M.Yu. Tretyakov

The study of microwave pressure lineshifts

Abstract The results of experimental investigations of pressure shifts and broadening of spectral lines of polar molecules performed in the submillimeter region by a microwave spectrometer RAD are reported. About 30 measurements were made of self-shift and foreign gas shift parameters of the lines of NH 3 , PH 3 , AsH 3 , and H 2 O molecules including lineshifts in excited vibrational states, lineshifts of transitions connected by common levels, lines with various values of J and K quantum numbers, and the “forbidden” |Δ K | = 3 lines. On the basis of the data obtained in this work and data available in the literature, new experimental dependences of lineshifts on molecular parameters are found. The results are well described by a simple “Stark effect” model of lineshifts. Some new directions of investigations are outlined.

Submillimeter-wave spectrum of H2Se : the evidence of fourfold clustering of rotational levels

Abstract The spectra of H 2 80 Se and H 2 78 Se were investigated in the region 300–650 GHz including transitions with high rotational excitation up to J = 20, K −1 = 20. The effect of fourfold clustering of rotational energy levels was observed and interpreted as a critical phenomenon. Very strong distortion of line strengths was also found for rotational transitions in the region of critical phenomena. The measured frequencies were fitted, together with available MW data and IR ground state combination differences, in the modified Watson Hamiltonian and the sets of rotational and centrifugal distortion constants were obtained.

A simultaneous analysis of the microwave, submillimeterwave, and infrared transitions between the ground and ν2 inversion-rotation levels of 15NH3

Abstract The submillimeterwave spectra of the pure inversion and inversion-rotation transitions in the ν2 excited state (79 transitons) and the diode laser spectra of the ν2 band (83 transitions) of 15NH3 have been measured. A simultaneous least squares analysis has been carried out of these data together with previously published wavenumbers of the pure inversion transitions and inversion-rotation transitions in the ground state measured by the microwave and Fourier spectroscopy, and the ν2 band transition frequencies obtained by the infrared-microwave two-photon technique. A theory of the Δk = ±3n interactions in the ground and ν2 excited states of ammonia (S. Urban, V. Spirko, D. Papousek, J. Kauppinen, S. P. Belov, L. I. Gershtein, and A. F. Krupnov, J. Mol. Spectrosc. 88, 274–282 (1981)) has been used in the analysis. The “smoothed” values of the ν2 band wavenumbers can be used for calibration purposes with better than 1 × 10−3 cm−1 precision.

Rotational spectrum of the H216O molecule in the (010) excited vibrational state

Abstract Five new MW lines of the pure rotational spectrum of water vapor in the (010) excited state have been measured. The analysis of the available data on the (010) vibrational state has been carried out using either a conventional model or a one-dimensional Borel approximation of a Watson-type Hamiltonian. In both cases a model with 24 parameters up to J 8 was used. The use of the Borel-approximated Watson Hamiltonian resulted in a fit of the experimental data within experimental uncertainty, whereas the fit using the conventional model produced a standard deviation 60 times worse.

Resonator spectrometer for precise broadband investigations of atmospheric absorption in discrete lines and water vapor related continuum in millimeter wave range.

The instrument and methods for measuring spectral parameters of discrete atmospheric lines and water-related continuum absorption in the millimeter wave range are described. The instrument is based on measurements of the Fabry-Pérot resonance response width using fast phase continuous scanning of the frequency-synthesized radiation. The instrument allows measurement of gas absorptions at the cavity eigenfrequencies ranging from 45 to 370 GHz with the highest to date absorption variation sensitivity of 4x10(-9) cm(-1). The use of a module of two rigidly bounded maximum identical resonators differing in length by exactly a factor of two allows accurate separation of the studied gas absorption and spectrometer baseline, in particular, the absorption by water adsorbed on the resonator elements. The module is placed in a chamber with temperature controlled between -30 and +60 degrees C, which permits investigation of temperature dependence of absorption. It is shown that systematic measurement error of discrete atmospheric line parameters does not exceed the statistical one and the achieved accuracy satisfies modern demands for the atmospheric remote sensing data retrieval. Potential systematic error arising from the neglect of the effect of water adsorption on mirror surfaces is discussed. Examples of studies of water and oxygen spectral line parameters as well as continuum absorption in wet nitrogen are given.

