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Physical Review A

Observation of large frequency shifts of the electron paramagnetic resonance of Rb87 atoms due to collisions with cell walls coated with RbH salt

Emily Ulanski, Zhen Wu

Published May 30, 2014• 8 citations
Publisher

Abstract

We report an observation of large frequency shifts on the order of several hundred Hz of the electron paramagnetic resonance (EPR) of the gas phase alkali-metal atoms ($^{87}\mathrm{Rb}$) in a millimeter-sized cell coated with RbH salt. We provide convincing evidence that the observed EPR frequency shift is due to collisions with RbH-coated cell walls. The RbH coating is nuclear spin polarized by spin exchange with optically pumped $^{87}\mathrm{Rb}$ vapor, and the observed frequency shift is caused by the Fermi contact interaction between the $s$ electron of the adsorbed $^{87}\mathrm{Rb}$ atom and the polarized nucleus on the salt surface. From the measured EPR frequency shift we calculated the ensemble-averaged phase shift $\ensuremath{\delta}{\ensuremath{\phi}}_{s}$ experienced by the spin polarized $^{87}\mathrm{Rb}$ atoms while adsorbed on the surface. Under our experimental conditions, we found the average phase shift $\ensuremath{\delta}{\ensuremath{\phi}}_{s}\ensuremath{\sim}70\phantom{\rule{0.28em}{0ex}}\mathrm{mrad}$.

DOI
10.1103/physreva.89.053431

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The study of wall interactions of spin polarized rubidium atoms using the frequency shift in the electron paramagnetic resonance of the rubidium atoms

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Wall-collision effect on optically polarized atoms in small and hot vapor cells

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In atomic vapor cells, atoms collide with the inner surface, causing their spin to randomize on the walls. This wall-depolarizing effect can be diffusive and it becomes more pronounced in smaller vapor cells under high temperatures. In this work we investigate the polarization of optically pumped alkali-metal atoms in a millimeter-sized cell heated to $150{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\text{C}$. We consider two extreme boundary conditions, fully depolarizing and nondepolarizing boundaries, and we provide an analytical estimation of the polarization difference between them. In the nondepolarizing case, the pump beam's absorption is proportional to the average atomic polarization. However, for fully depolarizing walls, the absorption peak may correspond to a polarization minimum. To mitigate the wall effect, we propose reducing the pump beam's diameter while maintaining the pump power to prevent illumination of the cell wall and increase the pump intensity in the central area. This is crucial for compact vapor-cell devices where the laser frequency cannot be detuned since it is locked to the absorption peaks. Additionally, we analyze the wall-depolarizing effect on the performance of an alkali-metal atomic magnetometer operating in the spin-exchange relaxation-free regime. We show that the signal strength is highly limited by wall collisions and we provide an upper bound for it.

arXiv:2305.11500•Physical Review A•Feb 16, 2024•7 citations
We report an observation of large frequency shifts on the order of several hundred Hz of the electron paramagnetic resonance (EPR) of the gas phase alkali-metal atoms (87Rb^{87}\mathrm{Rb}87Rb) in a millimeter-sized cell coated with RbH salt. We provide convincing evidence that the observed EPR frequency shift is due to collisions with RbH-coated cell walls. The RbH coating is nuclear spin polarized by spin exchange with optically pumped 87Rb^{87}\mathrm{Rb}87Rb vapor, and the observed frequency shift is caused by the Fermi contact interaction between the sss electron of the adsorbed 87Rb^{87}\mathrm{Rb}87Rb atom and the polarized nucleus on the salt surface. From the measured EPR frequency shift we calculated the ensemble-averaged phase shift \ensuremathδ\ensuremathϕs\ensuremath{\delta}{\ensuremath{\phi}}_{s}\ensuremathδ\ensuremathϕs​ experienced by the spin polarized 87Rb^{87}\mathrm{Rb}87Rb atoms while adsorbed on the surface. Under our experimental conditions, we found the average phase shift \ensuremathδ\ensuremathϕs\ensuremath∼70mrad\ensuremath{\delta}{\ensuremath{\phi}}_{s}\ensuremath{\sim}70\phantom{\rule{0.28em}{0ex}}\mathrm{mrad}\ensuremathδ\ensuremathϕs​\ensuremath∼70mrad.
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.
We report the first quantitative theoretical and experimental studies of frequency shifts of nuclear-magnetic-resonance (NMR) lines of noble-gas atoms and the corresponding frequency shifts of electron-paramagnetic-resonance (EPR) lines of alkali-metal atoms in mixtures of nuclear spin-polarized noble gases and vapors of spin-polarized alkali-metal atoms. The shifts are primarily due to the Fermi-contact hyperfine interaction. For the heavier noble gases there can be relatively large (\ensuremath{\approxeq}10%) contributions to the shifts from van der Waals molecules. The NMR frequencies of polarized 83Kr^{83}\mathrm{Kr}83Kr and 129Xe^{129}\mathrm{Xe}129Xe were measured in a vapor of spin-polarized alkali-metal atoms oriented parallel and antiparallel to the external magnetic field. The shift in the EPR frequency of the alkali-metal vapor due to the polarized noble-gas nuclei was measured simultaneously, thus providing a measurement of the absolute noble-gas nuclear polarization. The frequency shifts can be used as a convenient way to measure the absolute nuclear spin polarization of noble gases.
The spin-exchange rates due to binary collisions and due to the formation of van der Waals molecules between 83Kr^{83}\mathrm{Kr}83Kr nuclei and optically pumped rubidium-metal vapor have been determined using a technique that relies on the simultaneous observation of frequency shifts of nuclear magnetic resonance lines of noble-gas atoms and corresponding frequency shifts of electron paramagnetic resonance lines of alkali-metal atoms. In contrast, earlier studies have relied on the observation of the spin-relaxation rate of either the alkali-metal atoms or the noble-gas atoms.
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.
Methods commonly used in surface science have difficulties in studying surface interaction between spin polarized atoms and anti-relaxation coatings in sealed atomic vapour cells. The average dwell time of atom-wall collisions can provide important information for understanding the atom surface interaction. Here we present a simple method for directly measuring the average dwell time of spin polarized rubidium atoms on coated glass surfaces. The method relies on evanescent-wave induced light shift of the Zeeman resonance of Rb atomic spins, and does not depend on the microscopic details of surface interactions. This is the only reliable method currently available for measuring surface dwell time of spin polarized atoms sealed in vapor cells.
In atomic vapor cells, atoms collide with the inner surface, causing their spin to randomize on the walls. This wall-depolarizing effect can be diffusive and it becomes more pronounced in smaller vapor cells under high temperatures. In this work we investigate the polarization of optically pumped alkali-metal atoms in a millimeter-sized cell heated to $150{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\text{C}$. We consider two extreme boundary conditions, fully depolarizing and nondepolarizing boundaries, and we provide an analytical estimation of the polarization difference between them. In the nondepolarizing case, the pump beam's absorption is proportional to the average atomic polarization. However, for fully depolarizing walls, the absorption peak may correspond to a polarization minimum. To mitigate the wall effect, we propose reducing the pump beam's diameter while maintaining the pump power to prevent illumination of the cell wall and increase the pump intensity in the central area. This is crucial for compact vapor-cell devices where the laser frequency cannot be detuned since it is locked to the absorption peaks. Additionally, we analyze the wall-depolarizing effect on the performance of an alkali-metal atomic magnetometer operating in the spin-exchange relaxation-free regime. We show that the signal strength is highly limited by wall collisions and we provide an upper bound for it.