Physical Review A

Collisional energy transfer between excited Sr atoms

H. G. C. Werij, Martin A. Harris, J. Cooper, Alan Gallagher, James Floyd Kelly

Published Mar 1, 1991• 21 citations

Abstract

The following energy-transfer reactions have been observed in a Sr vapor following pulsed excitation of the 5 $^{1}$${\mathit{P}}_{1}$ state: ${\mathrm{Sr}}^{\mathrm{*}}$(5 $^{1}$${\mathit{P}}_{1}$)+${\mathrm{Sr}}^{\mathrm{*}}$(5 $^{1}$${\mathit{P}}_{1}$)\ensuremath{\rightarrow}Sr(5 $^{1}$${\mathit{S}}_{0}$)+${\mathrm{Sr}}^{\mathrm{*}\mathrm{*}}$(${\mathit{n}}^{2\mathit{S}+1}$L)-\ensuremath{\Delta}E, ${\mathrm{Sr}}^{\mathrm{*}}$(4 $^{1}$${\mathit{D}}_{2}$) +${\mathrm{Sr}}^{\mathrm{*}}$(4 $^{1}$${\mathit{D}}_{2}$)\ensuremath{\rightarrow}Sr(5 $^{1}$${\mathit{S}}_{0}$)+${\mathrm{Sr}}^{\mathrm{*}\mathrm{*}}$(${\mathit{n}}^{2\mathit{S}+1}$L)-\ensuremath{\Delta}E. The 4 $^{1}$${\mathit{D}}_{2}$ state was populated by a nonlinear stimulated process. By measuring the rate coefficients for these reactions for over 40 final states ${\mathit{n}}^{2\mathit{S}+1}$L, we look for insight into how these reactions depend on energy defect, spin, and angular momentum. It is found that there is not a very strong dependence on the energy defect, and that singlet and triplet final states are produced in comparable amounts. The final states tend to be populated more effectively as their angular momentum increases. Total rate coefficients are of the order of gas-kinetic rates.

DOI: 10.1103/physreva.43.2237

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Collision energy pooling for Sr(5 3 PJ )+Sr(5 J’3 ) → Sr(6 3,1 S)+Sr(5 1 S)

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Collisional energy transfer between excited-state strontium and noble-gas atoms

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Effects of alignment on strontium energy pooling collisions

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Physical Review A

Collisional energy transfer between excited Sr atoms

H. G. C. Werij, Martin A. Harris, J. Cooper, Alan Gallagher, James Floyd Kelly

Published Mar 1, 1991• 21 citations

Publisher

Abstract

The following energy-transfer reactions have been observed in a Sr vapor following pulsed excitation of the 5

^{1}$${\mathit{P}}_{1}

state:

{\mathrm{Sr}}^{\mathrm{*}}

^{1}$${\mathit{P}}_{1}

{\mathrm{Sr}}^{\mathrm{*}}

^{1}$${\mathit{P}}_{1}

)\ensuremath{\rightarrow}Sr(5

^{1}$${\mathit{S}}_{0}

{\mathrm{Sr}}^{\mathrm{*}\mathrm{*}}

(

{\mathit{n}}^{2\mathit{S}+1}

L)-\ensuremath{\Delta}E,

{\mathrm{Sr}}^{\mathrm{*}}

^{1}$${\mathit{D}}_{2}

) +

{\mathrm{Sr}}^{\mathrm{*}}

^{1}$${\mathit{D}}_{2}

)\ensuremath{\rightarrow}Sr(5

^{1}$${\mathit{S}}_{0}

{\mathrm{Sr}}^{\mathrm{*}\mathrm{*}}

(

{\mathit{n}}^{2\mathit{S}+1}

L)-\ensuremath{\Delta}E. The 4

^{1}$${\mathit{D}}_{2}

state was populated by a nonlinear stimulated process. By measuring the rate coefficients for these reactions for over 40 final states

{\mathit{n}}^{2\mathit{S}+1}

L, we look for insight into how these reactions depend on energy defect, spin, and angular momentum. It is found that there is not a very strong dependence on the energy defect, and that singlet and triplet final states are produced in comparable amounts. The final states tend to be populated more effectively as their angular momentum increases. Total rate coefficients are of the order of gas-kinetic rates.

