The spectrum of neutral krypton has been the subject of many investigations for its wide variety of applications. It is commonly used in astrophysics, plasma physics and laser physics. For example, krypton is one of the most abundant elements with
The spectrum of neutral krypton has been studied for decades, ranging from far ultraviolet to infrared. Meggers et al. [3, 4] investigated first the neutral spectra of krypton, and observed 460 spectral lines ranging from 3302 Å to 9751 Å. In 1933, Meggers and Humphreys  made further investigation and found 200 lines in the range of 7601 Å to 12124 Å. In 1949, Sittner and Peck  reported observations of neutral krypton lines between 1.2 and 2.2 μm, and three-fourths of these lines were classified with the known energy levels. In 1952, Humphreys et al.  investigated the spectra of Kr I in the infrared region from 12000 Å to 19000 Å, and reported 36 new lines of Kr I compared with the observations of Sittner and Peck . In 1952, Moore  first compiled the energy levels of Kr I, and published the results in "atomic energy levels". In 1969, Kaufman and Humphreys  reported 530 lines of Kr I in the region of 3300-40700 Å, and determined 45 even-parity and 66 odd-parity levels of neutral
In this work, we report the observation of a total of 120 spectral lines of Kr I in the region 7874-8425 Å (11870-12700 cm
This work utilizes the same experimental approach as for the former investigation of Bromine described in Ref.. A tunable single-mode cw Ti:Sapphire laser (Coherent Ring 899-29) is operated in the region from 11870 cm
Natural krypton is composed of six isotopes:
Considering that all lines are initially measured without exact calibration, we used 38 lines of neutral
All lines are listed in Tables Ⅰ, Ⅱ, and Ⅲ. Table Ⅰ shows 33 classified lines which have been reported in previous studies [26, 27]. Table Ⅱ and Table Ⅲ list new lines which have not been reported in literatures and can only be observed in the present work. Table Ⅱ shows 45 lines which can be classified with the known energy levels reported in Refs.[26, 27]. Table Ⅲ lists 42 new lines which cannot be classified with reported energy levels.
The observed wavelengths and the corresponding wavenumbers are given in the second and third columns of Table Ⅰ. In the columns 2 and 3 the information on the transitions is provided for air wavelengths (calculated by using formula in Ref.) and wavenumber, respectively. Columns 4 and 5 list the frequencies from NIST atomic database  and Ref., respectively. There is a difference of
All lines in our work are broadened as a result of isotope shift and hyperfine structure. FIG. 1 shows the one of our measured absorption spectra of Kr I of the 4s
Table Ⅱ and Table Ⅲ show our new observed lines. Table Ⅱ lists 45 new lines that can be classified with the known Kr I energy levels [26, 27]. The second column reports the observed wavelengths and the third lists corresponding observed vacuum wavenumbers. The rest six columns are the energies (Ref.), terms, and
Table Ⅲ lists 42 new lines which can't be classified with the known energy levels. However, these new lines indicate there are new possible energy levels which haven't been reported in previous studies. The discovery of new levels will help in reducing the number of unclassified lines of Kr I. So, we make further analysis of these new lines.B. Kr I energy levels
To obtain new energy levels of Kr, a special computer program called "Elements"  is employed here. The program can search possible unknown energy levels from unclassified lines via their expected hyperfine patterns based on selection rules and the venerable Ritz combination principle . The method is easily applied for the case where the hyperfine components are well separated. Moreover, if the magnetic dipole constant
Search strategies for finding new levels are implemented in the program with menu item "Seek New Level". Before computing, two files should be created: energy level file and wavelength file of Kr I. We assume a certain parity of the new level as odd and take the lowest level of even parity to get the energy of a predicted new odd level. Then we compare the calculated energy levels with the energy levels which are got by other wavenumbers of all other lines within our line list. If the energy obtained in this way coincides with that of predicted level, the wavelengths and even lower levels are listed. Then we repeat the process to get all the possible odd energy levels. If no new odd level is suggested, we try to find a new level having even parity. From one of the line groups of the calculated results, we can get the possible
Unfortunately, for most of our spectral lines, we cannot observe complete hyperfine patterns or hyperfine splitting of lines. That is to say, we need a new experiment like saturation absorption experiment which can obtain a higher resolution of observed lines. Running the program described above, we get more than 10 new possible even energy levels and odd energy levels for each line. Here we choose 8299.409 Å, 8288.110 Å, 8144.948 Å, 8110.991 Å and 8009.601 Å as examples and the results are shown in FIG. 3. The newly calculated energy level has an energy of 99474.4 cm
Limited by the line width of the Doppler-limited absorption spectrum, the hyperfine structure may be too congested to be identified. Due to the overlapping lines with almost unresolved components, we could not get hyperfine structure positions and obtain the constants, so there may be more than 3 new energy levels theoretically.Ⅳ. CONCLUSION
We investigated 120 lines of Kr I which extend from 11870 cm
This work was supported by the National Natural Science Foundation of China (No.11674096). The authors would like to thank Prof. L. Windholz at Graz University of Technology for making the program "Elements" accessible.
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