Knowledge Bank

The Cavity RingDown Laser Absorption Spectroscopy (CRLAS or CRDS) technique is a unique tool for probing weakly absorbing molecular species. High resolution (Doppler-limited broadening) is achievable in the blue-UV energy region by intracavity frequency doubling a CW laser source (here a Ti:Sa laser) externally. Moreover, absolute intensity and energy calibrations are available. The $H2O$ molecule in vapor phase exhibits a weak absorption spectrum in the visible-UV energy range. Fourier transform spectra are available up to $\sim 25000cm-1$; our experimental setup allows us to probe the UV energy range potentially up to $\sim 27000cm-1$ (i.e. the energy region of the polyad $8\nu ,8\nu +\delta$) with Doppler-limited linewidth and a sensitivity of $\sim 4ppb/cm/Hz$. The modelling of high vibrational energy levels is now available as part of the ab initio investigation of floppy molecules. The new experimental data show poor agreement compared to the available ab initio predictions.