| Summary: | 博士 === 國立清華大學 === 物理學系 === 91 === In this dissertation, we are interested in molecular ion HeH+. Second only to H2+, the helium hydride HeH+, hydrogen deuterium HD and their isotopes are the simplest heternuclear molecules. They are the good theoretical testing ground and also play important roles in the astrophysics. It is composed of the two most abundant elements in the universe. Hence it has been suggested presenting in the astronomical objects. At present, the accuracy in theoretical and experimental results is about 30 MHz. We have achieved an improved accuracy to a few MHz.
A stable and narrow linewidth DFG (Difference Frequency Generation) source in mid-IR was set up with power 1.3~1.5 mW and the accuracy of about 1 MHz. The DFG source was based on a Nd:YAG laser of power 1.2 W at 1064 nm, and a Ti:Sapphire laser with power >1.5 W at 780-870 nm. A multi-channel periodically poled LiNbO3 (PPLN) was used to generate the difference frequency within the tuning range 2.92~4.77 μm. The method which we used to get the frequency of the DFG source was to know the individual frequency of Ti:Sapphire and two YAG lasers, with the aid of hyperfine transitions of iodine molecule.
Before measuring the transition frequency of HeH+, we have tested the accuracy of system by measuring the second derivative saturation absorption of methane F2(2), P(7) line of the band at 3.39 μm. It is one of the recommended frequencies. Our measured result was 88376182.694(40) MHz and it is 1.1 MHz larger than the recommended value 88376181.60018(27) MHz. At present, we do not know the origin of the discrepancy. However, we can conclude that the frequency accuracy of our DFG source is ~1MHz. In addition to F2(2) P(7) line of the band, we have also measured the frequencies of E and A2 transitions of the P(7) line .
Five spectra, R(0) to R(4), in fundamental band of HeH+ in the electronic ground state were measured by the concentration modulation. The sensitivity in the present experimental setup was about 10^-8 cm-1/Hz^0.5 . Meanwhile, we have also investigated some physical properties of HeH+ in the discharge tube.
In the future, we expect to improve the accuracy of the transition frequency of HeH+ to ~1 MHz by observing the saturation spectroscopy with cavity enhancement technique. We plan to remeasure the transition frequencies of other simple molecules such as HD and H3+. Both HD and H3+ play import roles in the quantum mechanics calculation and in the astrophysics.
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