Nanosecond Laser Ionization Iodine Methane And Benzene High Ion Clusters Are Generated

The interaction of 532 nm nanosecond laser pulse with methyl iodide and benzeneclusters were studied using a homemade time-of-flight mass spectrometer with the laserintensities in the range of 10⁹¹0¹0W/cm². The mechanism of laser-cluster interaction wasexplained with“three-steps”model.The influences of cluster size and laser intensity on the production of multiply chargedions were studied systematically, when methyl iodide and benzene clusters were exposed tolaser pulse. The cluster size has an effect on the charge state distribution of multiply chargedions, large cluster helped to produce higher multiply charged ions. The influence of laserintensity is not on the charge state distribution of multiply charged ions, but on the intensities ofmultiply charged ions. With the raising of laser intensity, the multiply charged ions intensitiesincreased rapid first and then went to saturation.The electron energy distribution was measured by the retarding potential method. To getthe actual electron energy distribution, it should considered the effects of geometric parametersof the TOF mass spectrometer, extraction potential and electron initial kinetic energy on theacceptance efficiency of electrons. The energy of electron increased rapid first and then went tosaturation as the multiply charged ions when laser intensity rising.The laser polarization effect on the methyl iodide clusters was investigated. Changing thedirection of polarization, the produced multiply charged ions were isotropic, while thepolarization state affect the intensities of multiply charged ions, and it was found that circularlypolarization is benefit for producing higher charged ions. The influence of laser polarization onmethyl iodide molecule was also studied. The CH₃⁺ peak split into four peaks in multiphotonionization and dissociation （MPID）, when laser pulse is parallel linear polarization. And basedon the spectrum, the mechanism of MPID is multiphoton dissociation and ionization at A bandand parent-ion ladder mechanism. The influence of laser intensity on the full width at halfmaximum W_1/2of I⁺ ion peak and the peak position shifts of CH_q⁺（q=0-3） ions wereinvestigated in details. The W_1/2of I⁺ ion is proportional to I_I+^0.5, while the peak shifts of CH_q⁺（q=0-3） were proportional to I_I+, which can be well explained by space-charge effect betweenions of I⁺and CH_q⁺（q=0-3） in the acceleration region of the time of flight mass spectrometer. Benzene, cyclohexene and cyclohexane clusters are chosen to study the multiphotonionization （MPI） efficiency effect on the relative intensity of multiply charged ions, as theypossess similar molecular structure and the same element constitution. Higher MPI efficiency ofmolecules can result more than one molecules are ionized at the edge of laser pulse, theCoulomb repeller forces between adjacent ions would result early splitting of clusters to smallersize, which will prevents the production of the highly charged ions.The mechanism of laser-cluster interaction was described by the model of“multiphotonionization （MPI） triggered- inverse bremsstrahlung （IB） heating-electron impact （EI） ionized”in detail. Combined with screening effect, it can illustrate the production of multiply chargedions at relatively low electron energy. And the experimental results of cluster size, laserintensity, laser polarization and MPI efficiency also explained qualitatively with it.