arXiv subjects

Vinicius Silva de Oliveira

Publications and source records attributed to Vinicius Silva de Oliveira.

11 recordsLinked to original sources

arXiv2026

Triple-Resonance Spectroscopy Using a Cavity-Enhanced Frequency Comb Probe

Accurate experimentally verified models of molecular hot-band transitions are essential for interpreting high-temperature spectra in environments ranging from exoplanetary atmospheres to combustion systems. However, line-resolved measurements of infrared hot-band transitions reaching highly excited vibrational states above 10000 cm$^{-1}$ are missing because these transitions are too weak to observe at room temperature and become spectrally congested at elevated temperatures. Here, we introduce a nonlinear spectroscopic approach that enables simultaneous measurement of individual infrared hot-band transitions between four vibrational bands up to 12000 cm$^{-1}$ over a broad spectral range with sub-Doppler resolution and sub-MHz frequency accuracy (10$^{-10}$ relative line position accuracy). The methods is based on an all-optical triple-resonance (AOTR) scheme that combines stepwise mid-infrared pumping using an optical-frequency-comb-stabilized, double-seeded continuous-wave optical parametric oscillator with broadband highly sensitive near-infrared probing using a cavity-enhanced optical frequency comb. As a proof of principle, we measure transitions between high polyads (P) of methane - groups of strongly interacting, near-degenerate vibrational energy states arising from couplings between the C-H stretching and bending modes. In a single probe spectrum, we simultaneously resolve sub-Doppler P4$\leftarrow$P0, P6$\leftarrow$P2 and P8$\leftarrow$P4 transitions, reaching the poorly understood polyad P8 near 12000 cm$^{-1}$ and providing the first set of 41 experimentally observed lines in the P8$\leftarrow$P4 spectral region.Comb-based AOTR spectroscopy opens a new route for broadband exploration of highly excited molecular states, delivering extensive high-accuracy spectroscopic data needed to refine molecular models and improve predictions of high-temperature spectra.

physics.optics↗

arXiv2026

Measurement and assignment of $\it{J}$ $\geq$ 10 rotational energy levels in the 9510 to 9810 cm$^{-1}$ and 6590 to 6900 cm$^{-1}$ ranges of methane using optical frequency comb double-resonance spectroscopy

Accurate models of high temperature methane spectra are needed in astrophysics. Previous measurements of methane hot-band transitions in the $\it{P}$6 $\leftarrow$ $\it{P}$2 polyad range have been limited to final rotational numbers of $\it{J}$ $\le$ 9, with theoretical predictions at higher $\it{J}$s remaining unvalidated. Here, we use optical-optical double resonance spectroscopy (OODR) with a 3.3 $μ$m narrow linewidth pump to excite the $ν$${_3}$ P(12, A${_1}$$^{(2)}$) methane transition ($\it{P}$2 $\leftarrow$ $\it{P}$0) and a cavity-enhanced frequency comb centered around 1.68 $μ$m to probe the sub-Doppler ladder-type ($\it{P}$6 $\leftarrow$ $\it{P}$2) and V-type ($\it{P}$4 $\leftarrow$ $\it{P}$0) transitions, as well as Doppler-broadened collision-induced four-level transitions ($\it{P}$6 $\leftarrow$ $\it{P}$2). 49 ladder-type transitions with final rotational states $\it{J}$ = 10-12 in the range of 9510 to 9810 cm$^{-1}$ (i.e., the $\it{P}$6 polyad) were assigned to effective Hamiltonian predictions and the ExoMol database, of which 6 reached vibrational states that had not been observed experimentally before. 19 sub-Doppler V-type transitions with final states $\it{J}$ = 11-13 in the range of 6590 to 6900 cm$^{-1}$ (i.e., the $\it{P}$4 polyad) were observed and assigned to the Hamiltonian and ExoMol, while only 2 of these V-type transitions could be unambiguously assigned to WKLMC and HITRAN line lists. 170 Doppler-broadened four-level double-resonance (4LDR) lines were observed, 7 of which were newly observed compared with our previous work when pumping transitions starting from the $\it{J}$ = 7 level in the ground state [Lehmann et al., J. Chem. Phys. 163, 144304 (2025)]. We could not assign these lines as they did not form combination differences with other observed 4LDR transitions.

physics.chem-ph↗

arXiv2026

Rotational energy levels in the ground vibrational state of methane with kHz-level accuracy from comb-referenced double-resonance and Lamb-dip spectroscopies

Methane is a key spherical-top molecule, yet restrictive selection rules for one-photon transitions have prevented determination of its ground state (GS) energies with state-of-the-art kHz-level accuracy. We report the GS rotational energy level differences with kHz-level accuracy from two frequency-comb-referenced sub-Doppler methods: optical-optical double-resonance spectroscopy in the $Λ$-type configuration, and Lamb-dip spectroscopy of allowed and forbidden transitions. A Hamiltonian fit to the data yields GS term values with rotational numbers up to $\it{J}$ = 12 with kHz level accuracy.

