Chapter 1 Introduction of Spectroscopy
1.1 Introduction of Electromagnetic Radiation
1.1.1 Electromagnetic Spectrum[1]
Spectroscopy is a general term referring to the interactions of various types of electromagnetic radiation with matters[2].The electromagnetic spectrum encompasses a wide range of wavelengths from gamma rays[3] to radio waves[4]，all of which can be used in spectroscopy.Exactly how the radiation interacts with matter is directly dependent on the energy of the radiation.This book will be concerned with only the ultraviolet[5]，visible[6]，infrared[7]，and radio frequencies[8].Each of these frequencies reacts with matter in a different way.The higher energy ultraviolet and visible wavelengths affect the energy levels of the outer electrons[9].Infrared radiation is absorbed by matter resulting in rotation and/or vibration of molecules[10].Radio waves are used in nuclear magnetic resonance[11] and affect the spin of nuclei[12]in a magnetic field.（Fig.1-1）
Fig.1-1 The electromagnetic spectrum
图1-1 电磁波谱
Electromagnetic radiation is described by having both the properties of waves and the properties of particles.Both of these properties are important in understanding and using spectrometric methods.In the wave model，electromagnetic radiation is described by means of a classical sinusoidal wave[13]which has the parameters of wavelength，frequency，velocity，and amplitude（Fig.1-2，Table 1-1）.
Fig.1-2 Representation of a beam of monochromatic light showing wave properties
图1-2 显示波动性的一束单色光
Table 1-1 Terms used to describe electromagnetic radiation
表1-1 用于描述电磁辐射的术语
❶ 波长。
❷ 波数。
❸ 频率。
❹速度。
❺ 振幅。
When electromagnetic radiation is emitted or absorbed，a transfer of energy occurs.This phenomenon is best described by treating the radiation as a stream of discrete particles.Molecules exist in a certain number of possible states corresponding to definite amounts of energy.The lowest energy state of a molecule is called the ground state[14].At room temperature，most molecules are in the ground state.Molecules can absorb energy and change to a higher energy level called the excited state[15].The amount of energy absorbed in this transition is exactly equal to the energy difference between the states.This energy difference between the states is also related to the frequency or wavelength of the adsorbed energy.
1.1.2 Interaction with Electromagnetic Wave
Single atoms have only a few possible energy states and therefore absorb only a few discrete wavelengths of radiation.Fig.1-3 depicts an energy level diagram[16] for a sodium atom in a gaseous state[17].If a beam of energy with a wavelength of 590 nm is directed at a sodium atom it will absorb the radiation and an electron will move to a higher energy state.Eventually it will return to the ground state，usually by loosing the absorbed energy as heat，but the electron may re-release electromagnetic energy.If energy with a wavelength of 600 nm is directed at a sodium atom，it won’t absorb 590 with 10 left over！！！ It will not absorb the wavelength at all.It is also possible for an electron in a sodium atom to move to an even a higher energy state.Moving to this state required the absorption of energy with a wavelength of 330 nm.
Fig.1-3 Energy-level diagram for a sodium atom
图1-3 钠原子的能级图
More complex molecules can have many possible states and can adsorb many different wavelengths.The wavelengths adsorbed by a molecule is therefore a characteristic of that molecule and is the basis of spectroscopy[18].The energy states in a molecule are made up of three components，electronic，vibrational，and rotational.The electronic component is characterized by the energy states of bonding electrons[19]（outer shell electrons）.Vibrational states are associated with interatomic vibrations[20] present in molecules.A molecule generally has many more vibrational levels than it does electronic levels and the energy difference between these states is generally much smaller than the differences between electronic states.There are also a number of rotational states for each of the vibrational states and these also have lower energies of transition between states.In fact，rotation is often hindered in liquid or solid samples to the extent that these small energies are not ordinarily detected（Fig.1-4）.
Fig.1-4 Energy-level diagram for a simple molecule
图1-4 简单分子的能级图
Molecules in an excited state will spontaneously “relax” and return to the ground state.There are a number of ways a molecule can return to the ground state.An electron that has been excited from the ground state to an excited state can simply return back to the ground state releasing the same wavelength of radiation that was adsorbed in the transition to the excited state.This process is called resonance fluorescence[21] and is most common in atoms in the gaseous state that do not have vibrational energy states.Relaxation can also take place from higher to lower vibrational levels in a series of small steps，releasing a small amount of kinetic energy[22] and causing a tiny increase in temperature.Nonresonance fluorescence takes place when the molecule is excited to a higher electronic state[23] and a higher vibrational state[24].Since the lifetime of the vibrational is very short，the first thing that will happen is relaxation to a lower vibrational state（release of kinetic energy）and then relaxation to a lower electronic state.Since some energy is lost by dropping to a lower vibrational level，the drop to the lower electronic state will emit a smaller amount of energy and therefore the emitted wavelength will be longer than the adsorbed wavelength.For example，some molecules excited with ultraviolet light[25] will emit visible light[26].
1.1.3 Wavelength（λ），Frequency（ν）and Energy（E）
The electromagnetic radiation parameters λ，ν and E are related to each other.
The relationship between wavelength（λ）and frequency（ν）is shown below：
c=λν
Where λ is wavelength in meters；
ν is frequency in hertz，1/s or s-1；
c=3.0×108m/s（the speed of light in a vacuum）.
Energy（E）and Frequency（ν）Relationships-Energy is directly proportional to frequency.To calculate energy from frequency（or vice Versa），use the following equation
E=hν=hc/λ
Where E is Energy in Joules，J；
ν is frequency in hertz，1/s or s-1；
h=6.626×10-34J·s（Planck’s constant[27]）.
Using these equations λ，ν and E can be converted to each other.
Example 1 What is the frequency of red light with a wavelength of 690nm？
First λ is wavelength in meters，so convert nm to meters：690nm=6.90×10-7m.
Example 2 How much energy does a photon of red light with a wavelength of 690nm？
First λ is wavelength in meters，so convert nm to meters 690nm=690×10-7m.
