We used strong-field laser pulses that were tailored with closed-loop optimal control to govern specified chemical dissociation and reactivity channels in a series of organic molecules. Selective cleavage and rearrangement of chemical bonds having dissociation energies up to approximately 100 kilocalories per mole (about 4 electron volts) are reported for polyatomic molecules, including (CH3)2CO (acetone), CH3COCF3 (trifluoroacetone), and C6H5COCH3 (acetophenone). Control over the formation of CH(3)CO from (CH3)2CO, CF3 (or CH3) from CH3COCF3, and C6H5CH3 (toluene) from C6H5COCH3 was observed with high selectivity. Strong-field control appears to have generic applicability for manipulating molecular reactivity because the tailored intense laser fields (about 10(13) watts per square centimeter) can dynamically Stark shift many excited states into resonance, and consequently, the method is not confined by resonant spectral restrictions found in the perturbative (weak-field) regime.

The dependence of the ultrafast relaxation kinetics of the S 2 and S 1 states in β -carotene homologs and lycopene on conjugation length studied by femtosecond time-resolved absorption and Kerr-gate fluorescence spectroscopies

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The dependence of the ultrafast relaxation kinetics of the S 2 and S 1 states in β -carotene homologs and lycopene on conjugation length studied by femtosecond time-resolved absorption and Kerr-gate fluorescence spectroscopies

In intense laser fields, fragment ions can be produced from CH 3 COX (X ) CH 3 , CF 3 , and C 6 H 5 ) either by absorption and dissociation followed by ionization (ADI) or absorption and ionization followed by dissociation (AID). Electronic structure calculations were carried out using Hartree-Fock, density functional, and correlated levels of theory to understand the possible fragmentation pathways. The calculated ionization potentials are in very good agreement with the available experimental data. For acetone, the acetyl ion is predicted to be the most preferred dissociation product and can be produced by either mechanism. The very low C-CF 3 bond energy in the parent ion of trifluoroacetone provides a clear reason for the absence of CF 3 COCH 3 + and CF 3 CO + ion peaks from the mass spectrum of CF 3 COCH 3 after intense laser excitation and indicates that fragmentation occurs by AID. For acetophenone, both CH 3 CO + and C 6 H 5 CO + are stable fragments, with the latter being produced by an AID mechanism.