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DISSOCIATIVE EXCITATION SPECTROSCOPY OF $Cs_{2}: Cs_{2} + \gamma \to Cs_{2}{^{*}} \to Cs(6^{2}S) + Cs(5^{2}D)$

Jon A. Anderson, C. B. Collins, Pablo A. Vicharelli

Published Jan 1, 1979• 0 citations
OpenAlex

Abstract

A new method for studying dissociative molecular states, complementary to standard spectroscopy techniques, is discussed and its application to the specific case of $Cs_{2}$ is reported. Previous multi-photon ionization $experiments^{1,2}$ revealed the existence of hybrid absorption resonance, in which one photon causes a molecular transition to a dissociative state and a second photon is absorbed by one of the resulting dissociation fragments. The method of dissociation excitation spectroscopy uses two synchronized pulsed lasers to sequentially excite a dissociative molecular state and then detect the dissociation fragments through atomic excitation followed by collisional ionization. By scanning the wavelength of the laser probing the molecule it is possible to obtain a spectrum giving the relative probability that a specific dissociation product will be formed after photo-absorption in the molecule. The dissociation excitation spectrum, including the fine-structure branching ratio, has been obtained for the case \\[ Cs_{2} + \\gamma \\rightarrow Cs_{2}^{*} \\rightarrow Cs(6^{2}S_{1/2}) + Cs(5^{2}D_{3/2,5/2}) \\] over the wavelength region 4575-6050 $\\AA$. This spectrum shows marked similarities to the standard absorption spectrum of $Cs_{2}$. The implications for the molecular potentials of $Cs_{2}$ will be discussed.

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Jiguang zazhi•Jan 1, 2004•0 citations
DISSOCIATIVE EXCITATION SPECTROSCOPY OF Cs2:Cs2+γ→Cs2∗→Cs(62S)+Cs(52D)Cs_{2}: Cs_{2} + \gamma \to Cs_{2}{^{*}} \to Cs(6^{2}S) + Cs(5^{2}D)Cs2​:Cs2​+γ→Cs2​∗→Cs(62S)+Cs(52D)
A new method for studying dissociative molecular states, complementary to standard spectroscopy techniques, is discussed and its application to the specific case of Cs2Cs_{2}Cs2​ is reported. Previous multi-photon ionization experiments1,2experiments^{1,2}experiments1,2 revealed the existence of hybrid absorption resonance, in which one photon causes a molecular transition to a dissociative state and a second photon is absorbed by one of the resulting dissociation fragments. The method of dissociation excitation spectroscopy uses two synchronized pulsed lasers to sequentially excite a dissociative molecular state and then detect the dissociation fragments through atomic excitation followed by collisional ionization. By scanning the wavelength of the laser probing the molecule it is possible to obtain a spectrum giving the relative probability that a specific dissociation product will be formed after photo-absorption in the molecule. The dissociation excitation spectrum, including the fine-structure branching ratio, has been obtained for the case \[ Cs_{2} + \gamma \rightarrow Cs_{2}^{*} \rightarrow Cs(6^{2}S_{1/2}) + Cs(5^{2}D_{3/2,5/2}) \] over the wavelength region 4575-6050 AA\\AAAA. This spectrum shows marked similarities to the standard absorption spectrum of Cs2Cs_{2}Cs2​. The implications for the molecular potentials of Cs2Cs_{2}Cs2​ will be discussed.
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Population of the 6~2D state of Cs atoms by photodissociation of Cs_2 molecules through the use of the fixed frequency line 441.6 nm of a He-Cd~+ laser radiation is produced. Combining ratio of atomic to molecular fluorescence, the ratio of collisional rate coefficient to predissociation rate is estimated to be 9.6×10~(-18) cm~3. The branching ratio is defined as the ratio of 6D_(3/2) to 6D_(5/2) fluorescence. The branching is determined in the Cs density range between 1 and 9×10~(15) cm~(-3). The ratio of the dissociation rates produced in the Cs 6~2D fine-structure states is 0.41. Fine-structure changing cross section has been measured for wing excitation, the result is consistent with cross section obtained from resonant excitation of the 6~2D states.
The two-photon excitation of cesium to high-n Rydberg levels through resonant dissociative states of the dimer is investigated over the wavelength range 6000-5900A. Highly vibrationally excited sp1Sigmasbrmgsp+\\sp1\\Sigma\\sb{\\rm g}\\sp+sp1Sigmasbrmgsp+ ground state molecules are excited to the continuum of sp1Pisbrmu\\sp1\\Pi\\sb{\\rm u}sp1Pisbrmu bound states which dissociate to give 5D atoms. The 5D atoms are subsequently photoexcited to high-n Rydberg states which are detected by selective field ionization techniques. Series corresponding to both (5D to\\toto nF) and (5D to\\toto nP) transitions with components up to n = 150 were observed. The photoexcitation process appears to be very efficient and produces copious quantities of Cs Rydberg atoms.
Abstract In this and subsequent chapters, we consider are other laser methods for controlling atoms and molecules that, in contrast to all the methods discussed above in Chapters 2–8, are destructive. They result in the ionization of the atoms or molecules being controlled and the dissociation and fragmentation of molecules. All these methods are united by the capability of the particles to be ionized or dissociated in a photo selective manner, which makes it possible to detect and separate atoms or molecules of certain species. They are based on the fact that all the structural details of atoms and molecules manifest themselves in their spectra, for example, some details manifest in the spectra of atoms are the atomic weight (the number of nucleons), the number of protons (the nuclear charge and the type of element), the number of neutrons (the isotopic state), and even the excitation energy of the nucleus (the isomeric composition). This enables one to excite, and, in some cases, ionize atoms and molecules in a photo selective way and thus accomplish their photo selective excitation and separation. Figure 9.1 lists all types of photo selectivity. All of them have been successfully realized by the method of resonance multistep ionization of atoms that is considered in the present chapter. Our primary attention here is concentrated on the realization of laser control of the photo selectivity of ionization. A detailed description of this method can be found in the monographs by Letokhov (1987) and Hurst and Payne (1988).
The two-photon excitation and three-photon ionization of atomic cesium is investigated over the 6550-6950-\AA{} wavelength region with a tunable-dye-laser source having a 0.06-0.08-\AA{} linewidth and a space-charge ionization detector sensitive to a few ions per second. An online data-acquisition computer provides significant signal-to-noise recovery. The resulting dispersion curve for photoionization is interpreted in terms of the two-photon transitions from the ground 62S126^{2}S_{\frac{1}{2}}62S21​​ level to resonant n2Dn^{2}Dn2D and n2Sn^{2}Sn2S intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from n=9to13n=9\mathrm{to}13n=9to13 have been recorded for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}D_{\frac{5}{2},\frac{3}{2}}$ series and from n=11to14n=11\mathrm{to}14n=11to14 for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}S_{\frac{1}{2}}$ series.
Populations of the n 2L(nL=7p,6D)states of Cs atoms by photodissociation of Cs_2 molecules through the use of the fixed frequence line 441.6nm of a He-Cd laser radiation are produced.Combining ratios of atomic to molecular fluorescence at the Cs density range between 1 and 9×10 15 cm -3 ,the ratios of collisional excitation transfer rate coefficients and the predissociation rates are obtained.The cross section for the fine-structure-changing collision [i.e,Cs(7P_ 1/2 )+Cs(6S)Cs(7P_ 3/2 )+Cs(6S)]is 3.5×10 -14 cm 2.