Performance of a chirped-pulse Fourier transform millimeter wave spectrometer in the range of 75-110 GHz.

We present a home-built chirped-pulse Fourier transform millimeter wave (CP-FTMMW) spectrometer. The setup is devoted to the sensitive recording of high-resolution molecular spectroscopy in the W band between 75 and 110 GHz. We describe the experimental setup in detail, including a characterization of the chirp excitation source, the optical beam path, and the receiver. The receiver is a further development of our 100 GHz emission spectrometer. The spectrometer is equipped with a pulsed jet expansion and a DC discharge. Spectra of methyl cyanide as well as hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) products from the DC discharge of this molecule are recorded to characterize the performance of the CP-FTMMW instrument. The formation of the HCN isomer is favored by a factor of 63 with respect to HNC. Hot/cold calibration measurements enable a direct comparison of the signal and noise levels of the CP-FTMMW spectra to those of the emission spectrometer. For the CP-FTMMW instrument, we find many orders of magnitude of signal enhancement and a much stronger noise reduction due to the coherent detection scheme.

Ab initio potential and rotational spectra of the CO-N2 complex.

Ab initio calculations of the intermolecular potential energy surface (PES) of CO-N2 have been carried out using the closed-shell single- and double-excitation coupled cluster approach with a non-iterative perturbative treatment of triple excitations method and the augmented correlation-consistent quadruple-zeta (aug-cc-pVQZ) basis set supplemented with midbond functions. The global minimum (De = 117.35 cm-1) of the four-dimensional PES corresponds to an approximately T-shaped structure with the N2 subunit forming the leg and CO the top. The bound rovibrational levels of the CO-N2 complex were calculated for total angular momenta J = 0-8 on this intermolecular potential surface. The calculated dissociation energies D0 are 75.60 and 76.79 cm-1 for the ortho-N2 (A-symmetry) and para-N2 (B-symmetry) nuclear spin modifications of CO-N2, respectively. Guided by these bound state calculations, a new millimeter-wave survey for the CO-N2 complex in the frequency range of 110-145 GHz was performed using the intracavity OROTRON jet spectrometer. Transitions not previously observed were detected and assigned to the subbands connecting the K = 0 and 1, (jCO, jN2 ) = (1, 0) states with a new K = 1, (jCO, jN2 ) = (2, 0) state. Finally, the measured rotational energy levels of the CO-N2 complex were compared to the theoretical bound state results, thus providing a critical test of the quality of the PES presented. The computed rovibrational wave functions were analyzed to characterize the nature of the different bound states observed for the two nuclear spin species of CO-N2.

A laboratory heterodyne emission spectrometer at submillimeter wavelengths.

We present first results on a newly built broadband emission spectrometer for the laboratory making use of a double sideband (DSB) heterodyne receiver. The new spectrometer is perfectly suited for high-resolution emission spectroscopy of molecules of astrophysical importance. The current SIS receiver operates at RF frequencies between 270 and 390 GHz, coincident with Band 7 of the ALMA telescope. The instantaneous bandwidth is 5 GHz (DSB). In this work the full spectrometer and its components are described. Its performance, in particular its sensitivity, stability, reproducibility and systematic errors, is characterized in detail. For this purpose very broad band emission spectra of methyl cyanide have been recorded and compared to theoretical spectra. Isotopic variants are found in natural abundance and features attributed to vibrationally excited species are all recorded in the same spectrum. The performance of the new spectrometer is compared extensively to that of a traditional FM-absorption spectrometer and to recent versions of chirped-pulse spectrometers operated in the mm-wave regime. Further applications and future advancements of the current instrument are discussed.

Rotational study of the CH4-CO complex: Millimeter-wave measurements and ab initio calculations.

The rotational spectrum of the van der Waals complex CH4-CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 110-145 GHz. Newly observed and assigned transitions belong to the K = 2-1 subband correlating with the rotationless jCH4 = 0 ground state and the K = 2-1 and K = 0-1 subbands correlating with the jCH4 = 2 excited state of free methane. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the CH4-CO complex. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of CH4-CO have been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)-F12a] and an augmented correlation-consistent triple zeta (aVTZ) basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the CH4 face closest to the CO subunit and binding energy De = 177.82 cm(-1). The bound rovibrational levels of the CH4-CO complex were calculated for total angular momentum J = 0-6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 91.32, 94.46, and 104.21 cm(-1) for A (jCH4 = 0), F (jCH4 = 1), and E (jCH4 = 2) nuclear spin modifications of CH4-CO, respectively.

