arXiv subjects

Isak Silander

Publications and source records attributed to Isak Silander.

17 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 ${\mu}$m narrow linewidth pump to excite the ${\nu}$${_3}$ P(12, A${_1}$$^{(2)}$) methane transition ($\it{P}$2 $\leftarrow$ $\it{P}$0) and a cavity-enhanced frequency comb centered around 1.68 ${\mu}$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 ${\Lambda}$-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 ${\mu}$m continuous wave (CW) pump is used to populate selected states in the ${\nu}$${_9}$ vibrational mode. The sub-Doppler OODR transitions are then probed with two different cavity-enhanced probes tunable around 1.7 ${\mu}$m: a frequency comb probe that allows for broadband measurements and simultaneous detection of many OODR lines, and a CW probe that measures individual lines with higher signal-to-noise ratio and better frequency accuracy. We report center frequencies and relative intensities of 90 ladder-type hot-band transitions from three different states in the ${\nu}$${_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 ${\nu}$${_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 ${\mu}$m single-frequency pump and a cavity-enhanced 1.65 ${\mu}$m comb probe to measure 33 ladder-type (3${\nu}$${_3}$ ${\leftarrow}$ ${\nu}$${_3}$) and 8 V-type (2${\nu}$${_3}$) transitions in the 5880-6090 cm$^{-1}$ range of methane, reaching states with rotational E symmetry in the region of the P6 and P4 polyads, respectively. We assign the ladder-type transitions using new Hamiltonian predictions and the ExoMol line list, and the V-type transitions using the new Hamiltonian, ExoMol, HITRAN2020, and the WKLMC line lists. While 7 of the states in the 3${\nu}$${_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 ${\mu}$m range

We present an optical-optical double-resonance (OODR) spectrometer based on a 3.3 ${\mu}$m continuous wave pump and two cavity-enhanced probes: a frequency comb tunable in the 1.64 - 1.8 ${\mu}$m range, and a comb-referenced continuous wave (CW) laser tunable in the 1.6 - 1.75 ${\mu}$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 ${\nu}$${_3}$ band of methane and the comb probe covering the 5550 to 6070 cm$^{-1}$ interval, we detect 37 ladder-type transitions in the 3${\nu}$${_3}$ $\leftarrow$ ${\nu}$${_3}$ band region and 6 V-type transitions in the 2${\nu}$${_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${\nu}$${_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${\nu}$${_3}$ ${\Leftarrow}$ ${\nu}$${_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${\nu}$${_3}$ ${\Leftarrow}$ ${\nu}$${_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 ${\nu}$${_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

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 ${\nu}$$_3$ band, and a comb covering the 5800-6100 cm$^{-1}$ range probes sub-Doppler ladder-type transitions from the pumped levels with J' = 6 to 8, respectively. We report 118 probe transitions in the 3${\nu}$$_3$ $\leftarrow$ ${\nu}$$_3$ spectral range with uncertainties down to 300 kHz (1 x 10$^{-5}$ cm$^{-1}$), reaching 84 unique final states in the 9070-9370 cm$^{-1}$ range with rotational quantum numbers J between 5 and 9. We assign these states using combination differences and by comparison to theoretical predictions from a new ab initio-based effective Hamiltonian and dipole moment operator. This is the first line-by-line experimental verification of theoretical predictions for these hot-band transitions, and we find a better agreement of transition wavenumbers with the new calculations compared to the TheoReTS/HITEMP and ExoMol databases. We also compare the relative intensities and find an overall good agreement with all three sets of predictions. Finally, we report the wavenumbers of 27 transitions in the 2${\nu}$$_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↗

arXiv2024

Realization of the Pascal based on Argon using a Fabry-P\'erot refractometer

Based on a recent experimental determination of the static polarizability and a first-principles calculation of the frequency-dependent dipole polarizability of argon, this work presents, by use of a Fabry-P\'erot refractometer operated at 1550 nm, a realization of the SI unit of pressure, the pascal, for pressures up to 100 kPa, with an uncertainty of [(0.98 mPa)$^2 + (5.8 \times 10^{-6} P)^2 + (26\times10^{-12}P^2)^2]^{1/2}$. The work also presents a value of the molar polarizability of N$_2$ at 1550 nm of 4.396572(26)$\times 10^{-6}$m$^{3}$/mol, which agrees well with previously determined ones in the literature.

physics.optics↗

arXiv2023

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${\nu}$${_3}$${\leftarrow}$${\nu}$${_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↗

arXiv2023

Optical frequency comb Fourier transform spectroscopy of formaldehyde in the 1250 to 1390 cm$^{-1}$ range: experimental line list and improved MARVEL analysis