physics.chem-ph↗

arXiv2026

Optical frequency comb double-resonance spectroscopy of the 9030-9175 cm$^{-1}$ states of ethylene

We use optical-optical double-resonance (OODR) spectroscopy to measure for the first time hot-band transitions of ethylene (C${_2}$H${_4}$) between states in the 3000 cm$^{-1}$ and 9000 cm$^{-1}$ energy ranges. A 3.2 $μ$m continuous wave (CW) pump is used to populate selected states in the $ν$${_9}$ vibrational mode. The sub-Doppler OODR transitions are then probed with two different cavity-enhanced probes tunable around 1.7 $μ$m: a frequency comb probe that allows for broadband measurements and simultaneous detection of many OODR lines, and a CW probe that measures individual lines with higher signal-to-noise ratio and better frequency accuracy. We report center frequencies and relative intensities of 90 ladder-type hot-band transitions from three different states in the $ν$${_9}$ vibrational mode. We exploit combination differences and measurements of polarization-dependent intensity ratios to determine the final state rotational quantum numbers J. Comparison to theoretical predictions from ExoMol allows tentative assignments for 28 transitions. We report improved center frequencies for the three pump transitions in the $ν$${_9}$ band. Furthermore, we observe 18 sub-Doppler V-type transitions from the depleted ground state to the 6000 cm$^{-1}$ region and assign 14 of them to the variational line list of Mraidi et al. [J Quant Spectros Radiat Transfer. 2023;310:108734 doi:10.1016/j.jqsrt.2023.108734].

physics.chem-ph↗

arXiv2025

Measurement and assignment of E-symmetry states in the 6010-6110 cm$^{-1}$ and 8940-9150 cm$^{-1}$ ranges of methane using optical frequency comb double-resonance spectroscopy

We use sub-Doppler optical-optical double-resonance (OODR) spectroscopy with a 3.3 $μ$m single-frequency pump and a cavity-enhanced 1.65 $μ$m comb probe to measure 33 ladder-type (3$ν$${_3}$ ${\leftarrow}$ $ν$${_3}$) and 8 V-type (2$ν$${_3}$) transitions in the 5880-6090 cm$^{-1}$ range of methane, reaching states with rotational E symmetry in the region of the P6 and P4 polyads, respectively. We assign the ladder-type transitions using new Hamiltonian predictions and the ExoMol line list, and the V-type transitions using the new Hamiltonian, ExoMol, HITRAN2020, and the WKLMC line lists. While 7 of the states in the 3$ν$${_3}$ range have been previously observed either in earlier OODR work (without cavity enhancement) with 1.5 MHz accuracy or in FTIR measurements of cold bands with 150 MHz resolution, the states reported here have uncertainties down to 150 kHz (5 $\times$ 10$^{-6}$ cm$^{-1}$). The E-symmetry states exhibit first-order Stark splitting, which will be reported in our future work.

physics.chem-ph↗

arXiv2025

Combined frequency comb and continuous wave cavity-enhanced optical-optical double-resonance spectrometer in the 1.7 $μ$m range

We present an optical-optical double-resonance (OODR) spectrometer based on a 3.3 $μ$m continuous wave pump and two cavity-enhanced probes: a frequency comb tunable in the 1.64 - 1.8 $μ$m range, and a comb-referenced continuous wave (CW) laser tunable in the 1.6 - 1.75 $μ$m range. The comb probe provides broad spectral coverage (bandwidth up to 7 THz) for simultaneous detection of many sub-Doppler OODR transitions with sub-MHz line position accuracy, while the CW probe allows targeting individual transitions with kHz accuracy and higher signal-to-noise ratio in shorter time. Using the pump stabilized to the frequency of the R(0) transition in the $ν$${_3}$ band of methane and the comb probe covering the 5550 to 6070 cm$^{-1}$ interval, we detect 37 ladder-type transitions in the 3$ν$${_3}$ $\leftarrow$ $ν$${_3}$ band region and 6 V-type transitions in the 2$ν$${_3}$ band region and assign them using available theoretical predictions. Using the CW probe, we measure selected ladder- and V-type transitions with much higher precision. We also detect Lamb dips in the R(0)- R(3) transitions of the 2$ν$${_3}$ band and report their center frequencies with kHz level accuracy. The synergy effects of the comb- and CW-OODR open new possibilities in precision spectroscopy of levels that cannot be reached from the ground state.

physics.optics↗

arXiv2024

Sub-Doppler optical-optical double-resonance spectroscopy using a cavity-enhanced frequency comb probe

