Observations of a new dynamical process, absorption of two optical photons during a binary collision, have been made. Measurements on magnesium-rare-gas collisions display a rich phenomenology associated with the collisions, the relative linear polarization directions of the exciting light beams, and their separate detunings from atomic Mg 3s ^{1}$${\mathit{S}}_{0}\ensuremath{\rightarrow}3p ^{1}$${\mathit{P}}_{1}\ensuremath{\rightarrow}5s ^{1}$${\mathit{S}}_{0} stepwise resonance. Polarization spectra for Mg-Ne and Mg-Ar fractional collisions are reported, and are seen to contain information on short-range collision dynamics and on evolution of alignment produced in the process.
Extensive experimental studies of Mg-Ne and Mg-Ar fractional collisions are reported. In the fractional-collision process, two-color, two-photon absorption takes place during a single binary collision. Polarization and relative intensity spectra were measured in an approximately \ifmmode\pm\else\textpm\fi{}50-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range around the atomic Mg 3s
1S0\ensuremath{\rightarrow}3p
1P1\ensuremath{\rightarrow}5s
1S0 two-photon resonance. The generally-weak-field spectra show a remarkable phenomenology associated with the relative linear-polarization directions of the two exciting laser beams and with their separate detunings from atomic one-photon resonance. The polarization and relative absorption spectra display effects due to relative detuning, multiple Condon points, and laser intensity. Details of our experimental approach, and spectra for the process in Mg-Ar and Mg-Ne are presented and discussed. The data are seen to contain information on the short-range collision dynamics of the Mg--rare-gas pairs, and on the development of the alignment over the course of the collision.
Experimental, polarization-dependent excitation spectra for excited-state-Mg--rare-gas-atom optical collisions are reported. In these first studies of the process, polarized Mg atoms in the 3p
1P1 level are produced by absorption of linearly polarized light tuned to the 3s
1S0\ensuremath{\rightarrow}3p
1P1 resonance transition at 285.2 nm. Detuning-dependent, collision-induced polarization spectra are measured in a \ifmmode\pm\else\textpm\fi{}200-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range around the Mg 3p
1P1\ensuremath{\rightarrow}5s
1S0 transition at 571.2 nm. The spectra correspond to probing transient Mg-Ne and Mg-Ar molecules on 3p $^{1}\mathrm{\ensuremath{\Pi}}_{1}$\ensuremath{\rightarrow}5s $^{1}\mathrm{\ensuremath{\Sigma}}_{0}^{+}$ and 3p $^{1}\mathrm{\ensuremath{\Sigma}}_{0}^{+}$\ensuremath{\rightarrow}5s $^{1}\mathrm{\ensuremath{\Sigma}}_{0}^{+}$ electronic transitions. Measurements of these excited-state polarization spectra for Mg-Ne optical collisions reveal that for detunings to the red of the atomic Mg 3p
1P1\ensuremath{\rightarrow}5s
1S0 transition, electronic linear polarization greater than 50% survives far into the molecular regime. This represents a direct measure of the polarization important to alignment-dependent inelastic processes in alkaline-earth-metal--rare-gas-atom collisions. The polarization spectra are discussed in terms of existing information on the interatomic potentials and through an axial recoil limit for the polarization degree for parallel and perpendicular molecular transitions. Rate coefficients k for disalignment of Mg 3p
1P1 atoms by collisions with Ar [k=9.4(5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$
cm3/s] and with Ne [k=6.5(7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$
cm3/s] are also extracted from the data.
Experimental spectra for excited-state optical collisions between
Mg(3p1P1) atoms and rare-gas atoms are presented. Aligned Mg atoms are produced in the
3p1P1 level by excitation with linearly polarized light. Collisions between the excited metal atoms and Ne or Ar rare-gas atoms are probed in a $1000\ensuremath{-}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range in the vicinity of the Mg $3p{}^{1}{P}_{1}\ensuremath{\rightarrow}5s{}^{1}{S}_{0}$ and $3p{}^{1}{P}_{1}\ensuremath{\rightarrow}4d{}^{1}{D}_{2}$ transitions. The probe radiation is linearly polarized either along or perpendicular to the alignment axis, leading to a linear polarization for the optical collision process as a function of detuning of the probe from the atomic resonances. Substantial spectral variation of the linear polarization reflects the different molecular transitions responsible for the excitation process, and the evolution of the atomic alignment into the molecular regime. In the present case, the transitions correspond to excitation from the Mg--rare-gas $3p{}^{1}{\ensuremath{\Pi}}_{1}$ and $3p{}^{1}{\ensuremath{\Sigma}}_{0}^{+}$ molecular terms to the $5s{}^{1}{\ensuremath{\Sigma}}_{0}^{+},$ $4d{}^{1}{\ensuremath{\Delta}}_{2},$ $4d{}^{1}{\ensuremath{\Pi}}_{1},$ and $4d{}^{1}{\ensuremath{\Sigma}}_{0}^{+}$ terms. Experimental details of the approach, and the results obtained, will be described. Discussion of the polarization spectra is made through theoretical modeling focusing on the effects of the symmetry of the molecular states involved in the process. The modeling employs the semiclassical theory developed earlier. For a range of qualitatively correct interaction potentials and corresponding Condon points for the photo-excitation channels, good agreement between experiment and theoretical estimates is obtained.
