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.

The current research confirms that substrate temperature plays a dominant role in improving the detection sensitivity of surface-enhanced laser-induced breakdown spectroscopy (SE-LIBS) technology. In the experiment, SE-LIBS is used to detect trace heavy metal elements in aqueous solutions. The influence of the substrate temperature on the spectral intensity of Pb and Cr is discussed. Calibration curves are constructed for Pb and Cr at different substrate temperatures. The corresponding limit of detection (LoD) is calculated. The LoDs of Pb and Cr decrease from 31.7 ng mL⁻¹ and 4.6 ng mL⁻¹ to 8.0 ng mL⁻¹ and 1.2 ng mL⁻¹ as the substrate temperature increases from 25 °C to 200 °C. Increasing the substrate temperature can further improve the analytical ability and detection sensitivity of SE-LIBS for detecting heavy metal elements. The study provides a feasible scheme for the detection of heavy metal elements in wastewater.

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.

The influence of a columnar cavity with different shapes on laser-induced Cu plasma was investigated by optical emission spectroscopy. The circle, square, triangle, and bilateral cavities were used to confine the plasmas. The measured spectral lines were Cu (I) 510.5 nm, 515.3 nm, and 521.8 nm. An obvious enhancement in the emission intensity and plasma temperature was observed when the cavities with different shapes were used to confine the laser-induced Cu plasma. By comparing the spectral intensities under the cavities with different shapes, it was found that the emission intensity was the strongest and the plasma temperature was the highest under the spatial confinement with the cylindrical cavity. Therefore, the spatial confinement effect with the cylindrical cavity is the most appropriate. The spatial confinement effect is attributed to the plasma plume compressed by the reflected shock wave.