Laser Techniques for State-Selected and State-to-State Chemistry IV, 1998
ABSTRACT A comparative study of the hexamethyldisiloxane (HMDSO) molecule photofragmentation indu... more ABSTRACT A comparative study of the hexamethyldisiloxane (HMDSO) molecule photofragmentation induced by laser multiphotonic (MPI) and synchrotron monophotonic (SR) processes is presented. The HMDSO sample was effusively expanded into the vacuum chamber and fragmented by either laser or synchrotron irradiation. The resulting ions were detected by a time-of- flight spectrometer using both electron-ion and ion-ion coincidence techniques. The parent ion has not been observed in both processes suggesting its instability. MPI induced fragmentation is characterized by a high ionic yield (IY) in the lighter fragments region. The MPI atomization is severe generating ions like C+ and Si+ that are absent from the SR spectra. The doubly-charged ions SiOSi(CH3)2++ and SiOSi(CH3)4++ are observed in the SR spectra. SR and MPI fragmentation have a common main route: the methyl group ejection yielding m/q equals 147,148,149 and m/q equals 15. The first presents a higher IY suggesting that the positive charge stays preferentially with the more massive fragment. Through MPI there is another route: the Si-O bond breakage yielding m/q equals 73,74,75 and m/q equals 89 (Si(CH3)3+ and OSi(CH3)3+. The metastable doubly charged ions were SiOSiC1,2,3,6Hn++ and OSiC3Hn++ in the SR case; and a wider fragment mass range was observed through MPI.
A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the d... more A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the dissociative double ionization of benzene by UV synchrotron radiation. The threshold energy for the main dissociative processes, leading to CH 3 + + C 5 H 3 + , C 2 H 3 + + C 4 H 3 + and C 2 H 2 + + C 4 H 4 + ion pairs were characterized by exploiting a photoelectron-photoion-photoion-coincidence technique, giving 27.8 AE 0.1, 29.5 AE 0.1, and 30.2 AE 0.1 eV, respectively. The first reaction also proceeds via the formation of a metastable C 6 H 6 2+ dication. The translational kinetic energy of the ionic products was evaluated by measuring the position of ions arriving to the detector. Theoretical calculations of the energy and structure of dissociation product ions were performed to provide further information on the dynamics of the charge separation reactions following the photoionization event.
A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the d... more A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the dissociative double ionization of benzene by UV synchrotron radiation. The threshold energy for the main dissociative processes, leading to CH 3 + + C 5 H 3 + , C 2 H 3 + + C 4 H 3 + and C 2 H 2 + + C 4 H 4 + ion pairs were characterized by exploiting a photoelectron-photoion-photoion-coincidence technique, giving 27.8 AE 0.1, 29.5 AE 0.1, and 30.2 AE 0.1 eV, respectively. The first reaction also proceeds via the formation of a metastable C 6 H 6 2+ dication. The translational kinetic energy of the ionic products was evaluated by measuring the position of ions arriving to the detector. Theoretical calculations of the energy and structure of dissociation product ions were performed to provide further information on the dynamics of the charge separation reactions following the photoionization event.
We study the diffision of water in weakly-hydrated samples of the smectite clay Na-fluorohectorit... more We study the diffision of water in weakly-hydrated samples of the smectite clay Na-fluorohectorite, The quasi one-dimensional samples are dry compounds of nano-layered particles consisting of -80 silicate platelets. Water diffises into a sample through the mesoporosity in between the particles, and can subsequently intercalate into the adjacent particles. The samples are placed under controlled temperature. They are initially under low humidity conditions, with all particles in a 1 WZ, intercalation state, We then impose a high humidity at one sample end, triggering water penetration along the sample length. We monitor the progression of the humidity front by monitoring the intercalation state of the particles in space and time. This is done by determining the characteristic spacing of the nano-layered particles in situ, from synchrotron wide-angle X-ray scattering measurements. The spatial width of the intercalation front is observed to be smaller than 2mm, while its velocity decreases with time, as expected from a diffusion process,
A new method for the determination of dynamical features of the molecular dication dissociation p... more A new method for the determination of dynamical features of the molecular dication dissociation processes, following the single photon double ionization, investigated by time-of-flight mass spectrometry technique has been developed. The method is based on an extension of the generalized simulated annealing statistical methodology, previously applied in other fields. Here it is described and applied, as an example, to the case of the dissociation of the CO22+ dication giving CO+ +O+ ion fragments. The results are consistent with previous determination of the metastable lifetime of the dication, but the analysis also provides additional information about the dynamics of the reaction.
