Development of a cw-cavity ring down spectrometer and electronic spectroscopy of transient species

Abstract

The study of molecular absorption and emission of radiation is of great importance in basic and applied science. Much of our knowledge on the geometrical and electronic structure of various molecules and molecular clusters stems from optical absorption studies performed in either bulk samples or, in molecular beam expansions. For many applications involving large polyatomic molecules, however, absorption measurements are potentially superior to those based on emission since rapid quenching (through energy redistribution processes) of the excited state will occur resulting in a greatly reduced emission quantum yield. In the limit of weak absorption the transmitted optical intensity decreases exponentially with absorption path length, in accordance with Beer's law, where the exponential decay constant, k, is the absorption coefficient at the frequency of the incident beam. The ability to accurately measure the ratio of I to Io typically limits the measurement to minimum losses of 0.01% to 0.001% and, as a rule, such precision absorption measurements require sophisticated optical systems and sources (often laser based) which have a stable output intensity. The required intensity stability has been achieved using several types of continuous lasers (e.g. infrared lasers diode lasers and tunable continuous wave dye lasers) using experimental configurations which typically employ some form of frequency modulation to discriminate against low frequency noise. The same success has not yet been possible for experimental systems based upon pulsed laser sources for several reasons. First, the pulse to pulse amplitude variation is typically large, greater than 10%, requiring a larger detector dynamic range and reducing the effective signal resolution. In addition, the short pulse widths of such lasers, typically 10-30 nsec, make it very difficult to modulate the frequency for differential analysis. In our laboratory a very successful experiment was already developed, based on frequency production double modulation spectroscopy of static plasma generated in a discharge cell. However, here the temperature is high (Trot=150 K), which produces a temperature broadening. Three qualities: high resolution, low temperature and Doppler-free are required simultaneously to solve the problem with laboratory spectroscopy. The solution was to construct a cavity ring down experiment using a continuous laser and slit jet. A continuous wave cavity ring down spectrometer has been constructed with the aim to record the electronic spectrum of rotationally–cold carbon chain radicals at high spectral resolution in direct absorption. The radicals are generated in a discharge of a high pressure gas pulse of acetylene in helium in a multilayer slit nozzle. A passive cavity mode locking scheme has been developed to handle refractive index changes inside the cavity caused by gas pulse and plasma fluctuations. A continuous wave cavity ring down spectrometer has the advantage that it is easier to record weak signals due to a single mode laser in resonance with only one transversal mode at a time. When multimode pulsed laser linewidth is much larger than cavity free spectral range, pulse to pulse give a fluctuation of spectral energy distribution. In case of a cw laser, we have one cavity mode at the same optical frequency. High accuracy (0.007 cm-1) and a linewidth of typically 500 KHz, gives the possibility to resolve the rotational structure. Another big advantage of a cw spectrometer is the low intracavity optical power of a few W/cm2, giving a stable transverse distribution. (In contrast, pulsed lasers have a very high intracavity optical power: the fluxes are of the order of MW/cm2 and the energy is then distributed over several transverse and longitudinal modes.)