Microwave Spectroscopy

Research object

Microwave spectroscopy is a branch of physics that studies the behavior, structure and motion of matter through the resonance interaction of radio frequency or microwave electromagnetic fields with matter, referred to as spectroscopy. Its research objects can be atoms, molecules and their condensed matter, or neutrons, protons, electrons, atomic nuclei and plasma. The experimental observation can be carried out either in a steady state, or in a dynamic state or even in a short transient state. The frequency range of radio frequency and microwave electromagnetic waves is about 10 to 10 Hz. With the development of theory and experimental technology, spectroscopy is extending to higher frequency bands.

The research of spectroscopy is mainly divided into: ①resonant emission or absorption of atoms and molecules (rare gas, atomic beam, molecular beam); ②electron spin resonance (electron paramagnetic resonance); ③nuclear magnetic resonance ④Nuclear quadrupole resonance; ⑤Double resonance and multiple resonance (see optical magnetic resonance). This article only addresses important developments in atomic and molecular physics and the application of spectroscopy related to spectroscopy.

The sub-discipline of physics that studies the behavior, structure and motion of matter through the resonance interaction of radio frequency or microwave electromagnetic fields and matter. Referred to as spectroscopy. The research objects can be atoms, molecules and their condensed matter, but also neutrons, protons, electrons, atomic nuclei and plasma. Experimental observations can be performed in a steady state, or in a dynamic state or even in a short transient state. The spectrum frequency range is 109~1011 Hz.

Disciplinary history

Before the 1930s, spectroscopy experiments in atomic physics were mainly carried out in the visible light band, mainly measuring wavelengths, measuring the fine structure and hyperfine structure of the spectrum The accuracy of measuring molecular spectra is not high, and the accuracy of measuring molecular spectra is not high. After the Second World War, electronics and microwave technology have made great progress, the sensitivity and resolution of detection instruments have been greatly improved, and the experimental technology has also been innovated. Microwave spectroscopy is mainly based on measuring frequency. It uses oscillators, magnetrons, klystrons, etc. to generate single-frequency microwaves. Through parallel metal wires, coaxial wires or waveguides, they penetrate the resonant cavity containing the analyte to detect the presence of the substance. The response of radiation attenuation caused by a slowly changing electric or magnetic field over time. Using the microwave spectroscopy method, the ultrafine structure of some atoms, Lamb shift, anomalous magnetic moments of electrons and muons, molecular bond lengths, etc. were accurately measured.

The development of microwave spectroscopy has led to the emergence of microwave quantum amplification, the advent of lasers, the invention of atomic clocks, and the establishment of frequency standards, which opened up the emerging science of quantum electronics. The accurate measurement of frequency has led to a substantial increase in the accuracy of physical constants, which has played an important role in promoting the development of natural science, applied science and engineering technology.

Before the end of the 1930s, spectroscopy experiments in atomic physics were mainly carried out in the visible light band. It mainly measured wavelengths. At that time, only some nuclear magnetic hyperfine structures and a few The influence of the nuclear electric quadrupole moment on it (see the ultra-fine structure of the atomic spectrum), the measurement accuracy is not high; in molecular physics, because the molecular band spectrum is mainly in the infrared band, the sensitivity and resolution of the observation instrument at that time Inferior, it is more difficult to accurately measure molecular structure and ultra-fine effects.

In 1933, C.E. Clitton and N.H. Williams first explored the spectrum of ammonia molecules in the microwave band, which became the first in microwave spectroscopy. In 1938, the famous experiment of I.I. Rabbi et al. pioneered the study of the resonance of electromagnetic waves by atomic and molecular beams. After the Second World War, due to advances in electronics and microwave technology, the sensitivity and resolution of detection instruments have been greatly improved, and due to the innovation of experimental technology, atoms other than the collision method (see the collision of electrons with atoms) have been greatly improved. And important experiments of molecular physics are mainly carried out by resonance method in the microwave band. Zavojsky (1945) on electron spin resonance, F. Bloch and EM Persel (1946) on nuclear magnetic resonance, HG De Meert and H. Kruger (1951) on nuclear power quadrupole The success of the observation of the moment resonance experiment enabled spectroscopy to rapidly extend to the radio frequency band. A. Castler (1950), the beginning of optical pumping (see laser), and the emergence of radio interstellar spectroscopy (1951), enriched and enriched the content of spectroscopy. The measurement of spectroscopy is mainly based on frequency. The accuracy of this measurement is generally more than a million times higher than the results obtained by measuring wavelengths in the visible and infrared bands. Due to the improvement of measurement accuracy, new observations appeared one after another.

Disciplinary Achievements

Determination of Atomic Magnetic Hyperfine Structure

As early as 1927, people used bismuth (Bi) ion The magnetic hyperfine structure of atomic lines was discovered in the spectroscopy experiment. After the atomic microwave resonance method is used for measurement, the measurement accuracy is greatly improved. The outstanding achievement is the measurement of the ultra-fine transition frequency of the ground state of cesium [914-1], the accuracy can reach 1×10; and many unprecedented measurements have been made. Measured atom. In 1954, the influence of atomic magnetic octopole moments such as iodine (I), indium (In), and gallium (Ga) was also measured.

Lamb shift Another outstanding achievement of the microwave atomic spectroscopy experiment is to measure the influence of the radiation field on the atomic state, and find that the Lamb shift, such as the 2sS state of hydrogen The shift of the 2pP state is 1057.845±0.009MHz (the two states of Bohr and Dick theory are coincident), which led to the establishment of quantum electrodynamics theory. After the advent of the laser in 1960, using new technology, the 1sS Lamb shift of the ground state of the hydrogen atom was discovered and measured.

