Line Center Frequency Research Articles

High-precision HD spectroscopy near 1.53 μm

In this study, we report highly precise and accurate measurements of the P(5), P(6), and O(3) transitions of the 2–0 band of HD occurring at approximately 1.5 μm. HD spectra were acquired in the pressure range of 100–900 Torr using a cavity ring-down spectrometer linked to an optical frequency comb. The line-shape analysis was performed using the Hartmann–Tran profile in the so-called β-corrected version. For the P(5) transition, we improved the accuracy of previous line center frequency determinations by more than two orders of magnitude. Moreover, for the P(5) and P(6) line centers, the total standard uncertainty is below 3 MHz, similar to those provided by quantum electrodynamics predictions, making them useful for direct comparisons that might foster future improvements of the theoretical model.

A Network Approach for the Accurate Characterization of Water Lines Observable in Astronomical Masers and Extragalactic Environments.

The water molecule, crucial to the chemical composition and dynamics of the universe, is typically identified in its gas phase via radio and submillimeter transitions, with frequencies up to a few THz. To understand the physicochemical behavior of astronomical objects, accurate transition frequencies are required for these lines. From a set of 26 new and 564 previous Lamb dip measurements, utilizing our ultrasensitive laser-based spectrometers in the near-infrared region, ultrahigh-precision spectroscopic networks were set up for H2 16O and H2 18O, augmented with 40 extremely accurate frequencies taken from the literature. Based on kHz-accuracy paths of these networks, considerably improved line-center frequencies have been obtained for 35 observed or predicted maser lines of H2 16O, as well as for 14 transitions of astronomical significance of H2 18O. These reference frequencies, attached with 5-25 kHz uncertainties, may help future studies in various fields of astrochemistry and astrophysics, in particular when precise information is demanded about Doppler-velocity components, including the gas flows of galactic cores, the kinematics of planetary nebulae, or the motion in exoplanetary atmospheres.

High-resolution mid-infrared spectroscopy based on ultrafast Cr:ZnSe laser.

High-resolution broadband direct frequency comb spectroscopy in the mid-infrared spectral region is an extremely powerful and versatile experimental technique that allows study of the molecular structure of gaseous compounds with multiple applicative and scientific implications. Here we present the first implementation of an ultrafast Cr:ZnSe mode-locked laser covering more than 7 THz at around the emission wavelength of 2.4 μm, for direct frequency comb molecular spectroscopy with a frequency sampling of 220 MHz and a frequency resolution of ∼100 kHz. This technique is based on a scanning micro-cavity resonator with a Finesse of ∼12,000 and a diffraction reflecting grating. We demonstrate its application in high-precision spectroscopy of the acetylene molecule by retrieving line center frequencies of more than 68 roto-vibrational lines. Our technique paves the way for real time spectroscopic studies as well as for hyperspectral imaging techniques.

A stimulated Raman loss spectrometer for metrological studies of quadrupole lines of hydrogen isotopologues

We discuss layout and performance of a high-resolution Stimulated Raman Loss spectrometer that has been newly developed for accurate studies of spectral lineshapes and line centre frequencies of hydrogen isotopologues and in general of Raman active transitions. Thanks to the frequency comb calibration of the detuning between pump and Stokes lasers and to an active alignment of the two beams, the frequency accuracy is at a level of 50 kHz. Over the vertical axis the spectrometer benefits from shot-noise limited detection, signal enhancement via multipass cell, active flattening of the spectral baseline and measurement times of few seconds over spectral spans larger than 10 GHz. Under these conditions an efficient averaging of Raman spectra is possible over long measurement times with minimal distortion of spectral lineshapes. By changing the pump laser, transitions can be covered in a very broad frequency span, from 50 to 5000 cm−1, including both vibrational and rotational bands. The spectrometer has been developed for studies of fundamental and collisional physics of hydrogen isotopologues and has been recently applied to the metrology of the Q(1) 1–0 line of H2.

