During last decades many gas sensing mechanisms, based on different physical or chemical interactions with sensitive materials, have been developed, but the problem of precise analysis of gas mixtures still remains. New sensing mechanism has been suggested that is based on an adiabatically changing electric field interacting with the rotational structure of the molecules with dipole moments. A single low frequency gas detector, that can be used for sensing of gas mixtures with high selectivity, accuracy, and sensitivity, has been theoretically demonstrated. The enhancement of the population difference between corresponding molecular levels and reached the theoretical maximum of absorption have been shown.
Abstract We have developed and demonstrated a new mechanism of manipulation of population in molecular rotational levels in a weakly aligned molecules. The mechanism is based on an adiabatically changing electric field interacting with the molecules with dipole moments. Treating molecules as simple rotators, we have described their behavior using the density matrix taking into account the relaxation processes. We have considered the interaction of the weakly aligned molecules with a microwave field in a high finesse cavity. We have found that, on one hand, the population inversion can be reached in the ensemble of the weakly aligned molecules to be used for the maser operation at room temperature. On the another hand, we have found that the enhancement of the absorption can reach the theoretical limit and be used for gas sensing with high sensitivity and selectivity. Such sensors can efficiently analyze the multi-gas mixtures and be used for a huge range of applications – stretching from technology, sciences, control of environment, biology and medicine.
Gas sensing technology is crucial for a wide variety of applications [1]. Currently, traditional gas sensing materials are hindered by limitations in selectively detecting single-type gas due to the non-specific interactions between gas molecules and sensing materials [2,3]. Single atoms (SAs) have the potential to generate specific effects on a single-type gas due to the uniform active center configuration [4,5]. However, the means to stimulate the capacity of single atoms towards a single-type gas sensing are still lacking. Therefore, the design of highly sensitive and selective sensing single-atom materials for single-type gas detection needs more attention. Recently, by utilizing the dynamic changes in the coordination structure of cobalt single atoms, Wu and Feng et al. realized high sensitivity and high selectivity towards ammonia gas sensing. In this work, they developed a cobalt single-atom material with a bidentate coordination structure (Co-2MI-G), which exhibited ultra-high selectivity for NH3 sensing [6]. A high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) image showed that no aggregates were formed on the surface of graphene-like Co-2MI-G and the Co single atoms were uniformly dispersed (Fig. 1a). It is indicated that from the fitting of Fourier transformed-extended X-ray absorption fine structure (FT-EXAFS), the coordination number of Co single atoms changed from 2 in pristine Co-2MI-G to 4 after NH3 gas exposure, corresponding to a stable model of bonding with two NH3 molecules (Fig. 1b and c). In order to further clarify the special coordination evolution mechanism, quasi in-situ X-ray photoelectron spectroscopy (XPS) technology was used to explore the state changes of Co single atoms before and after exposure to NH3. As illustrated in Fig. 1d, the satellite peak intensity for Co 2p decreased significantly, while the 2p1/2–2p3/2 separation energy weakened, indicating the conversion of Co2+ to Co3+. As a result, the Co-2MI-G-based sensor showed ultra-high sensitivity (67.598% for 1 ppm NH3, Fig. 1e) and high selectivity for volatile organic compounds (VOCs) and other interfering gases (Fig. 1f). Theoretical simulation elucidated the chemisorption process of NH3 molecules on Co-2MI-G. Density of state (DOS) analysis indicated that the Co-2MI-G adsorbed with NH3 displayed a stronger insulation and higher oxidation state than pristine Co-2MI-G, which enlarged the band gap from 0.14 eV to 0.50 eV (Fig. 1g). (a) HAADF-STEM image of Co-2MI-G. (b, c) FT-EXAFS curves of the proposed structure and the measured Co-2MI-G and Co-2MI-G + NH3. Insets are the proposed model of Co-2MI-G and Co-2MI-G + NH3 architectures. (d) Quasi in-situ XPS spectra of Co-2MI-G before and after exposure to NH3, followed by NH3 evaporation. (e) The resistance response curves of the Co-2MI-G-based sensor. (f) Selectivity of Co-2MI-G to NH3 and other VOCs gases. (g) Demonstration of frontier molecular orbital structures of Co-2MI-G and Co-2MI-G + 2NH3. Reproduced from ref. [6] with permission. In summary, this paper verifies that single atoms with a dynamically changing coordination structure could be used as a single-type gas sensing material with superior sensitivity and selectivity. This work pioneers a novel application of single atoms to the field of single-type gas sensing and provides an innovative perspective on utilizing single atoms by dynamic coordination structure. Conflict of interest statement. None declared.