We use optical frequency comb Fourier transform spectroscopy to record high-resolution, low-pressure, room-temperature spectra of formaldehyde (H$_2$$^{12}$C$^{16}$O) in the range of 1250 to 1390 cm$^{-1}$. Through line-by-line fitting, we retrieve line positions and intensities of 747 rovibrational transitions: 558 from the ${\nu}_6$ band, 129 from the ${\nu}_4$ band, and 14 from the ${\nu}_3$ band, as well as 46 from four different hot bands. We incorporate the accurate and precise line positions (0.4 MHz median uncertainty) into the MARVEL (measured active vibration-rotation energy levels) analysis of the H$_2$CO spectrum. This increases the number of MARVEL-predicted energy levels by 82 and of rovibrational transitions by 5382, and substantially reduces uncertainties of MARVEL-derived H$_2$CO energy levels over a large range: from pure rotational levels below 200 cm$^{-1}$ up to multiply excited vibrational levels at 6000 cm$^{-1}$. This work is an important step toward filling the gaps in formaldehyde data in the HITRAN database.

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 $\mu$m continuous wave optical parametric oscillator to pump transitions in the fundamental $\nu_3$ band and a 1.67 $\mu$m frequency comb to probe ladder-type transitions in the 3$\nu_3$ 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

Gas modulation refractometry (GAMOR) - On its ability to eliminate the influence of drifts

Gas modulation refractometry (GAMOR) is a technique based on a dual-Fabry-Perot (FP) cavity (DFPC) for assessment of gas refractivity, density, and pressure that can alleviate significant limitations of conventional refractometry systems, predominantly those related to drifts. Repeated assessments of the beat frequency when the measurement cavity is evacuated provide conditions under which the methodology is immune to the linear parts of the drifts in the system, both those from length changes of the cavities and those from gas leaks and outgassing. This implies that the technique is solely influenced by the non-linear parts of the drifts. This work provides a description of the principle behind the GAMOR methodology and explicates the background to its unique property. Based on simple models of the drifts of the temperature in the cavity spacer and the residual gas in the reference cavity, this work predicts that a GAMOR system, when used for assessment of refractivity, can sustain significant temperature drifts and leakage rates without being affected by noticeable errors or uncertainties. The cavity spacer can be exposed to temperature fluctuations of 100 mK over 103 s, and the reference cavity can have a leakage that fills it up with gas on a timescale of days, without providing errors or uncertainties in the assessment of refractivity that are 3 x 10^(-12), which, for N2, corresponds to 0.01 ppm (parts per million) of the value under atmospheric pressure conditions, and thereby 1 mPa. Since well-designed systems often have temperature fluctuations and leakage rates that are smaller than these, it is concluded that there will, in practice, not be any appreciable influence from cavity length drifts, gas leaks, and outgassing in the GAMOR methodology.

physics.ins-det↗

arXiv2019

Gas equilibration gas modulation refractometry (GEq-GAMOR) for assessment of pressure with sub-ppm precision

A novel realization of Gas Modulation Refractometry (GAMOR) that outperforms the original realization [Single Cavity Modulated GAMOR (SCM-GAMOR)], is presented. The reference measurements are carried out by equalizing the pressures in the two cavities. By this, the time it takes to reach adequate conditions for the reference measurements has been reduced. This implies that a larger fraction of the measurement cycle can be devoted to data acquisition, which reduces white noise and improves on short-term characteristics. The presented realization also encompasses a new cavity design with improved temperature stabilization and assessment. This has contributed to an improved long-term characteristics. The system was characterized with respect to a dead weight piston gauge. For short integration times (up to 10 min) it can provide a response that exceeds that of the original SCM-GAMOR system by a factor of two. For longer integration times, and up to 18 hours, the system shows, at 4303 Pa, an integration time independent Allan deviation of 1 mPa (corresponding to a precision defined as twice the Allan deviation, of 0.5 ppm). This implies that the novel system shows a significant improvement with respect to the original realization for all integration times (by a factor of 8 for an integration time of 18 hours). When used for low pressures, it can provide a precision in the sub-mPa region; for the case with an evacuated measurement cavity, the system provided, up to ca. 40 measurement cycles (ca. 1.5 hours), a white-noise limited noise of 0.7 mPa sqrt(cycle), and minimum Allan deviation of 0.15 mPa. Furthermore, over the pressure range investigated, i.e. in the 2.8 - 10.1 kPa range, it shows, with respect to a dead weight piston gauge, a purely linear response. This implies that the system can be used for transfer of calibration over large pressure ranges with exceptional low uncertainty.

physics.ins-det↗

arXiv2017

Fast Switching Dual Fabry-Perot-Cavity-based Optical Refractometry for Assessment of Gas Refractivity and Density - Estimates of Its Precision, Accuracy, and Temperature Dependence

