High sensitivity femtosecond gas phase pump-probe experiments using a hollow waveguide: intramolecular redistribution processes in CH 3 I

Abstract

Pump-probe experiments using a delay line are one important approach in the investigation of molecular dynamics on the femtosecond to picosecond time scale. As the pulse energies for femtosecond pulses are usually small, the measured signal has to be obtained from the overlap region of two focused laser beams. Due to the low density in the gas phase high sensitivity experiments are essential, particularly if one or both laser pulses are in the infrared (IR) or near-IR range with much smaller absorption cross-section as compared to the visible or ultraviolet. To increase the interaction volume between pump- and probe-pulse, the two laser beams can be focused into a hollow waveguide with an inner diameter d_ID = 100 to 500 &mgr;m. We have calculated the focusing condition for a near-IR pump- and an UV probe-beam to excite nearly exclusively the lowest HE₁₁-mode within the waveguide. For molecular samples with a low absorption coefficient (alpha < 0.1 m^-1 for the probe beam in the ultraviolet) an enhancement of the measured probe signal of a factor of 9-10, relative to a confocal arrangement in a cell, is calculated from the intensity distribution within a hollow waveguide with an inner diameter d_ID = 250 &mgr;m and length L_wg = 500 mm. The theoretical calculations were confirmed in pump-probe experiments of intramolecular vibrational energy redistribution (IVR) in CH₃I vapour. In the experiments the first overtone of the CH-stretching vibration is excited with a near-IR pump-pulse and the redistribution of the vibrational energy to other vibrational degrees of freedom, especially to the CI-stretching vibration, is detected through a change of the UV-spectrum by a probe pulse around 310 nm.