High sensitivity detection of quantum absorption is demonstrated at a sensitivity of
1times1011 integrated absorption, in 1 sec, using 10 mTorr total gas pressure. Achieving highly sensitive detection relies on simple basic principles: 1) The molecule-generated electric field in the forward direction (the "darkness wave") is most precious, and contains the maximum available information. It is the physical manifestation of the -
alpha∗L term appearing when beer#### absorption law is expanded for low opacity, 2) After generating the molecular signal, the detected unabsorbed light sets the "shotnoise" measurement floor. We use an optical transmission cavity for the gas cell, since the high internal "build-up" field will elicit a strong molecular "darkness wave", for a given transmitted de light on the detector, A more conventional explanation of Gavity Enhancement would be the extension of the effective cell length by the factor finesse
∗2/pi.3) To avoid excess low-frequency noise, such small signals should be measured by ac methods, comparing on-resonant and off-resonant cases in quick succession. 4) By SIMULTANEOUSLY obtaining and subtracting these cases, one provides a siginal channel with NO OUTPUT unless there is a resonance. This is conveniently accomplished by using the FM detection method with the optical heterodyne sidebands and carrier being transmitted through the cavity via adjacent axial orders 5). A remarkable property of this configuration, when the modulation frequency= cavity mode spacing, is the suppression of ANY detection of laser frequency noise with the transmitted light. We can refer to this property as (laser FM-) noise-Immune detection. This enables profitable use of cavity finesse in the range above 10,000, without a nois penalty, Altogether, we refer to this new spectroscopy as Noise-Immune, Cavity-Enhanced, Optical Heterodyne Molecular spectroscopy, i.e. "NICE OHMS". We have measured excellent saturated dispersion signals from HCCH in the band near 790 mm (
nu1+3nu3), and recently, using a Nd:YAG source, from the HCCD P(5) line (
nu2+3nu3) at 1064 mm. The saturation peaks broaden from the transit limit of 270 kHz near zero pressure at the rate 34.7 kHz/m Torr. We observe interesting narrow linwidths 8-fold below the transit limit, basically using optical selection of slow molecules by low power and pressure (2 mW and 2 m Torr). The absolute frequency is measurred to be 281, 635,363,960 MHz
pm45 kHz.
We present theoretical and experimental progress towards a new approach for the precision spectroscopy, coherent manipulation and state-to-state chemistry of single isolated molecular ions in the gas phase. Our method uses a molecular beam for creating packets of rotationally cold neutrals from which a single molecule is state-selectively ionized and trapped inside a radiofrequency ion trap. In addition to the molecular ion, a single co-trapped atomic ion is used to cool the molecular external degrees of freedom to the ground state of the trap and to detect the molecular state using state-selective coherent motional excitation from a modulated optical-dipole force acting on the molecule. We present a detailed discussion and theoretical characterization of the present approach. We simulate the molecular signal experimentally using a single atomic ion, indicating that different rovibronic molecular states can be resolved and individually detected with our method. The present approach for the coherent control and non-destructive detection of the quantum state of a single molecular ion opens up new perspectives for precision spectroscopies relevant for, e.g., tests of fundamental physical theories and the development of new types of clocks based on molecular vibrational transitions. It will also enable the observation and control of chemical reactions of single particles on the quantum level. While focusing on N2+ as a prototypical example in the present work, our method is applicable to a wide range of diatomic and polyatomic molecules.
Single particle spectroscopy has become a source of valuable information on fundamental interactions between light and matter. Whereas trapping and cooling of ions and atoms has been sucessfully achieved, single molecules, because of their multilevel structure (high number of internal degrees of freedom) have not been observed in electromagnetic traps so far. However due to the presence of zero phonon lines in conjunction with inhomogeneous broadening the spectroscopic isolation and detection of single molecules ‘trapped in solids at very low temperatures' is made feasible [1,2]. Single molecule spectroscopy allows to study the distribution of molecular properties and not only the statistical average which is generally observed.