Physical Review A

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

EMIL I. TOADER, C. B. Collins, B. W. Johnson, M. Y. Mirza

Published Oct 1, 1977• 4 citations

Abstract

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.

DOI: 10.1103/physreva.16.1490

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Multiphoton excitation and ionization of atomic cesium with a tunable dye laser

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Physical Review AMar 1, 197470 citations

Multiphoton ionization of rubidium

C. B. Collins, Stephen M. Curry, B. W. Johnson +5 more

Physical Review ANov 1, 197649 citations

Doublet line-strength ratios for the principal series of cesium

Dragos Constantin Popescu, Iovitzu Popescu, J. Maurer +2 more

Physical Review AOct 1, 197512 citations

Ionization, excitation of high-lying atomic states, and molecular fluorescence in Cs vapor excited at λ =455.7 and 459.4 nm

John Huennekens, Z. Wu, Thad G Walker

Physical Review AJan 1, 198549 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

Physical ReviewJan 5, 196741 citations

Cs 8943 nm laser pumped by photoassociation of Cs-Kr pairs: excitation of the Cs D_2 blue and red satellites

Jason D. Readle, J. T. Verdeyen, James Gary Eden +4 more

Optics LettersNov 18, 200938 citations

Physical Review A

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

EMIL I. TOADER, C. B. Collins, B. W. Johnson, M. Y. Mirza

Published Oct 1, 1977• 4 citations

Publisher

Abstract

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.

DOI: 10.1103/physreva.16.1490

Related papers

Multiphoton excitation and ionization of atomic cesium with a tunable dye laser

D. Popescu, C. B. Collins, B. W. Johnson +1 more

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 $6^{2}S_{\frac{1}{2}}$ level to resonant $n^{2}D$ and $n^{2}S$ intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from $n=9\mathrm{to}13$ have been recorded for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}D_{\frac{5}{2},\frac{3}{2}}$ series and from $n=11\mathrm{to}14$ for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}S_{\frac{1}{2}}$ series.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 $6^{2}S_{\frac{1}{2}}$ level to resonant $n^{2}D$ and $n^{2}S$ intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from $n=9\mathrm{to}13$ have been recorded for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}D_{\frac{5}{2},\frac{3}{2}}$ series and from $n=11\mathrm{to}14$ for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}S_{\frac{1}{2}}$ series.

