Doppler-limited rotational spectrum of the NH radical in the 2 THz region

The Doppler-limited rotational spectrum of the NH radical in its electronic (X) and vibrational ground state has been measured using the frequency stabilized Cologne side-band spectrometer in the frequency region near 2 THz. The nitrogen ¹⁴N nuclear hyperfine patterns have been observed accompanying the resolved fine (J^′←J″) structure of the N=2←1 rotational transition. The observed peak frequencies were analyzed in detail together with the previously measured hyperfine frequencies of the N=1←0 rotational transition and with combination differences obtained from the high-resolution electronic spectra to derive precise rotational, centrifugal distortion, fine, and hyperfine parameters. In the numerical analysis the essential attention has been paid to partly resolved and unresolved hyperfine structures. The peak positions of the partly or fully overlapped lines were analyzed with the help of a profile simulation with estimated half-widths and calculated relative intensities and in this manner the least square fit of the unresolved and partly resolved lines was significantly improved. The NH radical is an extremely important species in nitrogen chemical reaction networks in the interstellar medium and atmospheric chemistry.     

The Lamb-dip spectrum of the J = 1 ← 0 transition of DF: Crossing resonances and hyperfine components

The Lamb-dip technique has been applied to the observation of the J = 1 ← 0 transition of DF: for the first time, the hyperfine structure due to D and F have been resolved by using microwave spectroscopy. The high accuracy of this technique allows us to provide hyperfine parameters that are in very good agreement with those obtained from molecular beam experiment. In addition, our frequencies together with the unresolved ones up to J″ value of 47 allow us to provide the most accurate ground state rotational constants of DF known at the moment. Furthermore, due to the presence of a relevant number of strong crossing resonances, the J = 1 ← 0 transition of DF can be considered an illustrative case to show how they modify the shape of Lamb-dip spectra.     

Improved ground state rotational parameters of deuterium fluoride, DF

The rotational spectrum of DF in the 1.3-3.3 THz frequency region has been observed by means of a tunable far-infrared spectrometer. The J + 1 ← J, with J = 1-4, rotational transitions of DF have been recorded with an accuracy of the order of 50-200 kHz. These measurements, in conjunction with the hyperfine components of the J = 1 ← 0 transition recently observed [Cazzoli and Puzzarini, J. Mol. Spectrosc. 231 (2005) 124-130] and the rotational transitions up to J = 47 [R.S. Ram, Z. Morbi, B. Guo, K.-Q. Zhang, P.F. Bernath, J. Vander Auwera, J.W.C. Johns, S.P. Davies, Astrophys. J. Suppl. Series 103 (1996) 247-254] consented to improve the ground state rotational parameters of DF.     

The rotational spectrum of the ground state of methylamine

Recent progress is reported in measuring, assigning, and fitting the rotational spectrum of the ground vibrational state of methylamine, CH₃NH₂, a spectrum complicated both by internal rotation of the methyl top and by inversion of the amino group. New measurements of 513 rotational transitions with J up to 30 and K up to 9 were carried out between 49 and 326 GHz using the millimeter-wave spectrometer in Kharkov. After removing the observed quadrupole hyperfine splittings, these new data along with previously published measurements were fitted to a group-theoretical high-barrier tunneling Hamiltonian from the literature, using 53 parameters to give an overall weighted standard deviation of 0.80 for 850 far-infrared and 673 microwave transitions in the ground state. The root-mean-square deviation of 0.018 MHz obtained for 346 millimeter-wave transitions measured with 0.020 MHz uncertainty represents an approximately 30-fold improvement in fitting accuracy over past attempts.     

The Lamb-dip spectrum of the J + 1 ← J (J = 0, 1, 3-8) transitions of HCN: The nuclear hyperfine structure due to H, C, and N

The pure rotational J + 1 ← J transitions, with J = 0, 1, 3-8, of H¹³CN have been observed in the millimeter- and submillimeter-wave region using the Lamb-dip technique to resolve the hyperfine structure due to H, ¹³C, and ¹⁴N. The present observations allow us to provide for the first time the spin-rotation constant of ¹³C and the spin-spin interaction constant S₁₂ (between H and ¹³C) as well as to remarkably improve the quadrupole coupling and spin-rotation constants of ¹⁴N. In addition, a good empirical estimation of C_I(H), based on ab initio calculations, has also been provided. Furthermore, our frequencies together with previous data permit to determine the most accurate ground state rotational parameters known up to now.     

