Spectral Measurements of Ionization and Dissociation in Intense Laser Pulses

Ionization and dissociation of molecules under ultrafast intense laser radiation could be used to study the structure of molecules and their dynamical response. Molecules or atoms are subjected to femtosecond laser pulses having different intensities and polarization orientations to be ionized or be dissociated. The work presented in this report can be separated into two main parts. The first part focuses on the interaction between femtosecond laser pulses and molecules, in particular, the dissociation of acetonitrile (CH₃CN). We study the production of positively charged fragment ions of acetonitrile (CH₃CN) in intense laser radiation having 800 nm wavelength, 50 fs pulse duration with a reflectron time-of-flight (TOF) ion mass spectrometer. The formation of positively charged fragments ejected from acetonitrile at different laser intensities, between 4.4×10¹³ W/cm² and 3.3×10¹⁴ W/cm², and with various polarization orientations was studied. Using the results on intensity dependent ion yields, appearance energies of fragments were determined and these results were compared with quantum chemical calculations, which were done with GAMESS. When the laser polarization direction was parallel or perpendicular to the TOF axis, the angular distributions of C+ and C²⁺ ions showed maxima in both cases, while other fragments showed yield maxima, when the laser radiation was parallel to this axis. In addition, kinetic energy releases (KER) of H⁺ ions ejected from acetonitrile were experimentally measured using the linear part of the TOF ion mass spectrometer. Angular distributions and intensity dependences of H+ ions showed that three different photoionization mechanisms exist. Furthermore, KER of H⁺, H₂⁺ and H₃⁺ ejected from hydrocarbon molecule ethane (C₂H₆) were investigated experimentally by operating the TOFMS in the linear mode. Laser intensity and polarization dependences of these hydrogen ions were also studied in detail. In the second section of this work, the interaction of laser radiation with sodium atoms is discussed. Photo-electron energy spectra for sodium atom is measured experimentally and it is found that electrons are mainly ejected from 5p, 6p and 7p states. Photo-electron angular distributions (PAD) from 5p state for zero, first, second and third order ATI is also determined. We observed that PADs of electrons depend on the number of photons absorbed by the electrons