A long-lifetime and maintenance-free atomic cell for laser spectroscopy experiment of ⁶Li atoms Open Access

One type of atomic sample is vaporized atomic species, in which the vapor is dilute and interaction between atoms is negligible. A dense atomic vapor is often the key element in atomic physics research, such as in the fields of quantum metrology^1–7 and quantum information,^8–11 as well as fundamental symmetries tests.^12–15 For laser cooling and manipulation of alkali atoms, high precision laser spectrum supplies stable references for locking laser frequency onto specific resonant atomic transitions.^16–19 For rubidium atoms, the spectroscopic cell can be as small as a coin since it has a low melting point close to room temperature.²⁰ For metal lithium, an atomic cell is preferred for longer service time. Since the lithium bulk has a high melting point of ∼181 °C,²¹ the cell is usually heated to a temperature higher than 350 °C, which challenges the endeavor for observing laser spectrum with a high signal-to-noise ratio (SNR). Several types of atomic cells have been developed to meet the challenge,^22–26 which is usually an independent instrument with a large axial dimension between 30 and 50 cm. In order to get a high enough vapor pressure, the center of the atomic cell needs to be heated to a temperature as high as 350 °C, putting the optical windows of the cell at high risk. One stringent design is executed to solve this problem by attaching water cooling components around the cell tube and very close to the windows.²⁴ This design ensures a wider axial range to be heated well above the melting point of lithium bulk while keeping the optical windows close to room temperature. However, the distance between optical windows and the region of dense atomic vapor is small. Therefore, light transmission of the viewports decreases gradually due to the progressive coating of lithium atoms. Such a kind of lithium cell requires frequent maintenance, and the successive time intervals may be as short as three months. Another method to delay this process is to fill low pressure (<10 Pa) noble gas for buffering the high speed lithium atoms drifting toward the viewports,^23,26 but this method leads to serious spectrum broadening and light transmission reduction. So the traditional atomic cell for ⁶Li is usually complicated and suffers inevitably shorter lifetime.

Here, we introduce an improved heat pipe for the spectroscopy experiment of ⁶Li atoms. The cell has a prolonged axial length while its main structure is kept simple. We study how the extra-length and the special arrangement of heating and cooling of the cell lead to good performance. High quality saturation absorption signal (SAS) of the D-line transitions of ⁶Li atoms is observed at a lower working temperature of 300 °C, and the risk of contamination of viewports without buffer gas protection is minimized. Several measurements are conducted, including monitoring the axial temperature profile by using thermal sensors distributed evenly along the axial direction and analyzing the accumulated outgas after the cell is kept at room temperature for a certain time length. Connections of these measurements to the good performance are discussed at the end.

A schematic drawing of the experimental setup is shown in Fig. 1. The experimental setup consists of a lithium cell and auxiliary assemblies for vacuum pumping. The rectangular area enclosed by the dotted border is the lithium cell, which has an overall axial length of 1.1 m. Such a long tube has several benefits and will be discussed in part III. The main part of the lithium cell is an 80-cm-long stainless-steel tube (hereafter it will be abbreviated as “tube”) with a 19.05 mm inner diameter, and the rest are standard vacuum components that are commercially available. The angle valve 1 is used for vacuum isolation after the cell is fully prepeared and the auxiliary part is detached. During the process of fabrication, the cell is attached to the pumping station (Agilent, TwisTorr 304 FS AG) through two CF35 standard vacuum tees. The side arm of each tee is fixed with a leakage valve (Agilent, 951-5106) and a residual gas analyzer (SRS, RGA200), respectively. The RGA is for analyzing the residual gas components. The leakage valve can be used for vacuum braking with highly purified Ar gas, and it should be fully closed during the vacuum pumping process.

