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图 2是激光诱导石墨等离子体在3Pa条件下的350nm~600nm的发射光谱。延时为55ns,门宽为20ns。图中碳离子谱线居多,C2 Swan带(Δν为-1,1,0)的谱线也比较明显,其中C2 Swan带(Δν=0)中516.5nm的峰最强,C2 Swan带谱线弱于离子谱线,未标出的谱线为杂质谱线,没有发现明显的碳原子谱线,WANG[16]和RUIZ[17]等人测出的发射光谱图中在350nm~600nm范围内也没有发现碳原子谱线。在100mJ激光能量条件下,C2的产生与碳离子-电子的辐射复合有关,其中,离子谱线中最强的为C Ⅱ 426.7nm,所以通过观察羽辉中426.7nm的C Ⅱ和C2 Swan带(Δν=0)的发射强度的变化与峰值位置的变化,来研究C2的动力学变化和形成机制。
脉冲激光烧蚀靶材表面并发生溅射,经过汽化电离形成了高温高密度的等离子体,其内部各微粒相互碰撞形成电磁辐射,形成等离子体羽辉。纳秒激光作用靶材期间,等离子体为等温膨胀[18]。在等温膨胀阶段,激光首先与靶材相互作用产生低温、低密度的等离子体,然后与剩余激光能量作用进一步加热和电离[19]。在激光作用结束后,等离子体为近似绝热膨胀,在真空条件下为自由膨胀[20]。当存在环境气体时,等离子体与气体发生碰撞,其扩散动力学随着气体压力的变化而变化。通过控制ICCD的内部延时变化可以拍摄不同时刻羽辉的膨胀图像,采用窄带通滤波片,可以拍摄波长对应的发射微粒的变化。本文中研究了10-2Pa, 3Pa, 50Pa, 130Pa 4个气压下的时域羽辉膨胀图。
在10-2Pa条件下,激光诱导石墨等离子体羽辉膨胀时间演化图如图 3所示。因为窄带通滤波片的透射只有80%左右,所以拍摄到的C+和C2羽辉图像的光强偏弱。可以看到,在真空中羽辉呈球形自由膨胀,C+的发射强度明显强于C2,并且C2发射强度峰值非常靠近靶面。C2主要有两种来源,一种是激光烧蚀碳靶产生,一种是气相重组反应。当烧蚀碳靶激光能量较高时,等离子体温度足够高能够使碳靶喷射出大团簇的Cn团簇分解为碳原子和碳离子[12]。在真空和低气压条件下时,碰撞过程主要发生在等离子体密度最高的靶材附近,由于气相重组反应形成C2可以被忽略,所以此时C2主要来自于碳靶的直接发射[15]。
图 4是在3Pa条件下的羽辉膨胀时域演化图。当气压从10-2Pa增加到3Pa时,气体压力对等离子体的扩散没有产生特别大的影响,羽辉自由膨胀,变化趋势与图 2类似。
图 5是50Pa条件下的羽辉膨胀时域演化图。从图 5a可以看到,随着压力的增加,气体对等离子体的缓冲作用更加明显,在早期阶段,等离子体前端为圆形,但随着时间的推移,在140ns时,羽辉前端出现了变形,这是因为高动能粒子逐渐靠近靶面法线方向发射[21]。观察图 5c中C2的发射特性, 发现在开始阶段的时候,发射峰值位于靶材表面,在35ns之后,羽辉膨胀前端也出现了另一个发射峰值,这可能是由于C2的不同形成机制。靠近靶面的C2主要来自于碳靶的直接发射,羽辉前端C2是通过气相反应重组形成[22]:
$ \text{C}+\text{C}+M\to {{\text{C}}_{2}}+M $
(1) 式中,M表示某物质。增大M密度的方式为增加空气密度和碳微粒密度。所以,随着环境中空气的增加,使得M的密度增加,加强了三体重组反应,使得靶材前端出现C2。羽辉膨胀开始阶段时,靠近靶材表面的C2发射峰值占主导地位,随着延时的增加,羽辉前端的发射峰值慢慢增加并占主导地位,这可能是因为随着羽辉的膨胀,碳原子和离子逐渐靠近羽辉前端,加强了C2的重组形成。通过比较C+和C2的羽辉膨胀图可以看到,C2等离子体前端发射峰值位置基本一致,说明C+对于C2的气相反应形成有重要作用。
图 6是130Pa条件下的羽辉膨胀时域演化图。当气压升至130Pa时,羽辉强度进一步增大,同时C+和C2的强度也增强。这是因为等离子体前端与气体的碰撞加强,使得等离子体内部碰撞加强,从而使发射光强增大[23]。从图 6c可以发现,C2的发射峰值出现现象和50Pa条件下相似,膨胀初期时峰值位于靶材表面,25ns时刻出现两个发射峰值,35ns时,靠近靶面的发射峰值消失,C2发射主要位于等离子体前端,此时,C2的发射峰值和C+的发射峰值位置一致。比较图 6b和图 6c,可以很明显地发现C+的运动速度要快于C2,在膨胀前期的时,C+的强度要大于C2,在390ns时,C2发射位置出现了两个,此时,C2的发射峰值和C+的发射峰值位置不同。并且随后靠近靶面的C2发射位置占主导,C2的强度逐渐大于C+。这是因为气体对等离子体的约束作用,使得等离子体移动速度变慢,C2发射位置与等离子体一致,而此时气相重组反应中的碳微粒的来源不是C+,作者在1000Pa条件下的气压下也发现了这个现象。通过1.3μs时的光强度可以判断出C2的寿命长于C+,这是因为分子振动和转动弛豫[16]。可以推测,等离子体羽辉膨胀的前期阶段时,碳离子发射占主导,后期时碳分子发射占主导,RUIZ等人使用1064nm Nd: YAG激光器在Ar背景下诱导石墨等离子体也发现了这一现象[17]。
