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Optics and Spectroscopy

Probability of Absorption and Emission by an Atom Interacting with Ultrashort Laser Pulses

I. A. Aleksandrov, D. V. Chubukov, N. N. Rosanov

Published Feb 1, 2024• 0 citations
Publisher

Abstract

We consider the process of transition of a two-level system to the excited state with subsequent photon emission in the presence of a laser pulse with a high degree of unipolarity. Within the framework of quantum electrodynamics, we obtain analytical expressions for the differential probability of the process depending on the temporal scales of the problem: laser pulse duration, excited-state lifetime, inverse transition frequency, and inverse frequency of the photon emitted. Besides, we calculate the total absorption probability by integrating over the three-dimensional photon momentum and summing over polarizations. We compare the results obtained for unipolar and bipolar (many-cycle) pulses.

DOI
10.1134/s0030400x24020012

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The paper is devoted to the theoretical investigation of two-photon excitation of atom in a discrete energy spectrum by ultrashort electromagnetic pulses of femto- and subfemtosecond ranges of durations. An analytical expression for the total probability of the process is derived. Numerical simulations are made for hydrogen and sodium atoms. It is shown that the total probability of the process is nonlinear function of pulse duration and character of this function depends strongly on the frequency detuning of pulse carrier frequency from two-photon resonance.
The paper is devoted to the investigation of the absorption of ultrashort laser pulses on atoms in plasmas, accounting for the different broadening mechanisms of atomic resonant transitions. The analysis is made in terms of the absorption probability during the entire interaction between the laser pulse and atom. Attention is mainly given to dependence of probability upon the pulse duration and the carrier frequency of the pulse. The results are presented via dimensionless parameters and functions describing the effect of finite pulse duration on atomic spectra for different broadening mechanisms, namely Doppler, Voigt, Holtsmark and their combinations, as well as the Stark line broadening of Rydberg atomic lines.
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Transitions in a two-level system irradiated by linearly polarized cosine and sine waveform sub-one-cycle pulses are considered. The probabilities of transition after the end of laser pulse are derived numerically as functions of two dimensionless variables: E τ and F τ, where τ is the pulse duration, E is the energy distance between considered levels, and F is the dipole matrix element of the transition (all quantities are presented in atomic units). We derive also the dipole moment of a two-level system after the pulse termination as a function of the same dimensionless parameters. There is no significant dependence of dipole moment and inversion population on the cosine, or sine waveform.
Some features of the excitation of multilevel quantum systems under the action of electromagnetic pulses which are shorter than the inverse frequency of interlevel transitions are considered. It is shown that the interaction is characterized by a specific type of selectivity which is not connected with the resonant absorption of radiation. The simplest three-level model displays the inverse population of upper levels. The effect of an ultrashort laser pulse on a multilevel molecule was regarded as an instant reception of the oscillation velocity by the oscillator and this approach showed an effective excitation and dissociation of the molecule. The estimations testify to the fact that these effects can be observed using modern femtosecond lasers.