A long standing problem in catalysis is the identification and characterization of the active sites, i.e. an atom or an ensemble of atoms spouse on the surface of a catalyst.[Taylor1925] One relevant case, that is treated in this thesis, is constituted by the Phillips catalyst.[Hogan1958] For several reasons, even though this catalyst has be applied at industrial scale for decades and is accounted for a majority of the high density polyethylene (HDPE) production, the identification and the mechanism of the active sites are still under debate. This work was initiated in the framework of the PARACAT project, which is dedicated to study the paramagnetic species in catalysis, and focuses on 'The role of Cr paramagnetic states in olefin polymerization over Phillips catalyst.' In the course of the study, I brought this research to a larger scale which included but was not limited to the Phillips catalyst itself. Considering the relevance of the interaction between transition metal ions (TMI) and the support to the catalytic activity, I worked on systems that cover a number of oxide-supported TMIs by means of electron paramagnetic resonance (EPR) spectroscopy. In this thesis I investigated the paramagnetic Cr(V) and Cr(III) species in the Phillips catalyst. The Cr(III) species were suggested with possible relevance to the catalytic reaction while Cr(V) species were suggested as just the spectators in the reaction.[McDaniel2010, Groppo2018] Nevertheless, Cr(V) species were used in this work as spin probes to provide more information on the overall system. Field-sweep methods including continuous wave (CW) EPR and echo detected field sweep (EDFS) showed that the there are two Cr(V) species with different local geometries. Quantitative analysis of the CW EPR showed that these two Cr(V) species have different reactivity with ethylene. The instantaneous diffusion analysis were performed on the Cr(V) species to provide information on the dispersion of the Cr on the silica surface and the results suggested clusters were formed locally. Besides studying the Phillips catalyst itself, I studied also the silica supported organometallic-Cr catalyst, Cr[CH(SiMe3)2]3/SiO2, which served as a model system to investigate the catalytic active Cr species with well-defined oxidation states and geometry. Two categories of the Cr(III) species were assigned to the active sites for olefin oligomerization and polymerization. The assignment were done by comparing their distortion of the local geometries with that of the different precursors. On the other hand, microporous materials including zeolites and zeotype materials such as aluminophosphate (AlPO) can be engineered with different physical and chemical properties in terms of chemical composition and provide a relevant example of structure sensitivity of a heterogeneous catalyst.[Hartmann2002, Hartmann1999] Such structure sensitivity is highly relevant in catalysis and can be very well studies with EPR spectroscopy. In this regard, I investigated a series of SAPO-5 materials doped with different TMI. In the first place, the incorporation of Cr in SAPO-5 was studied focuses on the discrimination of isomorphous substitution at framework sites and extra-framework sites. In the hyperfine sublevel correlation (HYSCORE) spectrum, large hyperfine interaction (hfi) of 27Al with the matrix 31P signal provide solid evidence for the isomorphous substitution of Cr(V) at framework sites. In addition to the Cr-incorporated SAPO-5, a method to prepare a bi-metallic Mo/V-SAPO-5 system was developed and the metal-metal synergy was validated with a single electron transfer reaction and the short range hyperfine interaction. HYSOCRE spectra showed large \textit{hfi} of both 27Al and 31P and suggested the V species grafted at extra-framework sites. Moreover, the HYSCORE spectrum showed signals at low frequency region which were attributed to the 95,97Mo species with large hfi, confirming the short range interaction. Finally, the surface properties of SAPO-5 were studied by adsorbing NO radicals in the pores and investigating their interaction with the surface. Different adsorption sites of NO molecules according to different activation conditions were first discriminated by the g-factors obtained from the CW EPR. From the 27Al HYSCORE spectra, it is observed that when the activation temperature is higher the NO molecules are situated in vicinity of some defect Al