Photoionization Yields in Intense fs-Laser Fields - A Systematic Investigation of Chirp Effects

摘要

The femtosecond laser ionization of several small hydrocarbon molecules (methane, ethane, propane) has been investigated as a function of the second order spectral phase (linear chirp) of laser pulses centered at lambda = 807 nm. Ion yields are demonstrated to depend markedly on the linear chirp parameter. At laser intensities exceeding 5 x 10(14) W/cm(2) the ionization characteristics resemble an intensity-driven process, i.e. the largest ion yield is observed for a close to transform-limited laser pulse. For smaller laser intensities a pronouneed sign effect in the chirp dependence appears, i.e. the maximum ion yield is observed for a negative linear chirp parameter of several hundred to thousand fs(2). We suggest that this sign effect is connected to the anharmonicity of the electronic potentials involved. This suggestion is supported by the fact that basically no sign effect in the chirp dependence is observed for the noble gases argon, krypton and xenon. We further suggest that the electronic polarizability and structure plays some role as methanol does not show a marked sign-dependent chirp effect.