Lamb-dip Technique Research Articles

Deuterium hyperfine splittings in the rotational spectrum of NH2D as revealed by Lamb-dip spectroscopy

In the context of radio-astronomical observations, laboratory experiments constitute a cornerstone in the interpretation of rich line surveys due to the concomitant presence of numerous emitting molecules. Here, we report the investigation of three different rotational transitions of mono-deuterated ammonia (NH2D), a species of astrophysical interest, for which the contribution of the deuterium nuclear spin to the rotational spectrum has been resolved for the first time in the millimeter- and submillimeter-wave domain. The effect of hyperfine interactions on the rotational spectrum has been unveiled by a combined theoretical and experimental approach.Quantum-chemical calculations based on coupled-cluster theory have been employed to evaluate the hyperfine parameters of nitrogen, hydrogen, and deuterium in NH2D. Subsequently, the Lamb-dip technique has been used to investigate the rotational spectrum of NH2D at high-resolution. In detail, three low-J transitions have been recorded at 86, 110, and 333GHz with a frequency-modulation millimeter-/submillimeter-wave spectrometer. From the line profile analysis of the recorded spectra, the main terms responsible for the rotational hyperfine structure have been determined with good accuracy.Our work allows a comprehensive analysis of the rotational features of NH2D in radioastronomical spectra and a more accurate evaluation of its column density, especially in non-turbulent regions showing narrow linewidths.

Laboratory millimeter wave spectrum of N-methylhydroxylamine

The pure rotational spectrum of N-methylhydroxylamine (CH3NHOH), a potential candidate in the interstellar medium, has been studied in the millimeter-wave region up to 360GHz. The molecule was liberated from its hydrochloride salt and the ground and six lowest excited vibrational states of the most stable trans conformation as well as the 13C isotopologue in natural abundance have been measured and analyzed. From the observed ground state A–E splittings, a barrier hindering the methyl internal rotation V3=1245.8 (46) cm−1 has been derived. The sub-Doppler Lamb-dip technique has been employed to resolve the 14N hyperfine structure of strong transitions. The spectroscopic constants reported in the present study should constitute a solid set of laboratory data to support future searches of N-methylhydroxylamine in the interstellar medium.

THE HYPERFINE STRUCTURE OF THE ROTATIONAL SPECTRUM OF HDO AND ITS EXTENSION TO THE THz REGION: ACCURATE REST FREQUENCIES AND SPECTROSCOPIC PARAMETERS FOR ASTROPHYSICAL OBSERVATIONS

The rotational spectrum of the mono-deuterated isotopologue of water, HD16O, has been investigated in the millimeter- and submillimeter-wave frequency regions, up to 1.6 THz. The Lamb-dip technique has been exploited to obtain sub-Doppler resolution and to resolve the hyperfine (hf) structure due to the deuterium and hydrogen nuclei, thus enabling the accurate determination of the corresponding hf parameters. Their experimental determination has been supported by high-level quantum-chemical calculations. The Lamb-dip measurements have been supplemented by Doppler-limited measurements (weak high-J and high-frequency transitions) in order to extend the predictive capability of the available spectroscopic constants. The possibility of resolving hf splittings in astronomical spectra has been discussed.

The hyperfine structure in the rotational spectra of D2(17)O and HD(17)O: Confirmation of the absolute nuclear magnetic shielding scale for oxygen.

Guided by theoretical predictions, the hyperfine structures of the rotational spectra of mono- and bideuterated-water containing (17)O have been experimentally investigated. To reach sub-Doppler resolution, required to resolve the hyperfine structure due to deuterium quadrupole coupling as well as to spin-rotation (SR) and dipolar spin-spin couplings, the Lamb-dip technique has been employed. The experimental investigation and in particular, the spectral analysis have been supported by high-level quantum-chemical computations employing coupled-cluster techniques and, for the first time, a complete experimental determination of the hyperfine parameters involved was possible. The experimentally determined (17)O spin-rotation constants of D2 (17)O and HD(17)O were used to derive the paramagnetic part of the corresponding nuclear magnetic shielding constants. Together with the computed diamagnetic contributions as well as the vibrational and temperature corrections, the latter constants have been employed to confirm the oxygen nuclear magnetic shielding scale, recently established on the basis of spin-rotation data for H2 (17)O [Puzzarini et al., J. Chem. Phys. 131, 234304 (2009)].

Impact of sub-Doppler measurements on centrifugal-distortion terms: rotational spectrum of methyl fluoride revisited.

