1、andFrequency Modulation Spectroscopyof Rubidium AtomsDan Lee Grade 98, School of Physics, Department of Technical Physics摘要在这个实验中，我们测量了85Rb和87Rb原子的饱和吸收光谱和频率调变光谱在饱和吸收光谱中，87Rb原子的|F=1|F=0,1,2和|F=2|F=1,2,3,85Rb原子的|F=2 |F=1,2,3和|F=3|F=2,3,4以及它们的交错信号都被完全的捕捉住.这里,F 表示的是5S1/2基态的超精细能级，而 F则表示的是5P3/2激发态的超精细能级.
2、87Rb 原子的F=2|F=1,2,3的谱线则被用于调频技术Abstract We have measured D2 transitions of 85Rb and 87Rb atoms with saturated absorption spectroscopy and frequency modulation spectroscopy. These saturated absorption spectra, |F=1 |F=0,1,2 and |F=2 |F=1,2,3 of 87Rb atoms, |F=2 |F=1,2,3 and |F=3 |F=2,3,4 of 85Rb atoms
3、, and their crossover lines are completely resolved, where F indicates the hyperfine level of the 5S1/2 ground state and F indicates that of the 5P3/2 excited state. The derivatives of the |F=2|F=1,2,3 spectra of 87Rb atoms are obtained with the technique of frequency modulation.As we know, the Rubi
4、dium atom is one kind of boson. It obeys the Bose-Einstein statistics. In 1995, Rubidium atom was successfully used to realize the Bose-Einstein condensation. In the nature, there are two types of isotopes of Rubidium: 87Rb and 85Rb. If we consider the hyperfine structure of the isotopes of 87Rb and
5、 85Rb, we can get the figures for their energy levels. The hyperfine structure is resulted from the spin of the nucleus, which is called Zeeman effect that can lead to the separation of the energy levels in magnetic field. Fig. 1 below shows us the hyperfine structure of the 87Rb.Fig. 1. The hyperfi
6、ne structure of the energy levels of 87RbFig. 2 below shows us the hyperfine structure of the 85Rb. Fig. 2. The hyperfine structure of the energy levels of 85Rb The two figures are similar to each other. The small difference between them is that the separation of energy levels of 85Rb is less than t
7、hose of 87Rb. Another difference between the two isotopes is that the spin of the nucleus of 87Rb is 3/2 and that of 85Rb is 5/2. I did such a following experiment to study the main energy level and the spectrum at first. The experimental setup is shown schematically in Fig. 3.Fig. 3. Experimental s
8、etup for the absorption spectrum of the Rubidium The diode laser is driven by the current from the laser diode driver and is controlled by the temperature controller. We choose the diode laser as the laser resource here because it has too many advantages: the inexpensive price, the small line width
9、that is less than 100 kHz , the high output power which can reach more than 10 mW, the large tunable range of wavelength which is more than 20 nm , the high stability and the high sensitivity. All above, the most important merit is that it can provide the laser whose frequency is just what we need i
10、n such experiments. In this experiment, we also use a function generator to output a triangular wave with appropriate frequency and amplitude. We input this wave into the laser diode driver and then make the laser current change in a proper range. The amplitude of the triangular wave decides the ran
11、ge. So the wavelength （or frequency） of the laser changes with the triangular wave. The Rubidium atom will absorb some photons from the laser when their frequency is proper. The spectrum is shown in Fig. 4, where the amplitude of the triangular wave is 200 mV and its frequency is 80 MHz.Fig. 4. The
12、absorption spectrum of the Rubidium One thing that we must emphasize is why we do not choose the square wave or serrated wave but triangular wave. The current from the laser diode driver that drive the laser cannot be changed too drastically. Otherwise the diode laser would be damaged. From the Fig.
13、 4, we can see the four apparent spectra lines. From right to left, we mark them as a, b, c, d. In fact, each line of a, b, c, d contains fine spectra. The a-line contains the spectra lines from 87Rb |5S1/2,F=2 to |5P3/2,F=1,2,3. The b-line contains the spectra lines from 85Rb |5S1/2,F=3 to |5P3/2,F
14、=2,3,4. The c-line contains the spectra lines from 85Rb |5S1/2,F=2. The d-line contains the spectra lines from 87Rb |5S1/2,F=1 to |5P3/2,F=0,1,2. But because of the Doppler broadening effect, we cannot distinguish the fine spectra lines. The reason is interesting. We know that only the atoms can abs
15、orb a certain kind of photons whose energy （or frequency） exactly matches the separation of the energy level of the static atom. In fact, all the atoms move in all directions. Due to Doppler effect, the atom can be excited by those photons whose frequency is slightly away from the exact ones; meanwhile the separation among the energy levels of the hyperfine structure of Rubidium is tiny, too. All of above lead to the result that we are not able to distinguish the fine spectra.If we want to distinguish these fine spectra, we can use the method to get the saturated ab
