M.A. Koshelev

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.

Speed dependence of collisional relaxation in ground vibrational state of OCS: Rotational behaviour

Accurate experimental data on pressure broadened profiles of (16)O(12)C(32)S pure rotational lines in a broad range of quantum number J have been analyzed taking into account the speed dependence of collisional relaxation. Refined values of collisional self-broadening coefficients are determined and compared to previously known data. New quantitative information on departures of observed line shapes from the traditional Voigt profile is obtained. It is shown that these departures result mainly from the speed dependence of collisional relaxation. Theoretical calculations of self-broadening parameters are performed in the framework of the semiclassical impact Robert-Bonamy formalism where the mean relative molecular speed as well as the Maxwell-Boltzmann distribution of relative speeds is considered. The necessity of allowance for the speed dependence in line shape models is confirmed and satisfactory results have been obtained by arbitrarily limiting the integration of the differential cross section to a finite value of the impact parameter. It is shown for the first time for the whole rotational spectrum that speed dependent models not only improve accuracy of modeling the observed line profiles but also give physically grounded values of collisional relaxation parameters.

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.

Modern Resonator Spectroscopy at Submillimeter Wavelengths

Classical resonator spectroscopy methods have been realized for the first time in the submillimeter wave range. A resonator spectrometer was developed earlier at the IAP RAS on the basis of an open Fabry-Perot resonator excited by the radiation of a backward wave oscillator whose frequency is stabilized by the phase lock loop system. This spectrometer is successfully used for high-precision measurements of the dielectric properties of solid, liquid, and gaseous dielectrics as well as for the metal and coating reflection measurements in the 36-370 GHz range. In this paper, we report an extension of the upper limit of the spectrometer operation frequency to 520 GHz. Features of operation of the main spectrometer systems in the extended frequency range are analyzed. The broadband measurements of absorption in modern MPCVD diamonds are presented. A continuous record of the absorption spectrum of the laboratory atmosphere in the 350-500 GHz range obtained for the first time by the high-sensitivity microwave method is demonstrated. Further prospects for extension of the spectrometer range to terahertz frequencies are discussed.

Low rotational transitions of the CF3H molecule: The pressure-induced shift and broadening

Using a microwave spectrometer with a radioacoustic signal detection, the absorption profiles in the multiplets of the low rotational transitions J’−J = 2−1, 3−2, 4−3, and 5−4 of the 12CF3H molecule in the ground vibrational state at pressures of pure gaseous CF3H from 0.1 to 1.3 Torr when all the K components of the multiplets merge into a single spectral line are studied. The parameters of the pressure-induced shift and broadening of the observed lines are determined by comparing the theoretically modeled absorption signal and the experimental spectrum. The model used takes into account weak lines corresponding to the excited vibrational states v3 and v6 of the 13CHF3 molecule and the instrumental features of the spectrometer. The observed multiplet is simulated as an isolated Lorentzian line and as a sum of the profiles of the K components with known unshifted positions and known amplitude ratio. The shift and broadening parameters obtained in both cases are shown to agree well with each other. The dependences of the shift and broadening parameters on the quantum number J are analyzed and compared with the previously obtained data for the lines J’−J = 1−0 and 2−1.

Modern technique for absorption investigation in atmosphere and condensed media in the MM wavelength range

A new type of installation for the investigation of dielectric absorption, including the atmosphere, in the wide range of frequencies 45-200 GHz is created. Special attention is paid both to the investigation of ultralow absorption in modern high-quality dielectrics, and to the atmospheric absorption, including the lines of oxygen and water vapour absorption.

Modern resonator spectroscopy at SubMM wavelengths

The resonance methods of investigations are widely used in precision spectroscopy in all frequency ranges due to their high sensitivity. But the resonance systems have different specific features in each range. In the millimeter (MM) and submillimeter (SubMM) ranges, classical open Fabry-Perot resonators with an empty resonator Q-factor of about 106 (for reasonable sizes) are used for the gaseous and condensed media investigations. The present work based on the previous development is to our knowledge the first advancement of high-sensitivity classical resonator spectroscopy to frequencies of up to 520 GHz. The resonator spectrometer earlier developed at the IAP RAS, methods of measurement, and the results of studying various materials from 40 GHz up to 370 GHz are well described. So in present paper, a detailed description of the resonator excitation system and the frequency phase lock system, which were essentially modified for operation at the 350-520 GHz, is presented. Also the first results obtained using the spectrometer in the extended frequency range are reported.

Atmosphere continuum absorption investigation at MM waves

Investigation of atmospheric gases spectral properties is demanded to develop absorption models. Such models are very important for the global Earths atmosphere monitoring. Most of atmospheric lines and continuum spectral parameters are obtained in laboratory experiments. Recovered atmospheric parameters accuracy of depends directly on accuracy of laboratory measurements. Parameterization of the continuum absorption, in particular, the water vapor continuum is one of the most difficult problems. The problem is related to yet unknown physical origin of the continuum, its weakness in comparison with absorption in regular discrete lines of water molecule, and general difficulties of the water-related measurements. The water-related continuum broadband measurement by the resonator spectrometer [1] revealed strong influence of water adsorbing on resonator elements on results of the continuum parameters measurements. Analysis [2] of the MM/SubMM continuum measurement methods used in the earlier studies leads to the conclusion that water adsorbing onto mirrors was a common factor influencing the water-related continuum parameters obtained in all well-known laboratory experiments. In all these cases gas cells with multiple reflections of radiation off inner mirrors were used as the most appropriate technique for weak continuum absorption investigation, although the problem of separation of water vapor absorption and absorption by water adsorbed on mirrors was not solved in the experiments. To overcome the problem it was proposed to use the cavity length variation method [1], which permits separation of these two types of absorptions and, as a result, the accuracy of continuum absorption measurements increases.

Investigation of MM and Sub-MM Waves Absorption using the Methods of Precise Resonator Spectroscopy

Investigation of continuum absorption (CA) depending on humidity in a mixture of nitrogen and water vapor at the atmospheric pressure has been conducted within 100-210 GHz frequency range at two temperatures: 297 K and 274.5 K using resonator spectrometer. Linear and square humidity parts of the continuum absorption have been separated from the experimental data and values of corresponding parameters have been determined for each temperature. Temperature dependence of the parameters obtained has been estimated. Comparative analysis of the data obtained and results of previous theoretical and experimental studies is presented. The estimation calculations of CA are presented for 269.5 K. The behavior of CA measured is in good agreement with previous measurements and modern conception, but CA temperature dependence is stronger than in previous publications [3-6]. It is concerned with CA properties.

The Absorption Investigation of MM and Submm Waves by Methods of Precise Resonator Spectroscopy

The results of absorption investigation of electromagnetic waves in atmosphere in the frequency range 100-370 GHz are presented. The precise information about integral absorption, broadening parameters and central frequency shifts of line absorption of oxygen and water vapour are obtained. For the first time the absorption data at negative temperatures was obtained