Methyl fluoride is a prototypical symmetric-top molecule. Despite the fact that its rotational spectrum has been largely investigated, centrifugal-distortion constants were determined only up to the sextic terms. The present investigation demonstrates the importance of sub-Doppler measurements in the terahertz region not only for deriving higher-order centrifugal-distortion terms but also for revising the parameters available in the literature. The Lamb-dip technique has been exploited for obtaining sub-Doppler resolution in the 102 GHz to 1.2 THz frequency range, thus allowing for the improvement of all spectroscopic parameters. Furthermore, the hyperfine structure due to fluorine and hydrogens has been resolved, thus enabling the determination of the corresponding spin-rotation constants with an accuracy rivaling that obtained by molecular-beam electric resonance measurements and of the dipolar spin-spin coupling constants for the first time. The prediction and analysis of hyperfine structures were guided and supported by high-level quantum-chemical calculations of the parameters involved.

The rotational spectrum of hydrogen sulfide: The H2[formula omitted]S and H2[formula omitted]S isotopologues revisited

The rotational spectra of two isotopic species of hydrogen sulfide have been revisited. For H233S, which was detected in natural abundance, accurate measurements were performed in the submillimeter-wave region, from 500GHz up to 1.56THz, thus allowing improvement of the spectroscopic parameters as well as determination of new high-order centrifugal-distortion constants. The rotational spectrum of the main isotopologue was investigated in the millimeter- and submillimeter-wave region up to 1.6THz, employing the Lamb-dip technique to obtain sub-Doppler resolution. As a consequence, transition frequencies at 1THz were retrieved with an accuracy of 1kHz and the hyperfine structure due to hydrogens was resolved, thus allowing the first determination of the spin–rotation tensor of H in H2S. Improved and new spectroscopic parameters were then provided that allow accurate predictions of rotational transitions up to 20THz; in particular, the newly determined constants permit prediction of rotational transitions with J<15, Ka<12 (up to about 10THz) with expected uncertainties of a few hundreds of kHz.

33 S hyperfine interactions in H2S and SO2 and revision of the sulfur nuclear magnetic shielding scale

Using the Lamb-dip technique, the hyperfine structure in the rotational spectra of H2(33)S and (33)SO2 has been resolved and the corresponding parameters--that is, the sulfur quadrupole-coupling and spin-rotation tensors--were determined. The experimental parameters are in good agreement with results from high-level coupled-cluster calculations, provided that up to quadruple excitations are considered in the cluster operator, sufficiently large basis sets are used, and vibrational corrections are accounted for. The (33)S spin-rotation tensor for H2S has been used to establish a new sulfur nuclear magnetic shielding scale, combining the paramagnetic part of the shielding as obtained from the spin-rotation tensor with a calculated value for the diamagnetic part as well as computed vibrational and temperature corrections. The value of 716(5) ppm obtained in this way for the sulfur shielding of H2S is in good agreement with results from high-accuracy quantum-chemical calculations but leads to a shielding scale that is about 28 ppm lower than the one suggested previously in the literature, based on the (33)S spin-rotation constant of OCS.

Sub-Doppler Resolution in the THz Frequency Domain: 1 kHz Accuracy at 1 THz by Exploiting the Lamb-Dip Technique

We report the first thorough investigation of the Lamb-dip effect in the THz region, which in turn allows sub-Doppler resolution to be exploited in this frequency region. It is demonstrated that an accuracy of 1 kHz, or even better (i.e., an accuracy better than 1 part in 10(9)), and a frequency resolution of 50 kHz (i.e., a resolution better than 5 parts in 10(8)) can be routinely obtained in our laboratory. It has also shown that Lamb-dip spectra can be recorded using either a Fabry-Perot interferometric cell or a free-space cell. Hydrogen sulfide (H2S), sulfur dioxide (SO2), deuterated water (D2O), and methyl fluoride (CH3F) have been selected as examples for demonstrating the accuracy and resolution reachable, thus providing the most accurate frequency values in the 1.0-1.2 THz frequency range for these molecules. Measurements for SO2 have also been employed in a global fit, thus improving its spectroscopic parameters for the vibrational ground state.

Rotational spectrum of silyl chloride: hyperfine structure and equilibrium geometry

The Lamb-dip technique was employed to record the rotational spectra of two isotopic species of silyl chloride, namely 28SiH3Cl and 29SiH3Cl, in order to investigate their hyperfine structure. High-accuracy quantum-chemical computations were employed to predict the hyperfine parameters involved and to support the experimental investigation. Analysis of the experimental spectra led to an improvement in the accuracy of the known spectroscopic constants as well as allowed us to determine additional spectroscopic parameters for the first time. Furthermore, the equilibrium structure of silyl chloride was reinvestigated using both theoretical and experimental data. The best theoretical and semi-experimental geometries were found to agree within their stated accuracy, leading to the following recommended structure: r(Cl–Si) = 2.046(1)Å, r(Si–H) = 1.469(1)Å, and ∠ClSiH = 108.43(1)○.