FTMW spectroscopy of jet-cooled 9-cyanoanthracene

Rotational spectrum of jet-cooled 9-cyanoanthracene has been observed in the 4-8 GHz region with a Fourier-transform microwave spectrometer. The present observation of 25 low-J transitions with J^′′?11 has confirmed the previous results on the rotational constants of the ground state determined by rotational coherence spectroscopy [J. Phys. Chem. A. 105 (2001) 1131] and provided the values with significantly improved precision. An accurate set of hyperfine splitting constants is also reported for the ¹⁴N nuclear quadrupole coupling. The electric dipole moment was determined from Stark effect measurements on several split components: μ_b(=μ)=4.406(7) D.     

High-resolution spectroscopy of the ν8 band of methylene bromide using a quantum cascade laser

A continuous wave cavity ringdown spectrometer with a Fabry-Perot quantum cascade laser has been used to collect a rotationally-resolved infrared spectrum of the ν₈ vibrational band of methylene bromide in a slit nozzle expansion. In our laboratory, previous observations of the vibrational band were limited by spectral coverage to only the P and Q-branches and by the 24 MHz step-size of the laser [1]. The issue of limited spectral coverage has been resolved using a Fresnel rhomb and a wire grid polarizer to protect the laser from the destabilizing effects of back-reflection from the ringdown cavity. The frequency step-size of the spectrometer has been reduced from 24 MHz to 2 MHz. With both of these instrument enhancements, we have been able to record the R-branch of the vibrational band, and can resolve many lines that were previously blended in spectra acquired using a pinhole expansion nozzle. Significant hyperfine splitting was observed for the low-J transitions in the P and R-branches. It was possible to neglect the effects of hyperfine splitting for transitions involving J″ > 2 in the spectral assignment, and simulations using the constants obtained by fitting to Watson’s S-reduced Hamiltonian for CH₂⁷⁹Br⁸¹Br, and the A-reduced form for CH₂⁷⁹Br₂ and CH₂⁸¹Br₂, provide a good match to experimental spectra. A total of 297 transitions have been assigned for all three isotopologues, with a standard deviation of 0.00024 cm⁻¹(∼7 MHz).     

Ground state spectrum of methylcyanide

The rotational spectrum of methylcyanide (acetonitrile) in the ground vibrational state was measured in the spectral region from 91 to 810 GHz using the Cologne and Tsukuba spectrometers operated in the Doppler-limited and sub-Doppler saturation layouts. The resolution of the saturation Lamb-dip measurements is estimated to be about 1 kHz at the best of circumstances and the measuring accuracy of 10-60 kHz depending very sensitively on the quality of the spectrum. In the cases of rotational transitions with the low quantum number J (J<18) and with a low difference of the rotational quantum numbers J−K, the resolved or partly resolved hyperfine structures of the rotational transitions were observed. Together with the most accurate data from the literature, the newly measured experimental data were analyzed using the traditional polynomial energy formula as well as the Padè approximant for the effective rotational Hamiltonian. The resulting rotational, centrifugal distortion, and hyperfine structure spectroscopic constants were obtained with a significantly higher accuracy than the ones listed in the literature. In addition, an anomalous accidental resonance was detected between the K=14 ground state levels and the K=12, +l levels in the excited v₈=1 vibrational state.     

Millimeter wave spectrum of glycine

We present new investigations of the millimeter wave spectra of the two lowest-energy conformers of glycine (NH₂CH₂COOH). Measurements of these spectra have been carried out between 75 and 260 GHz using the millimeter-wave spectrometer in Kharkov. The new data set involves rotational transitions with J up to 44 and K_a up to 15 for conformer I and transitions with J up to 43 and K_a up to 14 for conformer II. This represents a more than twofold expansion both in the frequency range and J quantum-number range in comparison with previous investigations. The improved sets of spectroscopic parameters obtained for both conformers provide accurate transition frequencies for the key lines necessary for radio astronomy searches for interstellar glycine.     