Physical Review AMar 1, 197470 citations

Multiphoton ionization of rubidium

C. B. Collins, Stephen M. Curry, B. W. Johnson +5 more

The multiphoton excitation of rubidium has been investigated over the 4600-6500-\AA{} wavelength region with a tunable dye-laser source having a linewidth better than 0.1 \AA{} and a space-charge ionization detector sensitive to a few ions per second. Multiphoton transitions have been observed to occur both through intermediate atomic states and through intermediate continuum states of the rubidium molecule. In the former case two-photon transitions have been observed from the $5^{2}S$ ground state of atomic rubidium to higher-lying $n^{2}D$ levels for values of $n$ ranging from 9 to 34 and to $n^{2}S$ levels for values of $n$ from 11 through 20. The fine-structure intervals of the $n^{2}D$ levels for $n=9 \mathrm{to}c 13$ were measured together with the line-strength ratios and were found to be in good agreement with the predictions of a simple theoretical model. At the shorter wavelengths hybrid two-photon resonances were observed to be excited through resonant intermediate continuum states of ${\mathrm{Rb}}_{2}$ . As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate $p\ensuremath{\pi}(^{3}\ensuremath{\Sigma}_{u1})$ and ${0}_{g}^{+}$ terms dissociating to give a $5{P}_{\frac{3}{2}}$ atom and resulting in the strong development of features corresponding to the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}D_{\frac{3}{2},\frac{5}{2}}$ part of the diffuse series, for $n\ensuremath{\le}50$ in absorption and the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}S_{\frac{1}{2}}$ part of the sharp series. Components to $32^{2}D$ and $30^{2}S$ were recorded to a precision of 0.3 ${\mathrm{cm}}^{\ensuremath{-}1}$ and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a $5^{2}P_{\frac{1}{2}}$ atom were not observed in the wavelength interval available in this experiment.The multiphoton excitation of rubidium has been investigated over the 4600-6500-\AA{} wavelength region with a tunable dye-laser source having a linewidth better than 0.1 \AA{} and a space-charge ionization detector sensitive to a few ions per second. Multiphoton transitions have been observed to occur both through intermediate atomic states and through intermediate continuum states of the rubidium molecule. In the former case two-photon transitions have been observed from the $5^{2}S$ ground state of atomic rubidium to higher-lying $n^{2}D$ levels for values of $n$ ranging from 9 to 34 and to $n^{2}S$ levels for values of $n$ from 11 through 20. The fine-structure intervals of the $n^{2}D$ levels for $n=9 \mathrm{to}c 13$ were measured together with the line-strength ratios and were found to be in good agreement with the predictions of a simple theoretical model. At the shorter wavelengths hybrid two-photon resonances were observed to be excited through resonant intermediate continuum states of ${\mathrm{Rb}}_{2}$. As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate $p\ensuremath{\pi}(^{3}\ensuremath{\Sigma}_{u1})$ and ${0}_{g}^{+}$ terms dissociating to give a $5{P}_{\frac{3}{2}}$ atom and resulting in the strong development of features corresponding to the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}D_{\frac{3}{2},\frac{5}{2}}$ part of the diffuse series, for $n\ensuremath{\le}50$ in absorption and the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}S_{\frac{1}{2}}$ part of the sharp series. Components to $32^{2}D$ and $30^{2}S$ were recorded to a precision of 0.3 ${\mathrm{cm}}^{\ensuremath{-}1}$ and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a $5^{2}P_{\frac{1}{2}}$ atom were not observed in the wavelength interval available in this experiment.

Physical Review ANov 1, 197649 citations

Doublet line-strength ratios for the principal series of cesium

Dragos Constantin Popescu, Iovitzu Popescu, J. Maurer +2 more

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 $19^{2}P$ .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 $19^{2}P$.

Physical Review AOct 1, 197512 citations

Ionization, excitation of high-lying atomic states, and molecular fluorescence in Cs vapor excited at λ =455.7 and 459.4 nm

John Huennekens, Z. Wu, Thad G Walker

We have studied in detail the ionization and population of high-lying atomic states which occur in cesium vapor excited at the second resonance transition, 6S-7P. We have developed a simple model which qualitatively explains the laser-power dependences of level populations and electron densities which we observed. Measurements of spatial dependences of fluorescence point out the critical role of radiation trapping in these phenomena. Similar observations on rubidium demonstrate the same type of behavior when the first (5S-5P) or second (5S-6P) resonance is excited. At higher densities and temperatures we found that the fluorescence was dominated by diffuse molecular bands. Some of these bands probably consist of transitions to the repulsive $^{3}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ state. Additionally four bands observed at 817.5, 826.6, 835.3, and 839.6 nm are tentatively identified as the cesium analog of the $^{3}\mathrm{\ensuremath{\Pi}}_{\mathrm{g}}$${\ensuremath{\rightarrow}}^{3}$${\mathrm{\ensuremath{\Sigma}}}_{\mathrm{u}}^{+}$ satellite bands which have previously been observed in the blue wing of the first resonance doublet of the lighter alkali metals.We have studied in detail the ionization and population of high-lying atomic states which occur in cesium vapor excited at the second resonance transition, 6S-7P. We have developed a simple model which qualitatively explains the laser-power dependences of level populations and electron densities which we observed. Measurements of spatial dependences of fluorescence point out the critical role of radiation trapping in these phenomena. Similar observations on rubidium demonstrate the same type of behavior when the first (5S-5P) or second (5S-6P) resonance is excited. At higher densities and temperatures we found that the fluorescence was dominated by diffuse molecular bands. Some of these bands probably consist of transitions to the repulsive $^{3}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ state. Additionally four bands observed at 817.5, 826.6, 835.3, and 839.6 nm are tentatively identified as the cesium analog of the $^{3}\mathrm{\ensuremath{\Pi}}_{\mathrm{g}}$${\ensuremath{\rightarrow}}^{3}$${\mathrm{\ensuremath{\Sigma}}}_{\mathrm{u}}^{+}$ satellite bands which have previously been observed in the blue wing of the first resonance doublet of the lighter alkali metals.

