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

Abstract

High-resolution threshold photoelectron study of the propargyl radical by the vacuum ultraviolet laser velocitymap imaging method J. Chem. Phys. 135, 224304 (2011) Competitive ionization processes of anthracene excited with a femtosecond pulse in the multi-photon ionization regime J. Chem. Phys. 135, 214310 (2011) Interaction of nanosecond laser pulse with tetramethyl silane (Si(CH3)4) clusters: Generation of multiply charged silicon and carbon ions AIP Advances 1, 042164 (2011) Photoelectron spectroscopy of the molecular anions, Li3O and Na3O J. Chem. Phys. 135, 164308 (2011) Interpretation of the photoelectron spectra of superalkali species: Li3O and Li3O J. Chem. Phys. 135, 164307 (2011) Additional information on J. Chem. Phys. 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 4 ϫ 10 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 ͑␤-parameter͒ of the various electron bands and fragment ions. We conclude that multiphoton excitation of the highest occupied 22aЈ and 8aЉ CH 2 BrCl molecular orbitals of Br-character are both involved in the five-photon ionization, however, only excitation of the 22aЈ orbital appears to be ͑mostly͒ involved in the chirped control dynamics leading to enhanced fragmentation to CH 2 Cl + ͑X AЈ͒ +Br͑ 2 P 3/2 ͒. We propose that a wavepacket following or a time-delay resonance mechanism between the two-photon excited n x ͑Br, 22aЈ͒ → ͑2AЈ͒ repulsive surface and the three-photon near-resonant n x ͑Br, 22aЈ͒ → Rydberg͑AЈ͒ state of the neutral CH 2 BrCl molecule is responsible for the enhanced excitation of the n x ͑Br, 22aЈ͒ molecular orbital with up-chirped pulses. This leads to enhanced ionization to a configuration in the CH 2 BrCl + ͑X AЈ͒ continuum just above the dissociation limit of the CH 2 Cl + +Br͑ 2 P 3/2 ͒ channel, resulting in enhanced fragmentation.

Adaptive strong-field control of chemical dynamics guided by three-dimensional momentum imaging

“…Although past work has combined strong field pulse shaping with imaging [23][24][25] , and a few previous efforts [26][27][28][29] used VMI for adaptive control, they were exclusively based on raw images, in which the 3D momentum distributions were projected onto the two-dimensional (2D) detector plane. The azimuthal ambiguity inherent in such raw, 2D images complicated the extraction of a robust feedback signal 26 .…”

Incorporating real time velocity map image reconstruction into closed-loop coherent control

“…Thus, VMI provides angle-resolved kinetic energy release data for a given ion species at a rapid enough pace to be a candidate for use as closed-loop feedback. In fact, imaging studies of pulse-shape-dependent strong-field processes have appeared, [18][19][20][21] and some initial image-based feedback techniques have already been used by various groups. [21][22][23][24] In these initial efforts, the raw two-dimensional (2D) image was used as feedback.…”

Strong Field Molecular Ionization in the Impulsive Limit: Freezing Vibrations with Short Pulses

“…It can also be used to track excited state molecular dynamics and for molecular imaging [5][6][7][8][9][10]. A detailed understanding of the ionization dynamics is crucial for developing these frontier areas of molecular science [11].…”