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.
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.
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
1S0\ensuremath{\rightarrow}3p
1P1\ensuremath{\rightarrow}5s
1S0 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.
The concepts of quantum efficiency and ion yield pertinent to a photon detector consisting of an atmospheric pressure oxygen–argon–acetylene (or hydrogen) flame containing magnesium atomic vapour, and also some published experimental results obtained with this detector, are critically discussed. In these experiments, collision-induced ionization and direct photoionization of magnesium atoms are performed. In the former instance, one dye laser excites the atoms from the ground state into the 3s3p–1P10 state and another laser, in temporal and spatial coincidence with the first, brings the excited atoms into the 3s5d–1D2 state, from which collisional ionization occurs. In the last instance, direct photoionization from the 1P10 level is accomplished either with the excimer laser radiation at 308 nm or by tuning the second laser into an autoionizing resonance at 300.9 nm. It is shown that the total loss rate from the 1P10 level can be calculated by time resolving the resonance fluorescence waveform, with and without the presence of the second laser, and that, by integrating this signal over the duration of the laser pulse, the fluorescence dip obtained can be related to the ion yield of the excitation–ionization scheme. It is also stressed that, because of the presence of a metastable level which can act as a trap for the excited atoms, owing to quenching collisions in the flame, the quantum efficiency of this detector will never exceed the ‘ion branching ratio’, i.e., the ratio between the ionization rate and the total loss rate from the excited level. By evaluating several experimental results obtained with different laser systems, it is shown that, in the collision-induced ionization mode, both fluorescence and ionization measurements are necessary to derive the quantum efficiency, whereas in the direct photoionization mode, fluorescence data suffice.
An anomalous resonant dip of two-photon ionisation with 3s3p 1 P as an intermediate state and a stimulated emission by the 3s4s 3 S-3s3p 3 P transition, at 518.37 nm following the pulsed laser excitation near the 3s 2 1 S-3s3p 1 P resonance at 285.21 nm in magnesium vapour are observed and studied. The mechanism of the ionisation dip is attributed to formation and predissociation of the A 1 Sigma + u Mg 2 dimer, which transfer the population from the laser-pumped 3s3p 1 P level to 3s3p 3 P levels. Re-excitation of the produced 3s3p 3 P atoms and the subsequent radiative processes are presumed to explain the generation of stimulated emission. A theoretical model is employed to interpret the experimental phenomena.
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.