Accurate Wavelength Measurements

One of the major tasks for spectroscopists is the measurement of wavelengths of spectral lines. This allows the determination of molecular energy levels and of molecular structure. The attainable accuracy of wavelength measurements depends not only on the spectral resolution of the measuring device but also on the achievable signal-to-noise ratio and on the reproducibility of measured absolute wavelength values. 

With the ultrahigh resolution, which can, in principle, be achieved with singlemode tunable lasers (Vol. 2, Chaps. 1-5), the accuracy of absolute wavelength measurements attainable with conventional techniques may not be satisfactory. New methods have been developed that are mainly based on interferometric measurements of laser wavelengths. For applications in molecular spectroscopy, the laser can be stabilized on the center of a molecular transition. Measuring the wavelength of such a stabihzed laser yields simultaneously the wavelength of the molecular 

Such a scheme reduces the determination of lengths to the measurements of times or frequencies, which can be measured much more accurately than lengths [180]. Recently, the direct comparison of optical frequencies with the Cs standard in the microwave region has become possible with broadband frequency combs generated by visible femtosecond lasers. These frequency combs represent equidistant frequencies, separated by about 100 MHz, which span a wide frequency range, typically over 10 Hz. They allow absolute frequency measurements. This method will be discussed in Vol. 2, Sect. 14.7. 

Another method measures the absolute frequency ul of a stabilized laser and deduces the wavelength Al from the relation = using the best average of experimental values for the speed of light [4.61-4.63], which has been chosen to define the meter and thus the wavelength A by the definition  

1 m is the distance traveled by light in vacuum during the time At = 1/299 792 458 s This defines the speed of light as 

Such a scheme reduces the determination of lengths to the measurements of times or frequencies, which can be measured much more accurately than lengths [4.60]. This method will be discussed in Sect. 14.7. 

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If more accurate wavelength measurements have been made with a spectrometer, repeat the calculations and compare the uncertainties with those of the simpler diffraction experiment described above. Using your best frequency results, solve the three force constant equations for k, and k i. You will find two possible sets of these constants since the solution of the equations yields a quadratic expression. Choose between these two by recognizing that the interaction constant k i is usually much smaller than k or k Note if the frequencies used are appreciably in error, the solution may yield imaginary roots.) Typically the force constants for single, double, and triple CC bonds are about 500,1000, and 1500 N m respectively. What does your value of A, imply about the CC bond type in benzene ... 

Another approach to accurate wavelength measurements of pulsed and cw lasers, which can be also applied to incoherent sources, relies on a combination of a small grating monochromator and three Fabry-Perot etalons [4.69-4.71]. The incoming laser beam is sent simultaneously through the monochromator and three temperature-stabilized Fabry-Perot interferom-... 

Accurate wavelength measurement due to calibration with an internal laser. 

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