The graphical processing unit (GPU) offers massive speed advantages in a variety of spectroscopic applications. Single and dual optical frequency comb experiments generally require massive amounts of repetitive signal transformations. We demonstrate continuous, effectively indefinite readout for a comb-based optomechanical accelerometer by performing millions of Fourier transforms and lineshape fit iterations per second with a scheme that is applicable to many comb and chirp-based experiments. Within the realm of molecular absorption and emission spectroscopy, Hartmann-Tran lineshape generation speed is also heavily accelerated with the GPU, with lineshape improvements of 3-4 orders of magnitude compared to standard Python packages. These two examples lead to the possibility of real-time broadband sensing applications for molecular targets using inexpensive, mass-produced hardware.

r0pt Figure Molecular beams have played an important role in chemical physics research, and beam density is crucial in determining beam properties, reaction rate, and differential cross-section in chemical dynamic experiments. However, few results present the absolute molecular beam density at the moment. We set up an experimental setup that combines the supersonic molecule beam and cavity-enhanced method. The absorption spectrum of carbon monoxide in the beam is continuously recorded when the beam is passing the cavity and we have demonstrated that the absolute molecular beam density can be measured. The absorption spectrum of the CO R₃(0) transition in the beam is shown in figure 1, the beam density can be obtained by measuring the absorption spectrum at different pressures and correcting the effective absorption length. It is expected that with the laser-locked cavity-enhanced method beam density of other molecules, such as C₂H₂, H₂O, CH₄, etc., can be measured quantitatively, which is significant in the crossed molecular beam experiments. This also provides a new method for measuring reaction products in chemical dynamic experiments.

High-resolution infrared reduced-Doppler absorption spectra of jet-cooled gas phase trans-formic acid at T_rot ≈ 10.9(5) K are reported for the first time in the ν₁ OH stretching fundamental region, obtained by supersonically expanding trans-formic acid/Ar mixtures through a slit jet nozzle source. Four rovibrational bands are observed, with origins at 3570.493(5) (a/b-type), 3566.793(5) (a/b-type), 3560.032(9) (b-type), and 3534.6869(2) (a-type) cm⁻¹, respectively. Based on previous Raman jet spectroscopic work by Nejad and SibertA. Nejad, E.L. Sibert III, The Raman jet spectrum of trans-formic acid and its deuterated isotopologs: Combining theory and experiment to extend the vibrational database, J. Chem. Phys. 154(6) (2021) 064301. these four bands have been assigned to ν₁, ν₂ + ν₇, ν₆ + 2ν₇ + 2ν₉ (tentatively by our work), and 2ν₃, respectively. Specifically, two of the three upper dark states 2¹7¹ (a′) and 6¹7²9² (a′) are close enough to the “bright” 1¹ (a′) state to facilitate strong anharmonic resonance interactions, resulting in intensity mixing into the two zero-order bands that would otherwise be “dark”. Furthermore, our high-resolution spectral analysis reveals that there are local rotational crossings between zero-order 1¹ and 2¹7¹ states. This motivates the development of a 3 coupled state (1¹, 2¹7¹, and 6¹7²9²) picture to aid in the spectral analysis, which is able to match all 3 observed band origins and relative band intensities, as well as indicate the necessity of multistate coupling. Though limited by the range of J and K_a levels (J′ ≤ 9 and K_a′ ≤ 3) populated at supersonic jet temperatures, this work offers the first precision spectroscopic analysis of trans-formic acid in the ν₁ OH stretching region, which should aid in the assignment of the more complete yet highly congested room temperature FTIR spectraD. Hurtmans, F. Herregodts, M. Herman, J. Liévin, A. Campargue, A. Garnache, A. Kachanov, Spectroscopic and ab initio investigation of the ν_OH overtone excitation in trans-formic acid, J. Chem. Phys. 113(4) (2000) 1535.

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Footnotes:

A. Nejad, E.L. Sibert III, The Raman jet spectrum of trans-formic acid and its deuterated isotopologs: Combining theory and experiment to extend the vibrational database, J. Chem. Phys. 154(6) (2021) 064301., D. Hurtmans, F. Herregodts, M. Herman, J. Liévin, A. Campargue, A. Garnache, A. Kachanov, Spectroscopic and ab initio investigation of the ν_OH overtone excitation in trans-formic acid, J. Chem. Phys. 113(4) (2000) 1535..