A SUS316 stainless-steel mesh is rolled up and placed tightly against the inner wall of the tube to form a 50-cm long mesh tube, which is used for achieving a recyclable source of lithium metal. The mesh has a ∼0.42 mm spacing weaved with stainless wires of diameter 0.15 mm. The mesh can re-gather the liquid lithium toward the center of the tube as the central area is at a high temperature above the melting point of lithium because of the capillary effect. The lithium sample (∼1 g) was cut into small grains and loaded into the center of the mesh through a 2 cm by 2 cm opening. This is achieved by pulling the mesh along the axial direction of the tube to expose the opening just beyond the CF35 flange. This procedure should be conducted within an environment that is protected by pure argon gas (abundance 99.999%). Several thermo-elements are fixed outside the wall of the tube and evenly spaced along the axial direction. Neighboring distance of center elements is 15 cm and the distance between two end elements is 8.7 cm, as shown in Fig. 2. They are used to monitor the temperature profile of the tube when it is heated. A heating tape is wound around the tube evenly, which is powered by a transformer for maintaining the cell temperature.

After winding the heating tape, several layers of aluminum foil are used to cover the heating tape around the tube to reduce thermal radiation loss. The total length of the heating zone wrapped in aluminum foil along the axial direction of the main tube is 50 cm. This results in a 15-cm separation between the heating zone and the flange at each end. The number of layers of aluminum foils gradually decreases from the middle to both axial ends. As a result, the temperature of the flanges is kept at ∼40 °C without placing additional cooling elements between the heating zone and the end flanges even though the center temperature of the cell is kept as high as 350 °C. This design enables the center of the cell to work at high temperature while keeping both ends at room temperature without additional cooling equipment.

Lithium is such a highly reactive metal that it is prone to react with various gases, such as oxygen (O₂), nitrogen (N₂), and hydrogen (H₂). So, preparing a lithium cell is relatively cumbersome. Before loading lithium metal into the tube, we need to clean the tube thoroughly. The preparation procedure can be divided into five major steps, namely, structure assembly, system degassing, sample loading, sample degassing, and system standby. Structure assembly can be conducted according to Fig. 1. The CF35 viewports are replaced by two blind flanges during steps of system and sample degassing to allow the tube to be baked at 350 °C. This ensures thorough degassing and protects the viewports from overheating. After sample degassing, the viewports will be attached back. This process is conducted under the condition that the tube is flushed with highly purified argon gas through the leakage valve. In fabrication, equal layers of aluminum foils are covered around the tube for even heating. In the step of system standby, we wrap the tube, thickest in the middle, and gradually reduce the number of layers toward the ends. After preparation, the reading of the vacuum gauge on the pumping station is below 10⁻⁵ Pa. The center part of the cell is still wrapped by heating tape and aluminum foils for thermal shielding. The auxiliary parts are cooled down to room temperature and removed after closing the angle valve 1.

Several properties of the atomic cell have been characterized, which can account for its good performance. The temperature profile of the cell has been measured based on the thermo-elements positioned on the tube, and it shows us the hot point of 280 °C in the center and low temperature points beside the two ends of the tube. Residual gas composition inside the cell is analyzed by the RGA. The reactions between residual gas and lithium can be the reason for vacuum recovery after being heated above the melting point of the lithium block. The lifetime of our cell is estimated according to the single pass transmission in four years of the 671-nm laser resonant with the transition of the D2 line of the ⁶Li atom. A high SNR SAS spectrum of ⁶Li with different pressures of Ar gas is observed, which demonstrates it is unnecessary to insert the noble gas.

For the routine experiment, the heating tape is powered by a voltage stabilized transformer to maintain a center temperature of ∼280 °C for long-term operation. The plotted data points in Fig. 2 are temperature readings measured by the thermo-elements described in the preceding text. The temperature profile can be approximated with a parabolic curve (red dashed curve in Fig. 2), exhibiting a good symmetry about the zero position, which represents the center of the tube. When the tube center is maintained at ∼280 °C, the temperature within an axial range of about 47.6 cm can be well kept above the melting point of lithium (181 °C) while both end flanges are just warmed up below 50 °C. Therefore, it requires no additional cooling components to protect the optical viewports from overheating, which simplifies the structure and maintenance of the whole spectroscopy setup.

