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Wavelength modulation laser spectroscopy of N$_2$O at 17 $μ$m

Y. Wang, J. Rodewald, O. Lopez, M. Manceau, B. Darquié, B. E. Sauer, M. R. Tarbutt

TL;DR

This work demonstrates high-precision mid-infrared spectroscopy of N2O near $17\ \mu\mathrm{m}$ using wavelength-modulation spectroscopy with a quantum cascade laser. A comprehensive lineshape model incorporating Doppler and collisional broadening, saturation, and modulation distortion enables accurate extraction of line centers, which are calibrated against a high-stability cavity and a few NIST HITRAN-calibration lines. The authors report 165 transitions across 12 vibrational bands, achieving MHz-level uncertainties and good agreement with HITRAN, while deriving vibrational term energies $G_v$ and refined rotational and $l$-doubling parameters for several bands. The results refine mid-IR N2O line lists and support future frequency standards in this spectral region, potentially leveraging ultracold molecular references such as CaF to enhance calibration precision.

Abstract

Using a mid-infrared quantum cascade laser and wavelength modulation absorption spectroscopy, we measure the frequencies of ro-vibrational transitions of N$_2$O in the 17 $μ$m region with uncertainties below 5 MHz. These lines, corresponding to the bending mode of the molecule, can be used for calibration of spectrometers in this spectral region. We present a model for the lineshapes of absorption features in wavelength modulation spectroscopy that takes into account Doppler broadening, collisional broadening, saturation of the absorption, and lineshape distortion due to frequency and intensity modulation. Combining our data with previous measurements, we provide a set of spectroscopic parameters for several vibrational states of N$_2$O. The lines measured here fall in the same spectral region as a mid-infrared frequency reference that we are currently developing using trapped, ultracold molecules. With such a frequency reference, the spectroscopic methods demonstrated here have the potential to improve frequency calibration in this part of the spectrum.

Wavelength modulation laser spectroscopy of N$_2$O at 17 $μ$m

TL;DR

This work demonstrates high-precision mid-infrared spectroscopy of N2O near using wavelength-modulation spectroscopy with a quantum cascade laser. A comprehensive lineshape model incorporating Doppler and collisional broadening, saturation, and modulation distortion enables accurate extraction of line centers, which are calibrated against a high-stability cavity and a few NIST HITRAN-calibration lines. The authors report 165 transitions across 12 vibrational bands, achieving MHz-level uncertainties and good agreement with HITRAN, while deriving vibrational term energies and refined rotational and -doubling parameters for several bands. The results refine mid-IR N2O line lists and support future frequency standards in this spectral region, potentially leveraging ultracold molecular references such as CaF to enhance calibration precision.

Abstract

Using a mid-infrared quantum cascade laser and wavelength modulation absorption spectroscopy, we measure the frequencies of ro-vibrational transitions of NO in the 17 m region with uncertainties below 5 MHz. These lines, corresponding to the bending mode of the molecule, can be used for calibration of spectrometers in this spectral region. We present a model for the lineshapes of absorption features in wavelength modulation spectroscopy that takes into account Doppler broadening, collisional broadening, saturation of the absorption, and lineshape distortion due to frequency and intensity modulation. Combining our data with previous measurements, we provide a set of spectroscopic parameters for several vibrational states of NO. The lines measured here fall in the same spectral region as a mid-infrared frequency reference that we are currently developing using trapped, ultracold molecules. With such a frequency reference, the spectroscopic methods demonstrated here have the potential to improve frequency calibration in this part of the spectrum.

Paper Structure

This paper contains 12 sections, 22 equations, 8 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Methods
  3. Results and analysis
  4. Example data
  5. Fitting to a model
  6. Data reduction
  7. Comparison with other data
  8. Molecular parameters
  9. Conclusions
  10. Shape of wavelength modulated spectral lines
  11. Frequency uncertainties
  12. Table of frequencies

Figures (8)

  • Figure 1: Schematic of the experimental setup. The 17 $\mu$m laser beam is divided into two parts, one passing through the N$_2$O cell for absorption spectroscopy, and the other through a cavity for frequency calibration. A visible laser is used as an alignment aid. The laser wavelength is modulated and the absorption signal at a harmonic of the modulation frequency is detected using a lock-in amplifier. OSC OUT is the modulation fed to the laser; SIG IN is the direct absortion signal; SIG OUT is the output of the lock-in amplifier.
  • Figure 2: Relevant energy levels of N$_2$O and allowed transitions near 17 $\mu$m. Vibrational states labelled by $\nu_2$ are split into levels according to the value of the bending mode angular momentum quantum number $l$, which is restricted to $0 \le l \le \nu_2$ and $l$ even (odd) when $\nu_2$ is even (odd). Each vibrational state has a ladder of rotational states labelled by the rotational angular momentum quantum number $J$ (we have only shown the first 3 rotational states for the lowest 2 values of $\nu_2$). All transitions we drive have $\Delta \nu_1 = \Delta \nu_3 = 0$, $\Delta \nu_2 = 1$, $\Delta l = \pm 1$. Rotational transitions are labelled as $P$, $Q$ and $R$ for $\Delta J = -1, 0 ,+1$ respectively. The quantum numbers $\nu_1, \nu_3$ are not shown since they do not change in the transitions.
  • Figure 3: Examples of measured $^{14}$N$_2^{16}$O rovibrational transitions detected at $2f_{\rm mod}$ (red), together with cavity fringes (blue). (a) Part of spectrum spanning 0.48 cm$^{-1}$. Vertical black line indicates the frequency of the CaF $v'=1 \leftarrow v=0$ P(1) transition Charron1995Kaledin1999 (b) Expanded region spanning 0.22 cm$^{-1}$, showing many smaller lines. Transitions are labelled in the format '(band)P/Q/R(J,e/f)' with vibrational bands denoted: (1): $(01^10)-(000)$; (2): $(02^00)-(01^10)$; (3): $(03^10)-(02^00)$; (4): $(02^20)-(01^10)$ (5): $(04^20)-(03^10)$.
  • Figure 4: An example fit. Blue dots: absorption signal detected at $2f_{\rm mod}$; Red line: fit to model described by Eqs. (\ref{['eq:wms_lineshape']}) and (\ref{['eq:alpha']}). Blue vertical line: (1)P(9,e) transition of $^{14}$N$_2^{16}$O used as a calibration line in this work. The bottom panel shows the fit residuals. Band labelling is the same as in figure \ref{['fig:n2o_spectra_freq']}.
  • Figure 5: Comparison between our measured N$_2$O transitions and those in the HITRAN Gordon2022 database. The red line is the fitted absorption spectrum converted from the original wavelength modulated spectrum. For this dataset, the left hand axis gives the fraction of incident power transmitted through the cell. The blue sticks are the N$_2$O transitions from HITRAN Gordon2022 and are associated with the line strength given on the right axis. The strongest $\nu_2$ lines are much stronger than the other lines, having a height about 35 times the vertical scale of this figure.
  • ...and 3 more figures