Click on the link in the table below to view the abstract. If
you have online access to the journal in which the article appears, there
is a link to a PDF file containing the article in its abstract information.
Physical Review A (Atomic, Molecular, and Optical Physics) -- September 2001 -- Volume 64, Issue 4, pp. 043413
G. D. Gillen, M. A. Walker, and L. D. Van Woerkom
Department of Physics, The Ohio State University, 174 W. 18th Avenue, Columbus, Ohio 43210-1106(Received 31 May 2001; published 18 September 2001)
Using 120-fs, 800-nm Ti:Sapphire laser pulses, ionic yields of single and doubly charged magnesium ions were measured as a function of intensity and laser field ellipticity. A clear "nonsequential" enhancement of the doubly charged ion is observed for circularly polarized light in addition to linearly polarized light. Over the entire intensity range the double-ionization yield is considerably higher for linear polarization than it is for circular polarization.
©1999 The American Physical Society
Laser Physics -- September 2001 -- Volume 11, Number 9, 2001, pp. 982-988
L. D. Van Woerkom, M. J. Nandor, M. A. Walker, G. D. Gillen
Department of Physics, Ohio State University, 174 W. 18th Avenue, Columbus, Ohio, 43210-1106 USAH. G. Muller
FOM Institut voor Atoom-en Molekuulfysica, Amsterdam, 1098 SJ Netherlands(Received May 30, 2001)
The study of above-threshold ionization (ATI) is now over 20 years old and much has been learned
about the interaction of a single atomic electron with intense laser fields. The development of stable, high repetition rate ultrashort pulse laser systems has allowed experiments to probe intricate details of the ionization process using photoelectron and photoion spectroscopies. To date the overwhelming majority of data show that intense laser fields interact with only a single electron at a time to produce the well known features in electron kinetic energy spectra. We will present an overview of state-of-the-art experiments in ATI and show what can be learned about multielectron effects using careful measurements. Argon data showing the dominance of single electron physics at laser intensities near and below 1014 W/cm2 will be shown. In addition, new data taken with a model two-electron system (magnesium) continues the search for detailed a understanding of intense field interactions in complex atoms. In all cases the current status of what we learn using ATI will be shown and discussed.
Physical Review A (Atomic, Molecular, and Optical Physics) -- September 1999 -- Volume 60, Issue 3, pp. R1771-R1774
M. J. Nandor, M. A. Walker, and L. D. Van Woerkom
Physics Dept, Ohio State University, 174 West 18th Avenue, Columbus, OH
43210-1106 USA
H. G. Muller
FOM?Institut
voor Atoom-en Molekuulfysica, 1098 SJ Amsterdam, The Netherlands
(Received
1 March 1999; revised 23 April 1999)
Experimental photoelectron spectra, with high resolution in both kinetic energy and intensity, have been obtained and compared to a high-precision integration of the Schrödinger equation for photoelectron kinetic-energy yields in argon. We find exceptional quantitative agreement between data and calculation over a wide range of kinetic energies and peak laser intensities. In this paper we conclusively show that the single active electron model describes the physics of high-intensity photoionization to a high degree of accuracy. Furthermore, while multiple electrons may be ionized, multielectron effects appear to be completely absent from above-threshold-ionization photoelectron spectra.©1999 The American Physical Society
[PDF]
J. Phys. B: At. Mol. Opt. Phys. 31 No 20 (28 October 1998) 4617-4629
M J Nandor, M A Walker and L D Van Woerkom
The Ohio State University, 174 W 18th Ave, Columbus, OH 43210, USA
(Received 22 June 1998)Using a 110 fs, 800 nm Ti:sapphire laser, we have collected angle-resolved, high-resolution (<25 meV) photoelectron kinetic energy spectra of xenon for multiple intensities in the range10-. For various ATI peaks, we present angular distributions, which show complex and varied structures not previously reported or predicted. For each intensity, abrupt variations in the structures of the angular distributions coincide energetically with the onset of the plateau region in the photoelectron spectra.
©1998 IOP Publishing
[PDF]
Physical Review A (Atomic, Molecular, and Optical Physics) -- February
1998 -- Volume 57, Issue 2, pp. R709-R712.
P. Hansch, M. A. Walker, and L. D. Van Woerkom
Department of Physics, The Ohio State University, Columbus, Ohio 43210 USA(Received 31 July 1997)
We have measured and analyzed the clear emergence of ac-Stark-shifted multiphoton resonances with successive photon orders in xenon. The remarkable quality of our data illustrates the unambiguous evolution through parity-allowed resonances at the eight- and subsequent nine-photon levels. This marks a significant advance in showing that the transient resonance model is valid and strong optical-field ionization remains multiphoton in character at higher intensities. Furthermore, a simple Landau-Zener picture is sufficient to understand the basic principles.
©1998 The American Physical Society
[PDF]
Intensity-resolved multiphoton ionization: Circumventing spatial averaging
Physical Review A (Atomic, Molecular, and Optical Physics) -- February
1998 -- Volume 57, Issue 2, pp. R701-R704
M. A. Walker, P. Hansch, and L. D. Van Woerkom
Department of Physics, The Ohio State University, Columbus, Ohio 43210-1106 USA(Received 19 June 1997; revised 29 October 1997)
A persistent problem in traditional high-field photoionization experiments is the intensity averaging caused by the use of focused laser beams. We show that it is possible to deconvolve ionization probabilities directly from data for experiments in which the detection volume is restricted. The inversion technique is outlined and as an example we apply it to multiphoton multiple-ionization measurements of xenon.
