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New Journal of Physics

Double-resonance lineshapes in a cell with wall coating and buffer gas

Svenja A. Knappe, H. G. Robinson

Published Jun 28, 2010• 26 citations
Publisher

Abstract

Microwave double resonances were measured in a wall-coated Rb vapor cell as a function of additional buffer-gas pressure. These data were compared with similar measurements in an uncoated cell. It was found that the linewidth in the coated spherical cell of diameter 1.6 cm displays a distinct maximum around 0.2 kPa. This agrees well with theoretical solutions of the diffusion equation, assuming a complex reflection coefficient at the wall. It was also found that at intermediate pressures the lineshapes of the microwave resonances become asymmetric with a low-frequency tail. This agrees with the explanation that where the alkali mean free path is substantially smaller than the average distance between wall collisions at zero pressure, there exist two classes of atoms in the cell. The atoms that get trapped near the walls accumulate much larger phase shifts than those toward the center of the cell. This effect is not seen in the longitudinal relaxation rate, indicating that it is related to a phase-shift effect.

DOI
10.1088/1367-2630/12/6/065021

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Spectral lineshape narrowing of atomic vapor emission in a bufferless, wall-coated absorption cell has been analyzed using the same formalism developed t3 describe sub-Doppler linewidths. Dicke-narrowing, in a buffered cell. Dramatic linewidth reductions in the bufferless cell, compared to the buffered cell, occur because of the high probability that Doppler shifts before and after a particular wall collision will cancel each other. The formalism described allows more physical insight than the classic onedimensional box model previously invoked to explain spectral narrowing in bufferless, wall coated cells. Additionally, this formalism reveals a correlation between post-collision velocities and intercollision times in the buffered cell resulting in modifications of the standard Dicke analysia of these cells. Finally, the computational power of the technique is demonstrated. The effects of varying cell size on Rb and CS hyperfine lineshapes and the results of the presence of an inhomogeneous magnetic field within the cell are analyzed.

