样品温度对飞秒激光诱导Al等离子体中AlO光谱的影响
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.
1 引言
激光诱导击穿光谱(LIBS)是一种利用激光诱导等离子体产生的光谱技术[1]。它通过将高强度激光脉冲聚焦在样品表面,产生极高温度和压力条件下的等离子体。当激光脉冲瞬间加热样品时,样品中的原子和分子被激发成高能态,随后经历自由电子碰撞和再结合等过程,形成光辐射。这些辐射被收集并传递给光谱仪,通过测量和分析,可以获得样品的光谱信息。
激光诱导等离子体光谱具有许多优点,如快速、非接触和无需样品准备等。它可以用于元素分析、化学成分检测和矿物研究等[2]。通过分析样品的光谱特征,可以确定元素的种类和含量,甚至可以检测微量元素。近年来,激光诱导等离子体光谱研究也存在一些挑战,研究者们不断探索和改进LIBS技术,以进一步提高其性能,拓宽其应用范围[3]。其中一个研究热点是LIBS增强技术,旨在提高LIBS的检测灵敏度和分辨率[4]。在LIBS技术中,有几种常见的信号增强方法可以提高信号质量和检测灵敏度,如改变激光参数(能量、脉宽和重复频率等)[5]、优化聚焦条件[6]、降低背景辐射[7]、数据预处理[8]以及多元分析方法[9]等。还可以通过附加额外的设备来增强信号强度,典型的一些方法有空间约束LIBS[10]、双脉冲LIBS[11]、磁场加强LIBS[12]、微波增强LIBS[13]以及放电辅助LIBS[14]等。这些方法都是为了增强LIBS信号强度和提高分析灵敏度。另外,样品温度对LIBS的强度有一定的影响。在LIBS分析过程中,样品的温度可以影响观测到的光谱信号的强度和形状。首先,样品温度的升高会导致样品内部的原子、离子和分子的热运动增加,从而增加了激发态的存在概率,提高了辐射发射的概率,进而增强了光谱信号的强度[15]。因此,通常情况下,样品温度的升高会使LIBS信号的强度增加。然而,当样品温度过高时,可能会发生蒸发、化学反应或其他样品物理性质变化,这可能会降低光谱信号的强度。此外,不同元素的激发态寿命各不相同,而样品温度的变化会对激发态寿命产生影响,导致光谱峰的宽度、形状和位置发生变化。
随着激光技术的发展,飞秒(fs)脉冲激光被引入到LIBS研究中[16],fs-LIBS相对于传统纳秒LIBS具有诸多优势。fs激光的脉冲宽度非常短,相对于纳秒脉冲,可以减少背景噪声的干扰,提高光谱信号的分辨率[17]。fs激光具有极窄的脉冲宽度和极高的峰值功率,样品吸收的热和能量相对较少[18],因此可以在不破坏或损伤样品的情况下利用fs激光进行光谱分析,这特别适用于具有较低耐受性的样品。纳秒激光产生的火花在光谱信号中可能产生较高的背景信号干扰,而fs激光由于具有短脉冲宽度,可以减少或避免这种背景信号干扰。fs激光的特性使得它能够更精确地进行材料表征和分析[19]。在纳秒LIBS难以应用的领域,如薄膜分析、材料组分测量和微观结构分析等,fs激光有着广泛的应用[20]。另外,在纳秒LIBS技术中,长的脉冲宽度会导致等离子体再次被加热,产生更高温度的等离子体,使得等离子体中的原子或离子占主导地位,而等离子体中的分子在高温下容易解离。相比之下,fs激光产生的等离子体温度较低,更适合分子的形成[21]。LIBS中的分子发射光谱可用于检测和分析有机材料,包括硝基化合物、聚合物、炸药和生物样品等[22]。它还可以利用分子同位素光谱中相对较大的同位素位移来分析同位素。而LIBS中的氧化铀(UO)分子光谱对于核废料和含铀材料的分析非常重要。因此,LIBS中的分子光谱在多个领域中具有很大的应用潜力。虽然许多研究已经探讨了样品温度对LIBS的影响,但很少有研究探讨样品温度对fs-LIBS中的分子光谱的影响。因此,本文旨在研究飞秒脉冲激发下Al等离子体中样品温度对AlO分子带谱的影响。
本文利用fs脉冲激光激发Al靶产生LIBS,通过设置不同的Al靶初始温度,探测fs-LIBS中的AlO分子光谱,讨论样品温度对fs-LIBS中的分子光谱的影响。
2 实验装置
3 结果与讨论
3.1 时间积分的光谱
首先,在不同初始样品温度(30、100、200 ℃)下利用fs激光烧蚀Al靶,测量得到的AlO分子光谱如
图 2. 不同样品温度下的AlO分子光谱。(a)激光能量为100 μJ;(b)激光能量为200 μJ
Fig. 2. Spectra of AlO molecules under different sample temperatures. (a) Laser energy is 100 μJ; (b) laser energy is 200 μJ
由
相对标准差(RSD)是一个用于评估数据一致性和可靠性的指标。它是标准差与平均值的比值,通常用百分比的形式表示。在LIBS信号中,较低的RSD值意味着分析结果的重复性较好,结果更加可靠。这对于确定元素含量、检测样品的一致性以及确定痕量元素等至关重要。
