Intensity dependence of multiply charged atomic ions from argon clusters in moderate nanosecond laser fields

Abstract

We report the laser intensity dependence of multiply charged atomic ions (MCAIs) Arn+ with 2 ≤ n ≤ 8 from argon clusters in focused nanosecond laser fields at 532 nm. The laser field, in the range of 1011–1012 W/cm2, is insufficient for optical field ionization but is adequate for multiphoton ionization. The MCAI sections of the mass spectra for clusters containing 3700 and 26 000 atoms are dominated by Arn+ with 7 ≤ n ≤ 9, extending to Ar14+. While the distributions of the MCAIs remain largely constant throughout the intensity range of the laser, the abundance of Ar+ relative to the abundances of the MCAIs increases dramatically with increasing laser intensity. Consequently, exponential fittings of the yields result in a larger exponent for Ar+ than for MCAIs, and the exponents of MCAIs with 2 ≤ n ≤ 8 are similar, with only slight variations for different charge states. The width of the arrival time and, hence, the corresponding kinetic energy of Ar+ also increases with increasing laser intensities, while the width of the arrival time of MCAIs remains constant throughout the range of measurements. These results call for more detailed theoretical investigations in this regime of laser–matter interactions.

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

“…The laser intensity is 3.4 × 10 12 W cm −2 , but the KE distribution is almost independent of the laser intensity: as long as MCAI can be observed, similar TOF spectra with similar TOF profiles for the MCAI can be observed, at least in the intensity range between 10 11 W cm −2 and 10 13 W cm −2 . This result is in 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 + .…”

“…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.…”

“…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.…”

“…In one of our previous publications, 15 we predicted that the KE distribution of Ar + should be very different from those of MCAI, based on the dramatically different intensity dependence of the ion yields of Ar + and MCAI. While the KE of MCAI is on the order of tens to hundreds of eV, the KE of Ar + is less than 4 eV regardless of the fitting method for the TOF profiles.…”

“…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.…”