Lamb-dip saturated-absorption cavity ring-down rovibrational molecular spectroscopy in the near-infrared

The high-detection-sensitivity saturated-absorption cavity ring-down (SCAR) technique is extended to Lamb-dip spectroscopy of rovibrational molecular transitions in the near-infrared region. Frequency-comb-referenced sub-Doppler saturation measurements, performed on the acetylene ( ν 1 + ν 3 + ν 4 ← ν 4 ) R(14)e line at 6562 cm − 1 , are analyzed by a SCAR global line profile fitting routine, based on a specially developed theoretical model. Compared to a conventional cavity ring-down evaluation, our approach yields dip profiles with a linewidth freed from saturation broadening effects, reduced by 40%, and a signal-to-noise ratio increased by 90%. Ultimately, an overall (statistical and systematic) fractional uncertainty as low as 7 × 10 − 12 is achieved for the absolute line-center frequency. At the same time, our method is also able to accurately infer the linear (non-saturated) behavior of the gas absorption, providing Lamb-dip-based line strength measurements with a relative uncertainty of 0.5%.

Lamb-dip cavity ring-down spectroscopy of acetylene at 1.4 μm

Doppler-free saturated-absorption Lamb dips are observed for weak vibration-rotation transitions of C2H2 between 7167 and 7217 cm−1, using a frequency-comb assisted cavity ring-down spectrometer based on the use of a pair of phase-locked diode lasers. We measured the absolute center frequency of sixteen lines belonging to the 2 band, targeting ortho and para states of the molecule. Line pairs of the P and Q branches were selected so as to form a ‘V’-scheme, sharing the lower energy level. Such a choice made it possible to determine the rotational energy separations of the excited vibrational state for J-values from 11 to 20. Line-center frequencies are determined with an overall uncertainty between 3 and 13 kHz. This is over three orders of magnitude more accurate than previous experimental studies in the spectral region around the wavelength of 1.4 μm. The retrieved energy separations provide a stringent test of the so-called MARVEL method recently applied to acetylene.

Optical frequency comb Fourier transform spectroscopy of 14N216O at 7.8 µm

We use a Fourier transform spectrometer based on a compact mid-infrared difference frequency generation comb source to perform broadband high-resolution measurements of nitrous oxide, 14N216O, and retrieve line center frequencies of the ν1 fundamental band and the ν1 + ν2 – ν2 hot band. The spectrum spans 90 cm−1 around 1285 cm−1 with a sample point spacing of 3 × 10−4 cm−1 (9 MHz). We report line positions of 72 lines in the ν1 fundamental band between P(37) and R(38), and 112 lines in the ν1 + ν2 – ν2 hot band (split into two components with e/f rotationless parity) between P(34) and R(33), with uncertainties in the range of 90-600 kHz. We derive upper state constants of both bands from a fit of the effective ro-vibrational Hamiltonian to the line center positions. For the fundamental band, we observe excellent agreement in the retrieved line positions and upper state constants with those reported in a recent study by AlSaif et al. using a comb-referenced quantum cascade laser [J Quant Spectrosc Radiat Transf, 2018;211:172-178]. We determine the origin of the hot band with precision one order of magnitude better than previous work based on FTIR measurements by Toth [http://mark4sun.jpl.nasa.gov/n2o.html], which is the source of the HITRAN2016 data for these bands.

Doppler-limited precision spectroscopy of HD at 1.4 μm : An improved determination of the R (1) center frequency

We report on the improved determination of the center frequency of a hydrogen deuteride vibration-rotation transition. As a result of a refined analysis of cavity ring-down absorption spectra, it was possible to quantify the influence of the hyperfine structure on the line center frequency of the weak 2-0 $R(1)$ line, as retrieved in the Doppler-limited regime. It turns out that ignoring the hyperfine structure leads to an underestimation of the center frequency of about 350 kHz. The updated determination gives an absolute frequency of the $R(1)$ centroid of 217 105 181.936 MHz with a global uncertainty of 76 kHz.