©1998 The American Physical Society
[PDF]
Soft x rays from high-intensity laser interactions with solids
Physical Review E (Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics) ? July 1997 -- Volume 56, Issue 1, pp. 1273-1275
M. J. Nandor and L. D. Van Woerkom
Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA(Received 12 February 1997; revised 7 April 1997)
High-density plasmas are produced by illuminating solids with ultrashort laser pulses of 120 fs at 800-nm light, focused down to an intensity of approximately 8 x 1016 W/cm2. Soft x-rays are thus generated from the subsequent cooling of the plasma. Spectral data are presented for aluminum, Pyrex, and quartz targets. Emission from up to the lithiumlike species of all targets was observed.
©1997 The American Physical Society
[PDF]
Resonant hot-electron production in above-threshold ionization
Physical Review A (Atomic, Molecular, and Optical Physics) -- April
1997 -- Volume 55, Issue 4, pp. R2535-R2538
P. Hansch, M. A. Walker, and L. D. Van Woerkom
Department of Physics, The Ohio State University, Columbus, Ohio 43210(Received 18 November 1996)
We have observed surprising detailed structure in the photoelectron kinetic-energy spectrum of xenon under high-intensity short pulse conditions. We show that most of the photoelectrons with kinetic energies from 0?50 eV result from resonant processes at intensities up to 1.9 x 1014 W/cm2. In particular, we find that the high-energy photoelectron structure in above-threshold ionization is actually composed of narrow individual peaks whose energy positions do not shift with intensity. The amplitudes of the structures change rapidly with intensity and turn on at different specific intensities. While those structures appear to be due to resonances, they cannot be attributed to traditional Rydberg transient resonances.
©1997 The American Physical Society
[PDF]
Multiphoton ionization with precise intensity control
Optics & Photonics News -- December 1996 -- Volume 7, Issue 12, pp. 23-24
Peter Hansch, Mark A. Walker, and Linn D. Van Woerkom
The Ohio State University, Columbus, Ohio
Spatially dependent multiphoton multiple ionization
Physical Review A (Atomic, Molecular, and Optical Physics) -- October 1996 -- Volume 54, Issue 4, pp. R2559-R2562
P. Hansch, M. A. Walker, and L. D. Van Woerkom
Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA(Received 5 June 1996)
We have measured spatially dependent multiple ionization of xenon in an intense ultrashort pulse laser field. The ion yields for various charge states are dramatically different due to selecting specific locations within a Gaussian laser focus. The data show a clear enhancement of higher charge states and the double ion exceeds the single ion yield at certain locations. A model based on space-dependent volume scaling for different charge states is presented.
©1996 The American Physical Society.
[PDF]
High-precision intensity-selective observation of multiphoton ionization: a new method of photoelectron spectroscopy
Optics Letters -- August 15, 1996 -- Volume 21, Issue 16, pp. 1286-1288
Peter Hansch and Linn D. Van Woerkom
Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA(Received March 18, 1996)
We have developed a novel method of time-of-flight (TOF) photoelectron spectroscopy that permits observation of multiphoton ionizations with extremely high precision, especially for low-probability events. By scanning the laser-produced ionization region across a pinhole we can select specific laser peak intensities. The volumes occupied by low intensities rise rapidly compared with traditional straight TOF spectroscopy, resulting in high signal gains. This technique presents a new way of observing fundamental laser?matter interactions.
©1996 Optical Society of America.
[PDF]
Soft-x-ray imaging from an ultrashort-pulse laser-produced plasma using a multilayer coated optic
Journal of the Optical Society of America B: Optical Physics -- February
1996 -- Volume 13, Issue 2, pp. 454-458
J. R. Norby and L. D. Van Woerkom
Department of Physics, The Ohio State University, 174 W. 18th Avenue, Columbus, Ohio 43210-1106, USA(Received March 3, 1995; revised May 19, 1995)
Measurements are presented of soft-x-ray images from a plasma produced by a high-intensity ultrashort-pulse laser. For the intensity range of 1015?1016 W/cm2 the soft-x-ray source appears to follow the spatial profile of the driving laser. A curved multilayer coated optic is used to collect 13.5-nm light and form a magnified image of the plasma. Knife-edge scans have been performed in the image plane and show a geometrically limited spot size of 280 µm.
©1996 Optical Society of America.
[PDF]
An ellipsoidal mirror time-of-flight photoelectron energy analyzer
Review of Scientific Instruments -- December 1995 -- Volume 66, Issue
12, pp. 5512-5515
P. Hansch, J. R. Norby, S. H. Evans, and L. D. Van Woerkom
Department of Physics, The Ohio State University, Columbus, Ohio 43210(Received 12 June 1995; accepted 30 August 1995)
A novel time-of-flight photoelectron energy analyzer that uses an electrostatic field to reflect and focus electrons has been designed and built. By means of high quality ellipsoidal grids, photoelectrons can be collected at high efficiency and focused onto a detector. The problems of space charge and extended electron sources are addressed in the context of the ellipsoidal mirror analyzer. This versatile spectrometer analyzes single and multiphoton ionization processes and operates over a wide energy range. The experimental data demonstrate the high collection efficiency. We have obtained good spectra at target densities of less than 10-7 Torr. For six-photon ionization of xenon a gain of 36 has been achieved.
©1995 American Institute of Physics.
[PDF]