SUBMILLIMETER-WAVE SPECTRAL LINES OF NEGATIVE IONS (SH- AND SD-) IDENTIFIED BY THEIR DOPPLER SHIFT

For the first time submillimeter-wave transitions of an anion have been firmly identified. We observed one transition of SH− (J=1←0, 564 421.869 MHz) and two of SD− (J=1←0, 292 359.129 MHz; J=2←1, 584 629.519 MHz) created in the positive column of an electric discharge set up in a mixture of H2S and argon. By studying the Doppler shift brought about by the motion of charged species in the electric field of the discharge we were able to distinguish between positively and negatively charged ions and neutrals. We have hence shown that velocity modulation techniques commonly used for infrared spectroscopy could also be successfully applied for the study of ions in the submillimeter-wave region.

Tunneling-rotation spectrum of the hydrogen fluoride dimer

Abstract The tunneling-rotation spectrum of the ground state of the hydrogen fluoride dimer, HF⋯HF, has been extended to the submillimeter wavelength region with the RAD-3 submillimeter spectrometer at Gorky in the frequency range 180–380 GHz at 210 K and pressures from 0.5 to 1.5 Torr. The spectrum has been reinvestigated at lower frequencies with a conventional Stark spectrometer at pressures between 0.1 to 0.2 Torr at the National Institute of Standards and Technology. Lines of the a -type K = 3 subband have been observed for the first time. The tunneling frequency for the K = 3 state is 114 306.35(53) MHz. The rotational constants of this subband are somewhat anomalous as compared with those for the lower K sublevels. For example, the B rotational constant for the B u state is larger than that for the A g state, while for all the lower K sublevels the opposite is true. Higher- J R - and P -branch lines of the K = 0 through K = 2 subbands have been identified and improved rotational constants for these states have been obtained. An atlas of all a - and b -type far infrared and microwave ground state tunneling-rotation transitions for K up to 3 giving frequency, uncertainty, and relative intensity has been prepared.

Study of microwave pressure lineshifts: Dynamic and isotopic dependences

Abstract The study of pressure lineshifts and line broadenings initiated earlier (J. Mol. Spectrosc. 94, 264–282 (1982)) is extended to some other perturbing gases and to isotopic species. New dependences of the lineshifts on the molecular parameters are found. One of them is the dependence, as well as the proportionality of the lineshift to the line frequency found before is explained in terms of a “dynamic Stark effect” model. The dependence of the lineshifts on the isotopic composition of the absorbing molecule is also reported. One of the main results of the study is the demonstration of higher sensitivity of lineshifts to the characteristics of molecular interactions as compared to the corresponding line broadening.

High-resolution Fourier transform and submillimeter-wave study of the ν6 band of 12CH3F

Abstract The ν 6 band of 12 CH 3 F has been measured in the range 1077–1278 cm −1 using a Fourier transform spectrometer with a resolution of 0.002 cm −1 . The wavenumbers of 1070 rovibrational transitions have been used simultaneously with 99 previously reported frequencies of the ground state rotational transitions and 135 frequencies of rotational transitions in the v 6 = 1 excited vibrational state to determine 24 parameters of the ν 6 band and 6 spectroscopic parameters of the ground vibrational state of 12 CH 3 F.

Precise measurements of collision parameters of spectral lines with a spectrometer with radioacoustic detection of absorption in the millimeter and submillimeter ranges

The factors influencing the accuracy of determining the parameters of molecular lines with the use of a spectrometer with radioacoustic detection of absorption are analyzed. In addition, the following instrumental features of the spectrometer that were not studied earlier are analyzed: the dependence of the sensitivity of the radioacoustic cell on the pressure of the cell-filling gas and the frequency dependence of the radiation power in the cell at gas pressures lower than 10 Torr. Methods of mathematical simulation of the spectrometer’s output signal at various gas pressures are considered. A technique for calibrating the dependence of the cell’s sensitivity on the pressure based on measurements of the integrated absorption coefficient of molecular spectral lines is presented. With allowance for the considered features of the spectrometer, the parameters of the lines of rotational multiplet J = 9 of fluoroform in the ground vibrational state are studied at gas pressures of 0.01–10 Torr.