PRECISE LABORATORY MEASUREMENTS OF TRANS-DCOOH AND TRANS-HCOOD FOR ASTROPHYSICAL OBSERVATIONS

The rotational spectra of the mono-deuterated isotopologues of trans-formic acid, trans-DCOOH and trans-HCOOD, were investigated. In the millimeter- and submillimeter-wave frequency regions the Lamb-dip technique was exploited to obtain sub-Doppler resolution and to resolve the hyperfine structure due to the deuterium and hydrogen nuclei, thus enabling the accurate determination of the corresponding hyperfine constants. The experimental determination was supported by high-level quantum-chemical calculations at the coupled-cluster level of theory using large atomic-orbital basis sets. The Lamb-dip measurements were also supplemented by THz Doppler-limited measurements in order to extend the predictive capability of the available spectroscopic parameters to higher frequencies.

Spectroscopic investigation of fluoroiodomethane, CH2FI: Fourier-transform microwave and millimeter-/submillimeter-wave spectroscopy and quantum-chemical calculations

Guided by theoretical predictions, the rotational spectrum of fluoroiodomethane, CH(2)FI, has been recorded and assigned. Accurate values are reported for the ground-state rotational constants, all quartic, sextic, and two octic centrifugal-distortion constants. The hyperfine structure of the rotational spectrum was thoroughly investigated using a Fourier-transform microwave spectrometer and the Lamb-dip technique in the millimeter-/submillimeter-wave region, thus allowing the accurate determination of the complete iodine quadrupole-coupling tensor and of the diagonal elements of the iodine spin-rotation tensor. Relativistic effects turned out to be essential for the accurate theoretical prediction of the dipole moment and quadrupole-coupling constants and were accounted for by direct perturbation theory and a spin-free four-component treatment based on the Dirac-Coulomb Hamiltonian. The relativistic corrections to the dipole moment amount to up to 34% and to the iodine quadrupole-coupling tensor to about 15-16% of the total values.

Microwave, High-Resolution Infrared, and Quantum Chemical Investigations of CHBrF2: Ground and v4 = 1 States

A combined microwave, infrared, and computational investigation of CHBrF(2) is reported. For the vibrational ground state, measurements in the millimeter- and sub-millimeter-wave regions for CH(79)BrF(2) and CH(81)BrF(2) provided rotational and centrifugal-distortion constants up to the sextic terms as well as the hyperfine parameters (quadrupole-coupling and spin-rotation interaction constants) of the bromine nucleus. The determination of the latter was made possible by recording of spectra at sub-Doppler resolution, achieved by means of the Lamb-dip technique, and supporting the spectra analysis by high-level quantum chemical calculations at the coupled-cluster level. In this context, the importance of relativistic effects, which are of the order of 6.5% and included in the present work using second-order direct perturbation theory, needs to be emphasized for accurate predictions of the bromine quadrupole-coupling constants. The infrared measurements focused on the ν(4) fundamental band of CH(79)BrF(2). Fourier transform investigations using a synchrotron radiation source provided the necessary resolution for the observation and analysis of the rotational structure. The spectroscopic parameters of the v(4) = 1 state were found to be close to those of the vibrational ground state, indicating that the ν(4) band is essentially unaffected by perturbations.

The rotational spectrum of trans-DCOOD: Lamb-dip measurements, THz spectroscopy and quantum-chemical calculations

The rotational spectrum of the bi-deuterated isotopologue of trans-formic acid, trans-DCOOD, was recorded at sub-Doppler resolution in the millimeter- and sub-millimeter-wave region using the Lamb-dip technique. The hyperfine structure due to the deuterium nuclei could be resolved and accurate hyperfine constants were derived. The experimental determination of the deuterium quadrupole-coupling constants was supported by high-level quantum-chemical calculations at the coupled-cluster level using large atomic-orbital basis sets. The Lamb-dip measurements were also supplemented by THz Doppler-limited measurements in order to extend the predictive capability of the available spectroscopic parameters to higher frequencies, i.e., up to 5 THz.

The hyperfine structure in the rotational spectra of D2O: Lamb-dip measurements and quantum-chemical calculations

Thirteen rotational transitions of the doubly deuterated isotopic species of water, D2 16O, have been recorded in the millimeter- and submillimeter-wave region. The Lamb-dip technique has been used to resolve the hyperfine structure due to deuterium quadrupole coupling, due to spin–rotation as well as dipolar spin–spin interactions, thus enabling the determination of the corresponding hyperfine parameters to a good accuracy. The experimental investigation has been assisted by high-level quantum-chemical calculations of the hyperfine parameters.

