Ivan R. Medvedev

Submillimeter spectroscopy for chemical analysis with absolute specificity

A sensor based on rotational signatures in the submillimeter (SMM) region is described. This sensor uses frequency synthesis techniques in the region around 10 GHz, with nonlinear diode frequency multiplication to 210-270 GHz. This provides not only a nearly ideal instrument function, but also frequency control and agility that significantly enhance the performance of the spectrometer as a sensor. The SMM frequencies provide significantly stronger absorptions and broader spectroscopic coverage than lower-frequency microwave systems. Among the characteristics of the sensor are absolute specificity, low atmospheric clutter, good sensitivity, and near-term paths to systems that are both compact and very inexpensive.

Compact Submillimeter/Terahertz Gas Sensor With Efficient Gas Collection, Preconcentration, and ppt Sensitivity

Although heretofore unrealized, it has long been known the rotational fingerprints of gases that occupy the submillimeter/terahertz (SMM/THz) spectral region can provide a basis for analytical systems with unique gas detection, identification, and quantification capabilities. Among these capabilities are near absolute specificity, even in complex mixtures, quantitative analysis, and excellent sensitivity to small samples. This paper describes such a system, self-contained in a 1 cu. ft. package. This system combines modern SMM/THz technology, sorbents to capitalize on the small sample requirements of the spectroscopic technique, and modern computational power to use the information contained in the complex rotational fingerprints. Moreover, the system and approach described show a clear path to future sensor systems that can be even smaller and more robust, as well as very inexpensive.

A rigorous detection of interstellar CH3NCO: An important missing species in astrochemical networks

The recent analysis of the composition of the frozen surface of comet 67P/Churyumov-Gerasimenko has revealed a significant number of complex organic molecules. Methyl isocyanate (CH3NCO) is one of the more abundant species detected on the comet surface. In this work we report extensive characterization of its rotational spectrum resulting in a list of 1269 confidently assigned laboratory lines and its detection in space towards the Orion clouds where 399 lines of the molecule have been unambiguously identified. We find that the limited mm-wave laboratory data reported prior to our work require some revision. The abundance of CH3NCO in Orion is only a factor of ten below those of HNCO and CH3CN. Unlike the molecular abundances in the coma of comets, which correlate with those of warm molecular clouds, molecular abundances in the gas phase in Orion are only weakly correlated with those measured on the comet surface. We also compare our abundances with those derived recently for this molecule towards Sgr B2 (Halfen et al. 2015). A more accurate abundance of CH3NCO is provided for this cloud based on our extensive laboratory work.

Laboratory characterization and astrophysical detection of vibrationally excited states of vinyl cyanide in Orion-KL

