Ground State Combination Differences Research Articles

Frequency comb assisted mid-infrared spectroscopy of cold molecular ions

A mid-infrared spectrometer consisting of a high power optical parametric oscillator, a frequency comb, and a cold ion trap is described and characterized. The idler frequency at 3 μm is measured accurately by analyzing the pump and signal beat frequencies with the comb. This is done via two spectrum analyzers, allowing for a wide and continuous scanning ideal for spectroscopy of cold molecules with unknown spectra. The potential of this approach is demonstrated by measuring a ro-vibrational line of CH(5)(+) in a 22-pole ion trap via action spectroscopy of only several thousand cold ions. The current setup limits the precision of the line center frequency determination to some 100 kHz with ample room for future improvements. Following this approach, ground state combination differences of molecular ions can be predicted in order to identify them in astronomical objects.

Analysis of the Rotational Structure in the High-Resolution Infrared Spectrum of trans-Hexatriene-1-13C1: A Semiexperimental Equilibrium Structure for the C6 Backbone of trans-Hexatriene

trans-Hexatriene-1-(13)C(1) (tHTE-1-(13)C(1)) has been synthesized, and its high-resolution (0.0015 cm(-1)) infrared spectrum has been recorded. The rotational structure in the C-type bands for ν(26) at 1011 cm(-1) and ν(30) at 894 cm(-1) has been analyzed. To the 1458 ground state combination differences from these bands, ground state rotational constants were fitted to a Watson-type Hamiltonian to give A(0) = 0.8728202(9), B(0) = 0.0435868(4), and C(0) = 0.0415314(2) cm(-1). Upper state rotational constants for the ν(30) band were also fitted. Predictions of the ground state rotational constants for tHTE-1-(13)C(1) from a B3LYP/cc-pVTZ model with scale factors based on the normal species were in excellent agreement with observations. Similar good agreement was found between predicted and observed ground state rotational constants for the three (13)C(1) isotopologues of cis-hexatriene, as determined from microwave spectroscopy. Equilibrium rotational constants for tHTE and its three (13)C(1) isotopologues, of which two were predicted, were used to find a semiexperimental equilibrium structure for the C(6) backbone of tHTE. This structure shows increased structural effects of π-electron delocalization in comparison with butadiene and some differences from the cis isomer of HTE. Structures predicted with the MP2/cc-pVTZ model are also compared.

Extended line positions, intensities, empirical lower state energies and quantum assignments of NH3 from 6300 to 7000 cm−1

Nearly 4800 features of ammonia between 6300 and 7000cm−1 with intensities ≥4×10−24cm−1/(molecule·cm−2) at 296K were measured using 16 pure NH3 spectra recorded at various temperatures (296–185K) with the McMath–Pierce Fourier Transform Spectrometer at Kitt Peak National Observatory, AZ. The line positions and intensities were retrieved by fitting individual spectra based on a Voigt line shape profile and then averaging the values to form the experimental linelist. The integrated intensity of the region was 4.68×10−19cm−1/(molecule·cm−2) at 296K. Empirical lower state energies were also estimated for 3567 absorption line features using line intensities retrieved from 10 spectra recorded at gas temperature between 185 and 233K. Finally, using Ground State Combination Differences (GSCDs) and the empirical lower state energy estimates, the quantum assignments were determined for 1096 transitions in the room temperature linelist, along with empirical upper state energies for 434 levels. The assignments correspond to seven vibrational states, as confirmed from recent ab initio calculations. The resulting composite database of 14NH3 line parameters will provide experimental constraints to ab initio calculations and support remote sensing of gaseous bodies including the atmospheres of Earth, (exo)planets, brown dwarfs, and other astrophysical environments.