After preparation, the atomic cell is isolated by closing the angle valve 1. The vacuum level of the cell can be maintained at a pressure below 1.0 Pa, while the tube center is heated to a temperature of 280 °C (working temperature for routine operation). Right after the fabrication and subsequent cooling of the cell to room temperature, the pressure reading on the resistance vacuum gauge was observed to be 0.1 Pa. However, if the cell was kept at room temperature for an extended period, during which the pressure might reach 1.0 Pa within a day and exceed 10 Pa within three days. One might suspect that a leakage might have occurred, but it was discovered that the vacuum could be restored by simply heating the tube to a high temperature (e.g., 300 °C). It was noticed that the pressure in the tube drops below 1.0 Pa when the center temperature is ∼10 °C higher than the melting point of lithium. It is reasonable to believe that specific chemical reactions between lithium and residual gas components have occurred, resulting in a pump effect that could potentially increase the vacuum level of the tube.

To confirm the aforementioned hypothesis, we have analyzed the residual gas inside the tube under both room temperature and high temperature with the RGA. First, the cell was maintained at room temperature for 14 days before conducting the analysis, during which time the pressure increased to 25 Pa. Figure 3 depicts the composition of the accumulated gas within the tube by comparing the RGA results obtained prior to and following the opening of angle valve 1. The black curve in Fig. 3 presents the background gas components within the auxiliary vacuum chamber when the angle valve 1 is closed. This result shows that the background gas components are mainly H₂ and water vapor, with partial pressure levels of ∼1.6 × 10⁻⁸ and ∼0.4 × 10⁻⁸ Torr, respectively. As the angle valve 1 was unscrewed slightly, the pressure reading on the resistance vacuum gauge of the tube decreased to 6.3 Pa in 3 min. The leakage from the tube was then analyzed by the RGA as shown by the red curve in Fig. 3. It shows clearly that the gas accumulated within the tube is mainly N₂ and H₂. Once the angle valve 1 was fully open, the partial pressure of N₂ and H₂ decreased to the background level rapidly, which suggests that they were removed by the vacuum pumping station. Second, to gain further insight into the outgassing composition within the tube, the RGA analysis was conducted at elevated temperatures. Figure 4 illustrates the evolution of the partial pressures of both H₂ and N₂ as the tube is heated gradually up to a high temperature of ∼320 °C, with both the angle valve 1 and angle valve 2 fully open while the pump station is kept running. The partial pressure of H₂ increases with the temperature, whereas for N₂, the increase requires a temperature higher than 260 °C. At temperatures below 260 °C, the partial pressure of N₂ is observed to be almost constant. This result reveals that the primary outgassing components within the tube due to heating are N₂ and H₂, which must be managed appropriately to prevent significant pressure rising. If not, the cell may have a shorter lifetime or require more frequent maintenance.

It is reasonable to speculate that some specific chemical reactions are responsible for reducing the partial pressure of the gas composition in the cell, ensuring that it can be heated to a high temperature (e.g., 300 °C) to serve as a good atomic vapor cell for a lithium spectroscopic experiment. The RGA results indicate that these specific chemical reactions may occur between Li atoms and both H₂ and N₂ molecules. The chemical reaction equations are probably as follows: H₂ + 2Li = 2LiH and N₂ + 6Li = 2Li₃N. In case the cell is in a cold state, only atoms on the surface of the lithium bulk can participate in the reactions. Given the relatively limited surface area of the lithium bulk, these chemical reactions will eventually cease, resulting in an increased pressure due to the accumulation of N₂ and H₂ over time. As the cell temperature increases and above the melting point, the surface area of the fluid lithium expands; the partial pressure of ⁶Li atoms in the tube is also elevated. It supplies enough ⁶Li atoms for these chemical reactions. Since the melting points of LiH and Li₃H are 686.5 and 845 °C, respectively, and they are stable within the vacuum environment, they therefore have a marginal impact on the gas pressure in the tube. This should be accountable to the aforementioned pressure reading below 1.0 Pa of the tube when the cell temperature is maintained at 280 °C.

