\title{NONLINEAR SPECTROSCOPY: EXPERIMENTS WITH MOLECULES IN A TWO COLOUR RESONANT IR RADIATION FIELD AND THE THEORETICAL MODELLING BASED ON THE OPTICAL BLOCH EQUATION.}
\author{M.~VAN OPBERGEN, A.~F.~LINSKENS, I.~HOLLEMAN, N.~DAM}
\address{Catholic University of Nijmegen, Dept. of Molecular and Laser Physics, Toernooiveld, NL-6525 ED Nijmegen, Netherlands}
\author{\underline{B.~SARTAKOV}\thanks{Support by NATO collaborative research grant is gratefully acknowledged}}
\address{General Physics Institute, Vavilov str. 38, 117942 Moscow, Russia}
\author{~and~J.~REUSS}
\address{Catholic University of Nijmegen, Dept. of Molecular and Laser Physics, Toernooiveld, NL-6525 ED Nijmegen, Netherlands}

\begin{document}
\maketitle Recent achievements in nonlinear optics opened new possibilities in IR high resolution spectroscopy. In this contribution a two colour spectroscopy of cooled SF$_6$ and C$_2$H$_4$ seeded in a He beam is presented. The molecules were excited by CW CO$_2$ laser beams crossing the molecular beam. The frequencies of laser beams were tuned around CO$_2$ lines and shifted by Optoacoustic Modulators (OAM) in the range $\pm$ 500 MHz. The spectral resolution of 2 MHz was achieved. The excitation spectra were modelled in the dressed state picture (DSP) and by optical Bloch equations (OBE).

In C$_2$H$_4$ the one quantum transition $(v=0,\{4,1,3\} \rightarrow v=1,\{5,0,5\})$ was investigated. At the moderate intensities of the laser fields ($\nu_1$ and $\nu_2$) the UP-DOWN-UP-... (UDU) scheme of excitation was realized. The peaks with the frequencies $\nu_1$ and $\nu_2$ satisfying the resonant condition $(N+1) \cdot \nu_1 - N \cdot \nu_2 = \nu_{transition}$ with N=1 and N=2 were observed. The AC-Stark shifts of peaks were measured being in agreement with the results of computer modelling based on OBE. The modelling showed that increasing the laser intensities leads to a transition from the UDU picture to the AC-Stark split one photon line picture.

In SF$_6$ the two photon ladder-type transition $(v_3=0,J=4 \rightarrow v_3=1,J=3 \rightarrow v_3=2,J=3)$ was investigated. The frequency $\nu_1$ was varied near the frequency of the first transition while $\nu_2$ was varied near to the frequency of the second transition and to the frequency of the two photon transition. The peaks of the dressed state transitions were observed and the AC- Stark shifts of different M-components were measured. The variation of intensities of peaks due to the interference between many paths of excitation was observed. The DSP was used for the qualitative analysis of the interference effect and the quantitative modelling was based on OBE. Good agreement between measured and simulated spectra was achieved.

\end{document}