic second-order perturbation theory. The dependence of the asymmetry parameters on the polarization and energy of the incident light as well as on the angular momentum properties of the ionized electron are investigated. While dynamic variations of the angular distributions at photon energies near intermediate level reso…
We theoretically study the pulse-width dependence of the photoelectron angular distribution (PAD) from the resonance-enhanced two-photon single ionization of He by femtosecond ($\lesssim 20$ fs) extreme-ultraviolet pulses, based on the time-dependent perturbation theory and simulations with the full time-dependent Schrödinger equation. In particular, we focus on the competition between resonant and nonresonant ionization paths, which leads to the relative phase $δ$ between the $S$ and $D$ wave packets distinct from the corresponding scattering phase shift difference. When the spectrally broade…
We demonstrate that the present superaccurate measurements of transition processes between atomic states in hydrogen atom reached the limit of accuracy when transition frequency cannot be defined anymore in a unique way. This was predicted earlier and is due to the necessity to include the nonresonant corrections in the description of resonant processes. The observed spectral line profile becomes asymmetric, and it becomes impossible to extract the value of transition frequency from this profile in a unique way. Nonresonant corrections depend on the type of experiment and on the experimental a…
In the present paper, we consider nonresonant corrections to $ 2s-ns/nd $ transition frequencies in hydrogen for the experiments based on two-photon spectroscopy. A detailed study of angular correlations of quantum interference effects within the framework of rigorous quantum electrodynamics is given. Closed expressions for the resonant two-photon scattering cross sections on an atomic level with dependence on all atomic quantum numbers including fine and hyperfine structure are derived. These expressions are applied for the description of experiments based on two-photon spectroscopy with fixi…
We theoretically study the photoelectron angular distributions (PADs) from two-color two-photon near-threshold ionization of hydrogen and noble gas (He, Ne, and Ar) atoms by a combined action of femtosecond extreme ultraviolet (EUV) and near-infrared (IR) laser pulses. Using the second-order time-dependent perturbation theory, we clarify how the two-photon ionization process depends on EUV-IR pulse delay and how it is connected to the interplay between resonant and nonresonant ionization paths. Furthermore, by solving the time-dependent Schrödinger equation, we calculate the anisotropy paramet…
In this review, we have investigated the asymmetry of the line profile in precision one- and two-photon spectroscopy of hydrogen and helium atoms within the framework of a rigorous QED approach. A detailed analysis of the angular correlations of the quantum interference effect has been carried out using various examples. Nonresonant effects are also considered in relation to some astrophysical problems. In particular, a rigorous QED derivation of the nonresonant extension for the Lorentz line profile is given using the Ly$_α$ transition as an example; such a QED derivation has been lacking in …
The resonant spectra of hydrogen and anti-hydrogen atoms in the presence of an external electric field are compared theoretically. It is shown that nonresonant corrections to the transition frequency contain terms linear in the electric field. The existence of these terms does not violate space and time parity and leads to a difference in the resonant spectroscopic measurements for hydrogen and anti-hydrogen atoms in an external electric field. The one-photon 1s-2p and the two-photon 1s-2s resonances are investigated.
The relativistic and nonrelativistic approaches for the calculations of the two-photon decay rates of highly excited states in hydrogen are compared. The dependence on the principal quantum number (n) of the ns, nd, and np initial states is investigated up to n = 100 for the nonresonant emissions. For the ns states together with the main E1E1 channel the contributions of higher multipoles (M1M1, E2E2, E1M2) are considered. For the np states the E1M1 and E1E2 channels are evaluated. Moreover, the simple analytical formula for the E1M1 decay is derived in the nonrelativistic limit.
