Thomas Witte

Controlling molecular ground-state dissociation by optimizing vibrational ladder climbing

To achieve large population transfer to high vibrational levels in a selected ground-state mode of a polyatomic molecule [Cr(CO)6], we apply chirped femtosecond mid-infrared laser pulses at 2000 cm−1 to optimize vibrational ladder climbing as an energy deposition mechanism, which in turn controls the outcome of a unimolecular dissociation process. Its dependence on excitation parameters (frequency, intensity, chirp) is investigated and found to be in excellent agreement with a theoretical calculation. In particular, it is shown that optimizing vibrational ladder climbing allows for coherently controlled excitation even in a polyatomic molecule.

Molecular dissociation by mid-IR femtosecond pulses

Abstract By focusing a MIR femtosecond laser in a cell containing gas-phase metal carbonyls, the resonant infrared multiphoton dissociation of molecules was observed. Cr(CO)6,Mo(CO)6,W(CO)6, and Fe(CO)5 could easily be dissociated, which requires an excitation to at least v=7 or 8 of the CO stretch vibration. After irradiation with ∼150 fs pulses at 5 μm the metal carbonyl practically disappears in favor of free CO, as detected by the IR spectrum. By comparing the power dependence of the total conversion with a model, we can infer that only few vibrational degrees of freedom are involved in the excitation process.

Getting ahead of IVR: A demonstration of mid-infrared induced molecular dissociation on a sub-statistical time scale

Gaseous diazomethane (CH2N2) has been irradiated with femtosecond laser pulses tuned to the CNN asymmetric stretch at 2100 cm−1 in the mid-infrared. Laser-induced fluorescence detection of 1CH2 [537 nm, b1B1(0 16 0)←a1A1(0 0 0)] confirms infrared (IR) multiphoton-induced scission of the C–N bond on two distinct time scales, 480±70 fs and 36±8 ps. The longer time scale is consistent with a statistical dissociation process; the shorter one is indicative of directed dissociation, which occurs more rapidly than statistical intramolecular vibrational energy redistribution because of direct coupling of the vibrational energy from the IR-excitation mode into the reaction coordinate. Anisotropy measurements demonstrate that the CH2 group bends significantly out of the molecular plane prior to the dissociation.

Programmable amplitude- and phase-modulated femtosecond laser pulses in the mid-infrared.

We present a scheme to produce programmable phase- and amplitude-modulated femtosecond laser pulses in the mid-infrared regime of 3-10mum by difference frequency mixing. The 80-fs signal output of an optical parametric amplifier is shaped with a liquid-crystal mask and mixed in an AgGaS(2) crystal with a temporally stretched idler pulse. Without changing the mechanical alignment, we produce programmable amplitude modulations and chirped pulses at lambda=3mum with energy as high has thas 1muJ . This scheme, further, allows the generation of controllable pulse sequences. The results are in good agreement with theoretical simulations.

Optical parametric amplification of a shaped white-light continuum

Phase-locked two-color sub-40-fs double pulses in the visible are produced by noncollinear parametric amplification of white light tailored in a pulse shaper with a liquid-crystal mask. The carrier phase between the pulses is conserved during the amplification process and can be adjusted, as can the temporal separation and the center of wavelengths of the pulses.

Femtosecond Infrared Vibrational Up-Pumping of Liquid Phase W(CO)6

CO-stretch excitation of W(CO)6 in room temperature n-hexane using 5 μm femtosecond pulses transfers vibrational population to v>5, as measured by transient mid-infrared spectroscopy and compared to Bloch model calculations. These results constitute significant steps towards controlling molecular ground state populations and reactions.

Infrared Femtochemistry: Vibrationally induced molecular dissociation and its control employing shaped fs MIR laser pulses

Vibrations drive chemical reactions. The barriers of chemical reactions are typically between 1 and 3 eV, in comparison with the energy regime of infrared quanta ranging from .03 to .3 eV. This relation shows that one generally has to assume multiphoton excitation in the relevant infrared modes. An additional obstacle for successful infrared laser chemistry is given by the very rapid (typically ps) intramolecular vibrational energy relaxation (IVR). Only if it is possible to keep the vibrational energy localized where it is beneficial for the reaction to occur, and if direct coupling between the initial and final state is possible, does infrared laser chemistry have a chance. This picture of directly driving chemical reactions by selective vibrational excitation is an old dream of IR laser-based chemistry. Different types of chemical reactions involve different types of vibrational modes, e.g. stretching vibrations, isomerizations by skeletal modes, may control dissociation reactions and so on. The argument that infrared quanta are relatively energy-poor and infrared transitions generally have low absorption cross sections, especially if multiphoton excitation is required, limits the choice of suitable molecular transitions. This chapter discusses dissociation involving molecules with maximal transition dipole moments, comparatively weak bonds to be broken, and vibrational excitation in the mid-infrared spectral range.