Transient infrared spectroscopy on the picosecond time-scale by coherent pulse propagation

A novel time-resolved IR spectroscopy is accomplished studying coherent propagation of ultrashort pulses at low-intensity level. A detailed theoretical discussion is presented for electric dipole interaction between a resonant coherent pulse and a molecular transition. Homogeneous broadening (single line) and also more realistic situations with several adjacent transition frequencies are considered. Drastic changes of the pulse wings are predicted depending on several important parameters; e.g., absorption length αl, pulse duration t_p, and a molecular dephasing time T₂. It is shown that coherent superposition of neighboring transitions leads to a beating phenomenon, which is readily observable for αl≲1 and T₂/t_p > 1. The theoretical results are substantiated by experimental data on HCl gas in the pressure range 0.5 to 3 bar. Tunable pulses of 4 ps produced by a parametric generator system are used and two ultrafast IR light gates. Quantitative agreement between theory and experiment is obtained. The dephasing time T₂ of the R(3) vibration-rotation transition is measured to be T₂×p=21.5 ps bar in good agreement with spectroscopic data. Collective beating of the R(3) transition of the two isotopic species HCl ³⁵ and HCl³⁷ is observed for the first time in coherent pulse propagation. The linear intensity dependence of the propagation effect favors a wide range of applications in spectroscopy.