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Physical Review A

Use of Space-Charge-Amplification Techniques in the Absorption Spectroscopy of Cs and Cs2

Dragos Constantin Popescu, Mihail–Lucian Pascu, C. B. Collins, B. W. Johnson, Iovitzu Popescu

Published Oct 1, 1973• 75 citations
Publisher

Abstract

Space-charge amplification of the direct and indirect ionization resulting from photoabsorption in alkali-metal vapor is shown to provide a useful complement to the conventional techniques of absorption spectroscopy. Because of its inherently greater sensitivity, this technique is particularly useful in spectroscopic applications involving weak transitions such as those to highly excited states or absorptive transitions from excited states of low population. This paper reports the use of this technique to extend prior measurements of the oscillator strengths of the ($6^{2}S\ensuremath{\rightarrow}n ^{2}P$) series in Cs, the $n^{2}D$ doublet separations, and the photoionization spectrum of ${\mathrm{Cs}}_{2}$.

DOI
10.1103/physreva.8.1666

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Space-charge amplification of the indirect ionization resulting from photoabsorption in cesium vapor has been shown to offer to conventional techniques of absorption spectroscopy a useful complement of highest sensitivity. It was used in this work together with a xenon continuum source filtered to a linewidth of 0.14 \AA{} to measure the relative absorption probabilities for transitions of the cesium-principal-series ($6^{2}S\ensuremath{-}n^{2}P_{\frac{1}{2},\frac{3}{2}}$) doublets. Anomalous pressure effects on the resulting line-strength ratios, resulting in part from polarization molecules of ${\mathrm{Cs}}_{2}$*, were examined over a range of cesium-atom concentration varying from 1.2\ifmmode\times\else\texttimes\fi{}${10}^{14}$ to 1.5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. From the corrected doublet line-strength ratios for $11\ensuremath{\le}n\ensuremath{\le}16$, the segment of the curve of Fano $X$ parameters corresponding to discrete energies was constructed. Interpolation between these results and those from electron-polarization experiments served to locate the pole in the principal-series, doublet line-strength ratio near $19^{2}P$.

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Molecular satellites of the 6S2−7P2 doublet of cesium

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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

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.

•Jan 1, 1979•0 citations

Inelastic Collisions between Excited Alkali Atoms and Molecules. I. Sensitized Fluorescence and Quenching in Cs- N2 and Cs- H2 Systems

D. A. McGillis, L. Krause

Excitation transfer between the $6 ^{2}P_{\frac{1}{2}}$ and $6 ^{2}P_{\frac{3}{2}}$ resonance states in cesium, induced by Cs-${\mathrm{N}}_{2}$ and Cs-${\mathrm{H}}_{2}$ collisions, was observed in the first experiment of its kind, in which techniques of sensitized fluorescence are used to investigate collisional mixing between resonance states of alkali atoms in mixtures with molecular gases. The cesium-vapor-gas system, in which the cesium vapor pressure was low enough to avoid radiation trapping, was irradiated with a single component of the resonance doublet. The fluorescent light, which contained both components of the doublet, was monitored at right angles to the direction of excitation. Measurements of relative intensities of the fluorescent components yielded the following cross sections for excitation transfer and for quenching of resonance radiation. For Cs-${\mathrm{N}}_{2}$ collisions: ${Q}_{12}(^{2}P_{\frac{1}{2}}\ensuremath{\rightarrow}^{2}P_{\frac{3}{2}})=3.6$ ${\mathrm{\AA{}}}^{2}$, ${Q}_{21}(^{2}P_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{3}{2}})=16.2$ ${\mathrm{\AA{}}}^{2}$, ${Q}_{10}(^{2}S_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{1}{2}})=55.2$ ${\mathrm{\AA{}}}^{2}$, ${Q}_{20}(^{2}S_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{3}{2}})=63.5$ ${\mathrm{\AA{}}}^{2}$. For Cs-${\mathrm{H}}_{2}$ collisions: ${Q}_{12}=6.7$ ${\mathrm{\AA{}}}^{2}$, ${Q}_{21}=29.6$ ${\mathrm{\AA{}}}^{2}$, ${Q}_{10}=11.7$ ${\mathrm{\AA{}}}^{2}$, ${Q}_{20}=6.7$ ${\mathrm{\AA{}}}^{2}$. The large sizes of the excitation-transfer cross sections, which are comparable to the corresponding Cs-Cs cross sections, are attributed to resonances with transitions in the molecules.

