Femtosecond Raman time-resolved molecular spectroscopy
Résumé
The applicability of several femtosecond time resolved non-linear coherent techniques such as Raman induced polarization spectroscopy (RIPS), degenerate four-wave mixing (DFWM) and coherent anti-Stokes Raman spectroscopy (CARS) for molecular spectroscopy is presented. All methods rely on the initial coherent excitation of molecular states producing wavepackets, whose time evolution is then measured. In the case of RIPS and DFWM only pure rotational transitions are involved, whereas in CARS vibrational states can be excited. First the methodology of concentration and temperature measurements using RIPS in gas mixtures involving N2, CO2, O2, and N2O is shown. In addition some applications are given for the two closely related techniques DFWM and CARS. DFWM is suitable to extract the rotational constants of molecules to a high accuracy as is demonstrated by measurements on CO2 and pyrimidine, which is a biological building block. CARS can be used to study higher order molecular constants and to sensitively determine temperature in, e.g., H2 up to 2000 K. Finally, CARS is applied for the investigation of pressure dependent lineshape models, which are important for the temperature evaluation from spectroscopic data.