Internal state populations and velocity distributions of monatomic species ejected after the 1064 nm laser irradiation of barium

ROSSA, M., RINALDI, C.A., FERRERO, J.C.

JOURNAL OF APPLIED PHYSICS

AIP

Año: 2009 vol. 105 p. 63306 - 63311

0021-8979

The plumes accompanying 1064 nm nanosecond pulsed laser ablation of barium in vacuum at threemoderate incident laser fluences in the range of 5.3–10.8 J /cm2 have been studied using bothwavelength and time resolved optical emission spectroscopy and time-of-flight laser-inducedfluorescence. Neutral atoms and both singly and doubly charged monatomic cations in excited statesup to near the corresponding ionization limits are identified in the optical emission spectra. Thepopulation distributions of low-lying

1.41 eV “dark” states of Ba atoms measured bylaser-induced fluorescence revel that the metastable 3DJ and 1D2 abundances in the plume are higherthan predictions based on assuming a Boltzmann distribution. The 3DJ and 1D2 populations are seen,respectively, to decrease slightly and nearly no vary with raising fluence, which contrasts with theincreasing trend that is observed in the ground-state Ba
1S0 population. At all fluences, thetime-of-flight distributions of the whole dark states and of various of the emitting levels are bimodaland well described by Maxwell–Boltzmann and shifted Maxwell–Boltzmann velocity functions,respectively, with different average translational temperatures T for each state. The T values forthe dark states are insensitive to the fluence, while for all emitting species marked variations of Twith fluence are found. These observations have been rationalized in terms of material ejection fromthe target being dominated by a phase explosion mechanism, which is the main contributor to theBa
1S0 population. Thermionic emission from the target surface can also release initial densities offree electrons and cations which, at the prevailing irradiances, will arguably interact with theincident laser radiation by inverse bremsstrahlung, leading to further excitation and ionization of thevarious plume species. Such a heating mechanism ensures that the energy injected to the plume willalter the propagation velocities of the primary inverse bremsstrahlung absorbers, i.e., cations, to amajor extent than those of neutral atoms with increasing fluence. Electron-ion recombinationoccurring early in the plume expansion can lead to the generation of both neutral and ionic speciesin a manifold of long-lived Rydberg states, from which a radiative cascade will likely ensue. Thedistinct fluence dependences of the Ba
3DJ and Ba
1D2 populations and velocity distributionsshow up the major complexity that distinguishes their populating mechanisms with respect to theremaining species. © 2009 American Institute of Physics. DOI: 10.1063/1.3089214