F.C. De Lucia

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.

Microwave and submillimeter-wave spectra of CH3OH

Abstract Frequency measurements and assignments have been made for CH 3 OH lines in the 15- to 400-GHz region. The a -type R -branch multiplets are reported up to J = 8 ← 7 for the v t =0 torsional ground state, and to J = 6 ← 5 for the v t =1 and v t =2 excited states. Several new Q branches are listed and many b -type P - and R -branch transitions have been identified over a wide range of J and k values.

Millimeter wave spectroscopic studies of collision-induced energy transfer processes in the 13 CH 3 F laser

Millimeter wave spectroscopic techniques have been used to study the collision induced energy transfer processes that are important for the understanding of the 1.2 mm13CH 3 F optically pumped FIR laser. In these experiments the microwave diagnostic probe is copropagated with the CO 2 laser pump beam through a gain cell. The continuous tunability and absolute frequency reference of the experimental technique have made possible a quantitative investigation of the emission/absorption profiles of approximately 25 rotational transitions. From these experimental results and a simple but accurate theoretical model, energy transfer parameters have been calculated by means of a nonlinear least squares fit. These parameters accurately account for all observations and are readily interpretable on the basis of our understanding of fundamental collisional and radiative processes.

A time-resolved study of rotational and vibrational excitation and relaxation in the 13 CH 3 F optically pumped far-infrared laser

The rotational and vibrational energy transfer processes of the13CH 3 F optically pumped far-infrared (OPFIR) laser have been studied in a time-resolved experiment. The experiment uses a tunable millimeter and submillimeter spectroscopic technique as a diagnostic probe. Included are observations of the fast \Delta J processes that closely connect other J states within K = 3 to the directly pumped J = 5 level, a vibrational swapping mechanism that transfers excitation from K = 3 to other K states, vibrational relaxation due to both wall collisions and molecule-molecule collisions, the nonunity probability of vibrational deactivation in a wall collision, and pump saturation and hole burning effects due to the CO 2 pump laser. All of these observations are accounted for in the context of a numerical simulation. This results in a complete map of all of the collision-induced rotation-vibration transitions of importance to this basic OPFIR system including quantitative cross sections for the relevant processes.

A Double Resonance Approach to Submillimeter/Terahertz Remote Sensing at Atmospheric Pressure

The remote sensing of gases in complex mixtures at atmospheric pressure is a challenging problem and much attention has been paid to it. The most fundamental difference between this application and highly successful astrophysical and upper atmospheric remote sensing is the line width associated with atmospheric pressure broadening, ~ 5 GHz in all spectral regions. In this paper, we discuss quantitatively a new approach that would use a short pulse infrared laser to modulate the submillimeter/terahertz (SMM/THz) spectral absorptions on the time scale of atmospheric relaxation. We show that such a scheme has three important attributes. 1) The time resolved pump makes it possible and efficient to separate signal from atmospheric and system clutter, thereby gaining as much as a factor of 106 in sensitivity. 2) The 3-D information matrix (infrared pump laser frequency, SMM/THz probe frequency, and time resolved SMM/THz relaxation) can provide orders of magnitude greater specificity than a sensor that uses only one of these three dimensions. 3) The congested and relatively weak spectra associated with large molecules can actually be an asset because the usually deleterious effect of their overlapping spectra can be used to increase signal strength.

THz spectroscopy - techniques and applications

Spectroscopy and related applications have accounted for a large proportion of the work in the THz spectral region. These have included laboratory studies of the radiative and collisional properties of molecules, ions, and radicals, as well as field applications such as atmospheric remote sensing and molecular radio astronomy. This paper will review established techniques and discuss new approaches.

Spectral purity and sources of noise in femtosecond-demodulation terahertz sources driven by Ti:sapphire mode-locked lasers

Direct measurements of the spectral purity in terahertz femtosecond-demodulation sources are reported and compared to theory. Because these sources operate at very high harmonics (/spl sim/102-10/sup 4/) of the mode-lock frequency, a high spectral purity source is very dependent on a low-jitter femtosecond laser. Conversely, the spectral content of the terahertz sources provides detailed information about timing jitter and stringent tests of models used to describe the jitter. We find that both the behavior of the central core, and the noise skirts of the power spectrum of our sources, can be quantitatively related to measured ripple and continuum amplitude noise on the Ar/sup +/ pump laser by use of modulation theory.

The spectrum of HNO3 in the region 550–800 GHz

Nouvelle analyse du spectre de NO 3 H de 550 a 800 GHz. Nouvelles valeurs des constantes de rotation et de distorsion centrifuge

Rotational Energy Transfer in Small Polyatomic Molecules1

Significant progress has been made in the study of state-specific collision-induced rotational energy transfer (RET) in polyatomic molecules since the early studies of Wilson and co-workers, Unland and Flygare, and Oka. In an earlier contribution to this series, Oka reviewed the general issue of RET and summarized the field to that date. Additionally, there has been extensive work on simpler collisions, especially those between homonuclear diatomic molecules and atoms. This work, which explored the relationships among formal scattering theories, semiempirical scaling and fitting rules, and modern state-resolved experimental results, provides a useful basis for the development of an understanding of the more complex RET problem of polyatomic molecules. Because the subject of RET has become very broad, we have chosen to focus on incoherent state-to-state rates which result from collisions between like molecules and on the relationship of these rates to fundamental molecular parameters. In general, we have excluded discussions of experimental methods, collisions with foreign gases, velocity subset effects, nonlinear processes, polarization, and coherent effects. 135 refs., 18 figs., 5 tabs.