DOI: 10.1103/physreva.43.2237

Related papers

Collision energy pooling for Sr(5 3 PJ )+Sr(5 J’3 ) → Sr(6 3,1 S)+Sr(5 1 S)

James Floyd Kelly, Martin A. Harris, Alan C. Gallagher

Energy pooling (EP) has been observed in Sr vapor following pulsed optical excitation to the 5 ${}^{3}$ ${P}_{1}$ state. From the time-dependent radiative decay of 6 ${}^{3}$ ${S}_{1}$ and of ${6}^{1}$ ${\mathit{S}}_{0}$ via the cascade resonance line, the EP rate coefficients have been investigated for the following: Sr(5 $^{3}P_{J}$ )+Sr(5 ${}^{3}$ ${\mathit{P}}_{\mathit{J}\mathcal{'}}$ ) ${(5}^{1}$ ${S}_{0}$ ) +Sr(6 $^{3}S_{1}$ or ${6}^{1}$ ${S}_{0}$ ). The values of ${k}_{11}$ have been measured, as well as ${k}_{\mathrm{JJ}\mathcal{'}}$ averaged over a thermal distribution of 5 $^{3}P_{J}$ populations. The time-dependent Sr( ${5}^{3}$ ${P}_{J}$ ) densities were measured by absorption of lines from a Sr lamp crossing the excitation region. The rate coefficients are surprisingly large; in spite of the small statistical weights of these product states, they are nearly gas-kinetic. The dependence of the rates upon spin, J, and energy defect \ensuremath{\Delta}E has also been examined.Energy pooling (EP) has been observed in Sr vapor following pulsed optical excitation to the 5${}^{3}$${P}_{1}$ state. From the time-dependent radiative decay of 6${}^{3}$${S}_{1}$ and of ${6}^{1}$${\mathit{S}}_{0}$ via the cascade resonance line, the EP rate coefficients have been investigated for the following: Sr(5 $^{3}P_{J}$)+Sr(5${}^{3}$${\mathit{P}}_{\mathit{J}\mathcal{'}}$) ${(5}^{1}$${S}_{0}$) +Sr(6 $^{3}S_{1}$ or ${6}^{1}$${S}_{0}$). The values of ${k}_{11}$ have been measured, as well as ${k}_{\mathrm{JJ}\mathcal{'}}$ averaged over a thermal distribution of 5 $^{3}P_{J}$ populations. The time-dependent Sr(${5}^{3}$${P}_{J}$) densities were measured by absorption of lines from a Sr lamp crossing the excitation region. The rate coefficients are surprisingly large; in spite of the small statistical weights of these product states, they are nearly gas-kinetic. The dependence of the rates upon spin, J, and energy defect \ensuremath{\Delta}E has also been examined.

Physical Review AAug 1, 198836 citations

Collisional energy transfer between excited-state strontium and noble-gas atoms

Debra Miller, L. You, J. Cooper +1 more

Inert-gas-induced collisional transfer between the first four excited states of Sr has been measured by determining excited-state densities following pulsed laser excitation of the 5 $^{1}$ P state. We observe 5 $^{1}$ P collisional transfer exclusively to the 4 $^{3}$ D state, with (\ensuremath{\sim}800 K) rate coefficients ranging from 0.24 to 2.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for the various gases. We also measure 4 $^{1}$ ${\mathit{D}}_{2}$ transfer primarily to the 4 $^{3}$ D state, with rate coefficients of 0.65 to 6.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. The 4 $^{3}$ D state radiative lifetime is observed to be 2.9\ifmmode\pm\else\textpm\fi{}0.2 \ensuremath{\mu}s, collisional mixing between the 4 $^{3}$ ${\mathit{D}}_{\mathit{j}}$ fine-structure levels is essentially gas kinetic (k\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$), and k${10}^{\mathrm{\ensuremath{-}}13}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for 4 $^{3}$ D\ensuremath{\rightarrow}5 $^{3}$ P collisional transfer.Inert-gas-induced collisional transfer between the first four excited states of Sr has been measured by determining excited-state densities following pulsed laser excitation of the 5 $^{1}$P state. We observe 5 $^{1}$P collisional transfer exclusively to the 4 $^{3}$D state, with (\ensuremath{\sim}800 K) rate coefficients ranging from 0.24 to 2.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for the various gases. We also measure 4 $^{1}$${\mathit{D}}_{2}$ transfer primarily to the 4 $^{3}$D state, with rate coefficients of 0.65 to 6.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. The 4 $^{3}$D state radiative lifetime is observed to be 2.9\ifmmode\pm\else\textpm\fi{}0.2 \ensuremath{\mu}s, collisional mixing between the 4 $^{3}$${\mathit{D}}_{\mathit{j}}$ fine-structure levels is essentially gas kinetic (k\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$), and k${10}^{\mathrm{\ensuremath{-}}13}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for 4 $^{3}$D\ensuremath{\rightarrow}5 $^{3}$P collisional transfer.

