Molecular frequency standards

One way an optical standard could be provided is by harmonic multiplication of a microwave frequency standard in a synthesis chain. By use of this technique, a laser at 88 THz (3.39 pm) has been made phase coherent with a microwave oscillator. - The best optical frequency standards may be made by locking a local oscillator (laser) to an atomic or molecular resonance line. State-of-the-art accuracies are characterized by measurements on methane stabilized He-Ne lasers in which reproducibilities in the 10— -- range have... 

Aman J, Hammersberg M, Pendrill L R, Talvitie H, Zarka A and Chartier J-M 1996 Laser spectroscopy of molecular iodine with a tuneable semiconductor laser around 633 nm , Proc. Symposium on Frequency Standards and Metrology, Woods Hole, MA (USA), (World Scientific Publishing, ed. J. C. Bergquist), 437 - 40... 

The comb of equally spaced (in cm-1) transmission maxima from an etalon is useful as relative frequency markers to linearize the scan of a tunable laser. This makes it possible to interpolate between irregularly distributed absolute frequency standard lines (e.g., atomic lines, Fourier Transform Spectrometric Atlases of Molecular I2, Br2, Te2, and small polyatomic molecules) and to measure frequency separations between features in the spectrum being recorded. Pattern recognition methods, such as Loomis-Wood (see Section 1.1.4) and searches for repeated combination differences, work best when applied to spectra in which the frequency separations between lines are determined at the highest possible precision (better than 1/10 the average line spacing). 

As mentioned in the introduction to this chapter, visible/UV Fourier transform instruments are still found mainly as unique, one-of-a-kind instruments in a few spectroscopy laboratories. The research topics being pursued with these Fourier transform instruments include atomic spectrochemical measurements, atomic and molecular emission spectroscopy from hollow cathode discharges, and molecular absorption spectroscopy for accurate frequency standards and molecular constants. In each of these research efforts, the Fourier transform method has proven useful. In part, the success of this method is derived from the fundamental advantage originally stated by Jacquinot, and to some extend from the advantage stated by Fellgett. 

The method used is quite general and can be applied to other molecular species. Furthermore, in the near absence of background gas collisions it allows one to directly relate the rotational temperature of the ions to the temperature of the ambient black-body radiation. This feature (among others, see Refs. [101,105]) suggests the use of molecular ions, such as HD+ or CO" ", for BBR thermometry with possible applications in frequency metrology that is, it may help to improve the accuracy of frequency standards based on trapped ions [104]. 

Koelemeij, J.C.J., Roth, B., and Schiller, S., Cold molecular ions for blackbody thermometry and possible application to ion-based frequency standards, Phys. Rev. A, 76, 023413, 2007. 

Fig. 6. A typical experimental apparatus used in sub-Doppler FLN laser spectroscopy of a molecular beam of Cs. In the above experiment photomultiplier PMl monitors the total fluorescence emitted at the interaction region while PM2 records the narrowed fluorescence after dispersal through analyzers. In this case the monochromator acts as a narrow band filter for the Fabry-Perot signals while the Th lamp serves as a frequency standard. After Demtroder (1981).

The system is prepared at t=0 in the quantum state Pik> and the question is how to calculate the probability that at a later time t the system is in the state Fjn>. By construction, these quantum states are solutions of molecular Hamiltonian in absence of the radiation field, Hc->Ho Ho ik> = e k Fik> and H0 Pjn> = Sjn xPJn>. The states are orthogonal. The perturbation driving the jumps between these two states is taken to be H2(p,A)= D exp(icot), where co is the frequency of the incoherent radiation field and D will be a time independent operator. From standard quantum mechanics, the time dependent quantum state is given by ... 

According to apphcation of Dunham s formalism to analysis of molecular spectra, as for GaH and H2, these radial coefficients of seven types represent many Dunham coefficients Ym and their auxiliary coefficients Zki of various types that collectively allow wave numbers of observed transitions to be reproduced almost within their uncertainty of measurement through formula 54. Mostly because of inconsistency between reported values of frequencies of pure rotational transitions [118,119], the reduced standard deviation of the fit reported in table 3 is 1.25, slightly greater than unity that would be applicable with consistent data for which uncertainty of each measurement were carefully assigned. 

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