Research on Experimental Methods of Atomic and Molecular Spectroscopy

Abstract: When absorption spectrum spectroscopy has a continuous spectrum of light passing through an atom or molecular system, some absorption lines or absorption bands (see absorption spectra) appear due to the absorption of specific wavelengths of light by specific atoms or molecules. These lines or bands reflect Information about the properties of atoms or molecules.

Experimental techniques and methods for obtaining atomic and molecular spectral data. Spectral data usually refers to the data of the wavelength, profile, intensity, polarization and spectral line intensity of spectral lines with time. The energy difference between the transition energy levels can be determined from the spectral line wavelength data, so that the position of the energy levels can be determined. The line profile data reflects the width of the energy level related to the transition, the movement of the luminescent atoms or molecules, and the influence of the surrounding environment (such as electric field, magnetic field, etc.) on the luminescent atoms or molecules. Spectral line intensity is divided into absolute intensity and relative intensity, and their measurement data are related to the transition probability between corresponding energy levels of atoms or molecules. The polarization of the spectral line reflects the state of the luminescent atoms or molecules (such as the orientation of the external magnetic field, see Zeeman effect). The data of spectral line intensity changes with time can give information about the lifetime of excited states of atoms or molecules.

The experimental methods are divided into emission spectroscopy, absorption spectroscopy and scattering spectroscopy.

Emission (and fluorescence) spectroscopy excites atoms or molecules to an excited state, and when the transition from the excited state to a lower energy state emits radiation that identifies the characteristics of the luminescent particles (see emission spectrum), by measuring the spectrum of these radiations Spectral data of corresponding atoms or molecules can be obtained.

There are many ways to excite atoms or molecules, such as inductively coupled plasma discharge, arc discharge, spark discharge and other forms of gas discharge, light or laser irradiation, particle bombardment, etc. At present, emission spectroscopy has become the basic method of spectrochemical analysis for detecting the composition of substances.

The light emitted by an atomic spectrum lamp with a characteristic spectral line (such as an elemental empty cathode lamp) passes through the sample to be tested in the atomizer, and the composition and content of the sample to be tested are determined by the absorption intensity, which is called atomic absorption spectroscopy, which is the spectrum The basic method of chemical analysis. One or more tunable lasers are used to excite the atom or molecular system. The atom or molecule absorbs one or more photons to transition from the ground or low-excited state to the highly-excited state, and then ionizes or photoionizes by collision (see Atoms and Ionization of molecules), generating electrons and ion pairs, measured with ionization detectors (such as ionization probes, electron multipliers, proportional counters, etc.) to obtain multi-photon ionization spectra, thereby obtaining spectral data of atoms or molecules. This method is widely used in isotope separation, atomic and molecular physics research, analytical chemistry and other fields. It is also an important research method to place the sample of atoms or molecules to be measured in or out of the cavity of the tunable laser, and to obtain the spectral data of atoms or molecules by absorption spectroscopy, saturation absorption spectroscopy, or photoacoustic spectroscopy.

Scattering spectroscopy When the monochromatic light irradiates the molecular system, the light is scattered by the molecular system. Due to acoustic vibration, the scattering spectrum that appears near the frequency of the incident light is called Rayleigh scattering spectrum; due to optical vibration, its scattered light appears in the spectrum Different from the component of the incident light frequency, this scattering is called Mann scattering. The difference between the frequency of the scattered light and the frequency of the incident light is different for different molecules. By measuring the scattering spectrum, information about the molecular structure can be obtained. The selection rules of infrared absorption of molecules and Mann scattering are different, so these two methods are complementary to each other (see "class = link> Mann effect).

Experimental equipment Experimental equipment for atomic and molecular spectroscopy usually has a light source, an interaction chamber, a spectrometer, and a detector. In the emission spectrometry measurement, the light source and the interaction chamber are combined.

The study of light source atomic and molecular spectroscopy has a long history, and people have developed a variety of continuous spectrum light sources and line spectrum light sources used in different wavelength bands. Commonly used light sources include spark light sources, arc light sources, various gas discharge light sources, and various thermal radiation light sources.