Polarization and intensity spectra have been obtained for excited-state magnesium (3s3p 1P1) - rare gas optical collisions. The magnesium was excited by a linearly polarized laser tuned to the 3s21s0 → 3s3p 1P1 atomic transition at 285 nm. A second linearly polarized laser was tuned near the 3s3p 1P1 → 3s5s 1S0 transition at 571 nm (experiment A) and near the 3s3p 1P1 → 3s4d 1D2 transition at 553 nm (experiment B). The polarization vector of the second laser was oriented either parallel or perpendicular to the polarization direction of the first laser. The absorption of nonresonant light from the second laser, possible only during a collision, was monitored by cascade fluorescence from the 3s21S0 ← 3s4p 1P1 transition at 203 nm. For each detuning of the second laser from resonance, the degree of linear polarization and the intensity were measured. For experiment A, argon and neon were used as rare gases. For experiment B, argon was the rare gas used. For experiment B, fluorescence from the 3s3p 1P1 ← 3s5s 1S0 transition, and the 3s3p3Pj ← 3s5s 3S1 transition at 333 nm were also measured in an attempt to observe the region of inelastic transitions in the molecular states. No inelastic branching was observed in this experiment.
We have analyzed by perturbation theory technique two-photon polarization-dependent fractional optical collisions. In its general form the cross section of the collision is expressed as the overlap integral of its spectral profile with the spectrum of the second order correlation function of the electromagnetic field. Based on the semiclassical expansion of radial wave functions and of a retarded Green function we have expressed the spectral profile of the cross section in terms of well understood semiclassical characteristics of the process such as the rotational transformations of the transition dipole moments and the transition amplitudes describing the adiabatic or nonadiabatic dynamics of the electronic subsystem along classical trajectories. As a practical example, we have calculated the Mg-He ${(3s}^{2}{}^{1}{S}_{0}\ensuremath{\rightarrow}3p{}^{1}{P}_{1}\ensuremath{\rightarrow}5s{}^{1}{S}_{0},4d{}^{1}{D}_{2})$ fractional optical collision cross section. The partial wave analysis for excitation up to the
5s1S0 state has shown the nonvalidity of quasistatic approximation based on successive single-photon transitions for some frequency detunings. For example, for the positive detuning from two-photon atomic resonance we have obtained the dominant contribution coming from the direct two-photon Franck-Condon transitions. Such a contribution gives the magnitude of the polarization ratio, characterizing the dependence of the cross section on mutual laser polarizations at the first and at the second steps of the photoexcitation, different than for successive single-photon Franck-Condon transitions. In the more complicated case of excitation up to the
4d1D2 state of magnesium the role of the interference between different photoexcitation channels becomes important. The polarization ratio has a stable value close to
71 in a broad spectral range in the blue wing of the second frequency detuning. There is a strong dependence of the polarization ratio in the red wing of the second detuning. Such spectral behavior can be explained by the partial selection in the red wing of the atomic resonance line, perturbed by the collisions of those Zeeman transitions in which contributions in the polarization ratio have different orders and different signs.
An experimental investigation of the atomic-Na 3s
2${\mathit{S}}_{1/2--}$5s
2S1/2 two-color two-photon polarization spectrum has been made. Measurements of the linear polarization degree of the two-photon transition were made for one laser tuned within a \ifmmode\pm\else\textpm\fi{}100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range of the atomic-Na 3p
2Pj levels, with the second adjusted to satisfy the two-photon resonance condition. The relatively short-pulse (\ensuremath{\sim}0.5 ns) lasers used eliminate effects on the spectrum due to atomic-Na hyperfine interactions and radiation trapping, while greatly reducing depolarization due to collisions with Ar gas. In the absence of collisions, the measured polarization spectrum is in excellent agreement with calculations. Measurements of the Ar pressure dependence of the spectrum revealed strong depolarization in the vicinity of resonant excitation through the 3p
2P3/2 level. A collisional depolarization cross section of 200(30) \AA{} for this process was deduced from the data.