Accurate assignments of highly-excited molecular vibrational states are needed for the verification of theoretical predictions of high-temperature spectra observed in astrophysics and combustion environments. We previously demonstrated optical-optical double resonance spectroscopy with a continuous wave 3.3 μm pump and a 1.67 μm centered frequency comb probe, which allows simultaneous measurement of multiple hot-band transitions from a selectively populated state of CH₄ with sub-Doppler resolution[1]. The probe spectrum is dispersed using a Fourier transform spectrometer with comb-mode-limited resolution, and interleaving of spectra measured with different f_\textrep produces a final spectrum with 2 MHz point spacing. More recently, we implemented an enhancement cavity for the comb probe that improved the absorption sensitivity by more than two orders of magnitude. In addition, the use of both co- and counter-propagating probe light eliminated errors in the final state term value due to errors in the Lamb-dip locking point of the pump, which limited the accuracy of the earlier single-pass measurements. Here we use this cavity-enhanced system to measure 3ν₃ ← ν₃ methane transitions from higher J states with much-improved frequency and relative intensity accuracy, which allows unambiguous assignments of the final states using two independent methods. The first method relies on combination differences in probe spectra observed when the pump is sequentially Lamb-dip locked to P, Q, and R ν₃ band transitions from a fixed rotational level of the ground state. We also observe spectra with probe polarization both parallel and perpendicular to that of the pump light and compare the intensity ratio with predictions for the case of a strongly saturated pump. The two experimental methods of final state assignment agree, and are largely confirmed by theoretical predictions from the TheoReTS database. 1. A. Foltynowicz, et al., Phys. Rev. Lett. 126, 063001 (2021); Phys. Rev. A 103, 022810 (2021).

Optical-optical double resonance spectroscopy is a powerful tool to unravel and assign complex molecular spectra, such as found when exciting the overtone transitions of methane. When essentially monochromatic radiation is used for both pump and probe waves, such spectra are often described as “Doppler-free”, but it is more accurate to describe them as having much reduced Doppler broadening. Common treatments describe the lineshape of individual transitions as homogeneously broadened, however, they have widths multiple times the power-broadened homogeneous broadening of the levels involved, even when treated using simplified Bloch equations of three coupled levels. The spatial variation of pump field intensity, both transverse to the propagation vector and due to pump absorption, as well as the M_\textJ dependence or transition dipole moments, further complicate the lineshape but are easily accounted for. More difficult to account for is the presence of inelastic scattering, which changes the Doppler shifts in the probe spectrum, and requires a full collision Kernel to properly model. This talk will present the results of an ongoing attempt to quantitatively model the double resonance spectra we observe using a 3.3 μ continuous wave pump and a 1.67 μm comb probe, in particular their dependence on pump power and sample pressure.

The oxygen A-band, centered at 760 nm, is commonly used in space- and ground-based remote sensing to determine air mass because of the constant mixing ratio of oxygen in Earth's atmosphere. Since the retrieved air mass has a direct impact on measured concentrations of target species like carbon dioxide, increasingly precise greenhouse gas measurements require high-accuracy spectroscopy of the A-band. Pressure broadening, which results from collisions between the absorbing species and other atoms or molecules, dominates line widths in the A-band and is dependent on the identity of the collision partner. Pressure broadening by argon, which comprises 0.934% of Earth's atmosphere by volume, is typically neglected in atmospheric and laboratory measurements of the A-band. To investigate the magnitude of pressure broadening by argon, we used cavity ring-down spectroscopy to measure lineshapes of high J" lines in the P-branch of the oxygen A-band over a range of pressures and argon concentrations. Argon foreign-broadening parameters for these lines were determined in combination with other lineshape parameters using a multi-spectrum fitting algorithm. In addition to comparing the results to literature values, we will discuss the implications of including argon pressure broadening in atmospheric and laboratory measurements of the A-band.

Beryllium containing molecules often exhibit intense multi-reference character that can lead to large discrepancies in computational results depending on the treatment of electron correlation. Exemplary of this multi-reference character is the BeOBe molecule, which has long been a source of frustration for computational methods due to is very small singlet-triplet energy splitting. While the molecule has been studied experimentally before by laser induced florescence, this singlet-triplet gap had yet to be rigorously determined. Here we present the first experimental values for the BeOBe singlet triplet gap, obtained via Slow Electron Velocity Map Imaging (SEVI) spectroscopy on the BeOBe anion, as well as comparison to previous experiment and theoretical calculations. This new data links the singlet and triplet manifolds obtained in previous studies, providing new insight into this strange molecule.

