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The Journal of Chemical Physics

Quantum flux studies of the mechanism of Ca(4s5p 1P)→Ca(4s5p 3P) collisions

Millard H. Alexander

Published May 1, 1992• 21 citations
Publisher

Abstract

We apply a new method [M. H. Alexander, J. Chem. Phys. 94, 8931 (1991)] for the study of the mechanism of inelastic collisions, to the analysis of spin-changing collisions of Ca atoms in the 4s5p Rydberg state. The method involves the determination of the current density associated with, separately, the incoming and outgoing scattering wave functions in a locally adiabatic basis. This yields a picture of how the incoming flux, initially associated with a given internal state, redistributes itself as a function of the interparticle separation both as the particles approach, and, subsequently, as the particles recede. By proper selection of the initial state, we explore the dependence on orbital orientation of the probability for the spin-changing Ca(4s5p 1P)→Ca(4s5p 3P) process. Further, we show how the distribution of population among the fine-structure levels of the 3P state depends on final-state interactions in the exit channel.

DOI
10.1063/1.462606

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The resonant mechanism of interaction of alkaline-earth atoms having a low electron affinity to Rydberg atoms in circular (l = |m| = n–1) and near-circular states has been studied. To describe the dynamics of resonant processes accompanied by nonadiabatic transitions between ionic and Rydberg covalent terms of a quasimolecule, an approach based on the integration of coupled equations for the probability amplitudes has been developed taking into account the possibility of the decay of an anion in the Coulomb field of the positive ionic core of a highly excited atom. The approach involves the specific features of the problem associated with the structure of the wavefunction of a Rydberg electron in states with high orbital angular momenta l ~ n–1. This approach provides a much more accurate description of the dynamics of electronic transitions at collisions between atoms than that within the modified semiclassical Landau–Zener model. In addition, this approach makes it possible to effectively take into account many channels of the problem. The cross sections for resonant quenching of Rydberg states of the Li(nlm) atom with given principal n, orbital l = n–1, and magnetic m quantum numbers at thermal collisions with the Ca(4s 2) and Sr(5s 2) atoms have been calculated. The dependences of the results on n, m, and angle α between the relative velocity of the atoms and the normal to the plane of the orbit of the Rydberg electron have been obtained. The influence of orientational effects on the efficiency of the collisional destruction of circular and near-circular states has been studied. The results indicate a higher stability of such states to their perturbations by neutral particles as compared to usually studied nl states with low values of l (l ≪ n).

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Laser studies of electronic excitation transfer in atomic calcium: Ca(4s5p /sup 1/P/sub 1/)+Ararrow-right-leftCa(4s5p /sup 3/P/sub 2,1,0/)+Ar

William H. Pence, S.R. Leone

Time-resolved nsec kinetic methods are used to measure state specific electronic excitation transfer for the near-resonant process: Ca(4s5p /sup 1/P/sub 1/) +Ar /sup k//sub 1/arrow-right-left/sub k//sub -/1 Ca(4s5p /sup 3/P/sub 2,1,0/)+Ar. In stimulated emission experiments it is found that the Ca(4s5p /sup 3/P/sub 2/) level is preferentially populated. The rate constant k/sub 1/ for the specific /sup 3/P/sub 2/ channel is 5.2 +- 0.5 x 10/sup -11/ cm/sup 3/ molecule/sup -1/ sec/sup -1/. These results are discussed in terms of near-resonant spin changing collision mechanisms. Radiative lifetimes for Ca(4s5p /sup 1/P/sub 1/) and Ca(4s5p /sup 3/P/sub 1/) are also determined to be 77 +- 2 nsec and 67 +- 2 nsec, respectively.