Rotational study of the NH3-CO complex: millimeter-wave measurements and ab initio calculations.

The rotational spectrum of the van der Waals complex NH3-CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 112-139 GHz. Newly observed and assigned transitions belong to the K = 0-0, K = 1-1, K = 1-0, and K = 2-1 subbands correlating with the rotationless (jk)NH3 = 00 ground state of free ortho-NH3 and the K = 0-1 and K = 2-1 subbands correlating with the (jk)NH3 = 11 ground state of free para-NH3. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. Some of these transitions are continuations to higher J values of transition series observed previously [C. Xia et al., Mol. Phys. 99, 643 (2001)], the other transitions constitute newly detected subbands. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the ortho-NH3-CO and para-NH3-CO complexes. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of NH3-CO has been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations and an augmented correlation-consistent triple zeta basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the N atom closest to the CO subunit and binding energy De = 359.21 cm(-1). The bound rovibrational levels of the NH3-CO complex were calculated for total angular momentum J = 0-6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 210.43 and 218.66 cm(-1) for ortho-NH3-CO and para-NH3-CO, respectively.

A comprehensive experimental and theoretical study of H2-CO spectra.

A detailed description of a new ab initio interaction potential energy surfaces for the H2-CO complex computed on a six-dimensional grid (i.e., including the dependence on the H-H and C-O separations) is presented. The interaction energies were first calculated using the coupled-cluster method with single, double, and noniterative triple excitations and large basis sets, followed by an extrapolation procedure. Next, a contribution from iterative triple and noniterative quadruple excitations was added from calculations in smaller basis sets. The resulting interaction energies were then averaged over the ground-state and both ground- and first-excited-states vibrational wave functions of H2 and CO, respectively. The two resulting four-dimensional potential energy surfaces were fitted by analytic expressions. Theoretical infrared spectra calculated from these surfaces have already been shown [P. Jankowski, A. R. W. McKellar, and K. Szalewicz, Science 336, 1147 (2012)] to agree extremely well, to within a few hundredth of wavenumber, with the experimental spectra of the para and orthoH2-CO complex. In the latter case, this agreement enabled an assignment of the experimental spectrum, ten years after it had been measured. In the present paper, we provide details concerning the development of the surfaces and the process of spectral line assignment. Furthermore, we assign some transitions for paraH2-CO that have not been assigned earlier. A completely new element of the present work are experimental investigations of the orthoH2-CO complex using microwave spectroscopy. Vast parts of the measured spectrum have been interpreted by comparisons with the infrared experiments, including new low-temperature ones, and theoretical spectrum. Better understanding of the spectra of both para and orthoH2-CO complexes provides a solid foundation for a new search of the bound H2-CO complex in space.

Terahertz and far-infrared synchrotron spectroscopy and global modeling of methyl mercaptan, CH3(32)SH.

In this work, terahertz and Fourier transform far-infrared (FTFIR) synchrotron spectra of methyl mercaptan, CH(3)SH, have been investigated in order to provide new laboratory information for enhanced observations of this species in interstellar molecular clouds and star-forming regions. Like its methanol cousin, methyl mercaptan has particularly rich spectra associated with its large-amplitude internal rotation that extend throughout the THz and FIR regions. We have recorded new spectra for CH(3)SH from 1.1-1.5 and 1.790-1.808 THz at the University of Cologne as well as high-resolution FTFIR synchrotron spectra from 50-550 cm(-1) at 0.001 cm(-1) resolution on the far-IR beam-line at the Canadian Light Source. Assignments are reported for rotational quantum numbers up to J ≈ 40 and K ≈ 15, and torsional states up to v(t) = 2 for the THz measurements and v(t) = 3 for the FTFIR observations. The THz and FTFIR measurements together with literature results have been combined in a global analysis of a dataset comprising a total of 1725 microwave and THz frequencies together with ~18000 FTFIR transitions, ranging up to v(t) = 2 and J(max) = 30 for MW∕THz and 40 for FTFIR. The global fit employs 78 torsion-rotation parameters and has achieved a weighted standard deviation of ~1.1. A prediction list (v(t) ≤ 2, J ≤ 45 and K ≤ 20) has been generated from the model giving essentially complete coverage of observable CH(3)(32)SH transitions within the bandwidths of major new astronomical facilities such as HIFI (Heterodyne Instrument for the Far Infrared) on the Herschel Space Observatory, ALMA (Atacama Large Millimeter Array), SOFIA (Stratospheric Observatory For Infrared Astronomy) and APEX (Atacama Pathfinder Experiment) to close to spectroscopic accuracy.

