Fundamental Physics Using Atoms, 2010/08/09, Osaka Light-Induced Atom Desorption in Alkali Vapor Cells A. Hatakeyama (Tokyo Univ. Agr. Tech.) K. Hosumi K. Kitagami
Alkali vapor cells UHV cell for laser cooling Sealed glass cell Walls are required to confine alkali vapors to keep the vacuum
Wall surfaces: indispensable but unwelcome Spin relaxation and dephasing magnetometry, EDM search, quantum information processing... Loss of atoms experiments using radioactive isotopes, e.g., Fr EDM
Surfaces: still many mysteries The surface is often a key of many experiments. However, physical processes at cell surfaces are not well understood and controllable. Detailed studies from a surface science point of view will be required.
Two major useful surface techniques Anti-spin-relaxation coatings New coating material: T 2 > 1 min, Barabas et al., arxiv:1005.1617 Light-induced atom desorption Today s topic: review and ongoing experiment
Light-indued atom desorption: LIAD Light non-thermal process
First observed LIAD in cells Polydimethylsiloxane coated cell Gozzini et al, Nuovo Cimento D 15, 709 (1993) CH 3 CH 3 CH 3 CH 3 Si O Si O Si CH 3 CH 3 CH 3 CH 3 n Na density is increased when the cell is irradiated (even by orders of magnitude)
LIAD everywhere Various kinds of coatings siloxane, silane, paraffin... Bare (uncoated) surfaces glass (quartz, Pyrex,...), stainless steel,.. Porous materials porous silica, porous alumina.. Caution: Do not mix them up!
LIAD from bare glasses Two types of LIADs from uncoated glass surfaces are reported in 2000-01. They are key techniques in the experiments to load cells with alkali vapors.
UHV cells Typical cells for laser cooling and trapping experiments. Cells are being evacuated to maintain ultrahigh vacuum (~10-8 Pa). LIAD is used to quickly increase alkali density on demand. Threshold at ~3 ev.
Low temperature cells Rb vapor cell containing He buffer gas is cooled to liquid He temperatures. LIAD is used to disperse Rb atoms into the He buffer gas. Resonant photon energy dependence at ~1.8 ev. Normalized loading efficiency 1.0 0.8 0.6 0.4 0.2 0.0 Rb (cell 1) Rb (cell 2) K 1.4 1.6 1.8 2.0 2.2 Photon energy (ev) 2.4
Adsorption of alkali atoms on clean SiO 2 : Knowledge from surface science studies Alkali atoms are first strongly bound (binding energy: a few ev) to surface defects (e.g. non-bridging oxygen) as ionic adsorbates by transferring the valence electrons to the substrate. They do not return to the gas phase thermally at room temperature. With the strong binding sites filled, the adsorption energy for additional adsorbates is low (~0.5 ev). Alkali atoms can thermally return to the gas phase. Further deposition of alkali atoms leads to the formation of alkali aggregates, which are however unstable under UHV conditions and eventually evaporate after the deposition is terminated.
Guess about the cell surface UHV cells: small amount of deposition ; low partial pressure of alkalis isolated ionic adsorbates mainly. LT cells: large amount of deposition; exposure to saturated alkali vapor at room temperature; Cooling to LHe temperatures. Aggregate formation is favored.
Search for the origin of atomic alkalis in planetary atmosphere Na and K atoms contained in the planet Mercury and Moon may result from light-induced desorption. Desorption form model mineral surfaces (amorphous SiO 2 ) is studied. The desorption is likely to be caused by charge transfer excitation from SiO 2 to ionic alkali atoms. Photon energy dependence has a threshold at 3-4 ev. Similar to LIAD in UHV cells
Resonant desorption from alkali nanoparticles Desorption from alkali nanoparticles formed on quartz substrate held at LN2 temperature. It is supposed that the localized electronic excitation of certain binding sites on alkali particles is responsible for the resonant desorption. The surface plasmon resonance of the nanoparticles may enhance the desorption rate. It exhibits resonant photon energy dependence. Similar to LIAD in LT cells
Guess: summary Two types of LIADs from bare glass surfaces in alkali vapor cells are likely to be understood in the context of the results of the surface science studies. UHV cells: The neutralization of isolated ionic adsorbates by photo-excited electron transfer from the substrate is the origin of LIAD induced by ultraviolet light. LT cells: Desorption is stimulated by localized electronic excitation on alkali metallic aggregates. AH et al, e-j. Surf. Sci. Nanotech. 4, 63 (2006)
Experiment It is likely that LIAD from bare glass occurs via various desorption mechanisms (at least two). The characterization of the surface conditions is very important.
Experimental procedure Deposit Rb atoms on quartz substrate. Estimate the surface conditions by the temperature programmed desorption (TPD) method. Observe LIAD by shining uv light (λ=365 nm (3.4 ev)). uv light Base pressure: 10-7 - 10-8 Pa Mass spectrometer Heater Heater & or LN2 LN2 Heating Rb dispenser
View from a chamber window Mass spectrometer Quartz substrate Rb dispenser (behind) Heater
TPD spectrum: surface conditions TPD spectrum Heating rate: 8 K/s 5-min deposition @ 80 Rb multilayers First monolayer? 30 sec 0 s 1 min
Dependence on the amount of deposited Rb 1 min deposition 5 min deposition Desorption rate: 10 11 atoms/cm 2 s Too much deposition makes LIAD less effective.
X-ray Photoelectron Spectroscopy Surface analysis by XPS XPS spectrum O C Rb Si
Penetration of Rb into quartz O Si C Rb Caution: Glass reacts with alkalis
Summary: Experiment going on Light-induced desorption of alkali atoms is observed in various types of vapor cells. Our first target is LIAD from bare glass, specifically Rb on quartz. The study of the dependence on the amount of deposited alkalis and photon energy is underway. Our study will hopefully contribute to the Fr EDM experiment at Tohoku University by increasing the number of trapped Fr atoms.