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arXiv:2504.07888 · Journal of Physics B Atomic Molecular and Optical Physics

The role of buffer gas in shaping the D1 line spectrum of potassium vapour

Sharaa A. Alqarni, Danielle Pizzey, Steven A. Wrathmall, Ifan G. Hughes

Published Jun 30, 2025• 1 citation
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Abstract

Abstract In this study, we investigate the effect of buffer gas and magnetic field on the spectral line shapes of the potassium D1 transition using sealed vapour cells filled with varying amounts of neon as a buffer gas. Employing a dual-temperature control system, we independently manipulate the cell body and stem temperatures to explore Doppler and collisional effects on the spectrum. Our results show how the Voigt spectral profile changes from Gaussian- to Lorentzian-dominated forms due to pressure broadening and shifts caused by collisions between potassium atoms and neon. Our measurements are in excellent agreement with the literature values for potassium-neon collisions. For the first time we were able to incorporate the buffer-gas shift and broadening into the modified Voigt profile via the ElecSus code, and found excellent agreement between the predicted and measured line profiles. We also analyse the potassium D1 spectral lines in the hyperfine Paschen–Back regime using strong magnetic fields, demonstrating how Zeeman splitting modifies the pressure-broadened line shape. This work provides valuable insights into collision-induced broadening and shifts, enhancing our understanding of potassium spectroscopy and its application in the development of advanced magneto-optical filters for solar physics and other applications.