A continuous-wave optical parametric oscillator around 5-µm wavelength for high-resolution spectroscopy.

We present a continuous-wave optical parametric oscillator (OPO) capable of high resolution spectroscopy at wavelengths between 4.8 µm and 5.4 µm. It is based on periodically poled lithium niobate (PPLN) and is singly resonant for the signal radiation around 1.35 µm. Because of the strong absorption of PPLN at wavelengths longer than 4.5 µm, the OPO threshold rises to the scale of several watts, while it produces idler powers of more than 1 mW and offers continuous tuning over 15 GHz. A supersonic jet spectrometer is used in combination with the OPO to perform measurements of the transient linear molecule Si(2)C(3) at 1968.2 cm(-1). Fifty rovibrational transition frequencies of the ν(3) antisymmetric stretching mode have been determined with an accuracy on the order of 10(-4) cm(-1), and molecular parameters for the ground and the v(3) = 1 state have been determined most precisely.

Rotational study of carbon monoxide isotopologues in small (4)He clusters.

High resolution microwave (a-type) and millimetre-wave (b-type) spectra of He(N)-(13)C(16)O, He(N)-(12)C(18)O, and He(N)-(13)C(18)O clusters (N

The nu(5) antisymmetric stretching mode of linear C(7) revisited in high resolution.

The nu(5) antisymmetric stretching mode of the linear carbon cluster C(7) has been revisited using a sensitive high-resolution spectrometer, including an external-cavity quantum cascade laser covering the range of interest of 1894-1901 cm(-1). 50 transitions of the nu(5)-band have been recorded and analyzed together with 45 transitions of the nu(4)-band measured by Neubauer-Guenther et al. [J. Chem. Phys. 127, 014313 (2007)]. We determined the band centers, rotational and centrifugal constants very precisely. In addition, 29 hot band transitions have been measured and tentatively assigned to the nu(5)+nu(11)-nu(11) hot band. A global fit of the hot bands nu(5)+nu(11)-nu(11) and nu(4)+nu(11)-nu(11) is presented. Derived l-type doubling constants allow for an experimental estimation of the nu(11)-band center.

Rotational study of carbon monoxide solvated with helium atoms.

High resolution microwave and millimeter-wave spectra of HeN-CO clusters with N up to 10, produced in a molecular expansion, were observed. Two series of J = 1-0 transitions were detected, which correspond to the a-type and b-type J = 1-0 transitions of He1-CO. The B rotational constant initially decreases with N and reaches a minimum at N = 3. Its subsequent rise indicates the transition from a molecular complex to a quantum solvated system already for N = 4. For N > or =6, the B value becomes larger than that of He1-CO, indicating an almost free rotation of CO within the helium environment.

High resolution infrared spectra of the linear carbon cluster C(7): the nu(4) stretching fundamental band and associated hot bands.

High resolution infrared spectra of the nu(4) fundamental antisymmetric stretching mode and associated hot bands of the linear carbon cluster C(7) were recorded using a tunable diode laser spectrometer in the frequency range of 2135-2141 cm(-1). Spectra of the nu(4) fundamental, nu(4)+nu(11)-nu(11), nu(4)+2nu(11)-2nu(11), and nu(4)+nu(8)-nu(8), bands have been analyzed and are compared to recent experimental results and high level ab initio calculations. In particular, the presented results give experimental evidence for the rigidity of C(7) and confirm theoretical predictions of a rather regular chain molecule, similar to the cases of C(4), C(5), and C(9). For the two energetically low-lying bending modes, nu(8) and nu(11), the rotational constants differ by less than 0.2%, from the ground state value, B(0)=0.030 624 4(28) cm(-1), in good agreement with the recent calculations by Botschwina [Chem. Phys. Lett. 354, 148 (2002)]. From the hot band analysis and the [script-l]-type doubling constant q, experimental values for the band origins of the nu(8) and nu(11) fundamentals have been derived.

Application of superlattice multipliers for high-resolution terahertz spectroscopy.

Frequency multipliers based on superlattice (SL) devices as nonlinear elements have been developed as radiation sources for a terahertz (THz) laboratory spectrometer. Input frequencies of 100 and 250 GHz from backward wave oscillators have been multiplied up to the 11th harmonic, producing usable frequencies up to 2.7 THz. Even at these high frequencies the output power is sufficient for laboratory spectroscopy. Comparisons to conventional high-resolution microwave spectroscopy methods reveal several superior features of this new device such as very high line frequency accuracies, broadband tunability, high output power levels at odd harmonics of the input frequency up to high orders, and a robust applicability.

