D. S. Makarov

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.

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.

Some consequences of high temperature water vapor spectroscopy: water dimer at equilibrium.

It is shown that the evolution of water vapor spectra in the 2500-5000 cm(-1) range recorded at 650 K and pressures up to 130 atms after subtraction of monomer contribution may be interpreted qualitatively well on the basis of experimental data on water dimer and trimer obtained from cold molecular beams and in He droplets. The proposed spectroscopic model considers water vapor as a mixture of nonideal monomers, dimers, and trimers at chemical equilibrium. The effect of line mixing is taken into account in the monomer spectrum modeling. Decomposition of the high temperature spectra allowed determining a dimer equilibrium constant that was compared with the previously known values. The contribution of water trimer is assessed. The performed analysis indicates that the number of bound dimers in water vapor is quite large, even at such a high temperature.

Frequency stabilization of a primary subterahertz oscillator by a frequency comb of a femtosecond laser

The frequency of a primary subterahertz oscillator has been phase locked with the use of the equidistant components of a broad spectrum produced by a femtosecond laser. The optical-to-terahertz down-conversion of the laser pulse train and its mixing with subterahertz radiation has been performed at a Schottky diode. This work provides the opportunity of creating a principally new generation of frequency synthesizers with the desired power and phase noise a few orders of magnitude lower than that of their traditional analogues.

Femtosecond laser comb based subterahertz synthesizer

Functioning layout of a frequency comb-based subterahertz synthesizer is demonstrated. A primary subterahertz oscillator was phase-locked against the Ti:Sapphire femtosecond laser frequency comb down-converted to the subterahertz range by the semiconductor mixer. Synthesizer operation is demonstrated through Fabri-Perot resonator response curve recording experiment. Spectral purity of the synthesizer was estimated. The advantage of the comb-based synthesizer over the synthesizer based on the Agilent E8257D device was shown.

Modeling of the absorption profile of the 60 GHz band of atmospheric oxygen using the memory function formalism

The profile of the oxygen absorption band centered at 60 GHz is modeled using the formalism of memory functions. The memory function determines the relation between the initial and final states of a molecule after its collision. The model profile takes into account the spectral exchange and contains two parameters, which depend on temperature and pressure. These parameters characterize the probabilities of spectral exchange between series of lines (branches) corresponding to molecular transitions of different types. To describe the absorption profile in the frequency range of 0–120 GHz in terms of the model proposed, the “+” and “−” branches of fine structure transitions and the series of nonresonance lines with frequencies on the order of 10−3–10−5 GHz are taken into account. The values of the parameters that take into account the spectral exchange are found from the condition of the best fit between the developed model profile and the profile calculated according to the millimeter-wave propagation model. The parameters found have physically substantiated and smooth temperature dependences, which confirms the adequacy of the model. The difference between these models does not exceed 2–3%, which corresponds to the declared uncertainty.