The millimeter/submillimeter spectrum of magnesium chloride (MgCl) has been recorded in electronic excited sates not previously measured, using direct absorption spectroscopy in the range of 210-450 GHz. The molecule was synthesized by reacting chlorine gas (Cl$_2$) with magnesium vapor, produced using a Broida-type oven, in the presence of argon carrier gas. Two distinct rotational patterns were observed, with significantly different rotational constants (B=5504 and 6349 MHz). The first vanishes at N=30$\leftarrow$29, while the latter only appears at and above N=26$\leftarrow$25, with this effect being observed across all vibrational states. There is no evidence of perturbations in either pattern prior to their disappearance. In the first pattern, at least seven rotational transitions in each of six isotopologues ($^{24}$Mg$^{35}$Cl, $^{24}$Mg$^{37}$Cl, $^{25}$Mg$^{35}$Cl, $^{25}$Mg$^{37}$Cl, $^{26}$Mg$^{35}$Cl, $^{26}$Mg$^{37}$Cl) were measured in the ground vibrational state, with numerous vibrationally excited satellite lines (v=1-13) also being observed for each species. For the second pattern, measurements were taken for v=0-2. From both sets of data, rotational and fine structure parameters were determined for $^{24}$Mg$^{35}$Cl, as well as equilibrium constants and the equilibrium bond lengths, r$_e$ = 2.54 \AA \ and 2.36 \AA. From the first pattern, constants for the other five isotopologues were also established, including $^{25}$Mg hyperfine parameters. Based on comparison with theoretical calculations, one excited state has been identified as the (3)$^2\Sigma^+$ state, and the other may arise from the (4)$^2\Sigma^+$ state. The excited state manifold of MgCl has already been the subject of several computational studies, and the unexpected formation of these states emphasizes the need to further investigate both this molecule, as well as other alkaline earth metal halides.

Pure beryllium clusters have been the subject of a number of theoretical studies, but experimental validation for these studies has been lacking due to experimental hazards. The beryllium trimer has been predicted to be a closed shell D₃h neutral molecule, but limited computational studies have been carried out on the anion. In this study we present the first experimental data for the beryllium trimer and its anion. These data provide information on two transitions from the anion to the neutral, namely the Be₃⁻ X² A₂^" → Be₃ X¹ A₁^′ and Be₃⁻ 1² A₁^′ → Be₃ X¹ A₁^′ transitions. Ab inito electronic structure calculations have been validated against the experimental data, confirming the D_3h geometry of the neutral and anion ground states. Electronic transition energies computed at the EOM-CCSDT level of theory agree well with experimental measurements, but the spectra show little excited vibrational activity, in contrast to Franck-Condon simulations.

We investigate the Zeeman effect in the e ⁶Π−a ⁶∆ system of FeH. The molecule has been produced by reaction of Fe(CO)₅ with H atoms, and examined by laser excitation with selective detection and by Fourier transform resolved fluorescence. The field-free spectrum of the e ⁶Π−a ⁶∆ system was extensively studied by the J.M. Brown group and collaborators in the 1990's Goodridge et al J . Chem. Phys. 106 (12), 4823 (1997); Wilson et al J. Chem. Phys. 115 (13), 5943 (2001) Their analysis located the low-lying excited a ⁶∆ state 1890 cm⁻¹ above the X ⁴∆ electronic ground state. One of the infrared systems of FeH already recognisedWallace & Hinkle, ApJ, 559, 424 (2001)n the spectra of cool stars around 1.6 μm, E ⁴Π−A ⁴Π, originates in an excited state lying only 920 cm⁻¹ lower in energy in energy than a ⁶∆, so it seemed not unreasonable to expect signatures from the known sextet systems e−a and g−a to appear in the spectra of cool stars as well. We found that the e−a system can indeed be used as a diagnostic for FeH. Cross-correlation functions between a mask of laboratory-measured e−a transitions and spectra from four M- dwarf stars taken on spectropolarimetric instruments ESPaDOnS (on Maunakea) and Narval (in the French Pyrenees), show that reliable radial-velocity information can be extracted for these objects. Our new Zeeman measurements are intended to improve reference data for cross-correlation calculations for M-type stars, whose magnetic fields are typically 0−5000 Gauss.

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Footnotes:

Goodridge et al J . Chem. Phys. 106 (12), 4823 (1997); Wilson et al J. Chem. Phys. 115 (13), 5943 (2001). Wallace & Hinkle, ApJ, 559, 424 (2001)i