Mads Hammerich

Photoacoustic detection of NH3 in power plant emission with a CO2 laser

The paper describes a photoacoustic spectrometer for detection of NH3 in power plant emission with a detection limit below 1 ppm. The radiation source is a tunable CO2 waveguide laser, and detection is performed at reduced pressure, where the vibration-rotation lines of NH3 are essentially Doppler broadened. Immunity against interference is ensured by recording a characteristic spectral profile, and problems associated with the high concentration of CO2, and the associated line center absorption are eliminated by utilizing the effect of kinetic cooling on the photoacoustic phase. A computerized spectrometer has been constructed and tested under realistic conditions at a Danish power plant operating a test facility for selective non-catalytic reduction of NOx. Results of this field test are given.

Mode structure of hollow dielectric waveguide lasers

The mode structure of hollow dielectric waveguide lasers with free space sections and flat mirrors is studied theoretically and experimentally. The study covers the fundamental mode and the three most important higher order modes, and graphs are given which identify regions of high mode discrimination in the parameter space. Calculated coupling losses are verified experimentally by detailed studies of the output power of CO2 lasers as a function of resonator geometry. The intensity profile inside and outside the resonator is calculated, and the profile outside the resonator is compared with experiments for the fundamental mode as well as for the higher order modes. It is shown that in general the fundamental mode is non-Gaussian, and that drastically different output characteristics are obtained for different choice of output plane. The paper identifies design criteria for obtaining single line and single mode oscillation over a wide tuning range, even in the densest region of the CO2 laser line spectrum, and this is exemplified by a spectroscopic application.

Photoacoustic spectroscopy of O3 with a 450-MHz tunable waveguide CO2 laser

Abstract Photoacoustic absorption signatures have been obtained for ozone in 450-MHz tuning windows of a waveguide CO 2 laser. Out of 42 observed absorption lines, 31 are assigned to the ν 1 and the ν 3 bands, and 7 to various hot bands of 16 O 16 O 16 O. Two lines are assigned to the ν 3 band of the isotopomers 16 O 16 O 18 O and 16 O 18 O 16 O, which were present in their natural abundance. Precise collision broadening measurements are reported for two lines of the 16 O 16 O 16 O ν 3 band.

Photoacoustic study of kinetic cooling

Abstract Kinetic cooling is studied through photoacoustic CO 2 laser monitoring on gas mixtures containing CO 2 in quantities typical of smoke. The photoacoustic response is recorded in the time domain as well as in the frequency domain, and results are given for the change in responsivity induced by small quantities of H 2 O. The importance of phase information is illustrated for a number of cases.

Photoacoustic spectroscopy of C 2 H 4 with a tunable waveguide CO 2 laser

A photoacoustic gas spectrometer using a wave-guide CO(2), laser, tunable over 500-MHz windows in 70 lines, is described. The spectrometer is used for mapping Doppler-limited spectral signatures of ethylene and for measuring pressure-broadening rates for collisions with N(2), Ar, and He. A total of 41 observed transitions are assigned to the nu(7), nu(10), and nu(4) bands of normal ethylene and to the nu(7) band of the (13)C(12)CH(4) isotopomer.

Trace Gas Detection with Infrared Gas Lasers

The growing concern about our environment has led to a demand for methods and equipment which can perform a wide variety of monitoring tasks. The detection of specific molecules in the atmosphere may be motivated by the need for monitoring the emission of toxic chemicals used by industry, or by the need to perform a general control of the air we breathe, in particular in areas which are subject to large scale emission caused by human activities, such as traffic, livestock breeding, and energy production. In addition, it may be motivated by the need to get a better understanding of global trends in the concentration of molecules which are of importance in connection with the greenhouse effect and ozone destruction in the stratosphere, or it may simply be necessary in order to get a deeper insight into the huge number of physical and chemical processes which occur in the atmosphere, and which act together to produce what we commonly denote as weather and climate.

A pulsed N/sub 2/O waveguide laser with extended tunability

An air cooled N/sub 2/O waveguide laser with pulsed longitudinal DC excitation is described. It can oscillate in 70 lines, and, at a pressure of 80 mbar, it can be tuned single line and single mode over a tuning range of 350 MHz in a number of stronger lines. The laser performance is studied for different gas mixtures, involving He, N/sub 2/, and CO, and for various resonator configurations and cooling schemes. Operation at the required pressure is possible only if CO is used in the gas mixture. The potential of the laser for Doppler limited spectroscopy is illustrated through measurements of NH/sub 3/. >

Ammonia in power plant emission

Ammonia monitoring is needed in most schemes for denitrification of power plant emission. In the PALAMON system we use a 500 MHz tunable, single mode, single line, CO2 laser as light source for a low pressure, high temperature, photoacoustic cell. With this cell we can resolve the sR(5,O) line of the ammonia spectrum, and suppress the interfering C02(9R30) absorption line down to a lppm NH3 detection limit. The validity of the measured ammonia concentrations is strongly dependent on details of the sampling system and on the reliability of the calibration routines. In particular calibration with certified mixtures of NH3:N2 has proved insufficient due to the multiple and long time constants caused by adsorption of ammonia to different materials in the system. Presence of water vapor in the gas greatly reduces these time constants. Therefore a number of methods for simple production of moist calibration gases from macroscopic amounts of NH3 are applied. The calibrations are translated to response from an easily managable absorber in order to allow automated recalibration of the photoacoustic response. Data from a field test of the system, and calibration data will be presented.