Stark Modulation Spectroscopy Research Articles

A Comprehensive Spectral Rotational Analysis of the Interstellar Methyl Isocyanate CH3NCO

Abstract Methyl isocyanate (CH3NCO) is a recently identified interstellar molecule giving rise to many detected lines. Interestingly, its delayed identification was due not to weak lines but to a very complex rotational spectrum. To date, the only published laboratory transitions for this molecule are those between rotational energy levels with K ≤ 3. In the present work, Stark-modulation spectroscopy was used to record the room-temperature rotational spectrum of CH3NCO in the spectral region from 32 to 90 GHz. Observation of characteristic Stark effects, measured at specific low-voltage modulation conditions, and 14N nuclear quadrupole hyperfine structure allowed unambiguous assignment of rotational transitions up to K = 10. These newly assigned transitions were subsequently followed up to 364 GHz with the aid of Loomis–Wood-type displays. Since there are no reports on astrophysical detection of 13C isotopic species, first laboratory measurements between 50 and 300 GHz have also been performed for CH3N13CO and 13CH3NCO isotopologues. A comprehensive spectral analysis undertaken in this work made it possible to extend the knowledge of the rotational spectrum of CH3NCO to more than 2500 new transitions. Furthermore, more than 1200 lines were identified and analyzed for each of the isotopologues. The extensive line lists and sets of molecular parameters reported in this work provide the basis for further astrophysical searches of CH3NCO.

A Comprehensive Rotational Study of Interstellar Iso-propyl Cyanide up to 480 GHz

Abstract A detailed analysis of the rotational spectra of the interstellar iso-propyl cyanide has been carried out up to 480 GHz using three different high-resolution spectroscopic techniques. Jet-cooled broadband chirped pulse Fourier transform microwave spectroscopy from 6 to 18 GHz allowed us to measure and analyze the ground-state rotational transitions of all singly substituted 13C and 15N isotopic species in their natural abundances. The monohydrate of iso-propyl cyanide, in which the water molecule bounds through a stronger O–H⋯N and weaker bifurcated (C–H)2⋯O hydrogen bonds in a C s configuration, has also been detected in the supersonic expansion. Stark-modulation spectroscopy in the microwave and millimeter wave range from 18 to 75 GHz allowed us to analyze the vibrational satellite pattern arising from pure rotational transitions in the low-lying vibrational excited states. Finally, assignments and measurements were extended through the millimeter and submillimeter wave region. The room temperature rotational spectra made possible the assignment and analysis of pure rotational transitions in 19 vibrationally excited states. Significant perturbations were found above 100 GHz in most of the observed excited states. Due to the complexity of the interactions and importance of this astrophysical region for future radioastronomical detection, both a graphical plot approach and a coupled fit have been used to assign and measure almost 10,000 new lines.

Microwave spectrum, tunneling motions, and quadrupole coupling hyperfine structure of ethylene diamine

New transitions are measured in the microwave spectrum of two conformers of the non-rigid ethylene diamine (CH 2NH 2–CH 2NH 2) molecule using both molecular beam Fourier transform and stark modulation spectroscopy. Ab initio calculations are carried out to obtain geometries and energies for several stationary points of the potential energy surface and to gain quantitative information on the tunneling paths along which the molecule tunnels. The microwave data are reproduced using an IAM-like approach which accounts for the rotational dependence of the tunneling splitting corresponding to the interconversion large amplitude motion. For one of the conformers, the results of this analysis are consistent with the ones obtained through ab initio calculations. Hyperfine patterns are analyzed with the help of a theoretical approach accounting for the large amplitude motion effects and for hyperfine matrix elements within and between tunneling sublevels.

Optical heterodyne–Stark modulation spectroscopy

A spectroscopic scheme, which combines the Stark modulation with the optical heterodyne detection technique, improves the detection sensitivity by a factor of about 30 over that of Stark modulation spectroscopy alone.

The direction of electric dipole moment of ICl A 3Π1 state through the analysis of the Stark modulation spectroscopy

Recently, two works were reported that the permanent electric dipole moment of the ICl A state has the same direction as that of the X state, through the analysis of electric and magnetic pendular spectra and through the direct analysis of the hyperfine structures under the Stark effect, respectively. We determined clearly and sensitively the direction of the dipole moment in the A (v=27) state of I35Cl, through the line shape analysis of the Stark modulation spectra (SMS). While the result was the same as those of previous two works, the analysis of the line shape of the SMS was so useful to determine the relative direction of the dipole moment of the upper electronic state to that in the lower state, whichever of the first-order or the second-order Stark effect the state may show.

Tunable diode laser Stark modulation spectroscopy for rotational assignment of the HNO 3 7.5 μm band

The technique of Stark modulation spectroscopy for unravelling and assigning rotationally resolved dense molecular spectra has been employed using a tunable diode laser (TDL) source. Doppler-limited absorption and Stark modulation spectra of the HNO 3 7.5 μm band near the 1326 cm −1 band origin are presented, with preliminary values of the excited-state rovibrational constants derived from both TDL and Bomem FTIR spectra.