Harmonic instability in a quantum cascade laser with Fabry–Perot cavity

A new state of instability called harmonic instability, which is characterized by the appearance of side modes separated by multiple of free spectral range from each other, was discovered in quantum cascade lasers (QCLs) a few years ago. However, a detailed analysis using a model beyond the two-level density-matrix (DM) equations as well as incorporating phenomena such as the detuning of the primary-mode frequency from the line-center frequency and the frequency dependence of the background refractive index, resulting in group velocity dispersion (GVD), has not been performed yet. In this paper, we present a comprehensive analysis of harmonic instability in a QCL with a Fabry–Perot (FP) cavity. Starting from three-level DM equations, which include the resonant tunneling phenomenon and scattering rates between all three states, and then by using Maxwell's equation, we derive a closed-form expression for the gain of the side modes, from which quantities pertinent to instability can be determined. We also take the aforementioned phenomena into account in our theory. By using our theory, we show the way of determining primary-mode detuning from the line center. Furthermore, we study the effects of GVD on instability in detail, showing that the output from an FP QCL demonstrates the characteristic of a frequency-modulated wave up to a certain value of dispersion. Above this value, because of the significant deviation of the side-mode amplitude ratio from unity, the output shows neither frequency-modulated-like nor amplitude-modulated-like behavior.

Absolute frequency metrology of the CHF3 8.6-µm ro-vibrational spectrum at [formula omitted] level

An optical-frequency-comb-referenced quantum cascade laser is used to perform wavelength-modulation Lamb-dip spectroscopy on a large number of ro-vibrational transitions falling in the CHF3υ5 fundamental band at 8.6 µm. The combined (statistical + systematic) fractional uncertainty in the absolute determination of the line-center frequencies ranges from 9×10−11 down to 2×10−11. This represents an improvement by more than two orders of magnitude, as compared to a recent and extensive study performed with a high-resolution FTIR spectrometer [1]. Our investigation realizes a sharpened knowledge of the CHF3 spectrum over a wide interval, also managing to accurately determine the positions of very close, previously unresolved multiple ro-vibrational components.

Comb-locked frequency-swept synthesizer for high precision broadband spectroscopy

Frequency combs have made optical metrology accessible to hundreds of laboratories worldwide and they have set new benchmarks in multi-species trace gas sensing for environmental, industrial and medical applications. However, current comb spectrometers privilege either frequency precision and sensitivity through interposition of a cw probe laser with limited tuning range, or spectral coverage and measurement time using the comb itself as an ultra-broadband probe. We overcome this restriction by introducing a comb-locked frequency-swept optical synthesizer that allows a continuous-wave laser to be swept in seconds over spectral ranges of several terahertz while remaining phase locked to an underlying frequency comb. This offers a unique degree of versatility, as the synthesizer can be either repeatedly scanned over a single absorption line to achieve ultimate precision and sensitivity, or swept in seconds over an entire rovibrational band to capture multiple species. The spectrometer enables us to determine line center frequencies with an absolute uncertainty of 30 kHz and at the same time to collect absorption spectra over more than 3 THz with state-of-the-art sensitivity of a few 10−10 cm−1. Beyond precision broadband spectroscopy, the proposed synthesizer is an extremely promising tool to force a breakthrough in terahertz metrology and coherent laser ranging.

Lamb-dip spectroscopy of buffer-gas-cooled molecules

Nowadays, buffer-gas cooling represents an invaluable option to produce cold stable molecules, both in view of secondary cooling/trapping strategies towards the achievement of quantum degeneracy and for fundamental studies of complex molecules. From this follows a demand to establish a pool of specialized, increasingly precise spectroscopic interrogation techniques. Here, we demonstrate a general approach to Lamb-dip ro-vibrational spectroscopy of buffer-gas-cooled molecules. The saturation intensity of the selected molecular transition is achieved by coupling the probe laser to a high-finesse optical cavity surrounding the cold sample. A cavity ring-down technique is then implemented to perform saturation sub-Doppler measurements as the buffer (He) and molecular gas flux are varied. As an example, the (ν1+ν3) R(1) ro-vibrational line in a 20 Kelvin acetylene sample is addressed. By referencing the probe laser to a Rb/GPS clock, the corresponding line-center frequency as well as the self and foreign (i.e., due to the buffer gas) collisional broadening coefficients are absolutely determined. Our approach represents an important step towards the development of a novel method to perform ultra-precise ro-vibrational spectroscopy on an extremely wide range of cold molecules. In this respect, we finally discuss a number of relevant upgrades underway in the experimental setup to considerably improve the ultimate spectroscopic performance.