The Lamb-dip spectrum of phosphine: The nuclear hyperfine structure due to hydrogen and phosphorus

For the first time, the hyperfine structure of the rotational J = 1 ← 0 (K = 0) and J = 2 ← 1 (K = 0, 1) transitions of phosphine has been resolved by using microwave spectroscopy. To this purpose, the Lamb-dip technique has been employed. In addition, the J = 3 ← 2 (K = 0, 1, 2) transition has been recorded at Doppler resolution. The present investigation allowed us to provide accurate values for most of the hyperfine constants as well as ground state rotational parameters.     

Rotational spectra of gauche perfluoro-n-butane, C4F10; perfluoro-iso-butane, (CF3)3CF; and tris(trifluoromethyl)methane, (CF3)3CH

The microwave spectra of the gauche conformer of perfluoro-n-butane, n-C₄F₁₀, of perfluoro-iso-butane, (CF₃)₃CF, and of tris(trifluoromethyl)methane, (CF₃)₃CH, have been observed and assigned. The rotational and centrifugal distortion constants for gauche n-C₄F₁₀ are: A = 1058.11750(7) MHz, B = 617.6832(1) MHz, C = 552.18794(1) MHz, Δ_J = 0.0257(5) kHz, δ_J = 0.0052(3) kHz. A C-C-C-C dihedral angle, ω, of ∼55° has been determined. These values agree well with those obtained from a coupled cluster (CCSD/cc-PVTZ) calculation. The rotational and centrifugal distortion constants for iso-C₄F₁₀ and iso-C₄HF₉ are: B_o = 816.4519(4) MHz, D_J = 0.023(2) kHz, and B_o = 903.6985(25) MHz, D_J = 0.043(4) kHz, respectively. The dipole moment of iso-C₄F₁₀ and iso-C₄HF₉ have been measured and found to be 0.0338(8) and 1.69(9) D, respectively.     

Lineshape analysis of the J = 3 ← 2 and J = 5 ← 4 rotational transitions of room temperature CO broadened by N2, O2, CO2 and noble gases

Collisional relaxation has been considered for millimeter lines of carbon monoxide at room temperature. Accurate measurements of carbon dioxide- and rare gases-broadened widths have been performed on the J = 3 ← 2 rotational line of ¹²CO by using a video-type spectrometer. Measurements of nitrogen-, oxygen-, and xenon-broadened widths of the J = 5 ← 4 rotational line of ¹³CO were also carried by using a frequency-modulated spectrometer. A lineshape study performed on all the investigated binary systems provide confirmation that Voigt profile is not a suitable model to analyse experimental lines in the millimeter-waves region. On one hand, using this profile in the low pressure range, i.e. in the Doppler regime, the retrieved collisional linewidths do not follow a linear variation with the perturbing gas pressure. On the other hand, regardless of the pressure, lineshapes exhibit a narrowed profile. An accurate analysis of the pressure dependence of relaxation rates show that the Galatry profile is not appropriate and that experimental lineshapes are actually Speed Dependent Voigt profiles. Accurate broadening parameters were retrieved from this profile and compared to previous reported values and predictions calculated from the Robert-Bonamy formalism. Finally a variation of the ratio of relaxation speed dependence to broadening parameters versus relative masses of the collision partners is presented.     

The microwave spectrum and structure of the argon trifluoroacetonitrile complex

The rotational spectrum of argon trifluoroacetonitrile complex has been studied by pulsed-nozzle, Fourier transform microwave spectroscopy. Both a-type and b-type transitions have been observed. The rotational constants are A = 3053.0903(2) MHz, B = 1039.9570(2) MHz, and C = 895.5788(1) MHz. The ¹⁴N nuclear quadrupole hyperfine components of the rotational transitions have been resolved, the ¹⁴N nuclear quadrupole coupling constants are χ_aa = 1.746(1) MHz, and χ_bb − χ_cc = −6.426(2) MHz. The complex is T-shaped, with the argon atom located 3.73 Å from the center of mass of the trifluoroacetonitrile molecule.     