Variation According to experimental determination and theoretical calculation, it is found that the electron and sub-spin [kg2][kg2] factor (should be [kg2]2) and the fine structure constant [kg2][ kg2] variation. The measurement of electrons is 2×(1.001159622±0.000000027) (see atomic magnetic moment), and the reciprocal of the fine structure constant of the hydrogen atom ground state transition is 137.0357±0.0008.

Accurate determination of the ultrafine structure of nuclear power quadrupoles There are many nuclear charge distributions in nature that deviate from spherical symmetry. It is found in the abnormal change of the hyperfine structure that [kg2] theoretically uses the energy correction of the interaction between the quadrupole moment of nuclear power and its surrounding electric field gradient (referred to as the coupling of nuclear power quadrupole moment) to get an explanation. After using the atomic beam to measure the frequency in the microwave band, the accuracy is improved, and many nuclear power quadrupole moment coupling constants have been measured. After using the radio frequency nuclear quadrupole resonance to directly measure the frequency, the work was carried out faster. In addition to greatly improving the accuracy of the measurement, it also measured the chemical structure of the nuclear power quadrupole coupling, the temperature of the solid lattice, the phase change, the dislocation, and the Defects, doping, purity, thermal vibration, etc. In 1954, radio frequency and microwave spectroscopy were also measured.

By studying the interaction between microwaves and matter, the science of obtaining molecular rotational energy levels (see molecular spectroscopy) and related transition information. Microwaves are waves with wavelengths ranging from 1 to 1000 millimeters, which are divided into several bands according to their wavelengths:

Microwave structure

The energy of microwave photons is very small, which is about the same in the movement of molecules. Rotational energy level difference of heavy atom molecules, or smaller, such as the inversion of NH3 (see molecular symmetry) motion energy level difference and some finer energy level differences. Like other electromagnetic waves, the absorption and emission of microwaves must be accompanied by changes in electric dipoles or transitions such as electric quadrupoles, Zeeman effects, and Stark effects.

Microwaves are different from far-infrared rays with shorter wavelengths and ordinary radio waves with longer wavelengths in generation, transmission and detection, and the detection instruments used in different wavebands are also different. This is Because the microwave is transported and transmitted in the waveguide.

The waveguide is a rectangular metal tube, and the inside of the tube is smoothly plated with silver to prevent energy loss. The cross-section of the pipe used in the S-band is 76.2 mm × 25.4 mm, and the cross-section of the R-band is 7.02 mm × 3.15 mm. Microwaves are generated by klystrons or magnetrons, and their monochromaticity is good, so there is no need to use spectroscopic equipment such as those used in optical spectroscopy. Microwaves are generally detected by crystal diodes; or Stark modulation method, which can also reduce noise and increase sensitivity; sometimes other modulation methods can also be used.

Application field

The microwave spectrum is highly accurate. For example, the ground state 1←0 rotation transition of carbon monoxide molecule has a frequency of 3.84553319 cm-1.

The energy resolution of the microwave spectrum is much higher than that of the general optical spectrum, so we first used it to obtain more accurate molecular moment of inertia data. These data, coupled with the use of isotope effects, can determine the distance between nuclei in a molecule. The internuclear distance obtained by this method is still the most accurate, and can reach the seventh and eighth significant digits. Generally, the nucleus distance of diatomic molecules can be obtained directly, and triatomic molecules can also be obtained. For molecules with more atoms, it must be obtained by isotopic molecules. This is because the rotational spectrum can only give three moments of inertia.

In addition to rotational motion in molecules, there are many other motions whose energy level difference is within the range of microwave energy, such as the most famous inversion parachute motion of ammonia. Ammonia NH3 is a cone-shaped molecule. Three H atoms are on a H3 plane to form an equilateral triangle, and the N atom is on the top of the cone. It takes energy to overcome the potential barrier when N passes through the H3 plane. This energy is not large. Therefore, when the temperature is not too low, the N atom can basically pass through the H3 plane, sometimes above it, and sometimes below it. According to quantum mechanics, the relevant energy level is split into two at this time. This movement is like an umbrella, so it is called an inversion umbrella movement. The energy level difference of this split can be observed from the microwave spectrum, thus starting the study of similar potential barriers in several molecules.

In the study of molecular structure, microwave can also be used in the analysis of electrical quadrupole fine structure and magnetic hyperfine structure, and nuclear magnetic moment can be obtained from the analysis of hyperfine structure. The results obtained by studying the Zeeman effect and the Stark effect can verify the conclusion of the quantum mechanics calculation. In the atomic spectrum, many spectral lines fall in the microwave region, so its application is not limited to molecules.

Because the microwave spectrum is highly sensitive and unique, microwave can be used for analysis and identification (see figure for example), as well as for the determination of free radicals and intermediate products of chemical reactions. The most prominent example is that interstellar space chemistry emerged from the study of microwaves. Initially, the transition of hydrogen atoms at a wavelength of 21 cm was observed in the radio telescope, and then the Λ double-line transition of the OH group was discovered. Later, CH, CH+, CN, NH3, H2O, CH2O, CO, HCN, CH3OH, HCOOH, CH3CCH, HNCO, OCS, etc. were discovered successively. These are measured based on laboratory data. Two unknown strong lines were discovered in 1971, and they have never been observed in the laboratory. After calculation and experimentation, it was proved by many means such as C2H that it was produced by the C2H group, which shows that there are very strange molecules in interstellar space. Later, a large number of interstellar space compounds such as N2H, HCO+, HNC, C3N, C4H and H(C2)nCN (n=0,1,2,3) were discovered. These strange molecules may be related to the origin of life.

Because the resolution of the microwave spectrum is much higher than that of the infrared spectrum, some people use a very stable frequency laser and microwave to form a double resonance spectrum, which is not only in the spectral region of the laser, but also has a higher resolution. .

Related Articles