The purpose of the Project is to develop an ultrasonic approach to mid-infrared-laser-based photoacoustic (PA) detection of trace gases and to demonstrate the approach capabilities for a number of biological and technical applications, which need the highly sensitive detection of gas leaks to be emitted by small-sized objects in atmospheric air.

The state of the art. The mid-infrared laser-based photoacoustics is one of the most sensitive techniques for the local non-contact analysis of trace amounts of various chemical compounds in gas medium. The principle of the technique is based on measuring the amplitude and the phase of acoustic pressure oscillation arising due to absorption of a modulated laser beam by molecules of gas inside a PA cell-resonator. Traditionally, the modulation is implemented at frequencies no more than 10 kHz (that is, at audio frequencies). The volume of the PA cell ranges from tens of cubic centimeters [Appl. Phys. B. 50, 137, 1990] to a few liters [Appl. Opt, 37, 3345, 1998]. The PA technique is distinguished by a short time resolution (down to a few seconds) and a high sensitivity at the (sub) ppb (parts per billion, 1:10⁹) level with the minimal detectable absorption of 10^-10 cm^-1. The technique is generally recognized to be a promising approach to the inexpensive in situ multi-component analysis of atmospheric air in environmental pollution monitoring, exhaust gas monitoring, industrial process control and leak detection. The technique finds an expanding application in biology (plant physiology, crop and fruit storage and medical diagnostics by exhalation analysis) for sensitive detection of CO₂, C₂H₄, CH₄, NO₂, N₂O, H₂S, O₃, NH₃, C₂H₆, C₂H₆O, C₅H₁₂ and others gases, which play an important role in the metabolic reactions.

Unfortunately, the measurement sensitivity achieved in the PA experiments is insufficiently high in order to meet the current needs of Technology and Life Sciences in analyzing trace gases emitted by individual small-sized objects. The developed models of PA detector specified by the minimal detectable emission rate of ~10^-9 cm³/s [Photoacoustic Spectroscopy in Trace Gas Monitoring, in Encyclopedia of Analyt. Chem., J. Wiley & Sons Ltd, Chichester, p.2203, 2000] are exceeded significantly by the best commercial gas-leak detectors in the sensitivity (~10^-11 cm³/s for helium mass-spectrometer-based systems [http://www.qualytest.com/]) or in the sizes (halogen leak detectors).

A possible way of enhancement of the leak detection sensitivity is to reduce the volume of PA cell. The way requires the laser beam switching at higher frequencies. Of special interest is the switching in the range of ultrasonic frequencies up to the submegahertz scale where the negative effect of dynamical cooling and sound absorption on the PA detection is slight. By now the detailed investigation of features for the PA trace-gas detection at the high-frequency modulation (including the modulation in the ultrasonic range) has not been carried out. The development of ultrasonic high sensitive PA gas-leak detector based on mid-infrared lasers has not been performed.

Impact of the Project on the Progress in the Field. For the mid-infrared lasers, the beam modulation can be efficiently implemented at frequencies up to 300 kHz. That allows to reduce the cell sizes down to a few millimeters. Combining the high sensitivity inherent in the traditional PA-approaches with an ability to probe the gas inside such a small-sized PA cell gives a possibility to analyze a number of compounds to be emitted by individual small-sized objects with an extremely low emission rate.

• A crude estimation shows that the application of the approach can provide detecting the gas leak emitted by an object with the rate down to ~10^-14 cm³/s for C₂H₄ or ~10^-10 cm³/s for CO₂. For comparison, in the aerobic reaction an individual cell of living organism emits CO₂ with a rate from 10^-9 to 10^-10 cm³/s. In the photosynthesis reaction an individual cell of plant can absorb CO₂ at a rate higher than 10^-8 cm³/s.
• The detection sensitivity will be increased 10³ – 10⁶ times compared to the traditional PA approaches and more than 100 times in relation to the mass-spectrometer-based leak-detection systems. In contrast to the systems, the ultrasonic PA leak detector needs no expensive vacuum chambers and can be applied to in situ localization of leak for a large number of substances to be emitted in atmospheric air.
• The proposed approach is a promising line in order to create compact models of the high-sensitive laser PA leak detector.