Accurate parameters of molecular hot-band transitions, i.e., those starting from vibrationally excited levels, are needed to accurately model high-temperature spectra in astrophysics and combustion, yet laboratory spectra measured at high temperatures are often unresolved and difficult to assign. Optical-optical double-resonance (OODR) spectroscopy allows the measurement and assignment of individual hot-band transitions from selectively pumped energy levels without the need to heat the sample. However, previous demonstrations lacked either sufficient resolution, spectral coverage, absorption sensitivity, or frequency accuracy. Here we demonstrate OODR spectroscopy using a cavity-enhanced frequency comb probe that combines all these advantages. We detect and assign sub-Doppler transitions in the spectral range of the 3$ν$${_3}$${\leftarrow}$$ν$${_3}$ resonance of methane with frequency precision and sensitivity more than an order of magnitude better than before. This technique will provide high-accuracy data about excited states of a wide range of molecules that is urgently needed for theoretical modeling of high-temperature data and cannot be obtained using other methods.

physics.chem-ph↗

arXiv2024

Measurement and assignment of J = 5 to 9 rotational energy levels in the 9070-9370 cm$^{-1}$ range of methane using optical frequency comb double-resonance spectroscopy

We use optical-optical double-resonance (OODR) spectroscopy with a continuous wave (CW) pump and a cavity-enhanced frequency comb probe to measure high rotational energy levels of methane in the upper part of the triacontad polyad (P6). A high-power CW optical parametric oscillator, tunable around 3000 cm$^{-1}$, is consecutively locked to the P(7, A$_2$), Q(7, A$_2$), R(7, A$_2$), and Q(6, F$_2$) transitions in the $ν$$_3$ band, and a comb covering the 5800-6100 cm$^{-1}$ range probes sub-Doppler ladder-type transitions from the pumped levels with J' = 6 to 8, respectively. We report 118 probe transitions in the 3$ν$$_3$ $\leftarrow$ $ν$$_3$ spectral range with uncertainties down to 300 kHz (1 x 10$^{-5}$ cm$^{-1}$), reaching 84 unique final states in the 9070-9370 cm$^{-1}$ range with rotational quantum numbers J between 5 and 9. We assign these states using combination differences and by comparison to theoretical predictions from a new ab initio-based effective Hamiltonian and dipole moment operator. This is the first line-by-line experimental verification of theoretical predictions for these hot-band transitions, and we find a better agreement of transition wavenumbers with the new calculations compared to the TheoReTS/HITEMP and ExoMol databases. We also compare the relative intensities and find an overall good agreement with all three sets of predictions. Finally, we report the wavenumbers of 27 transitions in the 2$ν$$_3$ spectral range, observed as V-type transitions from the ground state, and compare them to the new Hamiltonian, HITRAN2020, ExoMol and the WKMLC line lists.

physics.chem-ph↗

arXiv2023

Self-broadening and self-shift in the $\mathbf{3ν_{2}}$ band of ammonia from mid-infrared-frequency-comb spectroscopy

We report the broadband absorption spectrum of the $3ν_{2}$ band of $^{14}$NH$_{3}$ near 4 $μm$. The data were recorded using a mid-infrared frequency comb coupled to a homebuilt Fourier-transform spectrometer with a resolution of 0.00501 $cm^{-1}$. Line positions, line intensities, self-broadening, and self-shift parameters for six rovibrational lines were determined at room temperature ($T=296$ K). Comparison with HITRAN 2016 shows good agreement at improved precision. The high precision and the rapid tunability of our experiment enables advanced fast spectroscopy of molecular gases.

physics.chem-ph↗

arXiv2020

Sub-Doppler Double-Resonance Spectroscopy of Methane Using a Frequency Comb Probe

We report the measurement of sub-Doppler double-resonance transitions in methane using a 3.3 $μ$m continuous wave optical parametric oscillator to pump transitions in the fundamental $ν_3$ band and a 1.67 $μ$m frequency comb to probe ladder-type transitions in the 3$ν_3$ band over 200 cm$^{-1}$ of bandwidth. We detected 36 ladder-type resonances for 9 pumped transitions with average center frequency accuracy of 1.7 MHz, limited by the pump frequency stability. The lines are assigned using the TheoReTS line list, and the intensity ratios for different relative pump/probe polarizations. This method provides accurate assignment of highly excited energy levels that cannot be done using high temperature spectra.

physics.chem-ph↗

arXiv2019

Intensity Noise Optimization of a Mid-Infrared Frequency Comb Difference Frequency Generation Source

We experimentally demonstrate in a difference-frequency generation mid-infrared frequency comb source the effect of temporal overlap between pump- and signal- pulse to the relative intensity noise (RIN) of the idler pulse. When scanning the temporal delay between our 130 fs long signal- and pump pulses, we observe a RIN minimum with a 3 dB width of 20 fs delay and an RIN increase of 20 dB in 40 fs delay at the edges of this minimum. We also demonstrate active long-term stabilization of the mid-infrared frequency comb source to the temporal overlap setting corresponding to the lowest RIN operation point by an on-line RIN-detector and active feedback control of the pump-signal- pulse delay. This active stabilization set-up allowed us to dramatically increase the signal-to-noise ratio of mid-infrared absorption spectra.

physics.optics↗