不同气压条件下,羽辉膨胀方向的C2强度时间演化空间分布图如图 7所示。可以很清楚地观察到C2强度及位置的变化,在10-2Pa和3Pa时,发射峰值出现在靶材表面,随着时间强度逐渐增强,然后慢慢衰减。随着气压增大到50Pa,前期靠近靶面第1个峰值逐渐增大,35ns时等离子体前端第2个峰值出现。在50ns后,第2个峰值强度大于第1个峰值强度。当气压增大到130Pa时,开始阶段第1个峰值出现并增大,在25ns时,出现第2个峰值,并且强度远大于第1个峰值。这是因为气压增大,气相重组反应加强,使得等离子体前端的C2强度增大,这与上面羽辉图的结果保持一致。图中曲线顶端变平是因为出现了强度饱和。
脉冲激光诱导石墨等离子体羽辉特性研究
Study on characteristics of graphite plume induced by pulsed laser
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摘要: 为了研究C2的演化规律,采用增强型电荷耦合器件(ICDD)直接成像法,通过Nd:YAG激光器烧蚀石墨靶,使用窄带通滤波片分辨出C2和C+的发射位置,研究了在不同空气压力条件下,脉冲激光诱导石墨等离子体中C2和C+的发射特性。当空气气压为10-2Pa和3Pa时,C2发射峰值位于靶材附近,此时C2的形成主要为靶材的直接发射;气压增大至50Pa时,由于气相重组反应加强,等离子体前端出现另一个C2的发射峰值,其峰值位置与C+一致,并且其逐渐占C2发射的主导地位,此时C2的形成主要来源于重组反应,C+发射光强要大于C2;当气压进一步增大至130Pa时,气相重组反应增加,在等离子体前端出现C2的发射强度增强,在1.3μs之后,C2的发射强度大于C+。结果表明,随着气压的变化,C2的发射峰值位置和强度发生明显变化。这一结果对碳等离子体沉积碳纳米材料原理研究是有帮助的。Abstract: In order to study evolution of C2, emission characteristics of C2 and C+ in graphite plasma were studied through intensified CCD direct imaging method at various air pressures.Graphite plasma was produced by Nd:YAG laser, the launch positions of C2 and C+ were distinguished through a narrow band-pass filter. At low pressure of 10-2Pa and 3Pa, emission peak of C2 is located near the target and the formation of C2 is mainly generated by direct emission of target material. With the increase of gas pressure to 50Pa, another emission peak of C2 appears at the front of plasma plume due to the enhancement of gas phase recombination reaction. This peak position is consistent with the C+ and then becomes the dominant of C2 emission. The formation of C2 mainly comes from the recombination reaction. The emission intensity of C+ is larger than that of C2. With the increase of pressure to 130Pa, gas phase recombination reaction increases and emission intensity of C2 increases at the front of the plasma. After 1.3μs, the emission intensity of C2 is greater than that of C+. The results show that C2 emission peak position and intensity change significantly with the pressure. The result is helpful for the study of the principle of carbon nanomaterials deposited by carbon plasma.
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Key words:
- laser technique /
- graphite plasma plume /
- intensified CCD imaging /
- C2 free radical
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