Rotational spectroscopy of the isotopic species of silicon monosulfide, SiS.

Pure rotational transitions of silicon monosulfide ((28)Si(32)S) and its rare isotopic species have been observed in their ground as well as vibrationally excited states by employing Fourier transform microwave (FTMW) spectroscopy of a supersonic molecular beam at centimetre wavelengths (13-37 GHz) and by using long-path absorption spectroscopy at millimetre and submillimetre wavelengths (127-925 GHz). The latter measurements include 91 transition frequencies for (28)Si(32)S, (28)Si(33)S, (28)Si(34)S, (29)Si(32)S and (30)Si(32)S in upsilon = 0, as well as 5 lines for (28)Si(32)S in upsilon = 1, with rotational quantum numbers J''< or = 52. The centimetre-wave measurements include more than 300 newly recorded lines. Together with previous data they result in almost 600 transitions (J'' = 0 and 1) from all twelve possible isotopic species, including (29)Si(36)S and (30)Si(36)S, which have fractional abundances of about 7 x 10(-6) and 4.5 x 10(-6), respectively. Rotational transitions were observed from upsilon = 0 for the least abundant isotopic species to as high as upsilon = 51 for the main species. Owing to the high spectral resolution of the FTMW spectrometer, hyperfine structure from the nuclear electric quadrupole moment of (33)S was resolved for species containing this isotope, as was much smaller nuclear spin-rotation splitting for isotopic species involving (29)Si. By combining the measurements here with previously published microwave and infrared data in one global fit, an improved set of spectroscopic parameters for SiS has been derived which include several terms describing the breakdown of the Born-Oppenheimer approximation. With this parameter set, highly accurate rotational frequencies for this important astronomical molecule can now be predicted well into the terahertz region.

Isotope effects in the CO dimer: millimeter wave spectrum and rovibrational calculations of (12C18O)2.

The millimeter wave spectrum of the isotopically substituted CO dimer, (12C18O)2, was studied with the Orotron jet spectrometer, confirming and extending a previous infrared study [A. R. W. McKellar, J. Mol. Spectrosc. 226, 190 (2004)]. A very dilute gas mixture of CO in Ne was used, which resulted in small consumption of 12C18O sample gas and produced cold and simple spectra. Using the technique of combination differences together with the data from the infrared work, six transitions in the 84-127 GHz region have been assigned. They belong to two branches, which connect four low levels of A+ symmetry to three previously unknown levels of A- symmetry. The discovery of the lowest state of A- symmetry, which corresponds to the projection K=0 of the total angular momentum J onto the intermolecular axis, identifies the geared bending mode of the 12C18O dimer at 3.607 cm(-1). Accompanying rovibrational calculations using a recently developed hybrid potential from ab initio coupled cluster [CCSD(T)] and symmetry-adapted perturbation theory calculations [G. W. M. Vissers et al., J. Chem. Phys. 122, 054306 (2005)] gave very good agreement with experiment. The isotopic dependence of the A+/A- energy splitting, the intermolecular separation R, and the energy difference of two ground state isomers, which change significantly when 18O or 13C are substituted into the normal (12C16O)2 isotopolog [L. A. Surin et al., J. Mol. Spectrosc. 223, 132 (2004)], was explained by these calculations. It turns out that the change in anisotropy of the intermolecular potential with respect to the shifted monomer centers of mass is particularly significant.

Charging mechanisms of trapped element-selectively excited nanoparticles exposed to soft x rays.

Charging mechanisms of trapped, element-selectively excited free SiO2 nanoparticles by soft x rays are reported. The absolute charge state of the particles is measured and the electron emission probability is derived. Changes in electron emission processes as a function of photon energy and particle charge are obtained from the charging current. This allows us to distinguish contributions from primary photoelectrons, Auger electrons, and secondary electrons. Processes leading to no change in charge state after absorption of x-ray photons are identified. O 1s-excited SiO2 particles of low charge state indicate that the charging current follows the inner-shell absorption. In contrast, highly charged SiO2 nanoparticles are efficiently charged by resonant Auger processes, whereas direct photoemission and normal Auger processes do not contribute to changes in particle charge. These results are discussed in terms of an electrostatic model.

Experimental determination of the nu5 cis-bending vibrational frequency and Renner-Teller structure in ground state (X2Piu) C2H2+ using laser induced reactions.