sites. However, the dominant Al species were observed either in samples activated at lower temperature or by increasing the NO dosage. This is postulated as that the defect sites were blocked by residual water molecules or saturated by excessive NO molecules. The presence of water molecules were validated by 1H HYSCORE experiments and the coordination of NO-water was estimated from the hfi structure. [Taylor1925] Taylor, H. S. Proc. R. Soc. London. Ser. A, Contain. Pap. a Math. Phys. Character 1925, 108, 105-111 [Hogan1958] Hogan, J. P.; Banks, R. L. Polymers and production thereof US Patent 2,825,721, 1958. [McDaniel2010] McDaniel M. P. Advances in Catalysis, 1st Ed. 2010; Vol. 52, pp 123-606 [Groppo2018] Groppo, E.; Martino, G. A.; Piovano, A.; Barzan, C. ACS Catal. 2018, 8, 10846-10863 [Hartmann2002] Hartmann, M.; Kevan, L. Res. Chem. Intermed. 2002, 28, 625-695 [Hartmann1999] Hartmann, M.; Kevan, L. Chem. Rev. 1999, 99, 635-663

Atomic precision measurements provide a strong testing ground for new theoretical ideas and fundamental laws of physics. Measurement of the Lamb shift in the hydrogen atom is one of the best examples towards this -it resulted in the birth of QED in 1949 by Dyson, Feynman, Schwinger and Tomonaga. The precision measurements of the hyperfine structure in hydrogen and deuterium by Nafe, Nelson and Rabi indicated that the g-factor for the electron was not exactly 2 as predicted by Dirac, but slightly greater, due to QED effects. Thus the precision measurements are indispensable not only for developing new theory but also for the verification and fine-tuning of theoretical parameters. Precision measurement of hyperfine structure provide valuable information about the nucleus structure, which is helpful in fine tuning of atomic wave-functions used in theoretical calculations. The aim of the work reported in this thesis is the measurement of hyperfine frequency and the observation of hyperfine structure constant in alkali atoms and in Yb atom. This thesis is organized as follows. In Chapter 1, an introduction to the importance of Alkali atoms and Yb atom in the field of precision measurement will be discussed. The scope of this thesis is also discussed in this chapter. In Chapter 2, an introduction to hyperfine structure starting from the beginning of the atomic physics will be discussed. We have discussed about the LS-coupling, jj-coupling, and the influence of the atomic nucleus on atomic spectra. We have also discussed the Zeeman effect and Doppler broadening. In chapter 3, the detail of experimental technique used in this thesis as copropagating satabs, hyperfine frequency measurement using AOM scan, AOM lock and ring cavity has been discussed. Experimental technique to observe the EIT signal in two electron Yb system has been discussed, which can be improved the precision in frequency measurement because of the narrow line-width. In chapter 4, we describe the co-propagating saturated-absorption spectroscopy and its application in frequency measurement. Saturated-absorption spectroscopy (satabs) in a vapor cell is a standard technique used to stabilise the laser frequency. In normal satabs we are getting some extra peaks known as a crossover peaks because laser interact with different velocity group in a vapor cell. In satabs the crossover peaks are stronger and often swamp the true peaks. So we have developed a technique of co-propagating satabs to remove the spurious peak, which has several advantages over conventional satabs. The co-propagating satabs signal appears on a flat background (Doppler-free) with good signal-to-noise ratio and does not have the problem of crossover resonances in between hyperfine transitions. We have adapted this technique to make measurements of hyperfine intervals by using one laser along with an acousto-optic modulator (to produce the scanning pump beam). In chapter 5, we describe the measurement of the hyperfine interval in the 2P1/2 state of 7Li using the SAS technique in hot Li vapor. This technique produces spurious ground crossover resonances that are more prominent that the real peaks. So we have used this ground crossover to measure the hyperfine interval using AOM locking technique. We have developed a technique to measure the absolute frequencies of optical transitions by using an evacuated Rb-stabilized ring-cavity resonator as a transfer cavity. In chapter 6, we study the wavelength-dependent errors due to dispersion