Measurement of intensity-dependent rates of above-threshold ionization (ATI) of atomic hydrogen at 248 nm

Technical Report · Mon Apr 01 04:00:00 EST 1991

DOI:https://doi.org/10.2172/5847431· OSTI ID:5847431

Nichols, T D

Measured rates of multiphoton ionization (MPI) from the ground state of atomic hydrogen by a linearly polarized, subpicosecond KrF laser pulse at 248 nm wavelength are compared to predictions of lowest-order perturbation theory, Floquet theory, and Keldysh-Faisal-Reiss (KFR) theory with and without Coulomb correction for peak irradiance of 3 {times} 10{sup 12}W/cm{sup 2} to 2 {times} 10{sup 14}W/cm{sup 2}. The Coulomb-corrected Keldysh model falls closest to the measured rates, the others being much higher or much lower. At 5 {times} 10{sup 13}W/cm{sup 2}, the number of ATI electrons decreased by a factor of approximately 40 with each additional photon absorbed. ATI of the molecular hydrogen background and of atoms from photodissociation of the molecules were also observed. The experiment employed a crossed-beam technique at ultrahigh vacuum with an rf-discharge atomic hydrogen source and a magnetic-bottle type electron time-of-flight spectrometer to count the electrons in the different ATI channels separately. The apparatus was calibrated to allow comparison of absolute as well as relative ionization rates to the theoretical predictions. This calibration involved measuring the distribution of irradiance in a focal volume that moved randomly and changed its size from time to time. A data collection system under computer control divided the time-of-flight spectra into bins according to the energy of each laser pulse. This is the first measurement of absolute rates of ATI in atomic hydrogen, and the first measurement of absolute test of MPI in atomic hydrogen without a large factor to account for multiple modes in the laser field. As such, the results of this work are important to the development of ATI theories, which presently differ by orders of magnitude in their prediction of the ionization rates. They are also important to recent calculations of temperatures in laser-heated plasmas, many of which incorporate KFR theory.

• Research Organization: Los Alamos National Lab., NM (USA)
• Sponsoring Organization: DOE; USDOE, Washington, DC (USA)
• DOE Contract Number: W-7405-ENG-36
• OSTI ID: 5847431
• Report Number(s): LA-12060-T; ON: DE91012091
• Country of Publication: United States
• Language: English

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74 ATOMIC AND MOLECULAR PHYSICS
CORRECTIONS
COULOMB CORRECTION
DATA
DIFFERENTIAL EQUATIONS
DISSOCIATION
DYNAMIC MASS SPECTROMETERS
ELECTRON MULTIPLIERS
ELECTRON TUBES
ELEMENTS
EQUATIONS
EXCIMER LASERS
EXPERIMENTAL DATA
GAS LASERS
HYDROGEN
INFORMATION
IONIZATION
KRYPTON FLUORIDE LASERS
LASER-PRODUCED PLASMA
LASERS
MASS SPECTROMETERS
MEASURING INSTRUMENTS
MICROCHANNEL ELECTRON MULTIPLIERS
MULTI-PHOTON PROCESSES
NONMETALS
NUMERICAL DATA
PARTIAL DIFFERENTIAL EQUATIONS
PERTURBATION THEORY
PHOTOIONIZATION
PLASMA
SCHROEDINGER EQUATION
SPECTROMETERS
TIME-OF-FLIGHT MASS SPECTROMETERS
TIME-OF-FLIGHT SPECTROMETERS
VACUUM SYSTEMS
WAVE EQUATIONS