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Rotational transitions are unique identifiers of molecular species, including isotopologues. This article describes the rotational detections of two laser-volatilized salts, NaCl and KCl, made with a miniature Fourier transform millimeter-wave (FTmmW) cavity spectrometer that could one day be used to measure solid composition in the field or in space. The two salts are relevant targets for icy moons in the outer solar system, and in principle, other molecular solids could be analyzed with the FTmmW instrument. By coupling the spectrometer to a collisionally cooling laser ablation source, (a) we demonstrate that the FTmmW instrument is sensitive enough to detect ablation products, and (b) we use the small size of the FTmmW cavity to measure ablation product signal along the carrier gas beam. We find that for 532 nm nanosecond pulses, ablated molecules are widely dispersed in the carrier-gas jet. In addition to the miniature spectrometer results, we present several complementary measurements intended to characterize the laser ablation process. For pulse energies between 10 and 30 mJ, the ablation product yield increases linearly, reaching approximately 1012 salt molecules per 30 mJ pulse. Using mass spectrometry, we observe Li+, Na+, and K+ in the plumes of ablated NaCl, KCl, and LiCl, which implies dissociation of the volatilized material. We do not observe salt ions (e.g., NaCl+). However, with 800 nm femtosecond laser pulses, the triatomic ion clusters Li2Cl+, Na2Cl+, and K2Cl+ are produced. Finally, we observe incomplete volatilization with the nanosecond pulses: some of the ejecta are liquid droplets. The insights about ablation plume physics gleaned from these experiments should guide future implementations of the laser-volatilization technique.
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We report argon-broadened water vapor transition parameters and their temperature dependence based on measured spectra spanning 6801-7188 cm-1 from a broad-bandwidth, high-resolution dual frequency comb spectrometer. The 25 collected spectra of 2% water vapor in argon ranged from 296 K to 1305 K with total pressure spanning 100 Torr to 600 Torr. A multispectrum fitting routine was used in conjunction with a quadratic speed-dependent Voigt profile to extract broadening and shift parameters, and a power-law temperature-dependence exponent for both. The measurements represent the first broad bandwidth, argon-broadened water vapor absorption study, and are an important step toward a foreign-gas-perturbed, high-temperature database developed using advanced lineshape profiles.
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We measure speed-dependent Voigt lineshape parameters with temperature-dependence exponents for several hundred spectroscopic features of pure water spanning 6801-7188 cm-1. The parameters are extracted from broad bandwidth, high-resolution dual frequency comb absorption spectra with multispectrum fitting techniques. The data encompass 25 spectra ranging from 296 K to 1305 K and 1 to 17 Torr of pure water vapor. We present the extracted parameters, compare them to published data, and present speed-dependence, self-shift, and self-broadening temperature-dependent parameters for the first time. Lineshape data is extracted using a quadratic speed-dependent Voigt profile and a single self-broadening power law temperature-dependence exponent over the entire temperature range. The results represent an important step toward a new high-temperature database using advanced lineshape profiles.
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Retrievals of atmospheric composition from near-infrared measurements require measurements of airmass to better than the desired precision of the composition. The oxygen bands are obvious choices to quantify airmass since the mixing ratio of oxygen is fixed over the full range of atmospheric conditions. The OCO-2 mission is currently retrieving carbon dioxide concentration using the oxygen A-band for airmass normalization. The 0.25% accuracy desired for the carbon dioxide concentration has pushed the required state-of-the-art for oxygen spectroscopy. To measure O2 A-band cross-sections with such accuracy through the full range of atmospheric pressure requires a sophisticated line-shape model (Rautian or Speed-Dependent Voigt) with line mixing (LM) and collision induced absorption (CIA). Models of each of these phenomena exist, however, this work presents an integrated self-consistent model developed to ensure the best accuracy. It is also important to consider multiple sources of spectroscopic data for such a study in order to improve the dynamic range of the model and to minimize effects of instrumentation and associated systematic errors. The techniques of Fourier Transform Spectroscopy (FTS) and Cavity Ring-Down Spectroscopy (CRDS) allow complimentary information for such an analysis. We utilize multispectrum fitting software to generate a comprehensive new database with improved accuracy based on these datasets. The extensive information will be made available as a multi-dimensional cross-section (ABSCO) table and the parameterization will be offered for inclusion in the HITRANonline database.