Cavity ringdown spectroscopy of 13C2H2 in the 12 900–13 400 cm−1 region

We have utilized an acetylene gas sample to assess the performance of a newly constructed cavity ringdown laser absorption (CRDS) spectrometer at the Arkansas Center for Laser Applications and Science (ArCLAS). During this assessment process, four overtone combination bands were observed for the 13C2H2 species, three of which have not been previously reported in the literature. For two of these previously unreported bands, a total of 85 rovibrational transitions (over 40 in each band) have been assigned using ground state combination difference theory. The data have been fit to standard energy expressions, and a set of molecular constants for each overtone combination band have been obtained. Here we present a brief description of the ArCLAS CRDS instrument along with the complete rovibrational analysis of these two combination overtone bands.

High-resolution infrared spectrum of jet-cooled methyl acetate in the C=O stretching region: Internal rotations of two inequivalent methyl tops

The jet-cooled high resolution infrared (IR) spectrum of methyl acetate (MA), CH(3)-C(=O)-O-CH(3), in the C=O fundamental band region was recorded by using a rapid scan IR laser spectrometer equipped with an astigmatic multipass cell. No high resolution IR analyses of the ro-vibrational transitions between the ground and non-torsionally excited vibrational states have hitherto been reported for molecules with two inequivalent methyl rotors. Because of the two chemically different methyl tops in MA, i.e., the acetyl -CH(3) and methoxy -CH(3), each rotational energy level is split into more than two torsional sublevels by internal rotations of these methyl groups. We were able to assign ro-vibrational transitions of four torsional species by using the ground state combination differences calculated from the molecular constants of the vibrational ground state recently determined by a global fit of the microwave and millimeter wave lines [M. Tudorie, I. Kleiner, J. T. Hougen, S. Melandri, L. W. Sutikdja, and W. Stahl, J. Mol. Spectrosc. 269, 211 (2011)]. The assigned lines were successfully fitted using the BELGI-Cs-IR program to an overall standard deviation which is comparable to the measurement accuracy. This study is also of interest in understanding the role of methyl rotors in the intramolecular vibrational-energy redistribution processes in mid-size organic molecules.

Rotational analysis of bands in the high-resolution infrared spectra of cis,cis- and trans,trans-1,4-difluorobutadiene-2-d1

Pure samples of cis,cis- and trans,trans-1,4-difluorobutadiene-2-d1 have been synthesized, and high-resolution (0.0015cm−1) infrared spectra have been recorded for these nonpolar molecules in the gas phase. For the cis,cis isomer, the rotational structure in two C-type bands at 775 and 666cm−1 and one A-type band at 866cm−1 has been analyzed to yield a combined set of 2020 ground state combination differences (GSCDs). Ground state rotational constants fit to these GSCDs are A0=0.4195790(4), B0=0.0536508(8), and C0=0.0475802(9)cm−1. For the trans,trans isomer, three C-type bands at 856, 839, and 709cm−1 have been investigated to give a combined set of 1624 GSCDs. Resulting ground state rotational constants for this isomer are A0=0.9390117(8), B0=0.0389225(4), and C0=0.0373778(3)cm−1. Small inertial defects confirm the planarity of both isomers in the ground state. Upper state rotational constants have been determined for most of the transitions. The ground state rotational constants for the two isotopologues will contribute to the data set needed for determining semiexperimental equilibrium structures for the nonpolar isomers of 1,4-difluorobutadiene.

The high-resolution FTIR spectrum of the ν4 + ν8 band of trans-d2-ethylene ( trans-C 2H 2D 2)

The FTIR absorption spectrum of the hybrid A– B type ν 4 + ν 8 combination band of trans-C 2H 2D 2 centered at 1845.98737 cm −1 in the 1730–1940 cm −1 region was recorded at an unapodized resolution of 0.0063 cm −1. A total of 2725 a- and b-type transitions was assigned and fitted to upper state ( ν 4 + ν 8 = 1) rovibrational constants up to sextic terms using Watson’s A-reduced Hamiltonian in I r representation. The b-type feature of the band was analyzed for the first time. The root-mean-square deviation of the IR fit was 0.00059 cm −1. The most accurate set of ground state rovibrational constants up to sextic terms was also derived from the simultaneous fit of 3340 ground state combination differences from the present analysis and the ν 4 band of trans-C 2H 2D 2. The transition dipole moment ratio μ a μ b was found to be 1.95 ± 0.06.