It should be noticed that this process consumes lithium atoms, which means a sufficient lithium sample should be loaded in the tube. Two cells were constructed according to the same configuration, and the only difference between them is that one was loaded with ∼1 g and the other with ∼0.5 g of lithium sample. For the cell with a smaller quantity of lithium sample, it was observed that the pressure within the tube could reach several Pascal after a period of several days at room temperature right after fabrication. We tried to pump out the accumulated outgas three times; however, for long-term operation, such as maintaining the cell at 300 °C for months, the vacuum within the tube may reach several Pascals and stay there. Nevertheless, there is no observable broadening of the lithium spectrum or reduction in the SNR of the SAS signal; therefore, we conclude there is no more vacuum maintenance needed for it. The tube with a ∼1.0 g lithium sample is maintained at 280 °C since its fabrication, and we always observe a pressure below 1.0 Pa, despite the tube being kept at room temperature for several days due to an accidental blackout. It should be noted that the limited size of the opening of the mesh precludes the possibility of loading a greater quantity of lithium samples. No attempt was made to fabricate a cell with a greater quantity of lithium sample.

Laser transmission is a significant parameter in spectrum experiments based on atomic cells. The atomic tube is filled with the lithium vapor when being heated to above the melting point of Li metal. The transmission of the tube was measured with the laser beam tuned to the resonant wavelength of the D2-line transition of ⁶Li atoms. It was observed that the transmission changes with the concentration of lithium vapor, which can be increased by heating the cell to a higher temperature. Figure 5 shows that the transmission of the laser decays with the center temperature of the tube in an exponential trend. The rapid decline of laser transmission indicates the rapid atomic density growth. The laser transmission at room temperature without baking the tube is about 87%, almost equal to the one at 240 °C, which is nearly consistent with the trend of the curve. We have measured the laser transmission at the temperature of 280 °C occasionally during four years of the service time. It drops from 77.4% in November of 2020 to 74.6% in July of 2024, which means that lithium vapor is rarely coated on the glass windows. We attribute the reason for our cell’s good performance to both the increased length and the special temperature distribution of our cell tube. Our tube has a length of 80 cm, and the hottest point is located in the center, leading to a symmetric distribution of lithium vapor density about the axial center. So, the vapor density around the glass window is quite low and has little probability to coat. The first prepared cell has been used in our cold atom experiment for more than four years. An estimated operation time could reach ten years.

We have managed to lock the laser frequency by using SAS of ⁶Li atoms with the prepared atomic cell. The detailed layout of the SAS experiment can be referred to our previous work.²⁷ The cell is heated to a center temperature of about 300 °C, and a probing light of 220 μW is injected into the cell to drive the 2²S_1/2–2²P_3/2 transitions of ⁶Li atoms. The laser frequency is scanned across a wide range of 400 MHz. The hyperfine splitting of the 2S_1/2 ground state of ⁶Li atoms can be observed in the SAS spectrum, which is related to the separation between the two upward peaks. In a conventional lithium atomic cell, the injection of a noble gas into the cell is essential to prevent coating, which otherwise impairs the transmission of probe light. At a high temperature above the melting point of bulk lithium, the injected noble gas concentrated in the zone close to the viewports serves as a buffer to prevent the energetic ⁶Li atoms from approaching the viewports, thus avoiding coating. In order to ascertain the potential benefits of the injected buffer gas, we injected purified Ar gas into the atomic tube and observed the lithium spectrum at a high temperature (300 °C). Figure 6 displays the spectrum with different pressures of the Ar gas and the full widths at half maximum (FWHM) of the peaks without Ar gas. It can be learned that the FWHMs of both upward peaks are almost constant with different Ar pressures, while the crossover peak increases with Ar pressure due to collisional broadening. However, when the Ar pressure is as low as 0.5 Pa, the FWHM of three peaks reduces, compared with the data without Ar gas, which shows us the cooling effect of noble gas. The fluctuation of peak positions between different Ar pressures can be attributed to the frequency drift of light emitted by the free-running laser. As the Ar pressure increases, the SAS signal becomes weaker, resulting in a declining SNR. It is therefore determined that there is no need to inject a noble gas for observing a spectrum with high SNR, resulting in a more reliable laser frequency stabilization for long-term operation of the experiment.