Expected Results. In the course of implementation of four project tasks, the following results will be obtained:

1. An experimental mid-infrared-laser-based setup to be intended for analyzing trace gases will be created.
2. Features of the ultrasonic approach to the laser-based PA detection of a number of trace gases in the wavelength region from 3 to 11 microns will be studied in detail for modulation frequencies ranging up to the submegahertz scale.
3. Equipment and computing algorithms for the high sensitive ultrasonic mid-infrared-laser-based PA detection of gas leak will be developed. The sensitivity achieved in test experiments will be better than the one for the best commercial leak detectors.
4. Capabilities of the proposed approach will be demonstrated for biological and technical applications, which need the highly sensitive detection of gas leaks to be emitted by small-sized objects in atmospheric air.

Application.

• The highly sensitive leak detection is a promising commercial application for the proposed technique. Possible areas of the PA leak detector application are the automobile industry, the HVAC Technology, the Vacuum and Over Pressure Technology and the Packaging Technology.
• A mid-infrared-laser-based PA gas analyzer can be used in order to sense explosive substances (such as trinitrotoluene and cyclotrimethyltrinitramine) in airports and railway stations or to find a tightness imperfection for the tubing with extremely toxic compounds (stibin, diboran, phosgen, phosphin, arsin) in semiconductor plants.
• The approach is expected to find the application in Life Sciences (entomology, microbiology, cell biology, biochemistry etc.) focused into the metabolic processes occurring in small-scale biological samples: small animals and plants, their organs, tissue pieces or microscopic objects down to individual cells.
• The ultrasonic PA detector can be considered as a material-economy tool for analyzing a number of chemicals in nanochemistry and other nanotechnology applications, which operate with the smallest amounts of substances.

• the Project provides an opportunity for weapons scientists and specialists from the Institute of Physics of NASB to redirect their talents, knowledge, and previous experience to the peaceful activity in the field of development of means intended for air pollution monitoring;
• the Project promotes the integration of the participants from Belarus into the international scientific community by their close cooperation with foreign scientists and specialists (including the Collaborators) in the field of spectroscopy and biology;
• the Project supports the basic and applied research and technology development at Institute of Physics of NASB exclusively for peaceful purposes, in particular, for the applications of laser systems and technologies in life sciences and environmental protection;
• the Project stimulates the solution of national and international technical problems, in particular, it promotes the solution of problems of environmental pollution and terrorism protection.
• the Project makes a contribution to the conversion of a part of the military industry of Belarus and to the creation of new working places which are not related to the production of weapons;
• the Project reinforces the transition of the Project participants to a market-based economy answering to civil needs.

1. creation of an experimental mid-infrared-laser-based setup to be intended for analyzing trace gases;
2. study of characteristic features of the proposed ultrasonic approach to the mid-infrared-laser-based PA detection of trace gases;
3. development of equipment and computing algorithms to be intended for the high sensitive ultrasonic mid-infrared-laser-based PA detection of gas leak;
4. demonstration of the capabilities and the potential of the proposed approach for biological and technical applications.

• systematic information exchange in the course of Project implementation;
• providing comments to the technical reports (quarterly, annual, final) submitted by Project participants to the ISTC;
• participation in the technical monitoring of Project activities performed by ISTC staff;
• assistance for Project participants to join international meetings;
• holding joint annual workshops;
• participation in testing and using the devices, developed in the course of the Project, participation in demonstration experiments;
• providing the material assistance (equipment and chemicals) required for performing experiments at collaborator's (Dr. F. J.M. Harren) Institution;
• cross-checks of the results obtained in the course of Project implementation;
• preparation of joint scientific papers, reports, and patents.