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The extension of radio frequency complementary metal oxide semiconductor (CMOS) circuitry into millimeter wavelengths promises the extension of spectroscopic techniques in compact, power efficient systems. We are now beginning to use CMOS millimeter devices for low-mass, low-power instrumentation capable of remote or in situ detection of gas composition during space missions. We have chosen to develop a Flygare-Balle type spectrometer, with a semi-confocal Fabry-Perot cavity to amplify the pump power of a mm-wavelength CMOS transmitter that is directly coupled to the planar mirror of the cavity. We have built a pulsed transceiver system at 92-105 GHz inside a 3 cm base length cavity and demonstrated quality factor up to 4680, allowing for modes with 20 MHz bandwidth, with a sufficient cavity amplification factor for mW class transmitters. This work describes the initial gas measurements and outlines the challenges and next steps.
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We have updated the isotopically invariant Dunham fit of O2 with newly reported literature transitions to derive (1) the energy levels, partition sums, band-by-band molecular constants, and RKR potentials for the X(3)Σg(-), a(1)Δg, and b(1)Σg(+) states of the six O2 isotopologues: (16)O(16)O, (16)O(17)O, (16)O(18)O, (17)O(17)O, (17)O(18)O, and (18)O(18)O; (2) Franck-Condon factors for their a(1)Δg-X(3)Σg(-), b(1)Σg(+)-X(3)Σg(-), and a(1)Δg-b(1)Σg(+) band systems. This new spectroscopic parameterization characterizes all known transitions within and between the X(3)Σg(-), a(1)Δg, and b(1)Σg(+) states within experimental uncertainty and can be used for accurate predictions of as yet unmeasured transitions. All of these results are necessary to provide a consistent linelist of all transitions which will be reported in a followup paper.
Asunto(s)
Oxígeno/análisis , Termodinámica , Teoría Cuántica , Análisis EspectralRESUMEN
Rotational spectra of OH and its isotopologues have been precisely measured using high efficiency terahertz (THz) sources. The measurements are compared with existing data and are useful for global modeling. For the first time, microwave measurements of the Λ-doubling transitions of the (17)OH isotopologue are combined with THz data successfully. Precise rotational, fine-structure, and hyperfine structure parameters for the (17)OH isotopologue are reported. An isotopically independent Dunham model for all isotopologues of (2)Π OH v < 3 is presented.
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We report the first high spectral resolution laboratory measurements of simulated oxygen A-band night glow. Our static discharge system approximates the conditions of the mesospheric oxygen night glow--suggesting O((1)D) + O2 (X(3)Σg(-)) â O((3)P) + O2 (b(1)Σg(+)) â O2 (X(3)Σg(-)) + hν as the primary source of the emission. Additionally, use of the static cell has enabled us to collect spectra for all six molecular oxygen isotopologues using isotopically enriched samples. The (0,0), (0,1), and (1,1) b - X vibrational bands were observed for all six isotopologues. The (1,2) and (2,2) bands were also observed for (16)O2. The frequencies of the observed (0,1) transitions resolved discrepancies in Raman data for (16)O(17)O, (17)O2, and (17)O(18)O, enabling us to improve the vibrational parameterization of the ground electronic state global fit. Rotationally resolved intensities were determined for the (0,0), (0,1), and (1,1) bands. The experimental band intensity ratios I(0,0)/I(0,1) = 13.53(24); I(1,1)/I(1,0) = 11.9(65); I(0,0)/I(0,2) = 503(197); and I(1,1)/I(1,2) = 5.6(19) are in excellent agreement with the recent mesospheric remote sensing data and calculated Franck-Condon factors.
