We study strong-field molecular ionization as a function of pulse duration. Experimental measurements of the photoelectron yield for a number of molecules reveal competition between different ionization continua (cationic states) which depends strongly on pulse duration. Surprisingly, in the limit of short pulse duration, we find that a single ionic continuum dominates the yield, whereas multiple continua are produced for longer pulses. Using calculations which take vibrational dynamics into account, we interpret our results in terms of nuclear motion and non-adiabatic dynamics during the ionization process.

We study strong field molecular ionization using few-(four to ten) cycle laser pulses. Employing a supercontinuum light source, we are able to tune the optical laser wavelength (photon energy) over a range of about ∼200 nm (500 meV). We measure the photoelectron spectrum for a series of different molecules as a function of laser intensity, frequency, and bandwidth and illustrate how the ionization dynamics vary with these parameters. We find that multiphoton resonances and nonadiabatic dynamics (internal conversion) play an important role and result in ionization to different ionic continua. Interestingly, while nuclear dynamics can be "frozen" for sufficiently short laser pulses, we find that resonances strongly influence the photoelectron spectrum and final cationic state of the molecule regardless of pulse duration -even for pulses that are less than four cycles in duration.

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.

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Using a 45 fs pump pulse at 800 nm, a wave packet is created in a charge asymmetric dissociation channel of iodine, I(2)(2+)→I(2+)+I(0+) (2,0). As the molecule dissociates, a two-color (1ω2ω) probe pulse is used to study the dynamics as a function of internuclear separation R. We find a critical region of R in which there is spatially asymmetric enhanced ionization of the (2,0) channel to a counterintuitive (1,2) channel. In this region the I(0+) is ionized such that one electron is released to the continuum and another is transferred to the I(2+) resulting in I(0+)→I(2+) and I(2+)→I(1+). At larger R, the ionization is consistent with simple one-electron ionization in a double well where I(0+)→I(1+). We find qualitative agreement between simulations and experiment further highlighting the importance of multielectron effects in the strong-field ionization of molecules.