Synchrotron-based far-infrared spectra and analysis of the ν 13, ν 19 and ν 20 bands of β-propiolactone

The far-infrared spectrum of β-propiolactone has been investigated in the region below 830 cm −1 with a resolution of 0.000959 cm −1 using synchrotron radiation from the Canadian Light Source. A detailed analysis of the observed rotational structure of the ν 13 (490.955500(9) cm −1), ν 20 (512.341950(10) cm −1) and ν 19 (791.343000(10) cm −1) fundamental bands provides spectroscopic information for these low-lying vibrational modes which correspond to the C O in-plane, C O out-of-plane and α-CH 2 rock motions, respectively. The ground state spectroscopic parameters were accurately determined by fitting 1943 ground state combination differences from the observed rotational structure in the infrared region along with previously reported microwave transitions. The spectroscopic parameters for the excited states were subsequently determined by fitting a total of 9007 infrared transitions from the three newly assigned bands together with the previously reported transitions of the ν 12 (747.244704(10) cm −1) and ν 8 (1095.433607(14) cm −1) bands. The resulting global analysis of 13 311 infrared transitions provides an accurate determination of the excited state spectroscopic parameters of β-propiolactone including a first order a-type Coriolis interaction of the closely-spaced ν 13 and ν 20 bands.

The ν12 band of C 2D 4

The Fourier transform infrared (FTIR) absorption spectrum of the ν 12 fundamental band of ethylene- d 4 (C 2D 4) was recorded in the 1017–1137 cm −1 region with an unapodized resolution of 0.0063 cm −1. Upper state ( v 12 = 1) rovibrational constants consisting of three rotational and five quartic constants were improved by assigning and fitting 2103 infrared transitions using Watson’s A-reduced Hamiltonian in the I r representation. The band centre of the A-type ν 12 band is found to be 1076.98480 ± 0.00002 cm −1. The present analysis covering a wider wavenumber range and higher J and K c values yielded upper state constants including the band centre which are more accurate than previously reported. The rms deviation of the upper state fit is 0.00045 cm −1. Improved ground state rovibrational constants were also determined from the fit of 1247 ground state combination differences (GSCD) from the presently-assigned infrared transitions of the ν 12 band of C 2D 4. The rms deviation of the GSCD fit is 0.00049 cm −1. In the rovibrational analysis, local frequency perturbations were not detected even at high J and K a values. The calculated inertial defect Δ 12 is 0.32551 ± 0.00001 μÅ 2. The line intensities of the individual transitions in the ν 12 band were measured and the band strength of 39.8 ± 2.0 cm −2 atm −1 was derived for the ν 12 band of C 2D 4.

Transient laser absorption spectroscopy of CH 2 near 780 nm

Bands in the CH 2 b ˜ 1 B 1 – a ˜ 1 A 1 transition between 12 500 and 13 000 cm −1 were recorded at Doppler-limited resolution using a transient frequency-modulation (FM) laser absorption spectrometer. Rotational levels in seven upper vibronic states: a ˜ ( 0 , 11 , 0 ) 1 , 3 , a ˜ ( 2 , 6 , 0 ) 1 , b ˜ ( 0 , 2 , 0 ) 4 , b ˜ ( 1 , 1 , 0 ) 1 , 2 , and b ˜ ( 0 , 3 , 0 ) 1 were assigned with the assistance of optical–optical double resonance (OODR) and ground state combination differences. Perturbations due to rotation-vibration coupling, anharmonic couplings, ℓ-resonance, as well as the Renner–Teller effect are discussed as sources of the irregular rotational patterns observed in the b ˜ ( 0 , 3 , 0 ) 1 , b ˜ ( 1 , 1 , 0 ) 1 , and a ˜ ( 0 , 11 , 0 ) 3 vibronic levels.