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We have developed a simultaneous global fit to the MW, THz, infrared, visible, and UV transitions of all six oxygen isotopologues, (16)O(16)O, (16)O(17)O, (16)O(18)O, (17)O(17)O, (17)O(18)O, (18)O(18)O, with the objective of predicting all transitions below the O((3)P) + O((3)P) dissociation threshold as well as the B(3)Σ(u) (-) state from O((3)P)+O((1)D) within state-of-the-art experimental uncertainty. Here, we report an isotopically invariant Dunham fit for the lowest three electronic states, X(3)Σ(g)(-), a(1)Δ(g), and b(1)Σ(g)(+). Experimental transition frequencies involving these three states of all six O(2) isotopologues were critically reviewed and incorporated into the analysis. For the (16)O(16)O isotopologue, experimental data sample vibrational states v = 0-31 for X(3)Σ(g)(-), v = 0-10 for a(1)Δ(g), and v = 0-12 for b(1)Σ(g)(+). To the best of our knowledge, this is the first analysis that simultaneously fits spectra from all six O(2) isotopologues.
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As part of a comprehensive review on molecular oxygen spectroscopy, we have measured rotational spectra of isotopic forms of molecular oxygen in its a(1)Δ(g) electronic state with high-resolution terahertz spectroscopy. The data are recorded in close proximity to predicted positions. Due to the high resolution and good signal-to-noise ratio, the fundamental hyperfine parameters eQq and C(I) are determinable for (17)O-substituted species for the first time. A refined nuclear spin orbit coupling constant, a = -211.9328(283) MHz, was determined, and is roughly two orders of magnitude more precise than values determined from near infrared spectroscopy or electron spin resonance studies. Vibrationally excited oxygen in the a(1)Δ(g) electronic state was also observable with small signal levels for many of the rotational transitions.
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The parallel vibration-rotation band ν(4) of methyl isocyanide (CH(3)NC), with a band center at 944.9 cm(-1), was studied by FTIR spectroscopy between 890 and 980 cm(-1) in order to improve the ground-state rotational constants. Such improvement is essential for the scheduled studies of excited vibrational levels and their mutual anharmonic resonances occurring at higher values of the K rotational number. Ground-state combination differences generated from this band, spanning values of J/K from 0 to 85/13, were combined with rotational data from the literature and newly measured rotational transitions, extending the J/K range from 3/0 up to 31/14, and fitted simultaneously with a fully quantitative reproduction of the data. The infrared data of the ν(4) band were analyzed together with rotational data of the ν(4) = 1 level, spanning values of J/K from 4/0 to 14/12. The fit in the approximation of an isolated vibrational state, with the transitions perturbed by weak local resonances excluded, yields reproduction of the data within experimental uncertainties.
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The rotational spectrum of n-propanol (n-CH(3)CH(2)CH(2)OH) was studied with several techniques of contemporary broadband rotational spectroscopy at frequencies from 8 to 550 GHz. Rotational transitions in all five conformers of the molecule, Gt, Gg, Gg', Tt, and Tg, have been unambiguously assigned. Over 6700 lines of the Gt, Gg, and Gg' species, for quantum number values reaching K(a) = 33 and J = 67, were fitted in a joint analysis leading to the determination of DeltaE(Gg-Gt) = 47.82425(25) cm(-1) and DeltaE (Gg'-Gg) = 3.035047(11) cm(-1). Stark effect measurements in supersonic expansion were used to further confirm the assignment. The results are compared with those for the ethanol molecule and with ab initio calculations, allowing several inferences to be drawn concerning the differences in the large amplitude torsional potential of the hydroxyl group in the two molecules.
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Complete and reliable knowledge of the ammonia spectrum is needed to enable the analysis and interpretation of astrophysical and planetary observations. Ammonia has been observed in the interstellar medium up to J=18 and more highly excited transitions are expected to appear in hot exoplanets and brown dwarfs. As a result, there is considerable interest in observing and assigning the high J (rovibrational) spectrum. In this work, numerous spectroscopic techniques were employed to study its high J transitions in the ground and ν(2)=1 states. Measurements were carried out using a frequency multiplied submillimeter spectrometer at Jet Propulsion Laboratory (JPL), a tunable far-infrared spectrometer at University of Toyama, and a high-resolution Bruker IFS 125 Fourier transform spectrometer (FTS) at Synchrotron SOLEIL. Highly excited ammonia was created with a radiofrequency discharge and a dc discharge, which allowed assignments of transitions with J up to 35. One hundred and seventy seven ground state and ν(2)=1 inversion transitions were observed with microwave accuracy in the 0.3-4.7 THz region. Of these, 125 were observed for the first time, including 26 ΔK=3 transitions. Over 2000 far-infrared transitions were assigned to the ground state and ν(2)=1 inversion bands as well as the ν(2) fundamental band. Of these, 1912 were assigned using the FTS data for the first time, including 222 ΔK=3 transitions. The accuracy of these measurements has been estimated to be 0.0003-0.0006 cm(-1). A reduced root mean square error of 0.9 was obtained for a global fit of the ground and ν(2)=1 states, which includes the lines assigned in this work and all previously available microwave, terahertz, far-infrared, and mid-infrared data. The new measurements and predictions reported here will support the analyses of astronomical observations by high-resolution spectroscopy telescopes such as Herschel, SOFIA, and ALMA. The comprehensive experimental rovibrational energy levels reported here will permit further refinement of the potential energy surface to improve ammonia ab initio calculations and facilitate assignment of new high-resolution spectra of hot ammonia.
