Nonlinear Spectroscopy of Radicals
Thermodynamic data are indispensable for combustion modelling. In many combustion systems the temperature range spans from 300 to 2300 K and therefore, enthalpy, entropy and heat capacity data for each species is required for a large temperature range. In contrast to stable molecules, direct measurements of thermodynamic properties of radicals are generally impossible due to the high reactivity and transient character of these open-shell molecules. Therefore, spectroscopic experiments on combustion relevant species are required to determine their thermodynamic functions via the partition function and statistical mechanical relationships.
In this work, experiments are devised which aim to improve the fundamental knowledge on combustion relevant radicals. Apart from the more conventional spectroscopic techniques, like laser-induced fluorescence (LIF), cavity ring-down (CRD) and multi-photon ionization, the application of nonlinear four-wave mixing is of particular interest. These methods have been established at the PSI more than a decade ago and are constantly being improved by this group and others. A sensitivity has been achieved presently in this laboratory that is sufficient to detect and measure high-resolution, double-resonance spectra of minor species, like the carbon dimer, C2-, which is generated from a discharge source in a molecular beam environment. The excellent sensitivity combinded with the selectivity of a double-resonant method is applied to deperturbation studies of the C2 molecule. Weak 'extra lines' are accessible that originate from from nearby-lying states that gain transition strenth through the perturbation process. The deperturbation analysis of the complex spectral region in the (6,5) and (6,4) bands of the Swan system unveils the presence of the energetically lowest high-spin state of C2 in the vicinity of the d 3Πg, v = 6 state.
In addition, complementary experiments at the VUV beam line of the SLS, which is operated by members of this group, are envisaged.