Multiphoton ionization in intense ultraviolet laser fields

Abstract

The mechanism of collision-free multiphoton ionization of rare gases irradiated with ultraviolet radiation at an intensity of up to -10 16 W/cm 2 at 248 ran and a pulse length of ~ 0.5 ps and -lO 1^ W/cm 2 at 193 nm and -5ps, respectively, has been studied by observing the electron energy spectra and ion charge state threshold intensities. The formation of multiply, charged ions by a sequential process of ionization has been directly detected in the electron energy spectra in the form of a characteristic pattern of interwoven above threshold ionization (ATI) ladder line series. The threshold intensities for ion production have been compared with the Keldysh model and were found to be in good agreement for light ions (Ne) and consistently lower for heavier ions (Xe), scaling with the atonic number. These measurements, together with estimates of the ion population dynamics during the rise of the laser pulse can be reasonably understood on the basis of a single electron picture.

Physical Processes at High Field Strengths

“…A summary of the overall results for the rare gases[ll, 12,14] is contained in which are in marked contrast to the two lighter materials. A change in the behavior, at least in terms of the cross section for jo^srgy transfer leading to ionization, appears J» occur between Ne and Ar."…”

“…Photoelectron energy spectra[4, [12][13][14] for process (1), recorded under collision-free conditions, provide information complementing that given by the measurement of ion charge state distributions. Commonly, the observed electron spectra consist of a characteristic pattern of interwaven above threshold ionization (ATI) ladder line series.…”

Ultrafast Dynamics in Cluster Systems

“…Metastable evaporation of the solvating molecules can take the stable cluster to degrees of aggregation below critical size and subsequently lead to fission. Findings concerning the phenomenon of Coulomb explosion come from a plethora of investigations of the interaction involving intense laser fields with atoms, − molecules, − and clusters. − …”

An Ultrafast Glimpse of Cluster Solvation Effects on Reaction Dynamics

“…However, in general it has been difficult to investigate the process due to its fleeting nature. Recent observations ,− have shed considerable light on our understanding of this phenomenon, with new findings coming from investigations of the interaction of intense laser field with atoms, − molecules, and clusters. − Specifically, during the course of experiments to study reaction dynamics within clusters, Castleman and co-workers observed an unexpected phenomenon in which femtosecond laser pulses instantaneously created clusters with a high charge state at surprisingly low fluxes of light, enabling a new and detailed look at the actual Coulomb explosion process. We first observed this with clusters of ammonia; results we reported were at just about the same time as work on high charge states of xenon reported by Rhodes and co-workers …”