Context. We perform a laboratory characterization in the 18–1893 GHz range and astronomical detection between 80–280 GHz in Orion-KL with IRAM-30 m of CH2CHCN (vinyl cyanide) in its ground and vibrationally excited states. Aims. Our aim is to improve the understanding of rotational spectra of vibrationally excited vinyl cyanide with new laboratory data and analysis. The laboratory results allow searching for these excited state transitions in the Orion-KL line survey. Furthermore, rotational lines of CH2CHCN contribute to the understanding of the physical and chemical properties of the cloud. Methods. Laboratory measurements of CH2CHCN made on several different frequency-modulated spectrometers were combined into a single broadband 50–1900 GHz spectrum and its assignment was confirmed by Stark modulation spectra recorded in the 18–40 GHz region and by ab-initio anharmonic force field calculations. For analyzing the emission lines of vinyl cyanide detected in Orion-KL we used the excitation and radiative transfer code (MADEX) at LTE conditions. Results. Detailed characterization of laboratory spectra of CH2CHCN in nine different excited vibrational states: 11 = 1, 15 = 1, 11 = 2, 10 = 1 ⇔ (11 = 1,15 = 1), 11 = 3/15 = 2/14 = 1, (11 = 1,10 = 1) ⇔ (11 = 2,15 = 1), 9 = 1, (11 = 1,15 = 2) ⇔ (10 = 1,15 = 1) ⇔ (11 = 1,14 = 1), and 11 = 4 are determined, as well as the detection of transitions in the 11 = 2a nd 11 = 3 states for the first time in Orion-KL and of those in the 10 = 1 ⇔ (11 = 1,15 = 1) dyad of states for the first time in space. The rotational transitions of the ground state of this molecule emerge from four cloud components of hot core nature, which trace the physical and chemical conditions of high mass star forming regions in the Orion-KL Nebula. The lowest energy vibrationally excited states of vinyl cyanide, such as 11 = 1 (at 328.5 K), 15 = 1 (at 478.6 K), 11 = 2 (at 657.8 K), the 10 = 1 ⇔ (11 = 1,15 = 1) dyad (at 806.4/809.9 K), and 11 = 3 (at 987.9 K), are populated under warm and dense conditions, so they probe the hottest parts of the Orion-KL source. The vibrational temperatures derived for the 11 = 1, 11 = 2, and 15 = 1 states are 252 ± 76 K, 242 ± 121 K, and 227 ± 68 K, respectively; all of them are close to the mean kinetic temperature of the hot core component (210 K). The total column density of CH2CHCN in the ground state is (3.0 ± 0.9) × 10 15 cm −2 . We report the detection of methyl isocyanide (CH3NC) for the first time in Orion-KL and a tentative detection of vinyl isocyanide (CH2CHNC). We also give column density ratios between the cyanide and isocyanide isomers, obtaining a N(CH3NC)/N(CH3CN) ratio of 0.002. Conclusions. Laboratory characterization of many previously unassigned vibrationally excited states of vinyl cyanide ranging from microwave to THz frequencies allowed us to detect these molecular species in Orion-KL. Column density, rotational and vibrational temperatures for CH2CHCN in their ground and excited states, and the isotopologues have been constrained by means of a sample of more than 1000 lines in this survey.

Fast analysis of gases in the submillimeter∕terahertz with “absolute” specificity

A submillimeter∕terahertz point detector for gas monitoring and quantification is described. It is based upon the fast (∼15GHz∕s) sweeping of high spectral purity (<1∕107), high brightness (∼1014K) microwave sources and a scanning electronic reference for frequency measurement. This approach can quantify the complex rotational spectrum of gases at a rate of ∼105 spectral resolution elements∕second at high signal to noise. This resolution and the uniqueness of Doppler limited rotational spectra provide “absolute” specificity and “zero” false alarm rates even in complex mixtures. Moreover, the small size, low power consumption, and the potential of very low cost make this approach attractive for a number of important applications.

Chemical analysis of exhaled human breath using a terahertz spectroscopic approach

As many as 3500 chemicals are reported in exhaled human breath. Many of these chemicals are linked to certain health conditions and environmental exposures. This experiment demonstrated a method of breath analysis utilizing a high resolution spectroscopic technique for the detection of ethanol, methanol, and acetone in the exhaled breath of a person who consumed alcohol. This technique is applicable to a wide range of polar molecules. For select species, unambiguous detection in a part per trillion dilution range with a total sample size in a femtomol range is feasible. It compares favorably with other methods of breath analysis.

A NEW APPROACH TO ASTROPHYSICAL SPECTRA: THE COMPLETE EXPERIMENTAL SPECTRUM OF ETHYL CYANIDE (CH3CH2CN) BETWEEN 570 AND 645 GHZ

There is a general consensus that many of the unidentified features in astrophysical spectra are due to low lying excited vibrational and torsional states of a few molecules—commonly referred to as the astrophysical weeds. This is a challenging spectroscopic problem not only because there are many such states, but also because these states are often highly perturbed and difficult to analyze. We have previously described an alternative approach based on experimental, intensity-calibrated spectra taken at many temperatures. In this paper, we describe the procedures and results obtained with this approach for ethyl cyanide, strategies for archiving and disseminating these results, and the prospects for using these results to reduce the confusion limit in the powerful new observatories that are coming online.

