发射光谱和焰色试验                                                              Have you ever observed what happens when it is raining in one part of the sky and the sun shining in the other? In such a situation, we often spot a rainbow in the part where it is raining. It is so because each raindrop acts as a tiny prism, which refracts the path of the white light of the sun falling on it, and scatters it into a brilliant riot of colours – red, orange, yellow, green, blue, indigo and violet, merging into each other. The band of colours so formed is called a spectrum and here, as one colour merges into the other, it is called a continuous spectrum. Several such drops together result in the formation of a rainbow.

你是否观察过，当一部分天空正在下雨，而另一部分却有阳光照耀时会出现什么现象？在这种情况下，我们经常能够在下雨的那部分天空观察到彩虹。这是因为每个雨滴均可以作为一个棱镜，对照射在其上的白光进行了折射，并将白光散射为七色光，即红色光、橙色光、黄色光、绿色光、蓝色光、靛青色光和紫色光，这些颜色相互之间存在交融。这样形成的色带被称为光谱，由于色带之间存在交融，被称为连续光谱。许多这样的雨滴在一起便形成了彩虹。

Besides continuous, another type of spectrum is the line spectrum, which is further classified as the absorption and the emission spectra. In this topic, we will study the emission spectra of elements and their application in chemistry as the flame test.

除了连续光谱，还存在另外一种光谱即线状光谱，线状光谱又可以分为吸收光谱和发射光谱。在本课题中，我们将对元素的发射光谱及其在化学中的应用（如焰色反应）进行研究。

A common example of the emission spectrum is the sodium vapour lamps, adorning the city at night. Let us go inside the lamp for a while and see what is happening there. We observe that there is a thin sealed gas tube, filled with low density gaseous sodium. There are positive and negative terminals at the two ends, connected to a high voltage power supply. At the microscopic level, we can see the sodium atoms moving at a high speed inside the tube and electrons moving from one terminal to the other. As these electrons continuously collide with the sodium atoms crossing their path, the energy supplied by the colliding electron excites the outer electron of the atom to jump to higher energy levels. However, this electron is not stable in this excited state, and soon falls back to its original energy level, giving out the absorbed energy as photons. The energy of these emitted photons corresponds to that of the yellow light of the visible spectrum. This process of excitation and de-excitation of electrons is the secret behind the yellow light of the lamp.

发射光谱的一个常见例子是在城市的夜晚用于装饰的钠气灯。让我们进入钠气灯的内部对其工作原理进行一下研究。我们观察到在其内部存在一个密封的充气管，其中充满了低密度的气体钠。在其两边存在连接高压电源的正极和负极。在显微水平，我们能观察到钠原子在真空管中快速运动，并且电子从负极流向正极。在电子由负极流向正极时与在其运动路线上的钠原子发生连续碰撞，由电子的碰撞获得的能量使原子外层的电子跃迁至更高的能级上。但是，这个电子处于这种激发态时是不稳定的，随后电子会很快地回到最初的能级上，并且以光子的形式释放出吸收的能量。这些释放的光子的能量与可见光谱中的黄色光相同。电子的这种激发和去激发过程正是灯发出黄色光的原理。

Let's see what happens on passing the light emitted by this sodium lamp through a slit and resolving it through a prism. We observe distinct yellow coloured lines, of specific wavelengths, exactly 586 nm and 589 nm, in a dark background. This arrangement of distinct bright lines on a dark background is the emission spectrum of sodium.

让我们看一下当这种钠灯发射的光通过狭缝以及通过棱镜时会发生什么。我们观察到在黑暗的背景下有不同的黄色光线，其特定的波长分别为586 nm和589 nm。这种在黑暗背景下不同发射谱线的排列就是钠的发射光谱。

If we change the gas in the discharge tube from sodium to neon, we get a bright red light, while a discharge tube filled with argon gives out a different glow. This is so because; the emission spectrum of each of these elements is different. In general, the emission spectrum of every element is unique and serves as a tool for its identification.

如果我们将放电管中的钠变为氖，我们将得到亮红色光，如果放电管中充满氩则会发出另一种颜色。这是因为，每种元素的发射光谱是不同的。一般来说，每种元素的发射光谱均是独特的，其发射光谱能够作为鉴别该元素的工具。

This unique property of emission spectrum is widely used in chemistry labs and workshops as flame test to identify the atoms present in unknown compounds. Here, we will discuss the flame test of sodium. Let's dip a looped platinum wire first in concentrated HCl and then in the given sodium salt. You will notice that some of the salt sticks to the loop. When this wire is placed in a flame, the ions are reduced to gaseous metal atoms in the central region of the flame. The high temperature of the flame excites the valence electron, present in the 3s orbital to a higher energy 3p orbital. As this excited electron falls back to its original energy level, it emits energy in the form of visible light with a wavelength of 589 nm. The emitted energy causes the colour of the flame to turn bright yellow which is similar to the one observed in the street lights. In general, any unknown element can be identified by its flame colour, which depicts nothing but the emission spectrum of the element.

发射光谱的这种独特性被广泛用于化学实验室和工厂中，通过焰色试验对未知原子进行鉴定。这里，我们将对钠的焰色试验进行讨论。首先，使用铂丝圈蘸取浓HCl，然后用铂丝圈蘸取钠盐。这样钠盐会附着在铂丝圈上。当将铂丝置于火焰上时，离子将在火焰的中心区域变为气化的金属原子。火焰的高温激发价电子，使其从最初的3s轨道跃迁至能级更高的3p轨道。当这些激发电子返回至最初的能级时，能量将以可见光的形式发射，其波长为589 nm。发射的能量使火焰的颜色变为明亮的黄色，这与我们在街上看到的装饰用灯发出的颜色类似。一般来说，对任何未知元素都能通过其火焰颜色进行鉴别，能够对元素的发射光谱进行描述。

To summarize, when gaseous sodium atoms in a discharge tube are excited, they absorb certain radiations, which are given out during their de-excitation. The spectrum so obtained by passing the emitted light through a prism, is called the emission spectrum of the element. This concept of excitation and de-excitation of electrons is also applied in chemistry labs for detection of these elements. This test is called the flame test.

综上所述，当放电管中气化的钠原子得到激发后，它们会吸收某种放射能量，这种能量随即在去激发中得到释放。通过使发射的光线通过棱镜得到光谱，这种光谱被称为元素的连续光谱。这种电子的激发和去激发的概念在化学实验室同样用于这些元素的检测。这种试验被称为焰色试验。

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