Laser-induced breakdown spectroscopy (LIBS) is a spectroscopic technique that uses laser-induced plasma. In recent years, research has focused on spectral-enhancement techniques aimed at improving the detection sensitivity and resolution of LIBS. During LIBS analysis, the sample temperature can influence the intensity and shape of the observed spectral signals. Increasing the sample temperature increases the thermal motion of atoms, ions, and molecules within the sample, thereby increasing the probability of excitation and emission and ultimately enhancing the intensity of the spectral signals. Moreover, the sample temperature can affect the excited-state lifetimes of the elements, which influences the spectral signals. Different elements have different excited-state lifetimes, and changing the sample temperature can alter the width, shape, and position of the spectral peaks. With the advancement of laser technology, femtosecond (fs)-pulsed lasers have been introduced in LIBS research, offering several advantages over traditional nanosecond LIBS. Owing to the extremely short pulse duration of femtosecond lasers, they can reduce background noise interference and improve the spectral signal resolution compared to nanosecond pulses. The energy of femtosecond pulses is highly concentrated and short-lived, resulting in minimal heat and energy transfer to the sample. Thus, using femtosecond lasers for spectroscopic analysis does not damage or alter the sample, making them particularly suitable for samples with low tolerance. Spark-induced background signal interference, which often occurs in nanosecond LIBS, is reduced or avoided in fs-LIBS owing to the shorter pulse duration of femtosecond lasers. Furthermore, in nanosecond LIBS, longer pulse widths can lead to secondary heating of the plasma and plasma with a higher temperature, where atoms or ions dominate and molecules tend to dissociate. In contrast, femtosecond lasers generate plasma at a lower temperature, making it more favorable for molecular formation. Although numerous studies have explored the effect of the sample temperature on LIBS, there is limited research on the influence of the sample temperature on the molecular spectra of fs-LIBS. Therefore, this study aims to investigate the effect of the sample temperature on the AlO molecular band spectrum in Al plasma excited by femtosecond pulses.

The experimental setup of the fs-LIBS system comprises femtosecond laser system, laser energy attenuator, beam focusing system, sample heating and motion system, spectral acquisition system, and data acquisition and analysis system. Femtosecond laser system utilizes a Ti∶Sapphire femtosecond amplifier. Laser energy attenuation is achieved using a half-wave plate and Glan polarizer. The lens has a focal length of 100 mm. The sample is heated using a proportion-integration-differentiation (PID)temperature-controlled heating table, and motion control is accomplished using a three-dimensional motorized stage. The spectral acquisition system comprises a spectrometer and an intensified charge coupled device (ICCD) camera. The data acquisition and analysis system primarily includes a computer used for collecting and processing the measured spectral data. During the experiment, the sample temperature is initially increased using a heating stage, and then a femtosecond laser is focused on a pure aluminum target to produce plasma. Simultaneously, the motorized stage moves the heating stage and aluminum target to ensure laser ablation on a fresh sample surface. The plasma emission is collected by a lens and guided into the spectral detection system through an optical fiber. The acquired signals are transmitted to a computer. The experiment is conducted under atmospheric conditions.

First, a comparison is made of the ablative effects of femtosecond lasers on Al targets with three initial sample temperatures: 30, 100, and 200 ℃. The measurements focus on the AlO molecular spectral band from the B²Σ⁺ to the X²Σ⁺ transition. The experimental results show that at two laser energies (100 μJ and 200 μJ), the Al targets heated to 100 ℃ and 200 ℃ yield stronger AlO spectra compared to the spectra from the Al target at 30 ℃. To understand the influence of the Al target temperature on the AlO molecular spectrum in detail, the experiment records the intensity of the AlO (0-0) peak as a function of the Al target temperature under laser energies of 100 μJ and 200 μJ. The intensity of the AlO (0-0) peak increases monotonically with the increasing Al target temperature. This result indicates that using a lower laser energy and higher target temperature makes it possible to achieve the emission of molecules in laser-induced plasma at the same or even stronger levels. This implies that at higher target temperatures, stronger molecular emissions can be obtained with a lower laser energy, potentially affecting the optimization and application of laser-induced plasma spectroscopic analysis techniques. To further understand the effect of the Al target temperature on the AlO molecular spectrum in fs-LIBS, it is necessary to consider the influence of the sample temperature on the vibrational temperature of the AlO molecules. The experimental results demonstrate that the vibrational temperature of the AlO molecules increases with the Al target temperature. Clearly, greater laser energy produces stronger plasma, resulting in higher vibrational temperatures for the AlO molecules within the plasma. Moreover, in LIBS, the plasma generated by the laser ablation of a target changes dynamically. Therefore, a time-resolved spectroscopic analysis of the AlO molecules is essential to better understand the influence of the Al target temperature on the AlO molecular spectrum in fs-LIBS. The experimental results reveal that increasing the Al target temperature significantly enhances the spectral intensity and prolongs the lifetime of the AlO molecules in fs-LIBS. Thus, the time-integrated spectra of AlO at higher Al target temperatures are stronger than those at lower Al target temperatures.

This study investigates the influence of the Al target temperature on the AlO molecular spectrum in fs-LIBS. Increasing the Al target temperature effectively enhances the spectral signal of AlO molecules in fs-LIBS. This is because, at higher temperatures, the femtosecond laser can more efficiently excite the target material, leading to the generation of more electrons and greater energy for molecular excitation, thereby increasing the production and emission of AlO molecules. Furthermore, as the Al target temperature increases, the vibrational temperature of the AlO molecules also increases. This indicates that the molecules are subjected to higher thermal excitation and exist in higher energy states, which increases their spectral intensity and activity. Moreover, increasing the target temperature further enhances the molecular excitation and emission processes, thereby increasing the intensity and duration of the spectral signals. Time-resolved spectroscopy reveals that the AlO molecules exhibit longer lifetimes and higher spectral intensities at higher Al-target temperatures. This suggests that at higher Al target temperatures, AlO molecules can remain in an excited state for longer time, thereby increasing the intensity and duration of the molecular emission. Therefore, by adjusting the Al target temperature, the spectral intensity and vibrational temperature of AlO in fs-LIBS can be optimized, thereby improving energy utilization and analytical accuracy.