Résumé des résultats (Abstract)
(Anglais)
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Spectroscopic properties of carbon chain radicals have been of interest because of their occurence in astropohysical environments and combustion processes. Whereas pure rotational and vibration-rotation spectra have been obtained for a number of homologeous series of carbon chains, electronic information is still limited and mainly restricted to spectra recorded in matrices.
In this project we have tried to produce carbon chain radicals using a high pressure slit nozzle discharge and sampling the plasma with an ultrasensitive detection technique, known as cavity ring down (CRD) spectroscopy. This technique has become a standard method, but applications have been mainly limited to the ultraviolet/visible and near infrared. Some studies are reported in the infrared, but no results are available in the vacuum ultraviolet (VUV). One of the few setups in the world capable of producing tunable VUV is at the Laser Centre Vrije Universiteit Amsterdam (LCVU), an EC large scale facility. The goal of this project was to start a CRD experiment and to extend applicationis to the VUV where several carbon chain radicals (particularly pure carbon chains) absorb.
The spectra are recorded in direct absorption (with absorption pathlengths of the order of several km) and at high spectral resolution (better than 0.03 cm-1). For this purpose a new vacuum setup was assembled, consisting of a roots-blower system with a total pumping capacity of 1200 m3/hr pumping a chamber onto which a high Q cavity is mounted. Inside the cavity a planar plasma source is mounted, through which gas is expanded supersonically, using diluted mixtures in rare gases. Both vacuum setup, optical system (CRD unit) and data acquisition had to be constructed/developed for this project, in which the Physics and Physical Chemistry department of the Vrije Universiteit Amsterdam participated. The initial experiment were performed on the 2Pu - X2Sg+ electronic system of N2+, a molecular ion known to be important in atmospheric reaction schemes. This experiment allowed to get the timing correct for CRD-decay (70 ms), gas pulse (1 ms) and discharge pulse (500 ms). Apart from the well known (3,0) vibrational band system around 680 nm, also the (8,4) system was observed. The latter system has not been reported in literature so far and as soon as both spin orbit components are recorded these data will be put into literature.
The N2+ measurements were used to test the characteristics of the new setup. Rotational temperatures as low as 10 K have been achieved. Linewidths are limited by residual Doppler broadening in the expansion to approximately 0.1 cm-1, but a new multi-channel version of the slit body has been developed which allows resolutions below 0.03 cm-1, i.e. limited by the laser bandwidth.
First tests have been made on carbon plasmas, which are harder to generate as carbon dust tends to cause short-circuitries. It was possible with the source to get a stable carbon plasma on a length of 3 cm, expanding 0.3 % mixtures of acetylene in He with backing pressures as high as 10 bar and a 10 Hz repetition rate. It was also possible to observe small carbon species (C2 and C3), but within the 3-week period of the project it was not possible to extend the measurements to C5. However, in view of the progress achieved within this short time, the participating groups will continue this research direction. It is hoped that the first VUV CRD measurements will be performed in the first half of 2002.
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