The influence of molecular structure on strong field energy coupling and partitioning

Investigating the coupling of polyatomic molecules with intense laser pulses is necessary for understanding strong field photochemistry. Furthermore, the amount of energy deposited in laser/molecule interactions, the degree and pattern of fragmentation, and the energy partitioning between electronic and nuclear motion are of great interest for coherent control experiments in the strong field regime. As a step toward this goal we examine time-of-flight mass and photoelectron spectra for a series of aromatic hydrocarbons - benzene, naphthalene, anthracene and tetracene collected using 780 nm, 80 fs laser pulses of intensity up to 2 x 10(14) W cm(-2). For this series, the amount of energy coupled increases with molecular size. The degree of fragmentation increases for the larger molecules, and the kinetic energy distributions of H+ and C+ ions and of photoelectrons shift to higher energies. Appearance intensities for ionization and activation of similar fragmentation channels decrease with increasing molecular size. The total amount of energy coupled appears to scale supra linearly with size of these molecules. The measurements suggest that energy deposition into polyatomic molecules occurs by interaction of the laser field with the molecular structure as a whole and not as a result of an interaction of the laser field with a collection of individual atoms. (C) 2001 Published by Elsevier Science B.V.