Sensitive Intracavity Photoacoustic Measurements with a C02 Waveguide Laser

A photoacoustic intracavity configuration is presented; a resonant photoacoustic cell excited in its First longitudinal mode is placed inside the cavity of a C 0 2 waveguide laser. Due to the high laser power and the sharp intracavity focus, saturation effects occur in the excitation and relaxation process of absorbing C 2H 4 molecules. A more optimal configuration is applied to measure the C2H 4 emission of several Rumex species. A detection sensitivity of 6 ppt (parts per trillion) C 2H 4 is reached, equivalent to a minimal detectable absorption of 1.8 x 10locm-1. PACS: 07.60, 43.85, 42.60 The photoacoustic technique for trace gas detection has proved to be a sensitive and reliable method at wavelengths in the infrared and near infrared [1-6]. For practical applications, however, the photoacoustic method becomes significantly more interesting when its sensitivity exceeds by some orders of magnitude that of current conventional techniques such as a gas chromatograph equipped with a flame ionization detector or photoionization detector [7], Inserting a photoacoustic cell in the laser cavity results in a photoacoustic detection which becomes even more sensitive due to the high intracavity laser power. Resonant photoacoustic cells for intracavity laser operation are known [8-12]. They combine a high sensitivity with a small length, necessary for intracavity operation. However, the large volume of a resonant cell, operating at its lowest radial or azimuthal acoustical mode, is disadvantageous. The requirements for prac­ tical operation are usually a low gas consumption and a fast response [10]. An open resonant cell excited in its first longitudinal mode fulfills these requirements. It becomes possible to investigate the gaseous exudates of small biological samples, which are present as trace gas in ambient air. In this paper we present such a photoacoustic cell placed inside the cavity of a C 0 2 waveguide laser. We describe the development of the device for three intracavity configurations. As an application, the de­ tection of C2H4 at ppt level (parts per trillion, 1:1012) will be discussed and measurements will be presented on the C 2H 4 emission of small Rumex plants (fresh weight ± lgram , 19 days old) under the stress of inundation. 1. Outline of the Intracavity Set-Up The laser used is a C 0 2 waveguide laser which emits ■ radiation at wavelengths between 9 and 11 p.m. The vibrational P and R transitions of C 0 2 produce radiation in four branches yielding about 90 laser lines. A hollow dielectric waveguide tube of quartz permits transmission of the electromagnetic radiation with very small attenuation [13]. The waveguide tube has a inner diameter of 2.6 mm and a discharge length of 45 cm. An increased gain per unit length and a decreased physical size are advantages of operating a C 0 2 waveguide laser instead of a normal C 0 2 laser. The losses due to coupling the radiation from the waveguide into free space and back into the guide have 138 F. J. M. H arren et al.