Dual Fabry-Perot-Cavity-based Optical Refractometry (DFCB-OR) have been shown to have excellent potential for characterization of gases, in particular their refractivity and density. However, its performance has in practice been found to be limited by drifts. To remedy this, drift-free DFPC-OR (DF-DFCB-OR) has recently been proposed. Suggested methodologies for realization of a specific type of DF-DFCB-OR, termed Fast Switching DFCB-OR (FS-DFCB-OR), have been presented in an accompanying work. This paper scrutinizes the performance and the limitations of both DF- and FS-DFCB-OR for assessments of refractivity and gas density, in particular their precision, accuracy, and temperature dependence. It is shown that both refractivity and gas density can be assessed by FS-DFCB-OR with a precision in the 10$^{-9}$ range under STP conditions. It is demonstrated that the absolute accuracy is mainly limited by the accuracy by which the instantaneous deformation of the cavity or the higher order virial coefficients can be assessed. It is also shown that the internal accuracy, i.e. the accuracy by which the system can be characterized with respect to an internal standard, can be several orders of magnitude better than the absolute. It is concluded that the temperature dependence of FS-DFCB-OR is exceptionally small, typically in the 10$^{-8}$ to 10$^{-7}$/C range, and primarily caused by thermal expansion of the FPC-spacer material. Finally, this paper discusses means on how to design a FS-DFCB-or system for optimal performance and epitomizes the conclusions of this and our accompanying works regarding both DF- and FS-DFCB-OR in terms of performance and provides an outlook for both techniques. Our works can serve as a basis for future realizations of instrumentation for assessments of gas refractivity and density that can fully benefit from the extraordinary potential of FPC-OR.

physics.ins-det↗

arXiv2017

Fast Switching Dual Fabry-Perot Cavity Optical Refractometry - Methodologies for Accurate Assessment of Gas Density

Dual Fabry-Perot cavity based optical refractometry (DFPC-OR) has a high potential for assessments of gas density. However, drifts of the FP cavity often limit its performance. We show that by the use of two narrow-linewidth fiber lasers locked to two high finesse cavities and Allan-Werle plots that drift-free DFPC-OR can be obtained for short measurement times (for which the drifts of the cavity can be disregarded). Based on this, a novel strategy, termed fast switching DFPC-OR (FS-DFPC-OR), is presented. A set of novel methodologies for assessment of both gas density and flow rates (in particular from small leaks) that are not restricted by the conventional limitations imposed by the drifts of the cavity are presented. The methodologies deal with assessments in both open and closed (finite-sized) compartments. They circumvent the problem with volumetric expansion, i.e. that the gas density in a measurement cavity is not the same as that in the closed external compartment that should be assessed, by performing a pair of measurements in rapid succession; the first one serves the purpose of assessing the density of the gas that has been transferred into the measurement cavity by the gas equilibration process, while the 2nd is used to automatically calibrate the system with respect to the relative volumes of the measurement cavity and the external compartment. The methodologies for assessments of leak rates comprise triple cavity evacuation assessments, comprising two measurements performed in rapid succession, supplemented by a 3rd measurement a certain time thereafter. A clear explanation of why the technique has such a small temperature dependence is given. It is concluded that FS-DFPC-OR constitutes a novel strategy that can be used for precise and accurate assessment of gas number density and gas flows under a variety of conditions, in particular non-temperature stabilized ones.

physics.ins-det↗

arXiv2017

Drift-Free Fabry-Perot-Cavity-based Optical Refractometry -- Accurate Expressions for Assessments of Gas Refractivity and Density

Recent advancements in the field of Fabry-Perot cavity based optical refractometry (OR), FPC-OR, comprising the use of proposed novel methodologies based on drift-free Dual FPC-OR (DF-DFPC-OR), have alleviated much of earlier problems with FPC-OR, in particular drifts of the cavity spacer, and therefore open up for new levels of accurate characterization of gases. However, to be able to use these techniques for highly accurate assessments, general and explicit expressions are needed for how the gas density, ${\rho_n}$, is related to the refractivity of the gas, $n - 1$, and how the latter depends on the change in frequency of laser light that follows an evacuation of the cavity, $\Delta {\nu_l}$. This paper presents such relations that properly acknowledge the influence of all conceivable phenomena and higher order (non-linear) contributions to the assessment of these entities under drift-free conditions. The relative contributions of each of these phenomena to measurements of gas refractivity and density are individually assessed. It is shown, among other things, that, the influence of cavity deformation is larger for an open cavity (placed in a compartment in which gas is introduced) than a closed one; if not accounted for appropriately, under standard pressure and temperature conditions it can contribute to the accuracy of the technique with up to a few times 10$^{-3}$ if an open cavity is used, while it can be an order of magnitude smaller, i.e. in the order of 10$^{-4}$, for a well-designed closed cavity. It is also shown that the non-linearities from dispersion are smaller for the cases when relocking takes place than when it is not used. The expressions derived can serve as a basis for assessment of refractivity or gas density, and changes in such, in future realizations of OR in such a way that they can fully benefit from the extraordinary power of DF-DFPC-OR.

physics.ins-det↗