The fabricated cell has excellent performance in the following aspects. First, the vacuum is self-sustained while the center temperature is maintained at ∼280 °C, which is the working temperature, yielding the advantage that it can be used for spectroscopy experiments without warm-up time. Second, the lifetime of the cell is very long. After four years running a spectroscopy experiment in our lab, the single-pass optical transmission of a 671-nm laser beam through viewports is typically ∼75% and shows little weakening. No need is found to fill noble gas to protect the viewports from contamination. Third, it is suitable for observing high SNR saturation absorption signals, which can be employed to achieve long-term laser frequency stabilization by locking the laser frequency onto one of the D2-line spectroscopic peaks of ⁶Li atoms. This is crucial for the successful cooling and trapping of ⁶Li atoms in a magneto-optical trap (MOT).

In this work, we have demonstrated the design, preparation, and characterization of a lithium atomic cell, which is composed of an 80-cm-long stainless steel tube with a 19.05 mm inner diameter and several standard components. Increased length of the spectroscopic vapor cell allows for the implementation of lithium experiments at a relatively low temperature of about 280 °C. A mesh placed against the inner wall of the tube can recover the liquid lithium that flows away from the center in a hot situation. The symmetrical distribution of the thermal insulation layers beside the center point in the axial conduct leads to the same distribution of temperature as well as lithium vapor density, which makes the viewports barely coated with lithium. The temperatures of the flanges are kept below ∼50 °C even though the center temperature of the cell is kept as high as 350 °C, stopping the viewports from bursting. These features make the atomic cell tube sustain a long service lifetime of more than four years. The observed enhancement of the vacuum level within the tube may be attributed to the chemical reactions between lithium and residual gas. In addition, being free of buffer gas makes the spectrum experiment extremely simple. In one word, the atomic cell discussed in this work is structurally simple, maintenance-free, and can sustain a long lifetime.

An identical cell can be applied to generate vapor of another lithium isotope (⁷Li), which is a bosonic system and is widely studied. The mixed vapor of the two isotopes can be used to detect the isotope shift. Such a specially designed long atomic cell can be used in other kinds of alkali metal atoms, such as Na and K. Both the symmetrical distribution of temperature and the increased length can benefit the lifetime of the cell for the lower density at two viewports that can lead to a relatively lower probability of coating. Maybe the length of the cell used in different types of atoms can be changed on demand. In addition, both Na and K are chemically active, and they can also react with gases released from the cell’s inner walls, which may be successfully used to improve the vacuum, the same as lithium cells.

This work was supported by the National Natural Science Foundation of China (Grant Nos. 12035006 and 12205095), the National Key Research and Development Program of China (Grant No. 2020YFE0202002), and the Zhejiang Provincial Natural Science Foundation of China (Grant No. LQ21A040001).

The authors have no conflicts to disclose.

Hong-Fang Song (宋红芳): Conceptualization (equal); Funding acquisition (equal); Writing – original draft (equal). Jie Wu (吴解): Data curation (equal). Ke Li (李可): Funding acquisition (equal); Investigation (equal); Writing – review & editing (equal). Fu-Qiang Wang: Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon request.