Physical Review AAug 1, 199222 citations

Effects of alignment on strontium energy pooling collisions

Jonathan Andrew Neuman, Alan Gallagher

The dependence of energy pooling cross sections on the alignment of atomic angular momentum with respect to the collision axis is investigated. In a vapor cell, excitation with counter-propagating polarized laser beams that define the collision axis prepares aligned strontium atoms in the $5s5p{}^{3}{P}_{1}$ level. Excited-state energy pooling collision products from the endothermic $5s5p{}^{3}{P}_{1}+5s5p{}^{3}{P}_{1}\ensuremath{\rightarrow}5s6s{}^{1}{S}_{0}{+5s}^{2}{}^{1}{S}_{0}$ reaction and the exothermic $5s5p{}^{3}{P}_{1}+5s5p{}^{3}{P}_{1}\ensuremath{\rightarrow}5s5p{}^{1}{P}_{1}{+5s}^{2}{}^{1}{S}_{0}$ reaction are studied. Alignment effects are studied as a function of collision energy, using pulsed laser excitation followed by delayed fluorescence detection to lower the transverse collision velocity and laser detuning to vary the collision velocity in the laser-beam direction. The rate coefficient for energy transfer to the $6{}^{1}{S}_{0}$ state is enhanced by $38\ifmmode\pm\else\textpm\fi{}13%$ when the ${M}_{J}$ alignment of the collision partners is perpendicular to the collision axis and parallel to each other compared to when there is no alignment of the collision partners. No alignment dependence is observed for energy transfer to the $5{}^{1}{P}_{1}$ state. This experiment offers the unique capability of simultaneously and independently controlling collision velocity and angular momentum alignment.The dependence of energy pooling cross sections on the alignment of atomic angular momentum with respect to the collision axis is investigated. In a vapor cell, excitation with counter-propagating polarized laser beams that define the collision axis prepares aligned strontium atoms in the $5s5p{}^{3}{P}_{1}$ level. Excited-state energy pooling collision products from the endothermic $5s5p{}^{3}{P}_{1}+5s5p{}^{3}{P}_{1}\ensuremath{\rightarrow}5s6s{}^{1}{S}_{0}{+5s}^{2}{}^{1}{S}_{0}$ reaction and the exothermic $5s5p{}^{3}{P}_{1}+5s5p{}^{3}{P}_{1}\ensuremath{\rightarrow}5s5p{}^{1}{P}_{1}{+5s}^{2}{}^{1}{S}_{0}$ reaction are studied. Alignment effects are studied as a function of collision energy, using pulsed laser excitation followed by delayed fluorescence detection to lower the transverse collision velocity and laser detuning to vary the collision velocity in the laser-beam direction. The rate coefficient for energy transfer to the $6{}^{1}{S}_{0}$ state is enhanced by $38\ifmmode\pm\else\textpm\fi{}13%$ when the ${M}_{J}$ alignment of the collision partners is perpendicular to the collision axis and parallel to each other compared to when there is no alignment of the collision partners. No alignment dependence is observed for energy transfer to the $5{}^{1}{P}_{1}$ state. This experiment offers the unique capability of simultaneously and independently controlling collision velocity and angular momentum alignment.