Laser Techniques for State-Selected and State-to-State Chemistry IV, 1998
ABSTRACT A comparative study of the hexamethyldisiloxane (HMDSO) molecule photofragmentation indu... more ABSTRACT A comparative study of the hexamethyldisiloxane (HMDSO) molecule photofragmentation induced by laser multiphotonic (MPI) and synchrotron monophotonic (SR) processes is presented. The HMDSO sample was effusively expanded into the vacuum chamber and fragmented by either laser or synchrotron irradiation. The resulting ions were detected by a time-of- flight spectrometer using both electron-ion and ion-ion coincidence techniques. The parent ion has not been observed in both processes suggesting its instability. MPI induced fragmentation is characterized by a high ionic yield (IY) in the lighter fragments region. The MPI atomization is severe generating ions like C+ and Si+ that are absent from the SR spectra. The doubly-charged ions SiOSi(CH3)2++ and SiOSi(CH3)4++ are observed in the SR spectra. SR and MPI fragmentation have a common main route: the methyl group ejection yielding m/q equals 147,148,149 and m/q equals 15. The first presents a higher IY suggesting that the positive charge stays preferentially with the more massive fragment. Through MPI there is another route: the Si-O bond breakage yielding m/q equals 73,74,75 and m/q equals 89 (Si(CH3)3+ and OSi(CH3)3+. The metastable doubly charged ions were SiOSiC1,2,3,6Hn++ and OSiC3Hn++ in the SR case; and a wider fragment mass range was observed through MPI.
A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the d... more A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the dissociative double ionization of benzene by UV synchrotron radiation. The threshold energy for the main dissociative processes, leading to CH 3 + + C 5 H 3 + , C 2 H 3 + + C 4 H 3 + and C 2 H 2 + + C 4 H 4 + ion pairs were characterized by exploiting a photoelectron-photoion-photoion-coincidence technique, giving 27.8 AE 0.1, 29.5 AE 0.1, and 30.2 AE 0.1 eV, respectively. The first reaction also proceeds via the formation of a metastable C 6 H 6 2+ dication. The translational kinetic energy of the ionic products was evaluated by measuring the position of ions arriving to the detector. Theoretical calculations of the energy and structure of dissociation product ions were performed to provide further information on the dynamics of the charge separation reactions following the photoionization event.
A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the d... more A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the dissociative double ionization of benzene by UV synchrotron radiation. The threshold energy for the main dissociative processes, leading to CH 3 + + C 5 H 3 + , C 2 H 3 + + C 4 H 3 + and C 2 H 2 + + C 4 H 4 + ion pairs were characterized by exploiting a photoelectron-photoion-photoion-coincidence technique, giving 27.8 AE 0.1, 29.5 AE 0.1, and 30.2 AE 0.1 eV, respectively. The first reaction also proceeds via the formation of a metastable C 6 H 6 2+ dication. The translational kinetic energy of the ionic products was evaluated by measuring the position of ions arriving to the detector. Theoretical calculations of the energy and structure of dissociation product ions were performed to provide further information on the dynamics of the charge separation reactions following the photoionization event.
We study the diffision of water in weakly-hydrated samples of the smectite clay Na-fluorohectorit... more We study the diffision of water in weakly-hydrated samples of the smectite clay Na-fluorohectorite, The quasi one-dimensional samples are dry compounds of nano-layered particles consisting of -80 silicate platelets. Water diffises into a sample through the mesoporosity in between the particles, and can subsequently intercalate into the adjacent particles. The samples are placed under controlled temperature. They are initially under low humidity conditions, with all particles in a 1 WZ, intercalation state, We then impose a high humidity at one sample end, triggering water penetration along the sample length. We monitor the progression of the humidity front by monitoring the intercalation state of the particles in space and time. This is done by determining the characteristic spacing of the nano-layered particles in situ, from synchrotron wide-angle X-ray scattering measurements. The spatial width of the intercalation front is observed to be smaller than 2mm, while its velocity decreases with time, as expected from a diffusion process,
A new method for the determination of dynamical features of the molecular dication dissociation p... more A new method for the determination of dynamical features of the molecular dication dissociation processes, following the single photon double ionization, investigated by time-of-flight mass spectrometry technique has been developed. The method is based on an extension of the generalized simulated annealing statistical methodology, previously applied in other fields. Here it is described and applied, as an example, to the case of the dissociation of the CO22+ dication giving CO+ +O+ ion fragments. The results are consistent with previous determination of the metastable lifetime of the dication, but the analysis also provides additional information about the dynamics of the reaction.
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Papers by M. Mundim