We demonstrate that Casimir-Polder potentials can be entirely independent of temperature even when allowing for the thermal photon numbers to become large at the relevant molecular transition frequencies. This statement holds for potentials that are due to low-energy transitions of a molecule placed near a plane metal surface. For a molecule in an energy eigenstate, the temperature-invariance is a consequence of strong cancellations between nonresonant potential components and those due to evanescent waves. For a molecule with a single dominant transition in a thermal state, upward and downwar…
We consider the general scenario of an excited level |i> of a quantum system that can decay via two channels: (i) via a single-quantum jump to an intermediate, resonant level |bar m>, followed by a second single-quantum jump to a final level |f>, and (ii) via a two-quantum transition to a final level |f>. Cascade processes |i> -> |bar m> -> | f> and two-quantum transitions |i> -> |m> -> |f> compete (in the latter case, |m> can be both a nonresonant as well as a resonant level). General expressions are derived within second-order time-dependent perturbation theory, and the cascade contribution …
We demonstrate and study the enhancement of intermediate-field two-photon absorption by shaped femtosecond pulses having spectral phases of antisymmetric nature. The intermediate-field regime corresponds to pulse intensities, where the two-photon absorption is coherently induced by the weak-field nonresonant two-photon transitions as well as by additional resonance-mediated four-photon transitions. It is a regime of significant excitation yields, exceeding the weak-field yields by two orders of magnitudes, reaching about 10-20% population transfer. The considered antisymmetric nature is with r…
Two-electron one-photon transitions have been successfully observed for the Ne projectile and Al target at low energy regime. Experimental energy values of two-electron one-photon transitions are compared with previously reported theoretical and experimental values. Ionization cross-section of two-electron one-photon transition is reported.
On the basis of a quantum microscopic approach we study the dynamics of the afterglow of a dilute Gaussian atomic ensemble excited by pulsed radiation. Taking into account the vector nature of the electromagnetic field we analyze in detail the angular and polarization distribution of single-photon superradiance of such an ensemble. The dependence of the angular distribution of superradiance on the length of the pulse and its carrier frequency as well as on the size and the shape of the atomic clouds is studied. We show that there is substantial dependence of the superradiant emission on the po…
We present a method for measuring atomic parity violation in the absence of static external electric and magnetic fields. Such measurements can be achieved by observing the interference of parity conserving and parity violating two-photon transition amplitudes between energy eigenstates of zero electronic angular momentum. General expressions for induced two-photon transition amplitudes are derived. The signal-to-noise ratio of a two-photon scheme using the 6s^2 1S0 to 6s6p 3P0 transition in ytterbium is estimated.
A two-photon transition in laser-cooled and trapped calcium atoms is proposed as the atomic reference in an optical frequency standard. An efficient scheme for interrogation of the frequency standard is described, and the sensitivity of the clock transition to systematic effects is estimated. Frequency standards based on this transition could lead to compact and portable devices that are capable of rapidly averaging down to $< 10^{-16}$.
We establish the existence of spin-statistic selection rules (SSSR) for multi-equal-photon transitions in atomic systems. These selection rules are similar to those for systems of many equivalent electrons in atomic theory. The latter ones are the direct consequence of Pauli exclusion principle. In this sense the SSSR play the role of the exclusion principle for photons: they forbid some particular states for the photon systems. We established several SSSR for few-photon systems. 1) First rule (SSSR-1): two-equivalent photons involved in any atomic transition can have only even values of the t…
Energies of two-electron one-photon transitions from initial double K-hole states were computed using the Dirac-Fock model. The transition energies of competing processes, the K$α$ hypersatellites, were also computed. The results are compared to experiment and to other theoretical calculations.
Vibrational overtones in deeply bound molecules are sensitive probes for variation of the proton-to-electron mass ratio $μ$. In nonpolar molecules, these overtones may be driven as two-photon transitions. Here, we present procedures for experiments with ${\rm O}_2^+$, including state-preparation through photoionization, a two-photon probe, and detection. We calculate transition dipole moments between all $X\,^2Π_g$ vibrational levels and those of the $A\,^2Π_u$ excited electronic state. Using these dipole moments, we calculate two-photon transition rates and AC-Stark-shift systematics for the …
We present experiments demonstrating an enhancement of resonant two-photon transition rate in 87Rb utilizing spectral phase manipulation of the excitation frequency comb. By selectively removing certain comb teeth, the resonant two-photon transition rate can be improved, and reach a factor of more than 1.8. The femtosecond pulse-train excitation of two-photon transition is investigated theoretically based on general multiphoton transitions and the results are compared with the experiments. The theory presented here gives a clear insight of physical mechanism of this quantum coherent control an…
Dark photon is a massive vector particle which couples to the physical photon through the kinetic mixing term. Such particles, if exist, are produced in photon beams and, in particular, in laser radiation. Due to the oscillations between the physical photon and the dark photon, the latter may be, in principle, detected in the light-shining-through-a-wall experiment. We propose a variant of this experiment where the detection of dark photons is based on the atomic transitions. The key feature of this scheme is that the detection probability is first order in the coupling constant due to the int…
