Date of Award
Doctor of Philosophy (PhD)
Electrical and Computer Engineering
C. K. Campbell
Theoretical and experimental techniques, which are useful in molecular spectroscopic studies and in the development of Laser Absorption Spectrometers (LAS), are described. Both fixed-frequency and tunable lasers have been employed. All the measurements reported are based on either direct or second harmonic absorption techniques.
A commercial CO/CO₂ line-tunable laser has been used to obtain useful criteria for air pollution monitoring via the direct absorption scheme. CO₂ laser absorption measurements on ozone at reduced pressures are reported. Measurements on seventeen NO absorption lines with a CO laser are described. For these, the absolute absorption as a function of pressure has been determined. As a result we have been able to establish accurate values for the absorption of NO at the pertinent CO laser wavelengths. From the best fit between experimental measurements and theoretical calculations we have deduced the separation between the appropriate NO and CO wavelengths, the NO/N₂ pressure broadening, the NO band strength, and the individual NO line strengths.
Using a tunable lead-salt semiconductor diode laser, and employing the direct absorption technique, we have developed a simple method for accurate frequency, measurements of ozone absorption lines. This technique is based on employing a tunable semiconductor diode laser, an etalon, and a White cell. We report accurate frequency meaaurements of over 100 absorption lines of ozone in the ν₃ band, which are near the CO₂ laser transitions. Despite the relative simplicity of the technique, we achieved an accuracy of better than 10 MHz in our measurements. This accuracy compares well with that achieved by the more complicated heterodyne techniques of high-resolution spectroscopy. However, these are the first measurements over an entire absorption band using the full resolution of a tunable diode laser.
A LAS, which combines the sensitivity of the acousto-optical methods with the convenience of direct, long-path optical detection, is described. The diode laser is wavelength modulated and the second harmonic detection technique is applied. This technique enables us to detect atmospheric pollutant gases with extremely high sensitivity (3 ppb of a weak absorbing pollutant such as SO₂ in its ν₁ band or 3 x 10ˉ³ ppb of a strongly absorbing molecule such as CO). This sensitivity is achieved using a frequency-locking technique for the diode. We report on the detection of O₃, SO₂, NH₃, and N₂O pollutants at their ambient, levels in air. A very high specificity and virtual elimination of interference effects are obtained by sampling the atmospheric air at reduced pressures.
A remote LAS station, which is considered to be adjunct to the above LAS, has been built. A remote retroreflector and an off-axis telescope near the laser have been employed to achieve a total pathlength of 1.2 km in the atmosphere. We report theoretical and experimental investigations of SO₂, H₂O, NH₃ and H₂O absorption in the region 1100 - 1200ˉ¹ cm at atmospheric pressure. This enabled us to develop a technique, based on the second harmonic detection scheme, to detect SO₂ in the atmosphere with a sensitivity of 50 ppb employing a diode laser emitting in the ν₁ band of SO₂. We report also on spectroscopic studies and absorption measurements for SO₂ and H₂O in the region 1300 - 1400 cmˉ¹. These studies indicate that it is possible to detect SO₂ with ambient levels in the atmosphere by employing a diode laser radiating in the ν₃ band of SO₂, near 1331.5 cmˉ¹.
In all the above mentioned techniques, theoretical calculations of laser transmission through the atmosphere must be predicted prior to applying such techniques. For this reason, a computer package has been developed and tested. This package generates the absorption line parameters of any molecule, even asymmetric-top molecules, in the vibration-rotation infrared region. The absorption (transmission) can be calculated at any pressure.
Elsherbiny, Mohamed Mahmoud, "Atmospheric Pollution Measurements with Tunable Lasers Employing Resonance Absorption" (1980). Open Access Dissertations and Theses. Paper 3216.