Nitrogen-14 Quadrupole Coupling Constants in Propanedinitrile, CH2(CN)2, by Microwave Spectroscopy

Nitrogen-14 quadrupole hyperfine structure has been resolved in the microwave spectrum of propanedinitrile, CH2(CN)2. Measurements between 5 and 30 GHz by Fourier transform and Stark modulation spectroscopy have been analysed. The general matrix elements for nuclear quadrupole interaction in molecules containing two equivalent coupling nuclei have been derived as simple formulae. The quadrupole coupling constants determined for the inertial axis system of CH2(CN)2 are χaa = - 2.368(28) MHz, χbb = 0.318(20) MHz, and χcc = 2.050(20) MHz. These values are used to establish that the electric field gradient tensor at nitrogen is cylindrically symmetric about the C≡N bond with a value of χCN = - 4.09(4) MHz, close to values found for other organic cyanides.

Stark-modulation spectroscopy of NH_3 with a 123-μm semiconductor laser

Several combination-band transitions NH(3) are observed by Stark-modulation spectroscopy with a 1.23-microm semi-conductor laser. Some of them are assigned to the P-branch transitions of the (2nu(1) + nu(4)) band.

Spatially resolved optical Stark-modulation spectroscopy in flames

This Letter describes the application of spatially resolved optical Stark-modulation spectroscopy for making point-absorption measurements in combustion environments by monitoring transitions between electronic energy levels. The technique is demonstrated in a study of atomic sodium aspirated into a flame.

Stark effect observed in molecular iodine and its application to laser-frequency stabilization

The Stark shift in the visible absorption spectrum of a homonuclear diatomic molecule has been observed for the first time to the author's knowledge in molecular iodine through the use of Stark modulation spectroscopy. The Stark modulation technique is proposed for a new method of stabilization of a visible laser frequency on the I(2) or ICI absorption lines.

Stark modulation spectrometer using a wideband Zeeman-tuned He-Xe laser

A 200 GHz-tunable 3.51 μm He−Xe laser is employed in Stark-modulation spectroscopy, and good performance of the spectrometer is demonstrated by an exemplary measurement on the HDO spectrum. Various spectral features and the anomalous line shapes due to the Stark effect are interpreted and used as an important clue to identification of the absorption lines.

Pressure and power correlated dependence in microwave spectroscopy using a non-linear-effect modulation spectrometer

The modification of the envelope of a modulated wave resulting from the non-linear absorption of a rotational transition can be used as the basic principle for the detection of transitions in gas phase spectroscopy. A spectrometer working on this principle is briefly described. Experimentally each transition contributes to two spectrometer signals, both of which have a non-Lorentzian lineshape. Nevertheless, the linewidth Delta nu can be easily correlated to the usual Lorentzian linewidth and is found to show the same pressure (P) and power ( pi ) dependence as in Stark modulation spectroscopy. Studies of pure OCS and of an OCS/air mixture have been carried out. In the case of pure OCS, the product S( Delta nu )4 is proportional to P3/2 and to pi 2. From the point of view of sensitivity the effective use of a non-linear-effect spectrometer is limited to cases where the sample pressure is intrinsically low.

Broadband and low insertion loss Stark cell for microwave spectroscopy

A groove guide cell for Stark modulation spectroscopy has been constructed and tested. This structure with quasi-unique mode propagation between two isolated plates is well adapted to application of a modulation voltage. The insertion losses are low (0.6 dB for a 3-m-long cell, and R band) and the cell is free of internal reflection. A metallic casing enclosing the plates acts as a shield against electrical interference, prevents low-voltage breakdown, and reduces wall adsorption of the gas. The performance of the cell is not affected by temperature variation (-80 degrees to 25 degrees C). A high sensitivity has been obtained by converting the cell into a 3-m-long, high-Q cavity. This type of cell appears to particularly recommend itself for use in intensity measurements, broadband spectroscopy, analytical studies, and high frequency range work.

An optical multichannel detection system for modulation spectroscopy

A measurement system using a silicon target vidicon is described, which can greatly improve signal-to-noise ratio compared with the commonly used photomultiplier. Contrary to the photomultiplier, however, a relatively high light level is required because of the higher dark current level of the vidicon. Originally developed for Stark modulation spectroscopy, the system should also be suitable for other modulation-type experiments.

Excited state dipole moments of phenanthrene: Electric field modulation of the luminescence

The dipole moments of the lowest 3ππ * and 1ππ * excited states of phenanthrene have been determined by Stark modulation spectroscopy and are ≤0.12 D and 0.34 D respectively. The method is extremely sensitive and makes possible quantitative investigations of relatively small dipole moment changes of broad banded spectral systems.