Several ways of molecular excitation

(1) Thermal excitation to excite electrons in normal state molecules requires a large amount of energy, which can only be achieved in flames and high-temperature furnaces. The energy required to excite molecular vibrations is much smaller, but only a certain proportion of molecular vibrations in heated gases are excited. In normal temperature gases, molecules are in rotation, and the energy required to excite molecular rotation is even smaller. The characteristic of thermal excitation is that the number of molecules at each energy level is determined by the thermal equilibrium distribution law. After the gas molecules excited by thermal excitation are rapidly cooled through an ultrasonic nozzle, since the molecular vibrations generally have a long relaxation time, the molecules still remain in the vibrational energy level within a certain short period of time. At this time, even a reversal of the particle occupation number at the vibrational energy level can be formed.
(2) Electron collision excitation: Electron collision excitation is usually achieved by using thermal electrons or free electron beams in gas discharge. With sufficient energy, thermal electrons collide with molecules non-elastically, increasing the internal energy of the molecules and achieving excitation. Using a single-energy electron beam to excite molecules is a more effective method and has selectivity.
(3) Light excitation: Excitation is achieved by the interaction of photons with molecules, and the subsequent emission of the molecules is called light excitation. The traditional method for observing molecular fluorescence is to filter a mercury light source and use the obtained monochromatic light to excite the molecules. With the emergence of tunable dye lasers, it is possible to study the excitation of a series of vibration and rotational energy levels of molecules. There is also an indirect light excitation called photolysis excitation. It is by irradiating the molecules with light of an appropriate wavelength that a certain bond breaks to obtain excitation, and the energy is redistributed. Light excitation has good selectivity and is easy to control, and is mainly used in fluorescence spectroscopy experiments.
(4) Chemical energy excitation: In many exothermic chemical reactions, the released chemical energy can partially be converted into the internal energy of the system and excite molecules. Different from thermal excitation, the distribution of particles at each energy level during the reaction process is generally different from the particle distribution at thermal equilibrium, and even a reversal of the particle occupation number may occur.
(5) Energy transfer excitation: In gas discharge, some atoms or molecules are in an unstable state with a long lifetime. When they collide with other molecules, due to energy transfer, the former returns to the ground state, and the latter gains energy and is excited. Atomic or molecules excited by light excitation or chemical energy excitation, as well as unstable atomic groups (free radicals) produced in chemical reactions, if the lifetime of the excited state is long, can also be used to excite the predetermined molecules.