Single ionization of molecular iodine

Abstract

We performed a study of the single ionization of iodine, I 2 over a range of wavelengths. Single ionization of I 2 is unexpectedly found to have a contribution from inner molecular orbitals involving the 5s electrons. The I + I + dissociation channel was recorded through velocity map imaging and the kinetic energy release of each channel was determined with 2D fitting of the images. Most of the measured kinetic energy data were inconsistent with ionization to the X, A, and B states of I + 2 , implying ionization from deeper orbitals. A pump-probe Fourier transform technique was used to look for modulation at the X and A state vibrational frequencies, to see if they were intermediate states in a two step process. X and A state modulation was only seen for kinetic energy releases below 0.2 eV consistent with dissociation through the B state. From these results and intensity, polarization, and wavelength dependent experiments we found no evidence of bond softening, electron rescattering or photon mediation through the X or A states to higher energy single ionization channels.

Resonantly enhanced inner-orbital ionization in molecular iodine

“…In fact, the branching ratio for ionization of the inner orbitals can exceed 90% [4]. We have also recently observed an interesting wavelength dependence to the ionization rates of the different orbitals [5]. Both of these results run counter to a simple tunneling model of strong field ionization, in that tunneling ionization strongly prefers the least bound electron and predicts no wavelength dependence [6].…”

“…For this experiment we were unable to cover the ranges of 525-550 nm and 590-626 nm with sufficient energy for ionization. We use the same experimental configuration as described in [5]. In addition we have introduced a pickoff mirror and photodiode after the OPA's prism compressor to monitor the beam energy.…”

“…To achieve velocity map imaging (VMI) of ion fragments we use a 3 element ion lens in conjunction with a microchannel plate (MCP) and phosphor screen which is imaged by a CCD camera at a rate of 20 frames/s. [5,9]. The MCP is gated to slice the data and produces a temporal width of 15 ns.…”

“…(1,1) However, some of the X and A state population can be excited to the dissociating B state. Direct ionization of the HOMO-2 can also populate the B state [5]. Thus, I + 2 and low energy (1,0) constitutes the OOI signal.…”

“…We have previously shown that the high KER (1,0) signal cannot come from 1) direction ionization of an outer-orbital, using energy conservation, as the energy levels at the equilibrium internuclear separation and the separated atom limit are known; and 2) a two-step process through the X or A states, using pump-probe and Fourier transform spectroscopy. For double ionization into the low lying levels of the dication we include I 2+ 2 , which has a known thermal width of 0.025 eV, and the (1,1) channel [5]. The (2,0) channel corresponds to electronically excited dications.…”