Alignment dependent enhancement of the photoelectron cutoff for multiphoton ionization of molecules.

The multiphoton ionization rate of molecules depends on the alignment of the molecular axis with respect to the ionizing laser polarization. By studying molecular frame photoelectron angular distributions from N(2), O(2), and benzene, we illustrate how the angle-dependent ionization rate affects the photoelectron cutoff energy. We find alignment can enhance the high energy cutoff of the photoelectron spectrum when probing along a nodal plane or when ionization is otherwise suppressed. This is supported by calculations using a tunneling model with a single ion state.

Precise in-situ measurement of laser pulse intensity using strong field ionization.

Building on the work of Alnaser et al. [Phys. Rev. A 70, 023413 (2004)], we devise an improved method for an in-situ measurement of the peak intensity in a focused, femtosecond infrared laser pulse. The method is shown to be effective with both photoion and photoelectron imaging devices. The model used to fit the experimental data has no unphysical free parameters used in fitting. The accuracy of the fit is 4% and the overall accuracy of the measurement is 8%.

Partitioning of the linear photon momentum in multiphoton ionization.

The balance of the linear photon momentum in multiphoton ionization is studied experimentally. In the experiment argon and neon atoms are singly ionized by circularly polarized laser pulses with a wavelength of 800 and 1400 nm in the intensity range of 10(14)-10(15) W/cm2. The photoelectrons are measured using velocity map imaging. We find that the photoelectrons carry linear momentum corresponding to the photons absorbed above the field free ionization threshold. Our finding has implications for concurrent models of the generation of terahertz radiation in filaments.

Measurement and control of plasma oscillations in femtosecond filaments.

The short-lived longitudinal plasma oscillations generated during filamentation in argon and nitrogen gas are measured with a specially designed current monitor. The magnitude and initial direction of the corresponding currents depend sensitively on laser polarization and nature of the gas. The results are interpreted as resulting from the competition between two forces acting on free electrons born during the filamentation process: the Lorentz laser force and a Coulomb wake force resulting from a lateral expansion of the plasma.

Probing angular correlations in sequential double ionization.

We study electron correlation in sequential double ionization of noble gas atoms and HCl in intense, femtosecond laser pulses. We measure the photoelectron angular distributions of Ne+ relative to the first electron in a pump-probe experiment with 8 fs, 800 nm, circularly polarized laser pulses at a peak intensity of a few 10(15) W/cm2. Using a linear-linear pump-probe setup, we further study He, Ar, and HCl. We find a clear angular correlation between the two ionization steps in the sequential double ionization intensity regime.

Direct test of laser tunneling with electron momentum imaging.

Tunneling is often used to describe multiphoton ionization of rare gas atoms in infrared fields. We test the tunneling approximation and its nonadiabatic extension by measuring the unperturbed momentum distribution along the κ direction of a circularly polarized light pulse. We find substantial, but not total, agreement between our results and the predictions of the model. As predicted, the κ direction momentum distribution is Gaussian and its width increases with the square root of electric field strength. However, the width is 15% too large and we find no evidence of nonadiabatic effects as we approach the expected limits of the approximation.