Effects of aromatic molecules inside argon clusters on the formation of multiply charged atomic ions in moderately intense nanosecond laser fields

Abstract

We report the effect of two molecular species, fluorene (C13H10) and 1, 3, 5-trichlorobenzene (C6H3Cl3, 3ClB), embedded in atomic argon clusters, on the generation of multiply charged atomic ions (MCAI) in moderately intense nanosecond laser fields at 532 nm. The near resonant-enhancement of two photon absorption in the two aromatic species produces only a few low charge state (+2) atomic ions in a neat molecular cluster, but enclosure of the same cluster with layers of Ar can significantly increase the charge state of MCAI. The yields of singly charged atomic ions from the molecular species, such as H+, C+, and Cl+, are positively correlated to the number of molecules inside an Ar cluster, but the yields of the MCAI and Ar+ demonstrate opposite behaviors. A higher number of aromatic molecules is actually detrimental to the production of Ar+ and of MCAI. Results of exponential fittings of the yields of MCAI at different laser intensities reveal a systematic change for the exponent of Ar+: with increasing concentrations of 3ClB in Ar clusters, the exponent decreases and eventually reaches the same value as those of MCAI. These results are consistent with our previous hypothesis that the formation mechanism of MCAI may be different from that of singly charged species, and that the strong resonance of Ar3+ may play an important role in the overall energy absorption. Moreover, the effect of the molecular core seems to change the formation mechanism of Ar+ to that of MCAI.

Kinetic energy distributions of atomic ions from disintegration of argon containing nanoclusters in moderately intense nanosecond laser fields: Coulomb explosion or hydrodynamic expansion

“…agreement with our previous report on the intensity effect on the yield of the MCAI: 14,15 all MCAIs demonstrate similar dependence on the laser intensity within our experimental range, except for Ar + . Fig.…”

“…5: the KE distribution shows negligible dependence on the cluster size, but it is extremely sensitive to the addition of 3ClB. The former is in general agreement with our previous reports: [14][15][16][17] the TOF profiles of MCAI are similar and are only weakly dependent on the cluster size and laser intensity. This observation, however, is different from reports of strong laser fields, where larger clusters typically result in higher kinetic energies.…”

“…More work by Vasta's group ensued, 12,13 and average kinetic energies of both cations and electrons have been reported. Our group entered the foray and confirmed the observations of the pioneers, [14][15][16][17] and we expanded the previous study by measuring the size of the clusters, 18 by resolving an issue related to the space average effect in focused laser fields, 19 and by reporting on the detailed dependence of the charge distribution on the laser intensity and cluster composition. 14 Although we attempted to observe the kinetic energy distribution from photoelectrons using the velocity map imaging method, the image is circularly symmetric, and the radial intensity distribution demonstrates a monotonic decay peaked at zero kinetic energy.…”

“…For clusters of much smaller sizes, the size effect should be much more dramatic as reported in the literature. 27,28 In our previous work on the effect of organic dopants inside Ar clusters, we observed an increased overall ionization yield, 14 but the charge state distribution of the MCAI shifts to lower states. In neat Ar clusters as shown in Fig.…”

“…On the other hand, all ions passing through the aperture can land on the effective area of the microchannel plate (MCP) and be detected. Similar to our previous work, [14][15][16][17] the experiment is typically performed with active background subtraction: in one laser pulse, a background spectrum is taken when the pulse valve containing the Ar gas and molecular additives is off, and in the next pulse, a signal spectrum is taken when the pulse valve is on, and the difference between the signal and the background spectrum is used in this report as the net signal spectrum. This method can not only remove contributions from the residue gas in the chamber, but also assist with calibration of the mass spectrometer by relying on the assignment of the background mass spectrum.…”