Hyperfine structure in the rotational spectra of trans-formic acid: Lamb-dip measurements and quantum-chemical calculations

Context. Formic acid, HCOOH, is the simplest organic acid and the first that has been identified in the interstellar medium. Its astrophysical relevance has motivated this spectroscopic study. Aims. The aim of this investigation is to provide very accurate rest frequencies for the trans isomer of HCOOH as well as to improve the spectroscopic and hyperfine parameters available in the literature for this molecule. Methods. The Lamb-dip technique has been exploited in order to record the rotational spectrum of trans-HCOOH at sub-Doppler resolution in the millimeter- and submillimeter-wave frequency ranges and, when possible, to resolve the hyperfine structure due to the hydrogen nuclei. THz measurements have been carried out as well. The experimental investigation has been supported by high-level quantum-chemical calculations. Results. As a consequence, we report here the most accurate ground-state rotational parameters known so far for the main isotopic species of trans-HCOOH as well as an accurate and reliable set of hyperfine parameters. Conclusions. The improvement in the accuracy of the ground-state rotational and hyperfine parameters as well as of the rest frequencies of rotational transitions might therefore be useful for future radioastronomical observations.

The magnetic hyperfine structure in the rotational spectrum of H 2CNH

The hyperfine structure in the ground-state rotational spectrum of methanimine was studied in the frequency range of 64–172 GHz by means of the Lamb-dip technique. This allowed to resolve, in some hyperfine components due to the 14N nucleus, doublets separated by only some tenth of kHz. We explain the splittings as due to magnetic interactions of the three protons with their molecular environment. The analysis of the experimental spectrum has been guided by quantum-chemical calculations of the hyperfine parameters.

A new experimental absolute nuclear magnetic shielding scale for oxygen based on the rotational hyperfine structure of H2O17

The hyperfine structure in the rotational spectrum of water containing (17)O has been investigated experimentally and by means of quantum-chemical calculations. The Lamb-dip technique has been used to resolve the hyperfine structure due to spin-rotation as well as spin-spin interactions and allowed the determination of the corresponding hyperfine parameters with high accuracy. The experimental investigation and, in particular, the analysis of the spectra have been supported by quantum-chemical computations at the coupled-cluster level. The experimental (17)O isotropic spin-rotation constant of H(2)(17)O has been used in a further step for the determination of the paramagnetic part of the corresponding nuclear magnetic shielding constant, whereas the diamagnetic contribution as well as vibrational and temperature corrections have been obtained from quantum-chemical calculations. This joint procedure leads to a value of 325.3(3) ppm for the oxygen shielding in H(2)(17)O at 300 K, in good agreement with pure theoretical predictions, and in this way provides the basis for a new absolute oxygen shielding scale.

Rotational spectra of isotopic species of silyl fluoride. Part I: Lamb-dip measurements and quantum-chemical calculations

The Lamb-dip technique has been employed for recording the rotational spectra of three isotopic species of silyl fluoride, namely 28SiH 3F, 29SiH 3F, and 30SiH 3F, in order to improve the knowledge of their spectroscopic parameters as well as to try to resolve their hyperfine structure. High-level quantum-chemical computations using state-of-the-art coupled-cluster techniques together with core-polarized correlation-consistent basis sets have been employed to provide reliable reference values for the hyperfine parameters involved and have been used to guide the experimental investigation. Analysis of the experimental spectra allowed to improve the accuracy of the known spectroscopic parameters as well as to determine for the first time sextic and octic centrifugal-distortion constants.

The hyperfine structure in the rotational spectrum of water: Lamb-dip technique and quantum-chemical calculations

Seven ortho rotational transitions have been recorded for the main isotopic species of water in the millimeter- and submillimeter-wave region using the Lamb-dip technique in order to resolve the hyperfine structure due to the hydrogens and to provide accurate hyperfine constants. The experimental determination has been supplemented by high-level quantum-chemical calculations of the hyperfine parameters thereby focusing in particular on a systematic study of the basis-set convergence and on vibrational effects.

The hyperfine structure in the rotational spectra of bromofluoromethane: Lamb-dip technique and quantum-chemical calculations

The hyperfine structure in the rotational spectra of six isotopic species of bromofluoromethane, namely CH2 79BrF, CH2 81BrF, CDH79BrF, CDH81BrF, CD2 79BrF and CD2 81BrF, has been investigated using the Lamb-dip technique in the submillimeter-wave frequency range. Measurements and assignment procedures were supported by high-level quantum-chemical calculations of the hyperfine parameters at the coupled-cluster level. For all species, the accuracy of the determined rotational and centrifugal distortion constants and the bromine quadrupole-coupling constants were improved with respect to available literature data, whereas the full bromine quadrupole-coupling tensor for the monodeuterated species and the bromine spin–rotation constants for all isotopologues were determined for the first time.