at the cavity mirrors by measuring the frequency of the same transition in the Cs D 2 line (at 852 nm) at three cavity lengths. The spread in the values shows that dispersion errors are below 30 kHz, corresponding to a relative precision of 10−10 . We give an explanation for reduced dispersion errors in the ring-cavity geometry by calculating errors due to the lateral shift and the phase shift at the mirrors, and show that they are roughly equal but occur with opposite signs. In chapter 7, we describe precision measurement of hyperfine structure in the 3P2 state of 171,173Yb, and see an unambiguous signature of the magnetic octupole coefficient C in 173Yb. The frequencies of the 3P23S1 transition at 770 nm → are measured using a Rb-stabilized ring-cavity resonator with an accuracy of 200 kHz. In 173Yb we obtain the hyperfine coefficients as A = − 742.11(2) MHz and B = 1339.2(2) MHz, which represent a two orders-of-magnitude improvement in precision, and C = 0.54(2) MHz. Using atomic-structure calculations for two-electron atoms, we extract the nuclear moments quadrupole Q =2.46(12)b and octupole Ω = 34.4(21)b × µN . The observation of nuclear octupole moment in two-electron atoms, to the best of our knowledge, was never reported before. In 171Yb we obtain the hyperfine coefficient A = 2678.49(8) MHz. Using this measurement as well as the previous measurement of A coefficient from our lab, we have compared the hyperfine anomalies for 1P1, 3P1 and 3P2 states. In chapter 8, we describe the EIT in two electron system of 174Yb from 1S0(Fg = 0) 3P1(Fe = 1). We have observed the EIT in degenerate two level system and → after lifting the degeneracy by applying the magnetic field we are getting five peaks. We have also observed the EIT in 173Yb. In 173Yb there are three degenerate two level system Fg =5/2 Fe =3/2, Fg =5/2 Fe =5/2, Fg =5/2 Fe =7/2. →→→ We have observed the same type of EIT signal for all the three transitions Fg = FFe = F, ±F + 1. → In Chapter 9, we give a broad conclusion to the work reported in this thesis and suggest future avenues of research to continue the work started here.

Precise measurements of both absolute frequencies and small frequency differences of atomic energy levels have played an important role in the development of physics. For example, high precision measurements of absolute frequencies of the 2S½ → 2P ½ transition (D1 line) of alkali atoms form an important link in the measurement of the fine structure constant, α. Similarly, precise interferometric measurements of the local gravitational acceleration (g) rely on the knowledge of the absolute frequencies of the 2S½ → 2P 3/2 transition (D2line) in alkali atoms. Difference frequency measurements of hyperfine structure and isotope shifts of atomic energy levels provide valuable information about the structure of the nucleus, which in turn helps in fine tuning the atomic wave functions used in theoretical calculations. The work reported in this thesis starts with the development and refinement of high precision measurement of absolute frequencies using a ring-cavity resonator. The measurement technique is relatively simple and cost-effective, but the accuracy is comparable to that achieved with the frequency comb technique (10¯11) when the accuracy is limited by the natural linewidth of the transition being measured. The technique combines the advantages of using tunable diode lasers to access atomic transitions with the fact that the absolute frequency of the D2 line in87Rb is known with an accuracy of 6 kHz. A frequency-stabilized diode laser locked to this line is used as a frequency reference, along with a ring-cavity resonator whose length is locked to the reference laser. For a given cavity length, an unknown laser locked to an atomic transition has a small frequency offset from the nearest cavity resonance. We use an acousto-optic modulator (AOM) to compensate for this frequency offset. The measured offset is combined with the cavity mode number to obtain a precise value for the frequency of The unknown laser. We have used this technique for absolute frequency measurements Of the D lines in133Cs and 6,7Li, and the 398.8nm line in Yb. We have also developed a technique to measure the ‘difference frequency’ of atomic energy levels using a single diode laser and an AOM. In this technique, the laser is first locked to a given hyperfine transition. The laser frequency is then shifted using the