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

Abstract

The plumes accompanying 1064 nm nanosecond pulsed laser ablation of barium in vacuum at three moderate incident laser fluences in the range of 5.3-10.8 J / cm 2 have been studied using both wavelength and time resolved optical emission spectroscopy and time-of-flight laser-induced fluorescence. Neutral atoms and both singly and doubly charged monatomic cations in excited states up to near the corresponding ionization limits are identified in the optical emission spectra. The population distributions of low-lying ͑Յ1.41 eV͒ "dark" states of Ba atoms measured by laser-induced fluorescence revel that the metastable 3 D J and 1 D 2 abundances in the plume are higher than predictions based on assuming a Boltzmann distribution. The 3 D J and 1 D 2 populations are seen, respectively, to decrease slightly and nearly no vary with raising fluence, which contrasts with the increasing trend that is observed in the ground-state Ba͑ 1 S 0 ͒ population. At all fluences, the time-of-flight distributions of the whole dark states and of various of the emitting levels are bimodal and well described by Maxwell-Boltzmann and shifted Maxwell-Boltzmann velocity functions, respectively, with different average translational temperatures ͗T͘ for each state. The ͗T͘ values for the dark states are insensitive to the fluence, while for all emitting species marked variations of ͗T͘ with fluence are found. These observations have been rationalized in terms of material ejection from the target being dominated by a phase explosion mechanism, which is the main contributor to the Ba͑ 1 S 0 ͒ population. Thermionic emission from the target surface can also release initial densities of free electrons and cations which, at the prevailing irradiances, will arguably interact with the incident laser radiation by inverse bremsstrahlung, leading to further excitation and ionization of the various plume species. Such a heating mechanism ensures that the energy injected to the plume will alter the propagation velocities of the primary inverse bremsstrahlung absorbers, i.e., cations, to a major extent than those of neutral atoms with increasing fluence. Electron-ion recombination occurring early in the plume expansion can lead to the generation of both neutral and ionic species in a manifold of long-lived Rydberg states, from which a radiative cascade will likely ensue. The distinct fluence dependences of the Ba͑ 3 D J ͒ and Ba͑ 1 D 2 ͒ populations and velocity distributions show up the major complexity that distinguishes their populating mechanisms with respect to the remaining species.

Time-resolved FTIR emission spectroscopy of Cu in the 1800–3800 cm−1region: transitions involving f and g states and oscillator strengths

“…This allows us to record the the time profiles of the observed Cu lines, i. e. their emission intensities I(t) as functions of the delay time, t. Such information can be helpful for diagnostics of the electronic state populations of neutral atoms in plumes formed by pulsed laser ablation Furusawa et al (2004); however there are few reports of investigations on such population dynamics Rossa et al (2009). The temporal dynamics of several lines is shown in Figure 2.…”

Chemiluminescence from the Ba(P3)+N2O→BaO(A Σ1+)+N2 reaction: Collision energy effects on the product rotational alignment and energy release

“…While other components of the 3 P level have not been investigated, their velocity distributions should be very similar to that for 3 P 1 , as it was found previously for 3 D J . 9, 10 The amount of small Ba n species in the beam is considered to be negligible since neither laser-induced fluorescence nor emission other than those of monatomic Ba species were detected.…”

“…A detailed characterization of the resulting beam at the prevailing laser ablation conditions and in the absence of N 2 O scattering gas has been reported elsewhere. [8][9][10] The time-of-flight ͑TOF͒ technique through pulsed laser-induced fluorescence and time-resolved fluorescence detection was used to this goal. At 3.3 cm from the Ba disk surface where the chemiluminescence is probed, the barium beam is composed mainly of neutral atoms in the 6s 2 1 S 0 , 6s5d 3 D J , and 6s6p 3 P J states in an overall abundance Ͼ98% of that of the total Ba atoms.…”

“…Advantage was taken here of the ability to modify significantly the Ba͑ 3 P͒ velocity distribution by changing the laser ablation conditions of the atomic beam source used previously. [8][9][10] Average values for both the initial relative velocity and collision energy that have been reported in Ref. 8, where pure thermal conditions were erroneously assumed are recalculated here by applying a different procedure, which is appropriate to the present hyperthermal atomic beam-Maxwellian gas experiments.…”

Time-resolved optical emission spectroscopy of laser-produced air plasma

“…The velocity and kinetic distributions for the different species were obtained from the time-of-flight ͑TOF͒ measurements using time-resolved OES. 25,26 Possible mechanisms for the production of these distributions are discussed. Line intensities from N + were used for determining electron temperature.…”