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We present a system level description of a cavity-enhanced millimeter-wave spectrometer that is the first in its class to combine source and detection electronics constructed from architectures commonly deployed in the mobile phone industry and traditional pulsed Fourier transform techniques to realize a compact device capable of sensitive and specific in situ gas detections. The instrument, which has an operational bandwidth of 90-102 GHz, employs several unique components, including a custom-designed pair of millimeter-wave transmitter and heterodyne receiver integrated circuit chips constructed with 65 nm complementary metal-oxide semiconductor (CMOS) techniques. These elements are directly mated to a hybrid coupling structure that enables free-space interaction of the electronics with a small gas volume while also acting as a cavity end mirror. Instrument performance for sensing of volatile compounds is highlighted with experimental trials taken in bulk gas flows and seeded molecular beam environments.
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Spectroscopic studies of planetary atmospheres and high-temperature processes (e.g., combustion) require absorption line-shape models that are accurate over extended temperature ranges. To date, advanced line shapes, like the speed-dependent Voigt and Rautian profiles, have not been tested above room temperature with broadband spectrometers. We investigate pure water vapor spectra from 296 to 1305 K acquired with a dual-frequency comb spectrometer spanning from 6800 to 7200 cm-1 at a point spacing of 0.0033 cm-1 and absolute frequency accuracy of <3.3 × 10-6 cm-1. Using a multispectral fitting analysis, we show that only the speed-dependent Voigt accurately models this temperature range with a single power-law temperature-scaling exponent for the broadening coefficients. Only the data from the analysis using this profile fall within theoretical predictions, suggesting that this mechanism captures the dominant narrowing physics for these high-temperature conditions.
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Rotational spectra for two isotopomers of chloroferrocene were measured using pulsed-beam Fourier transform microwave spectroscopy. Transitions were observed for (35)Cl and (37)Cl isotopomers in the 4-10 GHz range. Chlorine quadrupole coupling strengths and rotational constants were obtained from the data. The rotational constants and quadrupole coupling strengths for (35)Cl are A = 1370.009(8) MHz, B = 767.342(1) MHz, C = 634.8834(6) MHz, eQq(aa)() = -9.06(4) MHz, and eQq(bb)() = -28.43(6) MHz and for (37)Cl are A = 1362.23(2) MHz, B = 751.623(2) MHz, C = 622.324(1) MHz, eQq(aa)() = -8.78(18) MHz, and eQq(bb)() = -20.76(9) MHz. These measured rotational constants were used to determine the following structural parameters of chloroferrocene: r(Fe-(C(5)H(5))) = 1.594(20) Å, r(Fe-(C(5)H(4)Cl)) = 1.699(13) Å, r(C-Cl) =1.7204(9) Å, and r(C-C) = 1.4329(7) Å. No evidence for internal rotation was observed in the microwave spectrum.
