arXiv subjects

Kevin K. Lehmann

Publications and source records attributed to Kevin K. Lehmann.

At least 19 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

Theory of Lineshapes in Optical-Optical Double Resonance Spectroscopy

This paper presents lineshapes for molecular Optical-Optical Double Resonance (DR) Spectroscopy with arbitrary strength for both pump and probe field using the steady-state solutions for the 3-level density matrix. When the Doppler broadening can be neglected, the results are analytical, and the probe spectrum is a pair of Lorentzian lines that display Autler-Townes splitting, and each has an angular frequency half-width half maximum equal to the relaxation rates, which are all assumed equal. When Doppler broadening is introduced, one must resort to numerical integration except for the limit of weak pump and probe fields. When the Doppler width is assumed much larger than the pump and probe Rabi Frequencies, the calculated DR lineshapes are found to be Lorentzian with a strong pump field limit that is proportional to the pump Rabi frequency, what is commonly known as power broadening. However, the width does not equal the Rabi frequency and is different for co- and counter-propagating pump and probe fields. Furthermore, that broadening is largely inhomogeneous, despite the Lorentzian shape. The saturation power is found to be about 4 times higher than for the bare probe transition with the same relaxation rate, dramatically lower than that expected if the width is interpreted as homogeneous.

physics.optics↗

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↗

arXiv2025

Assignment of collision-induced four-level double-resonance transitions in the 3$ν$${_3}$ ${\Leftarrow}$ $ν$${_3}$ spectral region of methane

Optical-optical double-resonance (OODR) spectroscopy using a narrow-linewidth pump and a frequency comb probe has previously been used to measure and assign sub-Doppler transitions in the 3$ν$${_3}$ ${\Leftarrow}$ $ν$${_3}$ spectral region [J. Chem. Phys. 161, 124311 (2024)] when pumping from the J = (7, A${_2}$) ground state. Doppler-broadened double-resonance transitions were also observed in those OODR spectra. In this paper, 68 of these Doppler-broadened transitions are assigned to four-level double-resonance transitions involving collisional transfer from the pumped A${_1}$ symmetry state to other A${_1}$ and A${_2}$ symmetry (I = 2 meta nuclear spin) levels of the $ν$${_3}$ fundamental state. Assignments are made using combination differences and comparison with the term values and intensities of lines predicted by a new effective Hamiltonian, the accuracy of which has been validated by the sub-Doppler transitions.

physics.chem-ph↗

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

Polarization-dependent Intensity Ratios in Double Resonance Spectroscopy

Double Resonance is a powerful method spectroscopic method that provides an unambiguous assignment of the rigorous quantum numbers of one state of a transition. However, there is often ambiguity as to the branch ($ΔJ$) of the transition. The dependence of the intensity of the double resonance signal on the relative polarization of pump and probe radiation can be used to resolve this ambiguity and has been used for this in the past. However, the published theoretical predictions for this ratio are based upon a weak (i.e. non-saturating) field approximation. In this paper, we present theoretical predictions for these intensity ratios for cases where the pump field is strongly saturating, in the two limits of transitions dominated by homogeneous and inhomogeneous broadening. While saturation, as can be expected, reduces the magnitude of the polarization effect (driving the intensity ratio closer to unity), polarization anisotropy remains even with a strongly saturating probe field in most cases. For the case of an inhomogeneously broadened line, as when Doppler broadening linewidth dominates over even the power broadened homogeneous line width, a large fraction of the low pump power anisotropy remains. Results are presented for both the case of linear and circular pump and probe field polarizations. The present predictions are compared with experimental measurements on CH$_4$ ground state $\rightarrow ν_3 \rightarrow 3ν_3$ transitions recently reported by de Oliveira et al and found to be in better agreement than the weak field predictions.

physics.chem-ph↗

arXiv2020

Doppler-Free Two-Photon Cavity Ring-Down Spectroscopy of a Nitrous Oxide (N$_2$O) Vibrational Overtone Transition

We report Doppler-free two-photon absorption of N$_2$O at $λ$ = 4.53 $μ$m, measured by cavity ring-down spectroscopy. High power was achieved by optical self-locking of a quantum cascade laser to a linear resonator of finesse $F$ = 22730, and accurate laser detuning over a 400 MHz range was measured relative to an optical frequency comb. At a sample pressure of p = 0.13 kPa, we report a large two-photon cross-section of $σ_{13}^{(2)}$ = 8.0 $\times$ 10$^{-41}$ cm$^4$ s molecule$^{-1}$ for the $Q$(18) rovibrational transition at a resonant frequency of $ν_0$ = 66179400.8 MHz.

