John P. Nibarger

Dispersion-free transient-grating frequency-resolved optical gating

We have developed a compact dispersion-free TG (transient-grating) FROG (frequency-resolved optical gating) by utilizing a mask that separates the input beam into three distinct beams focused into fused silica to create the FROG signal. Two of the beams are reflected off the same set of mirrors to ensure identical optical paths, eliminating the difficulty in establishing zero time delay between the beams. In addition, the use of only reflective optics avoids material dispersion in the FROG except for the mixing crystal. This TG FROG is capable of operating with an intensity of 1 x 10(11) W/cm(2) and has resolutions less than 0.5 and 1.3 fs for 25- and 10-fs input pulses, respectively.

A framework for understanding molecular ionization in strong laser fields

The aim of this paper is to examine the role of excited states and multi-electron interactions in molecular ionization by strong laser fields. We present new data on the ionization and dissociation of iodine molecules that reveal important aspects of the strong field-molecule interaction in the short pulse regime. Our data, along with previous studies, is inconsistent with the simplest and commonly accepted model of molecular ionization in a strong laser field and this has led us to examine closely the individual ionization steps. We have found that a molecule can ionize into several distinct configurations predominantly through multi-electron interactions and the abundance of such configurations is dependent on internuclear separation. The ionization appears to be dominated by pairs of states with gerade and ungerade symmetry, as they have a large dipole coupling and the transition is near resonant with the strong laser field. In a one-electron molecule, this pair consists of the ground and first excited state whereas in a two-electron molecule this corresponds to the lowest lying pair of ionic states. In this paper, we propose a framework for organizing the numerous ionization pathways based on the electronic configuration of the initial charge state.

Excited state fragments following molecular ionization and dissociation in strong fields

We have observed for the first time that charge asymmetric dissociation in diatomic molecules leaves one of the fragments in an electronically excited state. Using a new double pulse technique, we determine the state of the post dissociation fragments. For example, we observed the reaction I2+(pulse1)→(I22+)**→I0++(I2+)*+(pulse2)→I0++I3+ demonstrating that the I2+ fragment must have been in an excited state. More generally, just as asymmetric dissociation implies that the initial molecular ion is in an excited electronic state, the observation of asymmetric channels in the post-dissociation ionization shows that the ionic fragments are themselves electronically excited.

Direct observation of excited state fragments following molecular ionization and dissociation in strong fields

Summary form only given. Strong field ionization of molecules has only recently been systematically studied. It has widely been assumed, for the purposes of analysis, that the dissociation products of a molecule are in their electronic ground state. For the first time, in this paper we show evidence that the dissociation fragments can be in excited electronic states. These results were obtained using a new technique called correlated ion spectroscopy where the potential energy curve of a molecule can be mapped out by using a pump and probe pulse which allow us to identify certain dissociation pathways.

Observation of ultrafast electron behavior in femtosecond optical breakdown of dielectrics

Summary form only given. Many studies have been conducted on the ultrafast breakdown of dielectrics in order to understand the different processes involved. Such investigations are important to diverse fields such as micromachining, medical physics and solid state physics. Generally, laser induced breakdown in dielectrics is described in terms of three major processes: (i) multiphoton ionization (MPI) and/or tunneling causing the excitation of electrons to the conduction band, (ii) electron-electron collisional ionization (avalanche process) due to Joule heating, (iii) plasma energy transfer to the lattice. While the first two processes deposit energy in the plasma, the third process releases the deposited energy to the lattice thereby inducing the actual damage. This transfer of energy to the lattice is expected to occur after the laser pulse. Until recently, the above processes have been studied by measuring the pulse duration dependence of OBT. Although single pulse OBT measurements have been extended to the 5 fs-range, it is very difficult to extract information regarding electron dynamics from such measurements, especially the time scale for the plasma energy decay. We have developed a new pump-probe double pulse experiment, which is more sensitive to dynamic behavior.