Ultrafast carrier relaxation dynamics in fluorescent carbon nanodots is investigated by femtosecond transient absorption spectra at different pH environments so as to understand the mechanism of fluorescence for the first time. Utilizing multi-wavelength global analysis to fit the measured signal via a sequential model, four different relaxation channels are found, which are attributed to electron-electron scattering and surface state trapping, optical phonon scattering, acoustic phonon scattering and electron-hole recombination respectively. The results reveal that the surface states are mainly composed of different oxygen-containing functional groups (epoxy, carbonyl and carboxyl) and carbon atoms on the edge of the carbon backbone and can effectively trap a large number of photo-excited electrons. The deprotonation of carboxyl groups at high pH will change the distribution of π electron cloud density between the carbon backbone and surface states and consequently, compared with the excited electrons in the acidic and neutral environments, those in the alkaline environment can be more easily trapped by the surface within 1 ps, thereby giving rise to stronger fluorescence emission.

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Enhancement of laser-induced Fe plasma spectroscopy with dual-wavelength is investigated by using femtosecond double-pulse laser.

In femtosecond double-pulse laser-induced breakdown spectroscopy, collinear double-pulse performance is investigated experimentally using various laser wavelength combinations of 800 nm and 400 nm Ti: sapphire lasers. The induced plasma emission line collected by BK7 lenses is the Si (I) at 390.55 nm. The double-pulse time separation ranges from -300 ps to 300 ps. The line intensity is dependent on the time separation of the dual-wavelength femtosecond double-pulse, and its behavior is unlike that of single-wavelength femtosecond double-pulses. Optical emission intensity can be enhanced by selecting appropriate time separation between sub-pulses. This result is particularly advantageous in the context of femtosecond laser-induced breakdown spectroscopy.

This paper carries out the spatial confinement effect on laser-induced Cu breakdown spectroscopy in a cylindrical cavity via a nanosecond pulsed Q-switch Nd:YAG laser operating at a wavelength of 1064 nm. The temporal evolution of the laser-induced plasma spectroscopy is used to investigate the characteristics of spectral persistence. The atomic spectral persistence in plasma generated from Cu with spatial confinement is experimentally demonstrated, where the results indicate that the diameter of the confinement cavity plays a very important role in the persistence of an excited neutral Cu emission line, while the depth of the confinement cavity is almost independent of Cu (I) line persistence. As the diameter of the confinement cavity increases, the persistence of the Cu (I) line in the plasma grows longer under a certain limit. The likely reason for this phenomenon is that under spatial confinement, the reflected shockwave compresses the plasma and leads to an increase in the plasma temperature and density at a certain delay time, which causes further excitation of atomic population to higher excited levels. Finally, the collision rate between particles in the plasma plume is increased.