Knowledge Bank

A high resolution Fourier transform Raman spectroscopy of supersonic expansions is demonstrated. The technique applies stimulated Raman excitation of molecular transitions in a small frequency interval $(\sim 1cm-1)$ by the help of two synchronized pump pulses and subsequent coherent Raman scattering of delayed probing pulses. Measuring over a long delay time interval $(10-8s)$ numerical Fourier transformation of the scattering signal transient yields precise spectroscopic information with a resolution of 50 MHz. Experimental data will be presented for the Q-band of the $\nu 1$-vibration of $CH4$, resolving for the first time the tensor splitting of the J=2 transitions. As a first demonstration of a picosecond infrared spectroscopy coherent pulse propagation of ultrashort infrared pulses is discussed. The drastic reshaping of the investigated resonant pulses provides direct information on various interesting parameters, e.g. dephasing time $T2$ and molecular frequency differences. The technique is particularly interesting because of the different selection rules of the IR transitions as compared with Raman scattering. Experimental data will be presented for the R-branch of the system HCl: Ar and compared with corresponding time-resolved Raman measurements.""