图 4. 不同样品温度下AlO(0-0)峰强的RSD值
Fig. 4. RSD values of AlO (0-0) peak intensity under different sample temperatures
为了深入地理解Al靶温度对fs-LIBS中AlO分子光谱的影响,讨论了样品温度对AlO分子振动温度的影响。实验中我们使用现有的理论模型拟合AlO分子光谱以获得振动温度[27]。简而言之,从高态u到低态l,对应的分子谱线强度为
式中:a0是玻尔半径;e是基本电荷;c是光速;
图 5. 当激光能量为100 μJ时测量和拟合光谱的对比。(a)样品温度为30 ℃;(b)样品温度为200 ℃
Fig. 5. Comparison of measured and fitting spectra when laser energy is 100 μJ. (a) Sample temperature is 30 ℃; (b) sample temperature is 200 ℃
图 6. AlO分子振动温度随样品温度的变化
Fig. 6. AlO molecular vibration temperature versus sample temperature
3.2 时间分辨的光谱
在LIBS中,激光烧蚀靶产生的等离子体是动态变化的[28]。因此,为了更好地了解Al靶温度对fs-LIBS中AlO分子光谱的影响,需要获得AlO分子的时间分辨光谱。
图 7. 当激光能量为100 μJ时AlO分子的时间分辨光谱。(a)样品温度为30 ℃;(b)样品温度为200 ℃
Fig. 7. Time-resolved spectra of AlO molecules when laser energy is100 μJ. (a) Sample temperature is 30 ℃; (b) sample temperature is 200 ℃
图 8. 当激光能量为100 μJ时,不同样品温度下AlO(0-0)峰值强度随延迟时间的变化
Fig. 8. Peak intensity of AlO (0-0) versus delay time under different sample temperatures when laser energy is 100 μJ
类似于时间积分的光谱,通过拟合
图 9. 当激光能量为100 μJ时,不同样品温度下AlO分子振动温度随延迟时间的变化
Fig. 9. Vibration temperature of AlO molecule versus delay time under different sample temperatures when laser energy is 100 μJ
4 结论
探究了fs-LIBS中Al靶温度对AlO分子光谱的影响,通过增加Al靶温度,可以有效增强fs-LIBS中AlO分子的光谱信号。这是因为高温下fs激光能够更有效地激发靶材产生等离子体,为分子激发提供了更多的电子和能量,从而增加了AlO分子的生成和辐射概率。其次,随着Al靶温度的增加,RSD逐渐降低,升高Al靶温度能够有效低提高AlO分子光谱信号的稳定性。再次,随着Al靶温度的升高,AlO分子的振动温度也随之增加。这表明分子受到更强的热激发,处于更高能量的状态,分子的光谱强度和活性得到增加。另外,增加靶材温度可以进一步增强分子的激发和辐射强度,提高光谱信号强度并延长其持续时间。时间分辨的光谱显示,高Al靶温度下AlO分子的寿命较长,光谱信号强度较高。这表明在高Al靶温度下,AlO分子能够停留更长的时间,分子发射的强度和持续时间增加。因此,通过调节Al靶温度,可以优化fs-LIBS中AlO分子光谱的强度和振动温度,进一步提高能量的利用率和分析的准确性。
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Article Outline
代玉银, 孙艳, 冯志书, 于丹, 陈安民, 金明星. 样品温度对飞秒激光诱导Al等离子体中AlO光谱的影响[J]. 中国激光, 2024, 51(5): 0511003. Yuyin Dai, Yan Sun, Zhishu Feng, Dan Yu, Anmin Chen, Mingxing Jin. Effect of Sample Temperature on Spectra of AlO Molecules in Femtosecond Laser-Induced Al Plasma[J]. Chinese Journal of Lasers, 2024, 51(5): 0511003.