The presence of chemicals in gaseous systems is of extreme importance for many applications, including homeland security, environmental protection and oil and gas exploration and production. Moreover, there is an increasing demand to control air quality and human health which promoted the development of monitoring systems based on high-performance gas sensing structures. In most cases, the detection, analysis, identification and quantification of these chemical substances is carried out from a remote distance. The current technologies for remote sensing have proved effective in revealing the concentrations of target chemicals over open and long paths. Among these technologies, laser absorption spectroscopy (LAS) is widely used. It follows the Beer-Lambert law to measure the chemical concentration by relating the attenuation of the laser light through the substance to the concentration of the integral. Compared with non-laser-based techniques, LAS has remarkable sensitivity owing to the beneficial impacts of the collimated and bright laser sources. The growing interest in laser-based technologies has contributed to the development of remote laser diagnostic techniques with high detection sensitivity and suitable for numerous applications. Nevertheless, these methods provide path-averaged concentrations of the target chemicals, which may limit their efficiency and constrain the range of applications.
Previous findings revealed that transitions involving Rydberg states provide information about molecular structures that could facilitate the identification of molecules. Moreover, the Rydberg spectra are less sensitive to temperature and internal vibrational excitation, and their complexity does not scale with the molecular size. Thus, Rydberg-based spectroscopy holds great potential for chemical analysis. By combining the advantages of Rydberg Fingerprint spectroscopy (RFS), backward transient absorption spectroscopy (BTAS) has emerged as a robust and efficient alternative for overcoming the limitations of the previous methods. Specifically, it allowed the identification of the chemical species from a distance and its efficiency has been demonstrated in several applications. However, BTAS has not been applied in the simultaneous identification of multiple chemical species, which could be the case in most real-life scenarios.
Herein, scientists at Brown University: Dr. Xuan Xu, Professor Fedor Rudakov and Professor Peter M. Weber investigated the applicability of RFS-BTAS for identifying chemical species from a distance in a multi-component system. In their approach, the experiments involved using UV excitation and Rydberg-Rydberg transition to endow this method with its most crucial inherent molecule-specificity. The applicability of this method in identifying closely related compounds was demonstrated using three model systems: N,N-dimethylisopropylamine (DMIPA), N,Ndimethylethanamine (DMEA) and trimethylamine (TMA). Their work is currently published in Chemical Physics Letters.
The authors demonstrated the efficiency and effectiveness of the method for identifying multi-component chemical compounds with minor structural differences while maintaining the advantages of high sensitivity and millimeter-scale spatial resolution. The transition energies between Rydberg states reveal the identity of the molecular compounds, while the arrival time signatures of the photon pulses at the detector give precise information about the distance of a substance. The use of pulsed lasers enables the recording of time-domain signatures that add further differentiation between molecular systems and enhances the identification of the target compounds.
In summary, this is the first study to successfully demonstrate the identification of closely related compounds in a multi-component system using RFS-BTAS. The combination of the remarkable spatial resolution and high sensitivity makes BTAS a promising technique for analyzing and quantification of the spatial distribution and chemical composition of vapors from a distance. Looking forward, using a spectrally dispersed single shot and supercontinuum probe pulse could reduce the time required for scanning the near-infrared probe wavelength Implementation with femtosecond laser pulses could increase the spatial resolution into the micrometer scale, making the method interesting to microscale engineering and manufacturing. In a statement to Advances in Engineering, Professor Peter Weber stated that the new BTAS method would be of great importance in numerous applications involving analyzing traces of molecular vapor from a distance.
Xu, X., Rudakov, F., & Weber, P. (2022). Chemical analysis from a distance: Spatially resolved, remote sensing using backward transient absorption. Chemical Physics Letters, 793, 139435.