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Journal of Physics B Atomic and Molecular Physics

Electron impact excitation of the forbidden transitions in lithium and sodium atoms in the presence of a resonant laser field

R S Pundir, K. C. Mathur

Published Feb 14, 1985• 7 citations
Publisher

Abstract

Electron impact excitation of lithium (2s-3s) and sodium (3s-4s) atoms in the presence of a resonant laser field is studied. The frequency of the laser field is studied. The frequency of the laser field is chosen to match the 2s-2p (3s-3p) transition frequency in the lithium (sodium) atom. The variation of the total cross section with laser intensity and incident energy is presented.

DOI
10.1088/0022-3700/18/3/026

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Electron impact excitation of lithium (2s-3s) and sodium (3s-4s) atoms in the presence of a resonant laser field is studied. The frequency of the laser field is studied. The frequency of the laser field is chosen to match the 2s-2p (3s-3p) transition frequency in the lithium (sodium) atom. The variation of the total cross section with laser intensity and incident energy is presented.
Electron-impact excitation of the hydrogen metastable 2s state to the 3s, 4s, 3d, and 4d states is studied in the presence of a resonant laser field using the rotating-wave approximation. The frequency of the circularly polarized laser field is chosen to match the 2s-3p transition frequency in the hydrogen atom. The role of the laser field in individual magnetic substate (mf{\mathit{m}}_{\mathit{f}}mf​=0,\ifmmode\pm\else\textpm\fi{}1,\ifmmode\pm\else\textpm\fi{}2) excitation is analyzed. It is found that for these optically forbidden transitions (s-s, s-d), a significant increase in the excitation cross sections is obtained through joint collisions with electrons and photons.
An experimental investigation of electron - impact induced transitions from the laser - excited 3P state of sodium atom to the 4S and 3D excited states is presented, the main objective being measurement of the atomic collision parameters ( ACPs ) that are related to individual scattering amplitudes. These parameters are complementary to the differential excitation cross - sections and provide finer details on the collisional excitation process. The experiment involved the preparation of the 3P - state with coherent laser radiation of a particular polarisation and measuring the rate of electrons scattered from the laser - excited state as function of laser polarisations. The ACPs were then deduced from these measurements and optical pumping parameters, which account for the optical preparation of the 3P - state. To formulate different measurement schemes, a measurement theory based on the general description of transitions between states of different angular momentum using density matrix formalism is derived. The investigation of the 3P - 4S transition is very similar to the studies of the 3S - 3P excitation performed by superelastic method involving the 3P - 3S scattering in the 'time - inverse' geometry. 'Standard' ACPs are used and, similarly to the superelastic method, the measurements for the 3P - 4S excitation performed in the 'time - inverse' geometry also reveal ACPs for the 4S - 3P de -excitation process. For the more complex 3P - 3D transition, however, the 'standard' ACPs may be less informative. It is shown that certain sums of individual magnetic sublevel excitation probabilities can be measured which may be considered as atomic collision parameters for transitions between P and D - states.
We performed quasiresonant laser excitation of Li–Cd vapor mixtures at 460.3 nm (2p–4d) and 497.2 nm (2p–4s). The initial population of the Li(2p) level is caused by three processes: collisional dissociation or photodissociation of the Li 2 (B 1 Π u ) molecules or energy transfer in Li 2 (B 1 Π u ) and Li(2s) collisions. The Li(4d,4s) atoms radiatively decay towards Li(3p), from which collisional population transfer to Cd(5 3 P J ) occurs, followed by three-body reactive collisions which produce the LiCd(2 2 Π ) excimer states
We have studied the electron-impact excitation of an atom, in the presence of a laser field whose photon energy is tuned close to the energy difference between two excited final states. Both the laser-projectile and the laser-target interactions are treated nonperturbatively, while the electron-atom interaction is treated within the first Born approximation. As an application, we have analyzed the resonant laser-assisted excitation of the 21{2}^{1}21S and 21{2}^{1}21P states of helium. The agreement between the present nonperturbative results and previous perturbative ones is excellent, except for very small detunings. The present nonperturbative treatment also shows that the results given by perturbation theory on both sides of the resonance, when plotted as a function of the laser frequency, correspond to the excitation of different Floquet pseudostates. This is related to the presence of avoided crossings in the diagram of the Floquet pseudoenergies, as a function of the laser frequency.
We investigated the near resonant scattering and collision induced fluorescence of sodium atoms with doublet excited states in a weak laser field. By changing the frequency of the incident light and measuring the integral intensities of the components of the scattered light, the near resonant excitation cross-section of each sublevel was obtained as functions of the frequency detuning. Their variations are qualitatively explained by the interaction potential between the sodium atom and various collisional perturbing species. We have measured the variation of the induced D1 and D2 fluorescence with laser power, density of sodium atoms and pressure of buffer gas. It is shown that the relation is linear. The experimental results are in agreement with the theory.