Initiation of air ionization by ultrashort laser pulses: evidence for a role of metastable-state air molecules

Abstract

Transmission measurements for femtosecond laser pulses focused in air with spectral analysis of emission from the focal region have been carried out for various pulse energies and air pressures. The air breakdown threshold and pulse attenuation due to plasma absorption are evaluated and compared with calculations based on the multiphoton ionization model. The plasma absorption is found to depend on the pulse repetition rate and is considerably stronger at 1 kHz than at 1-10 Hz. This suggests that accumulation of metastable states of air molecules plays an important role in initiation of air breakdown, enhancing the ionization efficiency at high repetition rates. Possible 2 channels of metastable-state-assisted air ionization and the role of the observed accumulation effect in laser material processing are discussed.

Association Reaction of Gaseous C2H4 in Femtosecond Laser Filaments Studied by Time-of-Flight Mass Spectrometry

“…This suggests that the decrease in the laser intensity not only leads to the suppression of C 5 H m and C 6 H m but also affects the generation of C 3 H m . It is possible that C 3 H m is formed by the dissociation of C 4 H m or larger species by succeeding filamentation laser pulses at higher field intensity . Alternatively, the increased yields of C 3 H m could be interpreted in terms of the changes in the distribution of reaction intermediates formed in the filament.…”

“…It is possible that C 3 H m is formed by the dissociation of C 4 H m or larger species by succeeding filamentation laser pulses at higher field intensity. 46 Alternatively, the increased yields of C 3 H m could be interpreted in terms of the changes in the distribution of reaction intermediates formed in the filament. It is established that intermediates containing one carbon atom (CH m ) is formed more efficiently at a higher field intensity.…”

Melting of gold by ultrashort laser pulses: advanced two-temperature modeling and comparison with surface damage experiments

“…Most of the experiments were performed in ambient air, although some measurements were repeated in vacuum in order to be certain that the air environment does not affect the threshold determination. The studied fluence range is well below the threshold for laser air ionization (air breakdown) [31]. However, the presence of the highly reflective gold samples can significantly, up to fourfold, increase the local laser intensity in front of the target surface due to interference of the reflected beam with the incident pulse tail [32].…”

“…For the bulk samples, the absorption of the laser pulse was approximately twice the value of what would be expected for an ideal surface. It should be noted that, for rough surfaces with a reflectivity close to mirror quality (σ << λ), the following expression can be used to evaluate the surface roughness from specular reflectance Rrough [31,32]…”

Spatially resolved filament wavefront dynamics

“…The lack of plasma at 0.15 m is also predicted by estimating the peak laser intensity generated by the most energetic pulse at this location. This is done using the pulse energy, 1/e 2 spot size, and pulse duration as − ⋅ ⋅ − for femtosecond pulses at 800 nm in air [58][59][60][61] . The laser intensity under these conditions is roughly 2 orders of magnitude below the ionization threshold and any effects of plasma can be neglected.…”