CH2 b̃1B1−ã1A1 Band Origin at 1.20 μm

The origin band in the b̃(1)B(1)-ã(1)A(1) transition of CH(2) near 1.2 μm has been recorded at Doppler-limited resolution using diode laser transient absorption spectroscopy. The assignments of rotational transitions terminating in upper state levels with K(a) = 0 and 1, were confirmed by ground state combination differences and extensive optical-optical double resonance experiments. The assigned lines are embedded in a surprisingly dense spectral region, which includes a strong hot band, b̃(0,1,0) K(a) = 0 - ã(0,1,0) K(a) = 1 sub-band lines, with combination or overtone transitions in the ã(1)A(1) state likely responsible for the majority of unassigned transitions in this region. From measured line intensities and an estimate of the concentration of CH(2) in the sample, we find the transition moment square for the 0(00) ← 1(10) transition in the b̃(1)B(1)(0,0,0)(0)-ã(1)A(1)(0,0,0)(1) sub-band is 0.005(1) D(2). Prominent b̃(1)B(1)(0,1,0)(0)-ã(1)A(1)(0,1,0)(1) hot band lines were observed in the same spectral region. Comparison of the intensities of corresponding rotational transitions in the two bands suggests the hot band has an intrinsic strength approximately 28 times larger than the origin band. Perturbations of the excited state K(a) = 0 and 1 levels are observed and discussed. The new measurements will lead to improved future theoretical modeling and calculations of the Renner-Teller effect between the ã and b̃ states in CH(2).

Rotational and Rovibrational Spectroscopy of CH3NC of the Ground and ν4 = 1 Vibrational States

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.

High-resolution FTIR spectroscopy of the ν8and Coriolis perturbation allowed ν12bands of ketenimine

High resolution FTIR spectra have been recorded in the region 250–770 cm−1 using synchrotron radiation and over 2000 transitions to the ν8 and ν12 states of the short lived species ketenimine have been assigned. Ground state combination differences combined with published microwave transitions were used to refine the constants for the ground vibrational state. Rotational and centrifugal distortion parameters for the v8 = 1 and v12 = 1 levels were determined by co-fitting transitions, and treating a strong a-axis Coriolis interaction. Selection rules for the observed ν12 transitions indicate that they arise solely from “perturbation allowed” intensity resulting from this Coriolis interaction.

Improved rovibrational constants for the ν12 band of C2H3D

Abstract The Fourier transform infrared absorption spectrum of the ν12 fundamental band of ethylene-d (C2H3D) was recorded at an unapodized resolution of 0.0063 cm−1 in the 1330–1475 cm−1 region. Upper state (ν12 = 1) rovibrational constants inclusive of three rotational, five quartic, and four sextic centrifugal distortion constants were improved by assigning and fitting 1444 infrared transitions using Watson’s A-reduced Hamiltonian in the Ir representation. The present analysis yielded more higher-order upper state constants than previously reported. The rms deviation of the fit is 0.00055 cm−1. Improved ground state rovibrational constants were also determined from the combined fit of 2026 ground state combination differences (GSCD) from the assigned infrared transitions of the ν12, ν3 and ν6 bands and 21 microwave frequencies of C2H3D. The rms deviation of the GSCD fit is 0.00047 cm−1. The A-type ν12 band is centered at 1400.76262 ± 0.00004 cm−1. Local frequency perturbations were not detected in the analysis. The calculated inertial defect Δ12 is 0.20809 ± 0.00003 μA2.

