# NONLINEAR SPECTROSCOPY: EXPERIMENTS WITH MOLECULES IN A TWO COLOUR RESONANT IR RADIATION FIELD AND THE THEORETICAL MODELLING BASED ON THE OPTICAL BLOCH EQUATION.

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 Title: NONLINEAR SPECTROSCOPY: EXPERIMENTS WITH MOLECULES IN A TWO COLOUR RESONANT IR RADIATION FIELD AND THE THEORETICAL MODELLING BASED ON THE OPTICAL BLOCH EQUATION. Creators: Sartakov, Boris G.; Van Opbergen, M.; Linskens, A.; Holleman, I.; Dam, N.; Reuss, J. Issue Date: 1995 Publisher: Ohio State University Abstract: The recent achievements in the nonlinear optics opened the new possibilities in the IR high resolution spectroscopy. In this contribution a two colour spectroscopy of cooled $SF_{6}$ and $C_{2}H_{4}$ seeded to He beam is presented. The molecules were excited by $CW {CO_{2}}$ laser beam crossing the molecular beam. The frequencies of laser beams were tuned near $CO_{2}$ lines by Optoacoustic Modulators (OAM) in the range $\pm 500 MHz$. The spectral resolution 2 MHz was achieved. The excitation spectra were modelled in the frames of dressed state picture (DSP) and optical Bloch equations (OBE). In $C_{2}H_{4}$ the one quantum transition $(\nu = 0, \{4, 1, 3\} \rightarrow \nu = 1, \{5, 0, 5\}$ 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 P*** \marginpar{*Check**} with the frequencies $\nu_{1}$ and $\nu_{2}$ satisfying the resonant condition $(N + 1)\cdot \nu_{1} - N \cdot \nu_{2} = \nu_{transtition}$ at N = 1 and N = 2 were observed. The AC - Stark shifts of peaks were measured being in the agreement with the results of computer modelling based on OBE. The modelling showed that the increasing of the laser intensities leads to the transition from the UDU picture to the AC - Stark split one photon line picture. In $SF_{6}$ the two quantum ladder type transitions $(\nu_{3} = 0, J = 4 \rightarrow \nu_{3} = 1, J = 3\to \nu_{3} = 2, J = 3)$ were investigated. The frequency $\nu_{1}$ was varied near the frequency of the first transition while $\nu_{2}$ was varied near as the frequency of the second transition and the frequency of the two photon transition. The peaks of the dressed states were observed and the AC - Stark shifts of different M - components were measured. The variation of the intensities of peaks due to the interference of 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. The good agreement between measured and simulated spectra was achieved. Description: $^{\ast}$ General Physics Institute, Vavilov str. 38, 117942 Moscow, Russia $^{\ast}$ Support by NATO collaborative research grant is gratefully acknowledged Author Institution: Catholic University of Nijmegen, Toernooiveld, NL - 6525 ED Nijmegen, Netherlands URI: http://hdl.handle.net/1811/29663 Other Identifiers: 1995-RH-07