Investigation of Closed-Loop Control of Polyatomic Molecules

Abstract

ABSTRACT Investigation of Closed-Loop Control of Polyatomic Molecules Lalinda N. Palliyaguru Doctor of Philosophy Temple University, 2009 Doctoral Advisory Committee Chair: Prof. Robert J. Levis Experimental results that are based on mass spectral data, in this thesis includes ionization/fragmentation of DMMP and CH2BrI in strong femtosecond laser fields, coherent control using closed-loop optimization method for bond dissociation in both DMMP and CH2BrI, as well as application of closed-loop control as a detection method for air borne organophosphates such as DMMP. Investigation of mass spectral data as a function of laser field intensity, wavelength, and polarization provided details of ionization/fragmentation of DMMP and CH2BrI. Furthermore, quantum mechanical calculations carried out with Gaussian 03 Computational package were used to analyze the mass spectral data. The analysis of the peak splitting pattern of the molecular fragments reveals that Coulomb explosion as well as the photodissociation of CH2BrI produces the TOF mass spectral product distribution. We observed a considerable enhancement of photodissociation of CH2BrI in strong elliptically polarized femtosecond laser fields. The enhanced photodissociation of CH2BrI in circularly polarized fields could be due to the interaction with both parallel and perpendicular electronic transitions that cause the efficient dissociation of C-I and C-Br bonds. We also demonstrate the selective dissociation of C-I and C-Br bonds using closed-loop optimal control method. The fragmentation processes of polyatomic molecules induced by an intense laser field exhibit sensitive dependence on the laser characteristics such as intensity, pulse duration, wavelength, and shape of the temporal pulse envelope. Adaptive laser pulse shaping can control the fragmentation of methyl/methoxy groups in dimethyl methylphosphonate (DMMP), a simulant for nerve agent Sarin. The exploitation of the sensitivity of molecular fragmentation to laser pulse shapes represents a new way to discriminate molecular identity. Here we have shown manipulation of the branching M-(OCH3)+/M-(CH3)+, M-2(CH3)+/M-(CH3)+ and M-(OCH3)+/M-2(CH3)+ fragment ion ratios for DMMP in the presence of complex background in the extraction region of TOF spectrometer using tailored femtosecond laser pulses. We also investigated the TOF mass spectra of DMMP as a function of laser field parameters, such as intensity, wavelength, polarization, and linear chirp. We positively identified that non-sequential ionization plays a role in the fragmentation process of DMMP.