Radiative transfer of hydrogen lines from supernova remnant shock waves: contributions of 2s-state hydrogen atoms

Abstract Radiative transfer in hydrogen lines in supernova remnant (SNR) shock waves is studied taking into account the population of the hydrogen atom 2s-state. Measurements of Balmer line emission, especially of H α, are often relied on to derive physical conditions in the SNR shock. On the other hand, Lyman series photons, especially Ly β, are mostly absorbed by upstream hydrogen atoms. As a result, atoms are excited to the 3p state, and then emit H α by the spontaneous transition from 3p to 2s. Thus, the nature of H α depends on how many Ly β photons are converted to H α photons. Moreover, the Balmer lines can be scattered by the 2s-state hydrogen atoms, which are excited not only by collisional excitation but also by the Lyman–Balmer conversion. It is shown for example that the H α photons are scattered if the shock propagates into an H i cloud with a density of ∼30 cm−3 and a size of ∼1 pc. We find that the line profile of H α becomes asymmetric resulting from the difference between line centre frequencies among the transitions from 3s to 2p, from 3p to 2s, and from 3d to 2p. We also find that the broad-to-narrow ratio of H α, which is often used to estimate the ion-electron temperature equilibrium, varies at most ≃ 10 per cent depending on the ionization degree of the upstream medium because of incomplete conversion of Lyman lines to Balmer lines.

Precision spectroscopy of HD at 1.38μm

The technique of frequency-stabilized comb-calibrated cavity ring-down spectroscopy was used to observe the weak R(1) transition of the deuturated molecular hydrogen (HD) first overtone band in the near infrared. Like molecular hydrogen, HD is a benchmark system to test quantum electrodynamics, looking for new physics beyond the standard model. The spectral line shape was measured with an extremely high fidelity at relatively low gas pressures. The use of a very refined line-shape model allowed us to retrieve the unperturbed line-center frequency of the R(1) line with a global uncertainty of about 100 kHz ($\ensuremath{\delta}\ensuremath{\nu}/\ensuremath{\nu}=5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$). Our value may solve the ambiguity that recently emerged for this line from a pair of sub-Doppler experiments. We also determined other parameters of interest, such as the line strength, the transition dipole moment, and the pressure-broadening coefficient.

Rovibrational fine structure and transition dipole moment of CF3H by frequency-comb-assisted saturated spectroscopy at 8.6 µm

We report on rovibrational fine structure spectroscopy of room-temperature trifluoromethane with a saturated absorption method, using a quantum cascade laser at around 8.6 µm phase-locked to a self-referenced mid-infrared frequency comb synthesizer. The absolute line-center frequencies of 36 lines have been measured by means of the wavelength modulation method with a best fractional uncertainty of ∼3×10−9 in a single acquisition scan from 34.7426 to 34.7443 THz. In addition, an extensive investigation of the measured Lamb-dip profiles using both direct pump-probe spectroscopy method and wavelength modulation technique has been performed for two selected lines, rR40(64) and rR36(38), belonging to the υ5 vibrational band, as a function of either the gas pressure or the pump beam power. This approach allowed the measurement of the main spectroscopic parameters, such as saturation intensity, transition electric dipole moment, as well as self-pressure broadening coefficient.