The spectrum of the nu(5) cis-bending vibration of ground state (X(2)Pi(u)) C(2)H(2)(+) has been recorded applying the method of laser induced reactions in a low-temperature 22-pole ion trap. It is obtained by counting the number of products of the reaction C(2)H(2)(+) (v(5) = 1) + H(2)--> C(2)H(3)(+) + H as a function of the laser wavelength. The vibronic transitions Delta-Pi and Sigma-Pi with their corresponding spin-orbit and Renner-Teller substructure have been observed. Using a perturbative analysis, the vibrational frequency has been determined to omega(5) = (710 +/- 4) cm(-1) and the Renner-Teller parameter epsilon(5) is on the order of 3 x 10(-2).

Endometrial stromal cells regulate gap-junction function in normal human endometrial epithelial cells but not in endometrial carcinoma cells.

Gap junctions are transmembrane proteins comprised of six connexin subunits that facilitate direct solute transport between adjacent cells through gap junctions. Previous studies from other laboratories have documented a correlation between reduced gap-junction function and malignant transformation. In endometrial cancer, a characteristic finding is a reduction in the number of stromal cells surrounding the malignant epithelial cells. Thus, the focus of this study was to determine the effect of endometrial stromal cells on gap-junction function in normal and malignant endometrial epithelial cells. To perform these studies, we evaluated normal endometrial epithelial cells and human endometrial epithelial cells including FEEC (fetal endometrial epithelial cells immortalized with simian virus 40 large-T antigen), HEC-1A (endometrial carcinoma stage 1A), and RL-95-2 (endometrial carcinoma grade II). Gap-junctional intercellular communication (GJIC) could not be demonstrated for any of the cell lines. Low levels of GJIC were observed for normal epithelial cells and higher levels were found between stromal cells. Increased levels of GJIC were observed between the epithelial cells when they were cocultured with stromal cells. The transformed epithelial cells showed no GJIC when cultured alone or when in coculture with stromal cells. The results suggest that endometrial stromal cells may help to regulate this differentiated function of endometrial epithelial cells and that malignant endometrial epithelial cells are not responsive to these regulatory signals. Mol. Carcinog. 28:70-75, 2000.

Changes in connexin 43 protein expression in human endometrial carcinoma.

The expression of connexin 43 was studied using immunohistochemical and Western blot analyses on cell lines of endometrial epithelial origin. Connexin proteins were examined because decreases in their expression and function have been correlated with carcinogenesis. The cell lines were chosen to represent increasing grades of endometrial cancer progression starting from FEEC (fetal endometrial epithelial cells; transformed with SV40 large T antigen) to HEC-1A (stage 1A endometrial carcinoma) to RL-95-2 (grade 2 endometrial carcinoma). Parallel studies using connexin 43 polyclonal antibodies for both Western blots and immunofluorescence showed that the levels of connexin 43 expression were normal endometrial stromal cells = FEEC > HEC-1A > RL-95-2. Consequently, we applied the immunofluorescence assay to analyze paraffin-embedded uterine sections from hysterectomy specimens of patients with normal endometrium and from patients diagnosed with grade 1, 2, and 3 endometrial cancer. Using five different cases from each category, we found an inverse correlation between connexin 43 expression and tumor grade. Our data indicate that connexin 43 expression may be useful as an adjunctive marker of progression for endometrial carcinoma.

Single photon infrared emission spectroscopy: a study of IR emission from UV laser excited PAHs between 3 and 15 micrometers.

Single-photon infrared emission spectroscopy (SPIRES) has been used to measure emission spectra from polycyclic aromatic hydrocarbons (PAHs). A supersonic free-jet expansion has been used to provide emission spectra of rotationally cold and vibrationally excited naphthalene and benzene. Under these conditions, the observed width of the 3.3-micrometers (C-H stretch) band resembles the bandwidths observed in experiments in which emission is observed from naphthalene with higher rotational energy. To obtain complete coverage of IR wavelengths relevant to the unidentified infrared bands (UIRs), UV laser-induced desorption was used to generate gas-phase highly excited PAHs. Lorentzian band shapes were convoluted with the monochromator-slit function in order to determine the widths of PAH emission bands under astrophysically relevant conditions. Bandwidths were also extracted from bands consisting of multiple normal modes blended together. These parameters are grouped according to the functional groups mostly involved in the vibration, and mean bandwidths are obtained. These bandwidths are larger than the widths of the corresponding UIR bands. However, when the comparison is limited to the largest PAHs studied, the bandwidths are slightly smaller than the corresponding UIR bands. These parameters can be used to model emission spectra from PAH cations and cations of larger PAHs, which are better candidate carriers of the UIRs.