The laser has high brightness, high monochromaticity and spatial coherence. The emergence of tunable laser makes the output frequency of the laser can be adjusted to resonate with the energy level transition of the atom or molecule to be studied. It can also eliminate the influence of the Doppler effect on the broadening of the spectral line, so since the advent of the laser, especially after the appearance of the tunable laser, the laser light source has rapidly developed into an important light source in atomic and molecular spectroscopy experiments. The application of a laser light source has improved the spectral resolution by several orders of magnitude, and also improved the sensitivity of detecting trace substances. With a laser light source, spectral data of atoms or molecules can be obtained in real time.

Synchrotron radiation (see cyclotron radiation and synchrotron radiation) contains a continuous spectrum from infrared to vacuum ultraviolet, or even to X-ray band. Its intensity is large and the intensity distribution is known. It can also be tuned as a monochromatic light after being split by a monochromator Use of light source. At present, there is no strong tunable laser in some spectral regions of infrared and far infrared, ultraviolet and vacuum ultraviolet regions, so synchrotron radiation has also been fully utilized in atomic and molecular spectroscopy experiments.

Interaction chamber The traditional light and substance interaction chamber refers to the absorption cell and scattering cell of gas and liquid. For solid materials, it is usually heated and vaporized with a heat pipe furnace. The common drawback of this type of interaction chamber is the existence of interactions between atoms or molecules. In response to this problem, atomic beam and molecular beam devices have been used in many high-precision atomic and molecular spectroscopy experiments. Atomic beams and molecular beams are directional moving atoms or molecular flows, and there is almost no collision between particles. When the incident light interacts with them vertically, the effect of the first-order Doppler effect can be eliminated.

Beam-foil spectroscopy is a method to study ion spectroscopy. The ion beam is passed through a solid thin foil, and the atoms or molecules to be measured are stripped and excited. A luminous ion beam that appears behind the foil can be used for spectral measurement. By measuring the luminous intensity of the beam at different distances from the foil in a direction perpendicular to the beam, data on the corresponding energy level life of the ions can be obtained.

Spectrometers and detectors usually use prism spectrometers and grating monochromators as spectroscopic instruments in atomic and molecular spectroscopy experiments. In the infrared and far infrared regions, Fourier transform infrared spectrometer is also used as a spectroscopic instrument. To measure line width and line profile, Fabry-Perot interferometers are sometimes used. The photodetectors in the ultraviolet and visible bands are photomultiplier tubes (see phototubes and photomultiplier tubes), and the infrared region is infrared photodetectors and heat detectors. In multiphoton ionization spectroscopy, various ionization detectors (such as ionization probes, electron multipliers, etc.) are commonly used to measure spectral signals. After the optical signal is converted into an electrical signal, it can be processed with a lock-in amplifier, sampling integrator, or signal averager. For weak light signals, a photon counter can be used. In order to obtain the relevant data of energy level life, time-resolved spectroscopy measurement is required. At this time, choose a photoelectric converter with a short response time, and use a fast oscilloscope, sampling integrator or transient waveform recorder to obtain the data of the spectral line intensity with time. In order to shorten the time for recording the spectrum, multiple spectral measurements can be performed simultaneously. The optical multi-channel analyzer has the function of simultaneous multi-channel spectral measurement. Using microcomputers to control the experiment, collection and data processing of atomic and molecular spectroscopy can improve the efficiency of experiments.

After the appearance of tunable lasers, the light source and spectroscopic instruments were combined to improve the spectral resolution, spatial resolution and spectral detection sensitivity. The advent of ultrashort pulse lasers has improved the time resolution of the spectrum.

Bamboo Folder

Bamboo Folder,Bamboo Board A4 Folder,Bamboo Folder and Acceptance,Recyclable Bamboo Folder

Sichuan Shihai Import And Export Trade Co.Ltd , https://www.zgshzm.com

Posted on