The millimeter-wave rotational spectrum of CF3CCD in the excited vibrational state v10=2

The millimetre-wave rotational spectra of the excited vibrational state v₁₀=2 of the symmetric top molecule, CF₃CCD, have been recorded for J^′′=12 up to J^′′=25. The l=±2 and l=0 series have been assigned and the spectra analysed to give rotational parameters including x_ll=7716.975 MHz. The main interactions between states of different l are the r_t(2,−1)=0.158 MHz and q_t⁺(2,2)=3.308 MHz. Two type of l-resonance are identified, one of which is due to an avoided crossing between the l=0 and l=+2 series. The spectra are qualitatively similar to the corresponding ones of CF₃CCH.     

Rotational spectrum of three conformers of 3,3-difluoropentane: Construction of a 480 MHz bandwidth chirped-pulse Fourier-transform microwave spectrometer

The rotational spectra for three conformers of 3,3-difluoropentane have been measured using both a newly constructed narrow bandwidth chirped-pulse Fourier-transform microwave spectrometer and a Balle-Flygare resonant cavity Fourier-transform microwave spectrometer. The chirped-pulse instrument produces a microwave pulse spanning up to 480 MHz bandwidth in the 7-18 GHz region by mixing a 1 μs chirped pulse (of up to 240 MHz bandwidth) from an arbitrary function generator with the output from a microwave synthesizer.Rotational spectra for the normal isotopic species and all possible ¹³C single substitutions were observed for the gauche-gauche and anti-gauche conformers, allowing a Kraitchman substitution structure and an inertial fit structure to be determined. ¹³C isotopic species and dipole moment components were not measurable for the less intense anti-anti species as a result of partially resolved fine splitting. Details of the new chirped-pulse instrument will be described and the structural results will be presented and compared with ab initio data for 3,3-difluoropentane.     

Nuclear quadrupole coupling in chloroform and calibration of ab initio calculations

The complex hyperfine structures in the J = 1 ← 0, and J = 2 ← 1 ground state rotational transitions of ³⁵Cl₃CH and ³⁵Cl₂³⁷ClCH were resolved and measured at conditions of supersonic expansion. Accurate spectroscopic constants for the two isotopomers have been derived from global fits of the hyperfine structure together with hyperfine-free high-J millimetre wave data. The complete inertial and principal quadrupole tensors of the chlorine nuclei have been determined, and the symmetric top treatment for ³⁵Cl₃CH and the asymmetric top treatment for ³⁵Cl₂³⁷ClCH yield identical results for the principal tensor components of the ³⁵Cl nucleus. The availability of precise experimental splitting constants for many molecules allows benchmarking of ab initio field gradient calculations, and it is found that for the chlorine nucleus optimum predictive performance for molecules of moderate size is obtained at the B3LYP/aug-cc-pVDZ level by using a scaling factor of 1.0619(23).     

Rotational and hyperfine spectra for a sandwich titanium complex, C5H5TiC7H7

Microwave spectroscopy measurements and density functional theory calculations are reported for the cyclopentadienyl cycloheptatrienyl titanium complex, C₅H₅TiC₇H₇. Rotational transition frequencies for this symmetric-top complex were measured in the 4-13 GHz range using a Flygare-Balle-type pulsed beam spectrometer. The spectroscopic constants obtained for the normal C₅H₅⁴⁸TiC₇H₇ isotopomer are B = 771.78907(38), D_J = 0.0000295(41), and D_JK = 0.001584(73) MHz. The quadrupole hyperfine splittings for C₅H₅⁴⁷TiC₇H₇ were clearly observed and the measured constants are B = 771.79024(32) MHz, D_J = 0.0000395(33), D_JK = 0.001646(24), and eQq_aa = 8.193(40) MHz. Analysis of the experimental and theoretical rotational constants indicates that the η⁷-C₇H₇Ti and η⁵-C₅H₅Ti bond lengths in the gas phase are about 0.02 Å longer than those reported for the solid-state X-ray structure. The calculated Ti-C bond lengths are shorter for the C₇H₇ ligand (r(Ti-C) = 2.21 Å) than for the C₅H₅ ligand (r(Ti-C) = 2.34 Å), and the C₇H₇ H atoms are displaced 0.15 Å out of the C₇ plane, toward the Ti atom.     

