Laser spectroscopic measurements of fine-structure changing cross sections of Ca⁺and Sr⁺ions in collisions with He atoms

Abstract

Producing Ca+ and Sr+ ions by laser ablation of pure metal samples and detecting their laser induced fluorescences, cross sections of collision induced transitions between fine-structure levels in the 4p 2PJ state of Ca+ and the 5p 2PJ state of Sr+ due to collisions with He atoms at room temperature (298 K) have been measured. The cross sections determined are σ(Ca+: 4p 2P3/2→4p 2P1/2)= 1.17±0.05 Å2, σ(Ca+: 4p 2P1/2→4p 2P 3/2) = (7.92±0.44)×10-1 Å2 and σ(Sr+: 5p 2P3/2→5p 2P 1/2) = (1.44±0.10)×10-2 Å2.

Exciplexes with Ionic Dopants: Stability, Structure, and Experimental Relevance of M+(2P)4Hen (M = Sr, Ba)

“…Apart from the spectroscopic experiments in bulk helium and He droplets, the formation (and decomposition) of exciplexes was observed following the excitation of the D2 line of Ba + in cold (3−20 K) He gas, 7,8 perhaps as a byproduct of an investigation on fine-structure changing collisional processes involving alkali-earth-metal cations and He atoms. 9 As a net result of these experiments, it emerged that Ba + ( 2 P 3/2 ) is capable of binding a single He atom, the possibility of binding more being hampered by the low He density employed during the experiments. Indeed, two He atoms would be expected to bind strongly to the cation due to the anisotropic density as suggested by Dupont−Roc; 10 additional atoms would bind only via weak dispersion forces.…”

Fine Structure Changing Cross Section of Ba+ Ions in Collisions with He Atoms

“…Here, the total energy of each fluorescence is obtained by spatially and temporally integrating the time-dependent fluorescence signal, taking into consideration the geometry factor of the detection system, transmission efficiencies of the optics, and the detection efficiency of the photomultiplier at each fluorescence wavelength. 11) As is discussed in the previous paper, 8) the energy ratio W SF =W LIF is given by…”

“…6) The latter result is contradictory to the known fact that the FSC collision rate above room temperature inverse-exponentially decreases with an increase in the energy splitting between the fine structure levels. 7,8) Such an enhancement at low temperature might be attributed to many-body effects, such as exciplex formation and/or bubble like cavity vibration. 9) To understand these dynamics the two-body FSC collision process, which is expected to occur at higher temperature, is fundamental, but it has not been measured for Ba þ yet, possibly because of the difficulty in the precise determination of its small cross section.…”

“…The experimental setup is almost the same as in the previous study. 8,10) Therefore we only give its short description here: Ba þ ions are produced by the laser ablation of a BaTiO 3 chip placed in a vacuum chamber by using the fundamental output of a Nd 3þ :YAG laser (New Wave, Tempest). Since FSC cross sections for collisions with residual molecules are usually much larger than for collisions with He atoms, 7) special care is taken to keep high purity of He gas in the vacuum chamber, as is previously described in detail.…”

“…Since FSC cross sections for collisions with residual molecules are usually much larger than for collisions with He atoms, 7) special care is taken to keep high purity of He gas in the vacuum chamber, as is previously described in detail. 8) Helium gas of research grade (99.99995%) is introduced into the vacuum chamber through a liquid nitrogen trap and is thoroughly purified by a getter pump (Saes CapaciTorr-D 400) in the chamber. The gas pressure is monitored with a capacitance manometer (MKS Baratron 626A).…”