Collisional shift and broadening of Rydberg states in nitric oxide at room temperature

Fabian Munkes, Alexander Trachtmann, Patrick Kaspar, Florian Anschütz, Philipp Hengel, Yannick Schellander, Patrick Schalberger, Norbert Fruehauf, Jens Anders, Robert Löw, Tilman Pfau, and Harald Kübler

Phys. Rev. A 109, 032809 – Published 15 March 2024

Abstract

We report on the collisional shift and line broadening of Rydberg states in nitric oxide (NO) with increasing density of a background gas at room temperature. As a background gas we either use NO itself or nitrogen ( $N_{2}$ ). The precision spectroscopy is achieved by a sub-Doppler three-photon excitation scheme with a subsequent readout of the Rydberg states realized by the amplification of a current generated by free charges due to collisions. The shift shows a dependence on the rotational quantum state of the ionic core and no dependence on the principle quantum number of the orbiting Rydberg electron. The experiment was performed in the context of developing a trace-gas sensor for breath-gas analysis in a medical application.

Received 31 October 2023
Accepted 23 January 2024

DOI:https://doi.org/10.1103/PhysRevA.109.032809

Physics Subject Headings (PhySH)

Scattering of atoms, molecules, clusters & ions

Molecules

Optical spectroscopy

Atomic, Molecular & Optical

Authors & Affiliations

Fabian Munkes ¹, Alexander Trachtmann¹, Patrick Kaspar¹, Florian Anschütz¹, Philipp Hengel ², Yannick Schellander ³, Patrick Schalberger³, Norbert Fruehauf ³, Jens Anders², Robert Löw¹, Tilman Pfau¹, and Harald Kübler ^1,*

¹University of Stuttgart, 5th Institute of Physics, Pfaffenwaldring 57, 70569 Stuttgart, Germany
²University of Stuttgart, Institute of Smart Sensors, Pfaffenwaldring 47, 70569 Stuttgart, Germany
³University of Stuttgart, Institute for Large Area Microelectronics, Allmandring 3b, 70569 Stuttgart, Germany

^*h.kuebler@physik.uni-stuttgart.de

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Issue

Vol. 109, Iss. 3 — March 2024

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Images

Figure 1
Sketch of the setup and measurement principle used throughout this work. Gas mixtures of a certain concentration of NO diluted in $N_{2}$ , or pure NO, enter the measurement cell on the left. Excitation of NO to a Rydberg state is achieved by a cw three-laser excitation scheme shown in the upper left. Subsequent collisions ionize these molecules. Charges are collected via electrodes, and the current is amplified and converted to a voltage by employing a transimpedance amplifier (TIA). A lock-in amplifier referenced on the modulated second transition improves the signal-to-noise ratio. The cell's cuboid has a width of 35 mm, a depth of 13.5 mm, and a height of 8.4 mm. Pictures are shown in Fig. 2.
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Figure 2
Pictures of the readout cell. In the center of the upper photo we can see the quartz windows, which are glued to the glass frame, such that the UV light may pass through. On the left and right are the attachments to the MFCs and pumps. The lower photo shows the PCB with readout electronics on top, as well as a copper plate on the bottom, which realizes the counterelectrode.
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Figure 3
Exemplary measurement trace of a pure NO measurement of 32(4) at a density of $N_{NO} \approx 1.91 (57) \times 10^{15} {cm}^{- 3}$ . The inset shows the fit of the nonmoving centered line, which is used to obtain the relevant data. The fit aligns well with the overall shape of the peak and is as such sufficient to extract both the FWHM and its frequency position.
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Figure 4
FWHM $γ$ and relative shift $δ$ for NO being subject to an increasing density $N$ of the background gas, either NO itself on the left or $N_{2}$ on the right. Plotted are the results for different principal quantum numbers $n$ and rotational quantum numbers $N^{+}$ of the ionic core [Hund's case (d)], and the notation $n (N^{+})$ is used in the legend. The relative shift is moved such that the first measurement points are referenced to zero. The solid line indicates possible contributions by effects on the $H^{2} Σ^{+}$ state as explained in the text. For the measurements of NO perturbed by $N_{2}$ the NO density was $0.6 (3) \times 10^{15} {cm}^{- 3}$ .
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Figure 5
Broadening rate $γ / N$ and shift rate $δ / N$ of the perturbation of Rydberg states in NO. We explain the high broadening rate in comparison to alkalis as seen in Refs. [14, 15] by the additional degrees of freedom in a molecule. Some exemplary alkali values are given in Table 1. An uncertainty of 30% in the measured pressure is indicated by error bars. For some measurement points the marker covers the error bar. Within the error the values are constant, which is expected when compared to alkalis. The individual values are given in Table 2.
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