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•Jan 1, 2014•0 citations
Microwave double resonances were measured in a wall-coated Rb vapor cell as a function of additional buffer-gas pressure. These data were compared with similar measurements in an uncoated cell. It was found that the linewidth in the coated spherical cell of diameter 1.6 cm displays a distinct maximum around 0.2 kPa. This agrees well with theoretical solutions of the diffusion equation, assuming a complex reflection coefficient at the wall. It was also found that at intermediate pressures the lineshapes of the microwave resonances become asymmetric with a low-frequency tail. This agrees with the explanation that where the alkali mean free path is substantially smaller than the average distance between wall collisions at zero pressure, there exist two classes of atoms in the cell. The atoms that get trapped near the walls accumulate much larger phase shifts than those toward the center of the cell. This effect is not seen in the longitudinal relaxation rate, indicating that it is related to a phase-shift effect.
Spectral lineshape narrowing of atomic vapor emission in a bufferless, wall-coated absorption cell has been analyzed using the same formalism developed t3 describe sub-Doppler linewidths. Dicke-narrowing, in a buffered cell. Dramatic linewidth reductions in the bufferless cell, compared to the buffered cell, occur because of the high probability that Doppler shifts before and after a particular wall collision will cancel each other. The formalism described allows more physical insight than the classic onedimensional box model previously invoked to explain spectral narrowing in bufferless, wall coated cells. Additionally, this formalism reveals a correlation between post-collision velocities and intercollision times in the buffered cell resulting in modifications of the standard Dicke analysia of these cells. Finally, the computational power of the technique is demonstrated. The effects of varying cell size on Rb and CS hyperfine lineshapes and the results of the presence of an inhomogeneous magnetic field within the cell are analyzed.
An alkene-based antirelaxation coating for alkali-metal vapor cells exhibiting Zeeman relaxation times up to 77 s was recently identified by Balabas et al. The long relaxation times, two orders of magnitude longer than in paraffin- (alkane-) coated cells, motivate revisiting the question of what the mechanism is underlying wall-collision-induced relaxation and renew interest in applications of alkali-metal vapor cells to secondary frequency standards. We measure the width and frequency shift of the ground-state hyperfine $m{}_{F}=0\ensuremath{\rightarrow}m{}_{F}^{\ensuremath{'}}=0$ transition (clock resonance) in vapor cells with 85{}^{85}85Rb and 87{}^{87}87Rb atoms, with an alkene antirelaxation coating. We find that the frequency shift is slightly larger than for paraffin-coated cells and that the Zeeman linewidth scales linearly with the hyperfine frequency shift.
We present a microfabricated alkali vapor cell equipped with an anti-relaxation wall coating. The anti-relaxation coating used is octadecyltrichlorosilane and the cell was sealed by thin-film indium-bonding at a low temperature of 140 °C. The cell body is made of silicon and Pyrex and features a double-chamber design. Depolarizing properties due to liquid Rb droplets are avoided by confining the Rb droplets to one chamber only. Optical and microwave spectroscopy performed on this wall-coated cell are used to evaluate the cell's relaxation properties and a potential gas contamination. Double-resonance signals obtained from the cell show an intrinsic linewidth that is significantly lower than the linewidth that would be expected in case the cell had no wall coating but only contained a buffer-gas contamination on the level measured by optical spectroscopy. Combined with further experimental evidence this proves the presence of a working anti-relaxation wall coating in the cell. Such cells are of interest for applications in miniature atomic clocks, magnetometers, and other quantum sensors.
Spectral lineshape narrowing of atomic vapour emission in a bufferless, wall-coated absorption cell has been analysed using the same formalism developed to describe sub-Doppler linewidths, Dicke narrowing, in a buffered cell. Within this formalism lineshapes are generated using Monte Carlo techniques. Dramatic linewidth reductions in the bufferless cell, compared with the buffered cell, occur because of the high probability that Doppler shifts before and after a particular wall collision will cancel each other. The formalism described allows more physical insight than the classic one-dimensional box model previously invoked to explain spectra narrowing in bufferless, wall-coated cells. Additionally, this formalism reveals a correlation between post-collision velocities and intercollision times in the buffered cell resulting in modifications of the standard Dicke analysis of these cells.
We measured the rate of velocity-changing collisions (VCCs) between alkali atoms and background gas in buffer-gas-free anti-spin-relaxation-coated cells. The average VCC rate in paraffin-coated rubidium vapor cells prepared in this work was 1×1061 \times 10^{6}1×106 s−1^{-1}−1, which corresponds to $\sim$1 mm in the mean free path of rubidium atoms. This short mean free path indicates that alkali atoms do not travel freely between the cell walls. In addition, we found that a heating process known as increases the VCC rate, and also confirmed that ripening improves the anti-relaxation performance of the coatings.
I studied the effect of wall collisions on the electron paramagnetic resonance (EPR) of gas phase alkali metal atoms (87Rb). Two types of walls were studied: antirelaxation-coated walls and RbH-coated walls. I measured the dwell times of spin polarized Rb atoms on the two most common antirelaxation coatings, octadecyltrichlorosilane (OTS)- and paraffin. I found that at a cell temperature of 72 C the dwell times for OTS- and paraffin-coated walls are 0.9 ± 0.1 μs and 1.8 ± 0.2 μs, respectively. The implication of these dwell times, which are much longer than what was previously reported in the literature, is discussed in connection with the relaxing property of these coatings. In cells coated with RbH salt I made the first observation of the EPR frequency shift of the gas phase 87Rb atoms. Due to collisions with the RbH salt, which was nuclear spin polarized by spin exchange with optically pumped Rb vapor. The measured EPR frequency shifts are of the order of several hundred Hz in a millimeter-sized cell. By measuring the dependence of the EPR frequency shifts on cell length and by comparing the EPR frequency shifts in RbH-coated cells with those in OTS-coated cells, I provided convincing evidence that the observed EPR frequency shifts are due to collisions with RbH-coated walls, and not due to gas phase processes. The EPR frequency shift is due to the Fermi contact interaction between the s-electron of the adsorbed 87Rb atom and the polarized nucleus on the salt surface. The measured EPR frequency shifts allowed me to calculate the ensemble-averaged phase shift experienced by the Rb atom while adsorbed on the walls. Under our experimental condition this is about 70 mrad. My experiment opens up the possibility of studying surface NMR using gas phase EPR. The phase shift is proportional to the surface nuclear polarization, thus provides an interesting method for measuring the surface nuclear polarization using gas phase EPR. My observation will alert the high precision miniature atomic magnetometry community that collisions with cell walls may cause a systematic error and line broadening in the magnetic resonance frequency.