Sub-Doppler and Doppler spectroscopy of DCN isotopomers in the terahertz region: ground and first excited bending states (v1v2v3)=(0 1 0)

The rotational spectra of the deuterium cyanide isotopic species DCN, D¹³CN, DC¹⁵N, and D¹³C¹⁵N were recorded in the vibrational ground and first excited bending state (v₂=1) up to 2 THz. The R-branch transitions from J=3←2 to J=13←12 were measured with sub-Doppler resolution. These very high resolution (∼70 kHz) and precise (±3-10 kHz) saturation dip measurements allowed for resolving the underlying hyperfine structure due to the ¹⁴N nucleus in DCN and D¹³CN for transitions as high as J=10←9. Additional high JR-branch (J=25←24 to J=28←27) transitions around 2 THz and direct l-type (ΔJ=0, J=19 to J=25) transitions from 66 to 118 GHz were recorded in Doppler-limited resolution. For the ground state of D¹³C¹⁵N, the J=1←0 transition was measured for the first time. The transition frequency accuracies for the other deuterated species were significantly improved. These new experimental data, together with the available infrared rovibrational data and previously measured direct l-type transitions, were subjected to a global least squares analysis for each isotopomer. This yielded precise sets of molecular constants for the ground and first excited vibrational states, including the nuclear quadrupole and magnetic spin-rotation coupling constants of the ¹⁴N nucleus for DCN and D¹³CN. The hyperfine structure due to the D, ¹³C, and ¹⁵N nuclei have not been resolved, but led to a broadening of the observed saturation dips.     

Analysis of the first triad of interacting states (0 2 0), (1 0 0), and (0 0 1) of D2O from hot emission spectra

The far-infrared and middle-infrared emission spectra of deuterated water vapour were measured at temperatures 1370, 1520, and 1940 K in the ranges 320-860 and 1750-3400 cm⁻¹. The measurements were performed in an alumina cell with an effective length of hot gas of about 50 cm. More than 3550 new measured lines for the D₂¹⁶O molecule corresponding to transitions from highly excited rotational levels of the (0 2 0), (1 0 0), and (0 0 1) vibrational states are reported. These new lines correspond to rotational states with higher values of the rotational quantum numbers compared to previously published determinations: J_max = 29 and K_a(max) = 22 for the (0 2 0) state, J_max = 29 and K_a(max) = 25 for the (1 0 0) state, and J_max = 30 and K_a(max) = 23 for the (0 0 1) state. The extended set of 1987 experimental rotational energy levels for the (0 2 0), (1 0 0), and (0 0 1) vibration states including all previously available data has been determined. For the data reduction we used the generating function model. The root mean square (RMS) deviation between observed and calculated values is 0.004 cm⁻¹ for 1952 rovibrational levels of all three vibration states. A comparison of the observed energy levels with the best available values from the literature and with the global predictions from molecular electronic potential energy surfaces of water isotopic species [H. Partridge, D.W. Schwenke, J. Chem. Phys. 106 (1997) 4618] is discussed. The latter confirms a good consistency of mass-dependent DBOC corrections in the PS potential function with new experimental rovibrational data.     

Microwave and submillimeterwave spectra of CH2DI and CHD2I

The pure rotational spectra of CH₂DI and CHD₂I have been measured by microwave Fourier transform spectroscopy, millimeterwave spectroscopy and submillimeterwave spectroscopy. The quadrupole hyperfine structure due to iodine has been analyzed by direct diagonalization of the quadrupole tensor. For the J = 1-0 transition of the ground state of CH₂DI, the quadrupole hyperfine structure due to deuterium could be resolved and the quadrupole coupling constant eQq_aa(D) determined.Accurate rotational and centrifugal distortion constants (up to sextic terms) have been determined. They are compared to the constants derived from the ground state combination differences (GSCD). A good agreement is observed but it is also found that the two kinds of data (GSCD and microwave) are complementary and a combined fit allows us to significantly improve the accuracy of the constants.