AOM to another hyperfine transition and the AOM frequency is locked to this difference. Thus the AOM frequency directly gives a measurement of the hyperfine interval. Applying this AOM technique we have measured the hyperfine interval of the D1 lines of all alkali atoms with high precision. We have further developed a technique of coheren-tcontrol spectroscopy (CCS) using co-propagating control and probe beam that is useful for highresolution spectroscopy. In this technique, the probe beam is locked to a transition and its absorption signal is monitored while the control beam is scanned through neighbouring transition. As the control comes into resonance with another transition, the probe absorption is reduced and the signal shows a Doppler free dip. This technique allows us to resolve transitions that are otherwise swamped by crossover resonances in conventional saturated absorption spectroscopy (SAS). We have applied this technique to measure hyperfine intervals in the D2 line of several alkali atoms. Thus, we were able to do high-precision measurements of both absolute and difference frequency of atomic transitions. The precision of the absolute frequency measurement is finally limited by the accuracy of 6 kHz with which the reference frequency is known. The nearby two photon transition in Rb, i.e. the 5S1/2→5D3/2 transition at 778 nm, is known with an accuracy of 1 kHz. In future, we hope to improve the accuracy of our technique using this transition as the reference. This thesis is organized as follows: In Chapter1,we give a brief introduction to our work.. We review the importance of frequency measurements and precision spectroscopy, followed by a comparison of the frequency comb and our ring cavity technique. In Chapter2, we describe measurements of the absolute frequency of the D lines of 133Cs using the ring cavity. We give a detailed discussion of the technique, the Possible sources of errors, and ways to check for the errors. The measurement of the absolute frequency of the D lines of Cs allows a direct comparison to frequency comb measurements, and thus acts as a good check on our technique. In Chapter 3, we describe the absolute frequency and isotope shift measurements in the 398.8 nm line in Yb. We probed this line by frequency doubling the output of a tunable Ti:Sapphire laser. We obtained< 60 kHz precision in our measurements and were able to resolve several discrepancies in previous measurements on this line. In Chapter 4, we describe the measurement of hyperfine structure in the D1 lines of alkali atoms. We used conventional saturated-absorption spectroscopy in a vapor cell to probe different hyperfine transitions and then used our AOM technique to measure the hyperfine interval with high precision. In Chapter 5 we discuss our measurements of hyperfine structure in the D2 lines of several alkali atoms. In the case of 23Na and 39K, the closely-spaced hyperfine transitions are not completely resolved in conventional saturatedabsorption spectroscopy due to the presence of cross over resonances. We have used coherent control spectroscopy to obtain crossover-free spectra and then measured the hyperfine intervals using an AOM. This technique was also used for high resolution spectroscopy in the D2 line of 133Cs. Finally, we describe our measurements of hyperfine structure in the D2 line of Rb using normal saturated absorption spectroscopy. Chapter 6, describes the relative and absolute frequency measurements in the D lines of6,7 Li at 670nm. High-precision measurements in lithium are of special interest because theoretical calculations of atomic properties in this simple three electron system are fairly advanced. Lithium spectroscopy poses an experimental challenge and we describe our efforts in doing highresolution spectroscopy on this system. Chapter 7 describes the hyperfine spectroscopy on the1P 1 state of 173Yb. Measurement of hyperfine structure in 173Yb has a problem because two of the hyperfine transitions overlap with the transition in 172Yb. In our earlier work (described in chapter 4), we had solved this problem by using multipeak fitting to the partially resolved spectrum. Here, we directly resolve the hyperfine transitions by using transverse laser cooling to selectively deflect the 173Yb isotope. In Chapter 8 , we give a broad conclusion to the work reported in this thesis and suggest future avenues of research to continue the work commenced here.