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

Resonance enhanced two-photon cavity ring-down spectroscopy of vibrational overtone bands: a proposal

This paper presents an analysis of near-resonant, ro-vibrational two-photon spectroscopy and the use of cavity ring-down spectroscopy for its detection. Expressions are derived for the photon absorption rate of a three-level system, correct to all orders and the simpler expressions that result from various approximations. The analysis includes the angular momentum projection degeneracies and linear or circular polarization of the exciting field. Expressions are derived for the rate of two-photon power loss for light inside a resonant cavity. Explicit calculations are made for excitation of the $ν_3$ mode of $^{12}\textrm{C}^{16}\textrm{O}_2$ for which the two-photon excitation spectrum is dominated by a single $v_3 = 0 \rightarrow 2, Q(16)$ line at $\tildeν = 2335.826$\,cm$^{-1}$. This transition has an intermediate $v_3 = 0 \rightarrow 1,P(16)$ one-photon transition that is off resonance by 0.093 cm$^{-1}$ (2.8 GHz). At 1\,torr total pressure, the Q(16) two-photon transition has a calculated cross-section of $2.99 \cdot 10^{-38}$\,cm$^4$s per CO$_2$ molecule in the $J = 16$ state or $2.24 \cdot 10^{-39}$\,cm$^4$s per CO$_2$ molecule at 300\,K is calculated. Analysis of the sensitivity limits for 2-photon cavity ring-down spectroscopy predicts a theoretical detection limit of 32\,ppq ($10^{-15}$) Hz$^{-1/2}$ for $^{12}\textrm{C}^{16}\textrm{O}_2$, higher sensitivity than has been realized using one-photon absorption. The analysis predicts that most polyatomic molecules will have sparse, Doppler-Free two-photon absorption spectra, which will dramatically increase the selectivity of trace gas detection of samples with multiple components with overlapping absorption bands. This is demonstrated by the predicted mid-IR two-photon absorption spectrum of butadiene using theoretical spectroscopic constants.

physics.chem-ph↗

arXiv2018

Influence Spatial Degeneracy on Rotational Spectroscopy Three Wave Mixing and enantiomeric state separation of Chiral Molecules

Pulse flip angles are calculated for three wave mixing, three state cycles of chiral molecules to produce optimized free induction decay amplitudes proportional to the enantiomeric excess of a sample, and to produced optimized degree of state specific enantiomeric separation. These calculations account for the spatial degeneracy of the levels involved and the resulting inhomogeneous distribution of transition dipole moments. It is found that cycles of transitions that include R followed by Q followed by P branch transitions display only modest reductions of the calculated optimal signals if spatial degeneracy is ignored. Transitions cycles P - Q - R are only slightly worse, while the Q - Q - Q cycles are much worse, increasingly so as the rotational total quantum number increases.

physics.chem-ph↗

arXiv2015

Stark Field Modulated Microwave Detection of Molecular Chirality

Patterson, Schnell, \& Doyle, {\it Nature} {\bf 497}, 475 (2013) introduced a microwave experiment that produces a perpendicularly polarized molecular emission when chiral molecules are resonantly excited in the presence of a Stark Field that is then adiabatically switched off before observation of the emission. The sign of the signal is opposite for two stereo-enantiomers and thus the magnitude of the signal gives the enantiomeric excess of the sample. This paper presents a detailed presentation of the theory behind this and provides expressions for the absolute calculation of the expected signal strength for different transitions.

physics.chem-ph↗

arXiv2015

Proposal for Chiral Detection by the AC Stark-Effect

Recently, two related three wave mixing experiments have been demonstrated that use allowed rotational transitions to produce a free induction decay signal with an amplitude linearly proportional to the enantiomeric excess of a chiral molecule. In the present work, a formally five wave mixing experiment is proposed that will exploit near-resonant AC Stark shifts to differentially split a rotational transition of R and S versions of a molecule and thus will allow for the separate measurement of the densities of both enantiomers.

physics.chem-ph↗

arXiv2004

UV spectra of benzene isotopomers and dimers in helium nanodroplets

We report spectra of various benzene isotopomers and their dimers in helium nanodroplets in the region of the first Herzberg-Teller allowed vibronic transition 610 1B2u<-1A1g (the A00 transition) at ~260 nm. Excitation spectra have been recorded using both beam depletion detection and laser-induced fluorescence. Unlike for many larger aromatic molecules, the monomer spectra consist of a single "zero-phonon" line, blueshifted by ~30 cm-1 from the gas phase position. Rotational band simulations show that the moments of inertia of C6H6 in the nanodroplets are at least 6 times larger than in the gas phase. The dimer spectra present the same vibronic fine structure (though modestly compressed) as previously observed in the gas phase. The fluorescence lifetime and quantum yield of the dimer are found to be equal to those of the monomer, implying substantial inhibition of excimer formation in the dimer in helium.

physics.chem-ph↗