DOI
10.1088/1361-6455/adea01

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Abstract In this study, we investigate the effect of buffer gas and magnetic field on the spectral line shapes of the potassium D1 transition using sealed vapour cells filled with varying amounts of neon as a buffer gas. Employing a dual-temperature control system, we independently manipulate the cell body and stem temperatures to explore Doppler and collisional effects on the spectrum. Our results show how the Voigt spectral profile changes from Gaussian- to Lorentzian-dominated forms due to pressure broadening and shifts caused by collisions between potassium atoms and neon. Our measurements are in excellent agreement with the literature values for potassium-neon collisions. For the first time we were able to incorporate the buffer-gas shift and broadening into the modified Voigt profile via the ElecSus code, and found excellent agreement between the predicted and measured line profiles. We also analyse the potassium D1 spectral lines in the hyperfine Paschen–Back regime using strong magnetic fields, demonstrating how Zeeman splitting modifies the pressure-broadened line shape. This work provides valuable insights into collision-induced broadening and shifts, enhancing our understanding of potassium spectroscopy and its application in the development of advanced magneto-optical filters for solar physics and other applications.
We present experimental and theoretical work on atomic bandpass filters and spectroscopic characterisations of alkali vapour systems for magneto-optical applications. We demonstrate a new method for generating arbitrary angle magnetic fields in atomic filters using Rb vapour. This involves a fixed of permanent magnets in a Voigt geometry combined with a solenoid. We compare this method with the previously used method of rotating the permanent magnet pair. This setup offers more precise and flexible angle control. While both methods generate similar transmission profiles, the new setup allows larger angles and supports longer vapour cells, enhancing magneto-optical filters. Next, we investigate K D1 transition in the presence of neon buffer gas, in particular the pressure induced broadening and frequency shift, and the Zeeman splitting in the hyperfine Paschen-Back (HPB) regime. We use dual-control temperature systems to independently adjust Doppler and collisional effects, and we achieve excellent agreement with literature values for potassium-neon collisions. For the first time, buffer gas effects integrate into our theoretical model ElecSus, producing accurate prediction of modified Voigt profiles. We conduct a comprehensive experimental and theoretical study into the Stokes polarimetry of potassium atomic vapour with neon buffer gas, focusing on the temperature and magnetic field effects. This work represents the first application of ElecSus to model the buffer gas polarimetry of K D1 transitions. This study provides new insights into the effects of buffer gases on Stokes parameters, and indicates advances in a theoretical framework for understanding atom-light interactions in buffer gas environments.
Abstract This study presents a comprehensive experimental and theoretical characterisation of Stokes polarimetry in potassium (K) vapour on the D1 line. Measurements were performed in the weak-probe regime, investigating the influence of neon buffer gas in the presence of an applied magnetic field in the Faraday geometry. While previous Stokes polarimetry studies in alkali-metal vapours have been conducted, the specific effects of buffer gas-induced broadening and shifts on the observed Stokes parameters remained largely underexplored. Here, experimental measurements of absolute absorption and dispersion were compared with a theoretical model for the electric susceptibility of the vapour, calculated using the established software package ElecSus . This work marks the first application of ElecSus to model buffer gas polarimetry of the potassium D1 line, with validation performed against experimental spectra for magnetic fields up to 1.2 kG. Our findings provide new insight into how the presence of buffer gas influences the observed Stokes parameters, thereby enhancing the predictive capabilities of theoretical frameworks for atom-light interactions in buffer-gas environments.
Abstract Selective reflection of a laser radiation from an interface formed by a dielectric window and a potassium atomic vapour confined in a nano-cell with 350 nm gap thickness is implemented for the first time to study the atomic transitions of K D 2 line in external magnetic fields. In moderate B -fields, there are 44 individual Zeeman transitions which reduce to two groups (one formed by σ + , the other one by σ − circularly-polarised light), each containing eight atomic transitions, as the magnetic field increases. Each of these groups contains one so-called ‘guiding’ transition whose particularities are to have a probability (intensity) as well as a frequency shift slope (in MHz G −1 ) that are constant in the whole range of 0–10 kG magnetic fields. In the case of π -polarised laser radiation, among eight transitions, two are forbidden at B = 0, yet their probabilities undergo a giant modification under the influence of a magnetic field. We demonstrate that for B -fields > 165 G a complete hyperfine Paschen–Back regime is observed. Other peculiarities of K D 2 line behaviour in magnetic field are also presented. We show a very good agreement between theoretical calculations and experiments. The recording of the hyperfine Paschen–Back regime of K D 2 line with high spectral resolution is demonstrated for the first time.
Abstract We investigate fine structure changing collisions in 87Rb vapour upon D2 excitation in a thermal vapour at 75 ∘C; the atoms are placed in a 0.6 T axial magnetic field in order to gain access to the hyperfine Pashen-Back regime. Following optical excitation on the D2 line, the exothermic transfer 5P 3 / 2 → 5P 1 / 2 occurs as a consequence of buffer-gas collisions; the 87Rb subsequently emits a photon on the D1 transition. We employ single-photon counting apparatus to monitor the D1 fluorescence, with an etalon filter to provide high spectral resolution. By studying the D1 fluorescence when the D2 excitation laser is scanned, we see that during the collisional transfer process the m J ′ quantum number of the atom changes, but the nuclear spin projection quantum number, m I ′ , is conserved. A simple kinematic model incorporating a coefficient of restitution in the collision accounted for the change in velocity distribution of atoms undergoing collisions, and the resulting fluorescence lineshape. The experiment is conducted with a nominally ‘buffer-gas free’ vapour cell; our results show that fine structure changing collisions are important with such media, and point out possible implications for quantum-optics experiments in thermal vapours producing entangled photon pairs with the double ladder configuration, and solar physics magneto-optical filters.
Abstract Unusual magnetically-induced (MI) transitions Fg =1 → Fe = 3 of the 87Rb atoms, D2 lines, forbidden in the absence of magnetic field but becoming significant in magnetic fields >500 G have been studied. The effect of the buffer gas of neon on MI transitions was studied using the process of resonant absorption of laser radiation in a nanocell (NC) with Rb atomic vapors with an NC thickness L = 390 nm and a buffer gas of neon with the pressures of 6 and 20 Torr. The use of NC enables to achieve high spectral resolution and selectively study the MI transitions. It was found that the addition of neon to the cell results in a decrease in the amplitude of the MI transition and its spectral broadening (for some optical processes, the addition of a buffer gas results in an improvement of the parameters). The optimal power of a CW laser for the effective formation of the MI transitions was found to be equal to 10 μW.
We have studied the phenomenon of electromagnetically induced transparency (EIT) of 87Rb vapor with a buffer gas in a magnetic field at room temperature. It is found that the spectral lines caused by the velocity selective optical pump effects get much weaker and wider when the sample cell mixed with a 5-Torr N2 gas while the EIT signal kept almost unchanged. A weighted least-square fit is also developed to remove the Doppler broadening completely. This spectral method provides a way to measure the Zeeman splitting with high resolution, for example, the Λ-type EIT resonance splits into four peaks on the D2 line of 87Rb in the thermal 2-cm vapor cell with a magnetic field along the electric field of the linearly polarized coupling laser. The high resolution spectrum can be used to lock the laser to a given frequency by tuning the magnetic field.