Internal laser-induced dipole force at work in C60 molecule

Response of polyatomic molecules to ultrastrong laser- and ion-induced fields

The exposure of molecules to short, ultrastrong electric fields leads to multiple ionization and a subsequent Coulomb explosion. We present a comparative study where uracil molecules are exposed to fields generated by high-power laser pulses (tau approximately 75 fs, I > 10(16) W/cm2) or swift highly charged ions (0.5 MeV Xe25+) representing a half-cycle pulse of less than 10 fs duration. Molecular dynamics and structural information contained in the fragmentation pathways can be assessed separately. Despite the similar field strengths large differences in fragment kinetic energies are found which are related to field shape and duration with the aid of molecular dynamics simulations.

Fragmentation dynamics of fullerenes in intense femtosecond-laser fields: Loss of small neutral fragments on a picosecond time scale

The fragmentation dynamics of C60 irradiated with intense femtosecond laser pulses is studied with one-color pump-probe spectroscopy. Small neutral fragments (C, C2, and C3) are formed by an 800-nm pump pulse which are then postionized by a delayed probe pulse. The respective ion signals detected by the time-of-flight mass spectrometry dramatically increase on a time scale of 10-20 ps.

Absolute ionization rates of multielectron transition metal atoms in strong infrared laser fields

We report on nonresonant strong field ionization of the multielectron transition metal atoms V, Nb, Ta, Ni, and Pd. Operating in the adiabatic regime (lambda = 1.5 microm), we quantitatively determined both (i) the first charge state saturation intensities and (ii) the absolute ionization rates for intensities ranging from threshold up to 3 x 10(14) W/cm2. We observed a dramatic suppression of ionization relative to single active electron approximation expectations. We suggest that this derives from dynamic polarization or screening effects within the multielectron atom, stressing a need for many-body theories of strong field ionization.

Controlling vibrational excitations in C60 by laser pulse durations

Two similar off-resonant ultrafast laser experiments in C60 have reported two different vibrational modes that dominate the relaxation process: one predicts the A(g) modes while the other the H(g) modes. A systematical simulation presented here reveals that this experimental discrepancy results from the laser pulse duration. The numerical results show that since each mode nu has a distinctive optimal duration tau(nu)(o), the A(g) modes are strongly suppressed for durations longer than 40 fs, while the H(g) modes start to grow. For the off-resonant and low-intensity excitations, the period Omega(o)(nu) of the dominant mode and tau(nu)(o) satisfy the relation Omega(o)(nu)/tau(nu)(o) approximately 3.4. By carefully scanning the laser frequencies and pulse durations, a comprehensive excitation diagram is constructed, which can be used to guide experiments to selectively excite the A(g) and H(g) modes in C60 by an ultrafast laser. Its potential impact is also discussed.

Recollision during the High Laser Intensity Ionization of C60

Through C60, we address the role of electron recollision in the nonresonant, femtosecond laser ionization of large, highly polarizable molecules. We show how the electron trajectories are influenced by the laser field, the laser induced dipole field, and the Coulomb field of the ion core. Working at long wavelengths we observe recollision in C60 through the ellipticity dependence of the fragmentation it produces. The ionizing electron emerges from C60z+ (z = 3, 4) with a lateral velocity of approximately 12 angstroms/fs, approximately half its Fermi velocity. Despite the large lateral velocity and competing forces on the electron, recollision remains relatively probable for this scale of molecule.

Stopping a vibrational wave packet with laser-induced dipole forces

Intense near-infrared laser pulses can generate laser-induced dipole forces that are strong enough to influence or control vibrational motion of a small molecule. Generally, the force acts to pull the molecule apart. Our numerical simulations show that, by applying the laser-induced dipole force at an appropriate time within one vibrational period, the wave packet motion of H+2 or D+2 can be accelerated or decelerated. Using the wave packet formed by the rapid ionization of H2 or D2, we also show that it is possible to move the vibrational population almost entirely to the v=0 state. Coherent cooling of the molecular vibrational motion can be achieved.