Physical Review AJan 1, 198549 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.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 ReviewJan 5, 196741 citations

Cs 8943 nm laser pumped by photoassociation of Cs-Kr pairs: excitation of the Cs D_2 blue and red satellites

Jason D. Readle, J. T. Verdeyen, James Gary Eden +4 more

Lasing on the D(1) transition (6P1/22-->6S1/22) of Cs has been observed by photoassociating Cs-Kr atomic pairs with a tunable, pulsed dye laser. Pumping of the blue or red satellites of the Cs D(2) line (62P3/2<==>62S1/2), peaking at approximately 841.1 nm and approximately 853 nm (respectively) in Cs/Kr/C(2)H(6) gas mixtures, provides a photodissociation laser in which the CsKr excimer parent molecule is not, at any point in the pumping process, in a bound electronic state. Relative to the absorbed pump pulse energy, laser slope efficiencies greater than or approximately 5% have been measured when the Cs number density is in the range of 5x10(14)-1.5x10(15) cm(-3) and the pump wavelength is 841.1 nm. Direct photoexcitation of the Cs 6P3/22 state at 852.1 nm under these conditions is a less efficient pathway for pumping the 894.3 nm laser, presumably as a result of competing nonlinear optical processes such as 1+2 resonantly enhanced multiphoton ionization of the alkali atom.Lasing on the D(1) transition (6P1/22-->6S1/22) of Cs has been observed by photoassociating Cs-Kr atomic pairs with a tunable, pulsed dye laser. Pumping of the blue or red satellites of the Cs D(2) line (62P3/2<==>62S1/2), peaking at approximately 841.1 nm and approximately 853 nm (respectively) in Cs/Kr/C(2)H(6) gas mixtures, provides a photodissociation laser in which the CsKr excimer parent molecule is not, at any point in the pumping process, in a bound electronic state. Relative to the absorbed pump pulse energy, laser slope efficiencies greater than or approximately 5% have been measured when the Cs number density is in the range of 5x10(14)-1.5x10(15) cm(-3) and the pump wavelength is 841.1 nm. Direct photoexcitation of the Cs 6P3/22 state at 852.1 nm under these conditions is a less efficient pathway for pumping the 894.3 nm laser, presumably as a result of competing nonlinear optical processes such as 1+2 resonantly enhanced multiphoton ionization of the alkali atom.

Optics LettersNov 18, 200938 citations

6S\ensuremath{-}7P

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}

6^{2}S_{\frac{1}{2}}

level to resonant

n^{2}D

and

n^{2}S

intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from

n=9\mathrm{to}13

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=11\mathrm{to}14

for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}S_{\frac{1}{2}}$ series.

5^{2}S

ground state of atomic rubidium to higher-lying

n^{2}D

levels for values of

n

ranging from 9 to 34 and to

n^{2}S

levels for values of

n

from 11 through 20. The fine-structure intervals of the

n^{2}D

levels for

n=9 \mathrm{to}c 13

were measured together with the line-strength ratios and were found to be in good agreement with the predictions of a simple theoretical model. At the shorter wavelengths hybrid two-photon resonances were observed to be excited through resonant intermediate continuum states of

{\mathrm{Rb}}_{2}

. As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate $p\ensuremath{\pi}(^{3}\ensuremath{\Sigma}_{u1})$ and

{0}_{g}^{+}

terms dissociating to give a

5{P}_{\frac{3}{2}}

atom and resulting in the strong development of features corresponding to the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}D_{\frac{3}{2},\frac{5}{2}}$ part of the diffuse series, for $n\ensuremath{\le}50$ in absorption and the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}S_{\frac{1}{2}}$ part of the sharp series. Components to

32^{2}D

and

30^{2}S

were recorded to a precision of 0.3 ${\mathrm{cm}}^{\ensuremath{-}1}$ and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a

5^{2}P_{\frac{1}{2}}

atom were not observed in the wavelength interval available in this experiment.

{\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

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.