Photoionization of helium atoms irradiated with intense vacuum ultraviolet free-electron laser light. Part I. Experimental study of multiphoton and single-photon processes

Journal Article · Mon Aug 15 00:00:00 EDT 2005 · Physical Review. A

DOI:https://doi.org/10.1103/PhysRevA.72.023409· OSTI ID:20718165

Laarmann, T; Guertler, P; Laasch, W; Schulz, J; Wabnitz, H; Moeller, T ^[1]; Castro, A R.B. de ^[2]

---

1. Hamburger Synchrotronstrahlungslabor HASYLAB at Deutsches Elektronen Synchrotron DESY, Notkestr. 85, 22607 Hamburg (Germany)
2. Laboratorio Nacional de Luz Sincrotron LNLS 13084-971 Campinas SP (Brazil) and Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas IFGW-UNICAMP 13083-970 Campinas SP (Brazil)

The interaction of He atoms with intense vacuum-ultraviolet light of a free-electron laser is investigated using time-of-flight mass spectroscopy and photoelectron spectroscopy. The atoms were irradiated with 100 fs pulses at 95 nm wavelength, which corresponds to {approx}13 eV photon energy. The ionization of He atoms is observed at a peak intensity of 10{sup 10}-10{sup 13} W/cm{sup 2}, which is due both to nonlinear multiphoton ionization with the fundamental wavelength and single-photon ionization with third harmonic radiation of the free-electron laser. The observation of two sharp photoelectron peaks in the kinetic energy spectra, that are separated by the photon energy, is in agreement with the numerical solution of the time-dependent Schroedinger equation. The calculation was done using the fully quantized field and a limited but representative set of basis states. The ionization rate dependence on the laser peak intensity indicates that: (a) The low-energy peak in the photoelectron spectra is mainly due to two-photon absorption of the fundamental, but (b) the high-energy peak at 15.4 eV is probably due to third harmonic FEL radiation. The theoretically predicted contribution from three-photon absorption of the fundamental is of about the same order of magnitude and could not be separated from the third harmonic background signal. Particularly, the photoelectron spectra and He{sup +} time-of-flight data give evidence that the intensity of third harmonic light is high enough to perform single-shot spectroscopy on gas phase samples.

• OSTI ID: 20718165
• Journal Information: Physical Review. A, Vol. 72, Issue 2; Other Information: DOI: 10.1103/PhysRevA.72.023409; (c) 2005 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 1050-2947
• Country of Publication: United States
• Language: English

Related Subjects

74 ATOMIC AND MOLECULAR PHYSICS
ABSORPTION
ATOMS
EV RANGE
FREE ELECTRON LASERS
HELIUM
HELIUM IONS
IRRADIATION
KINETIC ENERGY
MASS SPECTRA
MASS SPECTROSCOPY
MULTI-PHOTON PROCESSES
NONLINEAR PROBLEMS
NUMERICAL SOLUTION
PHOTOELECTRON SPECTROSCOPY
PHOTOIONIZATION
PHOTON-ATOM COLLISIONS
PHOTONS
PULSES
SCHROEDINGER EQUATION
TIME DEPENDENCE
TIME-OF-FLIGHT METHOD
ULTRAVIOLET RADIATION
VISIBLE RADIATION
WAVELENGTHS