High resolution infrared study of SbHD 2: The ground state and the Sb–H stretching bands ν1 and 2 ν1

High resolution infrared spectra of 121SbHD 2 and 123SbHD 2 have been studied in the region of ν 1 , the Sb–H stretching fundamental, from 1780 to 1990 cm −1. The 2 ν 1 stretching overtone band of 123SbHD 2, located in the 3640–3790 cm −1 range, has also been investigated. The SbHD 2 molecule is an asymmetric rotor of C s symmetry with the asymmetry parameter κ = 0.61. The ν 1 band is of hybrid type, formed by strong C-type and weak B-type transitions, and almost unperturbed. For 123SbHD 2, 2092 transitions have been assigned: 70% of these belong to the C component, the other 30% are of B-type. The assigned transitions have been fitted using a Watson type S-reduced Hamiltonian in the III l representation, with a standard deviation of the fit σ = 0.45 × 10 −3 cm −1. In order to determine the ground state parameters all possible ground state combination differences (GSCD) have been generated from the ν 1 transitions. In total, 3942 GSCD up to J ″ = 27, K a ″ = 25, and K c ″ = 20 have been fitted with σ = 0.52 × 10 −3 cm −1. Only C-type transitions have been observed in the weak 2 ν 1 overtone band. The 556 assigned transitions have been fitted with σ = 2.6 × 10 −3 cm −1 using the same Hamiltonian as for ν 1 . In the ν 1 band of 121SbHD 2 771 C-type transitions have been assigned, and the v 1 = 1 spectroscopic constants obtained from a fit with σ = 0.70 × 10 −3 cm −1. Using 618 GSCD the ground state spectroscopic constants of 121SbHD 2 have been derived with σ = 1.0 × 10 −3 cm −1. The molecular parameters for the ground and the v 1 = 1 states of the two isotopologues agree well. The quartic theoretical ab initio force field of SbH 3 has been used to predict all relevant spectroscopic parameters for 123SbHD 2, 121SbHD 2, 123SbH 2D, and 121SbH 2D. Relations between the harmonic frequencies and between the anharmonicity constants obtained in the expanded local mode theory, for the XH 3 → XH 2D/XHD 2 isotopic substitution, have been compared with those obtained in the present study.

A reevaluation of the assignment of the vibrational fundamentals and the rotational analysis of bands in the high-resolution infrared spectra of trans- and cis-1,3,5-hexatriene

Assignments of the vibrational fundamentals of cis- and trans-1,3,5-hexatriene are reevaluated with new infrared and Raman spectra and with quantum chemical predictions of intensities and anharmonic frequencies. The rotational structure is analyzed in the high-resolution (0.0013–0.0018 cm −1) infrared spectra of three C-type bands of the trans isomer and two C-type bands of the cis isomer. The bands for the trans isomer are at 1010.96 cm −1 ( ν 14), 900.908 cm −1 ( ν 16), and 683.46 cm −1 ( ν 17). Ground state (GS) rotational constants have been fitted to the combined ground state combination differences (GSCDs) for the three bands of the trans isomer. The bands for the cis isomer are at 907.70 cm −1 ( ν 33) and 587.89 cm −1 ( ν 35). GS rotational constants have been fitted to the combined GSCDs for the two bands of the cis isomer and compared with those obtained from microwave spectroscopy. Small inertial defects in the GSs confirm that both molecules are planar. Upper state rotational constants were fitted for all five bands.

High-resolution infrared analysis of the ν7 band of cis-ethylene- d2 ( cis-C 2H 2D 2)

The spectrum of the ν 7 band of cis-ethylene- d 2 ( cis-C 2H 2D 2) has been recorded with an unapodized resolution of 0.0063 cm −1 in the 740–950 cm −1 region using a Bruker IFS 125 HR Fourier transform infrared spectrometer. By fitting 2186 infrared transitions of ν 7 with a standard deviation of 0.00060 cm −1 using a Watson’s A-reduced Hamiltonian in the I r representation, accurate rovibrational constants for ν 7 = 1 state have been derived. The band center of ν 7 has been found to be 842.20957 ± 0.00004 cm −1. In a simultaneous fit of 1331 infrared ground state combination differences from the present ν 7 transitions, together with 22 microwave frequencies, ground state constants have been improved. The rms deviation of the ground state fit was 0.00027 cm −1.