HOW COMPLETE ARE ASTROPHYSICAL CATALOGS FOR THE MILLIMETER AND SUBMILLIMETER SPECTRAL REGION

With the growth in sensitivity and angular resolution of millimeter and submillimeter telescopes, the number of unidentified molecular spectral lines in surveys of the interstellar medium has grown rapidly. While some of these unidentified lines are due to as yet unidentified astrophysical species, it is the general consensus that most are due to lines from a limited number of well-known interstellar species, the interstellar weeds. These unidentified lines do not appear in astrophysical line catalogs, which are based on quantum mechanical models and are incomplete primarily because of the difficulty of performing the usual bootstrap assignment and analysis process in their often highly perturbed low-lying vibrational states. To address this problem, we have proposed and demonstrated an alternative catalog approach that is based on the analysis of intensity-calibrated spectra taken over a range of temperatures in the laboratory. These analyses also make it possible to quantitatively address the astrophysical completeness of existing catalogs. In this Letter, we use extensive new experimental data in the 210-270 GHz window to address this question for eight molecules that are considered to be the leading candidates for astronomical weeds—methyl formate, methanol, dimethyl ether, acetaldehyde, sulfur dioxide, methyl cyanide, vinyl cyanide, and ethyl cyanide. Additionally, for each of the eight molecules, we use these results and knowledge of the molecular vibrational/torsional energy levels to predict completeness as a function of astronomical source temperature.

An Experimental Approach to the Prediction of Complete Millimeter and Submillimeter Spectra at Astrophysical Temperatures: Applications to Confusion-limited Astrophysical Observations

Unidentified features in interstellar spectra (U lines) have persisted almost from the beginning of the field. In recent years, the number of such lines has rapidly increased in parallel with the sensitivity and frequency range of new observational facilities. Initially, the U lines often were from species considered exotic at the time, such as HCO+, but now the origins of these unidentified weeds are overwhelmingly from previously observed large molecules with dense spectra. The origin of the weeds problem lies in the nature of the spectroscopic approach that has typically been used in the millimeter and submillimeter spectral region: the bootstrap narrowband-observation, assignment, and theoretical prediction cycle. Unfortunately, the weeds arise from complex spectra involving many low-lying and often interacting vibrational states that are not typically part of the bootstrap process. This paper describes a purely experimental approach to this problem that does not require spectra assignment, but rather relies on the observation of complete spectra over a range of temperatures. It also discusses the potential for the use of these complete spectra in conjunction with the multiplex capabilities of modern radio telescopes for the detection of large species whose spectra consist of many relatively weak and ordinarily unobservable lines. This latter application will be made particularly challenging by the inhomogeneities and nonequilibrium characteristics of the interstellar medium. This approach is enabled by the FASSST (fast scan submillimeter spectroscopy technique) spectroscopic system and the use of collisional cooling cells to provide reference spectra at low temperature. Experimental and theoretical results are presented.

THE MILLIMETER- AND SUBMILLIMETER-WAVE SPECTRUM OF METHYL CARBAMATE (CH3OC(:O)NH2)

The rotational-torsional spectrum of the syn conformer of methyl carbamate [CH3OC(:O)NH2], an isomer of the essential amino acid glycine [NH2CH2C(:O)OH], has been recorded at room temperature in the spectral region from 79 to 371 GHz. Methyl carbamate possesses a methyl group internal rotor, which gives rise to A and E torsional substates, and associated splittings in the rotational spectrum. Almost 6000 new rotational transitions arising from the vibrational ground state have been assigned, about half of them belonging to the E torsional substate. Along with some earlier data, the newly measured lines were assigned and analyzed efficiently by the integration of two program packages: CAAARS, a suite for visual, interactive mouse-assisted line assignment of asymmetric rotor spectra; and ERHAM, a program that solves the effective Hamiltonian for molecules with up to two periodic large-amplitude internal motions. This Hamiltonian was used to fit 28 spectroscopic parameters for the methyl carbamate ground vibrational state to the observed line positions with a standard deviation of 0.081 MHz. With the determined spectroscopic constants and the available dipole moment components, we are able to predict the transition frequencies and intensities of many additional lines through 400 GHz. Methyl carbamate can now be searched for over a wide frequency range in appropriate interstellar sources such as hot molecular cores.