•May 15, 1981•0 citations
We apply a new method [M. H. Alexander, J. Chem. Phys. 94, 8931 (1991)] for the study of the mechanism of inelastic collisions, to the analysis of spin-changing collisions of Ca atoms in the 4s5p Rydberg state. The method involves the determination of the current density associated with, separately, the incoming and outgoing scattering wave functions in a locally adiabatic basis. This yields a picture of how the incoming flux, initially associated with a given internal state, redistributes itself as a function of the interparticle separation both as the particles approach, and, subsequently, as the particles recede. By proper selection of the initial state, we explore the dependence on orbital orientation of the probability for the spin-changing Ca(4s5p 1P)→Ca(4s5p 3P) process. Further, we show how the distribution of population among the fine-structure levels of the 3P state depends on final-state interactions in the exit channel.
We present a detailed study of the mechanism of spin–changing transitions between 1P and 3P electronic states corresponding to the nsn′p Rydberg state of an alkaline earth atom in collision with a structureless, spherical partner. This type of process takes place on four potential curves and involves collisional coupling between the electronic angular momentum L of the electronically excited atom, the spin S of the atom in the final 3P state, and the orbital angular momentum l associated with the relative motion of the collision partners. We review the use of Hund’s case coupling schemes in describing the collision and present a careful analysis of the initial state preparation under conditions of laser excitation in a crossed beam experiment. This underlies the development of a new statistical model for the dependence of the spin–changing cross section on the alignment of the pump laser. Finally, we describe a new technique to follow the redistribution of flux onto locally adiabatic states as the initially excited atom is approached by the closed-shell partner. This is used to show that dynamical corrections to our simple statistical model are small.
A quasiclassical analysis of the differential inelastic scattering of oriented Ca(4s5p,1P1)\mathrm{Ca}{(4s5p,}^{1}{P}_{1})Ca(4s5p,1P1​) atoms on He at a collision energy of 1 eV is presented. Inelastic channels correspond to spin-changing events populating different Zeeman sublevels of three fine-structure states of Ca(4s5p,3Pj),\mathrm{Ca}{(4s5p,}^{3}{P}_{j}),Ca(4s5p,3Pj​), j=0,j=0,j=0, 1, and 2. The magnitude of the right-left scattering asymmetry in the helicopter plane is shown to depend on the locking dynamics. The relative values of different Zeeman-specific cross sections are determined by the diffraction scattering on the attractive ${}^{1}\ensuremath{\Pi}$ state in the entrance channels, by the Coriolis mixing between degenerate components of the repulsive ${}^{3}{\ensuremath{\Sigma}}^{+}$ intermediate state, and by the radial coupling between 1-1 and ${0}^{\ensuremath{-}}{\ensuremath{-}0}^{\ensuremath{-}}$ fine-structure components of the ${}^{3}{\ensuremath{\Sigma}}^{+}{\ensuremath{-}}^{3}{\ensuremath{\Pi}}_{\ensuremath{\Omega}}$ states in the exit channels. The predicted large right-left scattering asymmetry suggests that it can be measured experimentally.
The quantum theory of atomic collisions in the presence of a weak radiation field is used to describe the process of redistribution of light in Ca–He collisions. This work is devoted to the simulation of the recent experiment of Lin and co-workers [J. Chem. Phys. 89, 4771 (1988)] in which, the absorption profile of the (4s2 1S→4s5p 1P) transition and the dependence on the detuning of the spin-changing (4s5p 1P→4s5p 3P) transition were determined. Close-coupled calculations are carried out, based on model interaction potentials for the ground and the excited states of the Ca–He system. The calculated absorption profile for the production of Ca atom in the (4s5p 1,3P) states is in qualitative agreement with the experimental results. The blue wing/red wing asymmetry in the relative transfer to the 3P state, is interpreted in terms of flux transfer among the molecular states, which occurs when the Ca atom is approached by the closed-shell partner.
The far wing absorption profiles for excitation on the Ca(4s2 1S0–4s5p 1P01) atomic transition, broadened in collisions with He are measured. We observe strong absorption in both wings and a blue wing satellite near Δ∼125 cm−1. We tentatively identify this satellite as due to a maximum in the CaHe(4s2 1∑+–4s5p 1∑+) difference potential. These line-broadening techniques are used to study electronic energy transfer in the spin-changing collisions of Ca with He: Ca(5p 1P01) +He→Ca(5p 3P0J)+He+ΔE. Measurements of the ‘‘single collision’’ triplet–singlet branching ratio as a function of laser detuning from the atomic resonance frequency indicate a clear red wing/blue wing asymmetry. We interpret this asymmetry in terms of a preferential orbital alignment effect in the energy transfer process [Phys. Rev. Lett. 53, 2296 (1984)]. No clear structure is observed in the range of detunings probed that might indicate the curve crossing responsible for the energy transfer.
The resonant mechanism of interaction of alkaline-earth atoms having a low electron affinity to Rydberg atoms in circular (l = |m| = n–1) and near-circular states has been studied. To describe the dynamics of resonant processes accompanied by nonadiabatic transitions between ionic and Rydberg covalent terms of a quasimolecule, an approach based on the integration of coupled equations for the probability amplitudes has been developed taking into account the possibility of the decay of an anion in the Coulomb field of the positive ionic core of a highly excited atom. The approach involves the specific features of the problem associated with the structure of the wavefunction of a Rydberg electron in states with high orbital angular momenta l ~ n–1. This approach provides a much more accurate description of the dynamics of electronic transitions at collisions between atoms than that within the modified semiclassical Landau–Zener model. In addition, this approach makes it possible to effectively take into account many channels of the problem. The cross sections for resonant quenching of Rydberg states of the Li(nlm) atom with given principal n, orbital l = n–1, and magnetic m quantum numbers at thermal collisions with the Ca(4s 2) and Sr(5s 2) atoms have been calculated. The dependences of the results on n, m, and angle α between the relative velocity of the atoms and the normal to the plane of the orbit of the Rydberg electron have been obtained. The influence of orientational effects on the efficiency of the collisional destruction of circular and near-circular states has been studied. The results indicate a higher stability of such states to their perturbations by neutral particles as compared to usually studied nl states with low values of l (l ≪ n).
Time-resolved nsec kinetic methods are used to measure state specific electronic excitation transfer for the near-resonant process: Ca(4s5p /sup 1/P/sub 1/) +Ar /sup k//sub 1/arrow-right-left/sub k//sub -/1 Ca(4s5p /sup 3/P/sub 2,1,0/)+Ar. In stimulated emission experiments it is found that the Ca(4s5p /sup 3/P/sub 2/) level is preferentially populated. The rate constant k/sub 1/ for the specific /sup 3/P/sub 2/ channel is 5.2 +- 0.5 x 10/sup -11/ cm/sup 3/ molecule/sup -1/ sec/sup -1/. These results are discussed in terms of near-resonant spin changing collision mechanisms. Radiative lifetimes for Ca(4s5p /sup 1/P/sub 1/) and Ca(4s5p /sup 3/P/sub 1/) are also determined to be 77 +- 2 nsec and 67 +- 2 nsec, respectively.