Toward elucidating the mechanism of femtosecond pulse shaping control in photodynamics of molecules by velocity map photoelectron and ion imaging.

The control of photofragmentation and ionization in a polyatomic molecule has been studied by femtosecond chirped laser pulse excitation and velocity map photoelectron and ion imaging. The experiments aimed at controlling and investigating the photodynamics in CH(2)BrCl using tunable chirped femtosecond pulses in the visible wavelength region 509-540 nm at maximum intensities of about 4x10(13) W/cm(2). We observe that the time-of-flight mass spectra as well as the photoelectron images can be strongly modified by manipulating the chirp parameter of ultrashort laser pulses. Specifically, a strong enhancement of the CH(2)Cl(+)/CH(2)BrCl(+) ion ratio by a factor of five and changes in the photoelectron spectra are observed for positively chirped pulses centered near 520 nm. These changes are only observed within a narrow window of wavelengths around 520 nm and only for positively chirped pulses. From the combination of the photoelectron spectra and the ion recoil energy of the CH(2)Cl(+) fragment we can deduce that the parent ionization and fragmentation is induced by a multiphoton excitation with five photons. The photoelectron images and the fragment ion images also provide the anisotropy (beta-parameter) of the various electron bands and fragment ions. We conclude that multiphoton excitation of the highest occupied 22a(') and 8a(") CH(2)BrCl molecular orbitals of Br-character are both involved in the five-photon ionization, however, only excitation of the 22a(') orbital appears to be (mostly) involved in the chirped control dynamics leading to enhanced fragmentation to CH(2)Cl(+)(X A(')) + Br((2)P(3/2)). We propose that a wavepacket following or a time-delay resonance mechanism between the two-photon excited n(x)(Br,22a(')) --> (2A(')) repulsive surface and the three-photon near-resonant n(x)(Br,22a(')) --> Rydberg(A(')) state of the neutral CH(2)BrCl molecule is responsible for the enhanced excitation of the n(x)(Br,22a(')) molecular orbital with up-chirped pulses. This leads to enhanced ionization to a configuration in the CH(2)BrCl(+)(X A(')) continuum just above the dissociation limit of the CH(2)Cl(+) + Br((2)P(3/2)) channel, resulting in enhanced fragmentation.