Physical Review•Jan 5, 1967•41 citations
Space-charge amplification of the direct and indirect ionization resulting from photoabsorption in alkali-metal vapor is shown to provide a useful complement to the conventional techniques of absorption spectroscopy. Because of its inherently greater sensitivity, this technique is particularly useful in spectroscopic applications involving weak transitions such as those to highly excited states or absorptive transitions from excited states of low population. This paper reports the use of this technique to extend prior measurements of the oscillator strengths of the (62S\ensuremath→n2P6^{2}S\ensuremath{\rightarrow}n ^{2}P62S\ensuremath→n2P) series in Cs, the n2Dn^{2}Dn2D doublet separations, and the photoionization spectrum of Cs2{\mathrm{Cs}}_{2}Cs2​.
Space-charge amplification of the indirect ionization resulting from photoabsorption in cesium vapor has been shown to offer to conventional techniques of absorption spectroscopy a useful complement of highest sensitivity. It was used in this work together with a xenon continuum source filtered to a linewidth of 0.14 \AA{} to measure the relative absorption probabilities for transitions of the cesium-principal-series ($6^{2}S\ensuremath{-}n^{2}P_{\frac{1}{2},\frac{3}{2}}$) doublets. Anomalous pressure effects on the resulting line-strength ratios, resulting in part from polarization molecules of Cs2{\mathrm{Cs}}_{2}Cs2​*, were examined over a range of cesium-atom concentration varying from 1.2\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 to 1.5\ifmmode\times\else\texttimes\fi{}1015{10}^{15}1015 ${\mathrm{cm}}^{\ensuremath{-}3}$. From the corrected doublet line-strength ratios for $11\ensuremath{\le}n\ensuremath{\le}16$, the segment of the curve of Fano XXX parameters corresponding to discrete energies was constructed. Interpolation between these results and those from electron-polarization experiments served to locate the pole in the principal-series, doublet line-strength ratio near 192P19^{2}P192P.
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.
The method of space-charge ion detection has been extended to the case of lithium. This was accomplished by the insertion of a space-charge-limited diode into a heat-pipe oven. Lithium vapor was probed with a N2{\mathrm{N}}_{2}N2​ laser-pumped tunable dye laser from 7150 to 4400 \AA{} and with a Xe arc lamp from 7500 to 2300 \AA{}. One-through four-photon events leading to ionization were observed. The atomic transitions observed were the principal series to n=10n=10n=10 and the 2s2s2s to 3d3d3d, 4s4s4s, and 4d4d4d two-photon transitions. The two-photon absorption signals depended upon the third and fourth powers of the light intensity, indicating a new ionization process not heretofore considered important in diode signals. This new process appears to be excitation transfer from excited-state atoms to other excited-state atoms or molecules. For the high excited-state densities achieved in this experiment, this process appears to predominate over photoionization of excited-state atoms and other collisional processes involving excited-state and ground-state species. The $B\ensuremath{\leftarrow}X$, $C\ensuremath{\leftarrow}X$, and $D\ensuremath{\leftarrow}X$ molecular transitions, investigated by previous workers, were observed here.
The use of in situ ion detection in alkali vapors coupled with multiphoton ionization provides a technique of highest sensitivity for the investigation of the spectroscopy of excimer states involving alkali atoms. This technique has been applied in this work to the molecular components of the spectrum of the $6S\ensuremath{-}7P$ absorption doublet of cesium. The dispersion curve was obtained with a tunable dye laser for the two-photon ionization of cesium over the 4500-4600-\AA{} wavelength range as a function of vapor pressure and source intensity. Strongly developed satellite bands were observed to be associated with the atomic lines $6^{2}S_{\frac{1}{2}}\ensuremath{\rightarrow}7^{2}P_{\frac{1}{2}}$ and $6^{2}S_{\frac{1}{2}}\ensuremath{\rightarrow}7^{2}P_{\frac{3}{2}}$, and were found to have well-defined band heads at 29.4 and 31.4 ${\mathrm{cm}}^{\ensuremath{-}1}$ to longer wavelengths, respectively. Sensitivity was observed to be an order of magnitude in excess of that obtained from classical absorption techniques, and the observation of a quadratic dependence of the molecular components confirmed the multiphoton nature of the detection sequence.
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.
Excitation transfer between the 62P126 ^{2}P_{\frac{1}{2}}62P21​​ and 62P326 ^{2}P_{\frac{3}{2}}62P23​​ resonance states in cesium, induced by Cs-N2{\mathrm{N}}_{2}N2​ and Cs-H2{\mathrm{H}}_{2}H2​ collisions, was observed in the first experiment of its kind, in which techniques of sensitized fluorescence are used to investigate collisional mixing between resonance states of alkali atoms in mixtures with molecular gases. The cesium-vapor-gas system, in which the cesium vapor pressure was low enough to avoid radiation trapping, was irradiated with a single component of the resonance doublet. The fluorescent light, which contained both components of the doublet, was monitored at right angles to the direction of excitation. Measurements of relative intensities of the fluorescent components yielded the following cross sections for excitation transfer and for quenching of resonance radiation. For Cs-N2{\mathrm{N}}_{2}N2​ collisions: ${Q}_{12}(^{2}P_{\frac{1}{2}}\ensuremath{\rightarrow}^{2}P_{\frac{3}{2}})=3.6$ A˚2{\mathrm{\AA{}}}^{2}A˚2, ${Q}_{21}(^{2}P_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{3}{2}})=16.2$ A˚2{\mathrm{\AA{}}}^{2}A˚2, ${Q}_{10}(^{2}S_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{1}{2}})=55.2$ A˚2{\mathrm{\AA{}}}^{2}A˚2, ${Q}_{20}(^{2}S_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{3}{2}})=63.5$ A˚2{\mathrm{\AA{}}}^{2}A˚2. For Cs-H2{\mathrm{H}}_{2}H2​ collisions: Q12=6.7{Q}_{12}=6.7Q12​=6.7 A˚2{\mathrm{\AA{}}}^{2}A˚2, Q21=29.6{Q}_{21}=29.6Q21​=29.6 A˚2{\mathrm{\AA{}}}^{2}A˚2, Q10=11.7{Q}_{10}=11.7Q10​=11.7 A˚2{\mathrm{\AA{}}}^{2}A˚2, Q20=6.7{Q}_{20}=6.7Q20​=6.7 A˚2{\mathrm{\AA{}}}^{2}A˚2. The large sizes of the excitation-transfer cross sections, which are comparable to the corresponding Cs-Cs cross sections, are attributed to resonances with transitions in the molecules.