High-resolution FTIR spectrum of CH2D79Br: the ground, v 5=1 and v 9=1 state constants

The first high-resolution infrared spectrum of CH2D79Br has been investigated in the range 710–1080 cm−1 by Fourier transform infrared spectroscopy employing a synchrotron radiation source at an unapodized resolution of 0.0025 cm−1. This spectral region accounts for the fundamental bands ν 5 (ν 0 = 768.809 cm−1) and ν 9 (ν 0 = 930.354 cm−1), having the structure of hybrid a/b- and c-type bands of near-prolate asymmetric tops, respectively. The rovibrational analysis was performed over a wide range of transitions with quantum numbers J ≤ 72 and Ka ≤ 16. The experimentally determined ground-state constants including four sextic coefficients were obtained for the first time by merging the ground-state combination differences of the two bands. The A-reduced Watson Hamiltonian in the Ir representation was adopted to fit the data. Only weak interaction effects ascribed to a-type Coriolis coupling influenced the Ka = 10–14 stacks of the bands. The residuals were adjusted by including one higher-order interaction term in a dyad model.

Tunneling dynamics and spectroscopic parameters of monodeuterated hydronium, H2DO+, from a combined analysis of infrared and sub-millimeter spectra

The infrared laser direct absorption spectrum of H(2)DO(+) in the OH stretching region was reported quite recently revealing large amplitude tunneling dynamics. The large rotational constants make the jet-cooled spectrum relatively sparse at low rotational temperatures and assignments thus challenging. Transitions were assigned through ground state combination differences, with additional tentative assignments made via comparison of predicted/observed spectra. More recently, 9 rotation-inversion transitions were recorded in the sub-millimeter (sub-mm) region, which yielded tunneling splittings and rotational constants differing slightly from IR results. This has prompted the present reinvestigation of the H(2)DO(+) spectra, which now takes full advantage of the combined data from both studies. While previous analyses considered each tunneling state as independent and non-interacting, the present analysis is based on a tunneling-Hamiltonian model for the well studied, isoelectronic NH(2)D molecule, modified to account for the larger tunneling splitting. The combined analysis revealed rotational interaction between tunneling states as well as between the two OH stretching modes and permitted a substantial number of new assignments to be made, including one sub-millimeter transition while only few IR assignments had to be corrected or omitted. It leads to improvement in parameters for both the ground as well as for the OH stretching states of this important molecular ion, which reproduce the assigned lines within experimental uncertainties, provides guidance, e.g., for the spectral search in the OD stretch region, and yields deeper insight into the tunneling dynamics.

High resolution analysis of the rotational levels of the (0 0 0), (0 1 0), (1 0 0), (0 0 1), (0 2 0), (1 1 0) and (0 1 1) vibrational states of 34S 16O 2

A high resolution (0.0018 cm −1) Fourier transform instrument has been used to record the spectrum of an enriched 34S (95.3%) sample of sulfur dioxide. A thorough analysis of the ν 2, 2 ν 2 − ν 2, ν 1, ν 1 + ν 2 − ν 2, ν 3, ν 2 + ν 3 − ν 2, ν 1 + ν 2 and ν 2 + ν 3 bands has been carried out leading to a large set of assigned lines. From these lines ground state combination differences were obtained and fit together with the existing microwave, millimeter, and terahertz rotational lines. An improved set of ground state rotational constants were obtained. Next, the upper state rotational levels were fit. For the (0 1 0), (1 1 0) and (0 1 1) states, a simple Watson-type Hamiltonian sufficed. However, it was necessary to include explicitly interacting terms in the Hamiltonian matrix in order to fit the rotational levels of the (0 2 0), (1 0 0) and (1 0 1) states to within their experimental accuracy. More explicitly, it was necessary to use a Δ K = 2 term to model the Fermi interaction between the (0 2 0) and (1 0 0) levels and a Δ K = 3 term to model the Coriolis interaction between the (1 0 0) and (0 0 1) levels. Precise Hamiltonian constants were derived for the (0 0 0), (0 1 0), (1 0 0), (0 0 1), (0 2 0), (1 1 0) and (0 1 1) vibrational states.