Absolute spectroscopy near 7.8 \u03bcm with a comb-locked extended-cavity quantum-cascade-laser

We report for the first time the frequency locking of an extended-cavity quantum-cascade-laser (EC-QCL) to a near-infrared frequency comb. The locked laser source is exploited to carry out molecular spectroscopy around 7.8 μm with a line-centre frequency combined uncertainty of ~63 kHz. The strength of the approach, in view of an accurate retrieval of line centre frequencies over a spectral range as large as 100 cm−1, is demonstrated on the P(40), P(18) and R(31) lines of the fundamental rovibrational band of N2O covering the centre and edges of the P and R branches. The spectrometer has the potential to be straightforwardly extended to other spectral ranges, till 12 μm, which is the current wavelength limit for commercial cw EC-QCLs.

Frequency-comb-assisted absolute calibration and linestrength of H12C13CH ro-vibrational transitions in the 2ν3 band

We report on a precision spectroscopic study of room-temperature H12C13CH in the 6463–6520 cm−1 range, using a continuous-wave diode laser phase-locked to a near-infrared optical frequency comb synthesizer stabilized, in turn, against a GPS-disciplined rubidium clock. By tuning the comb repetition frequency, several ro-vibrational transitions in the P and R branches of the 2ν3 overtone band are recorded. Accuracy levels as low as 100 kHz, 5·10−10 in fractional terms, are obtained in the determination of absolute line-center frequencies. In addition, line intensities are measured for the first time, with a best fractional uncertainty of 1%, and used to retrieve the purely vibrational transition dipole moment in conjunction with the first-order Herman–Wallis factor. Finally, the pressure broadening coefficients γself are also derived for the studied transitions.

Conjugating precision and acquisition time in a Doppler broadening regime by interleaved frequency-agile rapid-scanning cavity ring-down spectroscopy.

We propose a novel approach to cavity-ring-down-spectroscopy (CRDS) in which spectra acquired with a frequency-agile rapid-scanning (FARS) scheme, i.e., with a laser sideband stepped across the modes of a high-finesse cavity, are interleaved with one another by a sub-millisecond readjustment of the cavity length. This brings to time acquisitions below 20 s for few-GHz-wide spectra composed of a very high number of spectral points, typically 3200. Thanks to the signal-to-noise ratio easily in excess of 10 000, each FARS-CRDS spectrum is shown to be sufficient to determine the line-centre frequency of a Doppler broadened line with a precision of 2 parts over 1011, thus very close to that of sub-Doppler regimes and in a few-seconds time scale. The referencing of the probe laser to a frequency comb provides absolute accuracy and long-term reproducibility to the spectrometer and makes it a powerful tool for precision spectroscopy and line-shape analysis. The experimental approach is discussed in detail together with experimental precision and accuracy tests on the (30 012) ← (00 001) P12e line of CO2 at ∼1.57 μm.

Absolute frequency measurements of CHF3 Doppler-free ro-vibrational transitions at 8.6 μm.

We report on absolute measurements of saturated-absorption line-center frequencies of room-temperature trifluoromethane using a quantum cascade laser at 8.6 μm and the frequency modulation spectroscopy method. Absolute calibration of the laser frequency is obtained by direct comparison with a mid-infrared optical frequency comb synthesizer referenced to a radio-frequency Rb standard. Several sub-Doppler transitions falling in the υ5 vibrational band are investigated at around 1158.9 cm-1 with a fractional frequency precision of 8.6·10-12 at 1-s integration time, limited by the Rb-clock stability. The demonstrated frequency uncertainty of 6.6·10-11 is mainly limited by the reproducibility of the frequency measurements.

Spectral line-shape study by cavity-enhanced complex refractive index spectroscopy

We present a unique experimental setup that enables measurement of high-resolution complex refractive index spectra of gas sample using several cavity-enhanced methods: frequency-stabilized cavity ring-down spectroscopy, cavity mode-width spectroscopy and one-dimensional cavity mode-dispersion spectroscopy. Presented examples of applications cover molecular spectral line-shape investigations, line intensities and unperturbed line center frequency measurements and high-precision detection of systematic instrumental errors of various spectroscopic methods.