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We report the first demonstration of a continuous wave coherent source covering 2.48-2.75 THz, with greater than 10% instantaneous tuning bandwidth and having 1-14 µW of output power at room temperature. This source is based on a 91.8-101.8 GHz synthesizer followed by a power amplifier and three cascaded frequency triplers. It demonstrates for the first time that purely electronic solid-state sources can generate a useful amount of power in a region of the electromagnetic spectrum where lasers (solid state or gas) were previously the only available coherent sources. The bandwidth, agility, and operability of this THz source have enabled wideband, high resolution spectroscopic measurements of water, methanol, and carbon monoxide with a resolution and signal-to-noise ratio unmatched by any other existing system, providing new insight in the physics of these molecules. Furthermore, the power and optical beam quality are high enough to observe the Lamb-dip effect in water. The source frequency has an absolute accuracy better than 1 part in 10(12) and the spectrometer achieves sub-Doppler frequency resolution better than 1 part in 10(8). The harmonic purity is better than 25 dB. This source can serve as a coherent signal for absorption spectroscopy, a local oscillator for a variety of heterodyne systems and can be used as a method for precision control of more powerful but much less frequency agile quantum mechanical terahertz sources.
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We present highly accurate laboratory measurements on the pure rotational spectrum of doubly deuterated water, D2O, in selected frequency regions from 10 GHz up to 2.7 THz. Around 140 rotational transitions in both the vibrational ground and first excited bending states (upsilon2=0,1) were measured in total, involving energy levels with unexcelled high J and Ka rotational quantum numbers. The data give valuable information for the spectroscopic analysis of this molecule. In the case of the light and non-rigid water molecule, standard methods for its analysis are limited due to large centrifugal distortion interactions. Here, we present a global analysis of rotational and rovibrational data of the upsilon2=0 and 1 states of D2O by means of an Euler expansion of the Hamiltonian. In addition to the newly measured pure rotational transitions, around 4000 rotational and rovibrational lines have been included from previous work. It was possible to reproduce the extensive dataset to nearly its experimental uncertainty. The improved predictive capability of the model compared to previous work will be demonstrated.
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The rotational spectrum of cis-cis HOONO has been studied over a broad range of frequencies, 13-840 GHz, using pulsed beam Fourier-transform microwave spectroscopy and room-temperature flow cell submillimeter spectroscopy. The rotational spectrum of the deuterated isotopomer, cis-cis DOONO, has been studied over a subset of this range, 84-640 GHz. Improved spectroscopic constants have been determined for HOONO, and the DOONO spectrum is analyzed for the first time. Weak-field Stark effect measurements in the region of 84-110 GHz have been employed to determine the molecular dipole moments of cis-cis HOONO [mu(a) = 0.542(8) D, mu(b) = 0.918(15) D, mu = 1.07(2) D] and DOONO [mu(a) = 0.517(9) D, mu(b) = 0.930(15) D, mu = 1.06(2) D]. The quadrupole coupling tensor in the principal inertial axis system for the 14N nucleus has been determined to be chi(aa) = 1.4907(25) MHz, chi(bb) = -4.5990(59) MHz, chi(ab) = 3.17(147) MHz, and chi(cc) = 3.1082(59) MHz. Coordinates of the H atom in the center-of-mass frame have been determined with use of the Kraitchman equations, /aH/ = 0.516 A and /bH/ = 1.171 A. The inertial defects of HOONO and DOONO are consistent with a planar equilibrium structure with significant out-of-plane H atom torsional motion. Comparisons of the present results are made to ab initio calculations.
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Gas-phase structural parameters for ferrocenecarboxaldehyde have been determined using Fourier transform microwave spectroscopy. Rotational transitions due to a-, b-, and c-type dipole moments were measured. Eighteen rotational constants were determined by fitting the measured transitions of various isotopomers using a rigid rotor Hamiltonian with centrifugal distortion constants. Least-squares fit and Kraitchman analyses have been used to determine the gas-phase structural parameters and the atomic coordinates of the molecule using the rotational constants for various isotopomers. Structural parameters determined from the least-squares fit are the Fe-C bond lengths to the cyclopentadienyl rings, r(Fe-C)=2.047(4) A, and the distance between the carbon atoms of the cyclopentadienyl rings, r(C-C)=1.430(2) A and r(C1-C1')=1.46(1) A of ring carbon and aldehyde carbon atom. Structural parameters were also obtained using density-functional theory calculations, and these were quite helpful in resolving ambiguities in the structural fit analysis, and providing some fixed parameters for the structural analysis. The results of the least squares and the calculations indicate that the carbon atoms of the Cp groups for ferrocenecarboxaldehyde are in an eclipsed conformation in the ground vibrational state.