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

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.)