R.K. Vatsa
Bhabha Atomic Research Centre, India
Title: Photoionization of xenon clusters: Does the increasing laser intensity help in multiple ionization ?
Biography
Biography: R.K. Vatsa
Abstract
In the photoionization of CH3I clustersby laser pulses with an intensity of 109 W/cm2 (ï¬= 532 nm), formation of energetic, multiply charged atomic ions was reported by us[1].The C+3 ions,detected in TOF-MS,require a total energy of about 100 eV for formation (starting from CH3I) and in addition they had kinetic energy of ~ 300 eV. Increasing the size of the cluster resulted in enhancement of charge state as well as kinetic energy of the ions. Similar observations were made for other molecular as well as atomic cluster systems. For a given average cluster size, a further enhancement in charge state can be achieved by increasing the intensity of ionising laser. In the case of atomic clusters, we have previously reportedformation of multiply charged xenon ions up to Xe+10 state using 8 ns, 532 nm laser pulses with anintensity of~ 5x 109 W/cm2 [2]. We now report our results on ionization of xenon clusters (of similar average size) using 30 ps pulses with an intensity of ~1012 W/cm2. The ionization wavelengths(266 and 532 nm) were selected to probe if wavelength plays any role in multiple ionization. For the case of high intensity ionization at 266 nm, a strong mass peak for Xe+, followed by a mediumintensity peak for Xe+2 and a small peak for Xe+3 were observed. For these three charged states, theabundance of given state follows Boltzmann distribution. For photoionization at 532 nm, charge state up to Xe+10 could be observed with Xe+9 being the most abundant ionic species.For the lower charge states (+2 to +7), a non- Boltzmann distribution of ionic states was observed. The main cause of such a behavior is absorption of high amount of energy from the laser pulse leading to extended depletion of lower charge state which act as feedfor higher charged states.Theseresults at 1012 W/cm2 intensity once again confirm previous observations that the total energy absorbed by the cluster is much higher at longer wavelength. During the presentation, results obtained on photoionization of Xenon clusters at 1012 W/cm2 will be compared with earlier results obtained using 109 W/cm2. The apparent absence of intensity effects on overall ionization state of xenon will be discussed taking into considerationinitial multiphoton ionization by the laser pulse and subsequent electron induced ionization.