Physical Review AMar 1, 19982 citations

Velocity dependence of energy pooling collisions in strontium

Jonathan Andrew Neuman, J. Cooper, Alan C. Gallagher

The threshold behavior of the cross section of an endothermic energy pooling reaction is investigated. In a vapor cell experiment, pulsed excitation followed by delayed fluorescence detection narrows the transverse velocity distribution of colliding pairs of laser-excited strontium atoms in the ${5s5p}^{3}{P}_{1}$ level, and laser detuning establishes the collision velocity in the direction of counterpropagating laser beams. A probe laser detects the excited-state collision product of the endothermic ${5s5p}^{3}{P}_{1}{+5s5p}^{3}{P}_{1}\ensuremath{\rightarrow}{5s6s}^{1}{S}_{0}{+5s}^{21}{S}_{0}$ reaction. The cross section rises steeply at threshold and is nearly flat for 1.2--2 times the threshold energy. This behavior can be partly explained by the shape of the molecular potential curve of the collisionally produced state. From the measured $1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}{\mathrm{cm}}^{3}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ thermally averaged rate coefficient, the above-threshold cross section is found to be approximately $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}{\mathrm{cm}}^{2}.$ The thermally averaged rate coefficient for the highly exothermic energy pooling reaction ${5s5p}^{3}{P}_{1}{+5s5p}^{3}{P}_{1}\ensuremath{\rightarrow}{5s5p}^{1}{P}_{1}{+5s}^{21}{S}_{0}$ is also reported and measured to be $1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1}.$ The cross section for this exothermic reaction is found to be nearly independent of collision velocity.The threshold behavior of the cross section of an endothermic energy pooling reaction is investigated. In a vapor cell experiment, pulsed excitation followed by delayed fluorescence detection narrows the transverse velocity distribution of colliding pairs of laser-excited strontium atoms in the ${5s5p}^{3}{P}_{1}$ level, and laser detuning establishes the collision velocity in the direction of counterpropagating laser beams. A probe laser detects the excited-state collision product of the endothermic ${5s5p}^{3}{P}_{1}{+5s5p}^{3}{P}_{1}\ensuremath{\rightarrow}{5s6s}^{1}{S}_{0}{+5s}^{21}{S}_{0}$ reaction. The cross section rises steeply at threshold and is nearly flat for 1.2--2 times the threshold energy. This behavior can be partly explained by the shape of the molecular potential curve of the collisionally produced state. From the measured $1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}{\mathrm{cm}}^{3}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ thermally averaged rate coefficient, the above-threshold cross section is found to be approximately $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}{\mathrm{cm}}^{2}.$ The thermally averaged rate coefficient for the highly exothermic energy pooling reaction ${5s5p}^{3}{P}_{1}{+5s5p}^{3}{P}_{1}\ensuremath{\rightarrow}{5s5p}^{1}{P}_{1}{+5s}^{21}{S}_{0}$ is also reported and measured to be $1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1}.$ The cross section for this exothermic reaction is found to be nearly independent of collision velocity.

Physical Review AJul 1, 19975 citations

The collision cross sections for excitation energy transfer in Rb*(5P 3/2) + K(4S 1/2) → Rb(5S 1/2) + K*(4P J ) processes

Vlasta Horvatić, Cedomil Vadla, Mladen Movre

Zeitschrift für Physik D Atoms Molecules and ClustersJun 1, 19938 citations

Energy-pooling collisions in barium

Jonathan Andrew Neuman, Alan C. Gallagher, J. Cooper

Rate coefficients for energy-pooling collisions between low-lying excited states of Ba have been measured. The 6s6p ${(}^{3}$ ${\mathit{P}}_{1}$ ) level in Ba is pumped by a cw diode laser, and radiative decay and collisions with buffer-gas atoms also populate the 5d6s ${(}^{1}$ D) and 5d6s ${(}^{3}$ D) metastable levels. The densities of these low-lying excited states are measured by the absorption of lines from a Ba hollow-cathode lamp, and the states populated by energy-pooling collisions are studied by comparing their fluorescence intensity to that of the laser-excited level. The rate coefficients are on the order of gas-kinetic rates, and they are not strongly dependent on energy defect, spin changes, or angular-momentum changes.Rate coefficients for energy-pooling collisions between low-lying excited states of Ba have been measured. The 6s6p${(}^{3}$${\mathit{P}}_{1}$) level in Ba is pumped by a cw diode laser, and radiative decay and collisions with buffer-gas atoms also populate the 5d6s${(}^{1}$D) and 5d6s${(}^{3}$D) metastable levels. The densities of these low-lying excited states are measured by the absorption of lines from a Ba hollow-cathode lamp, and the states populated by energy-pooling collisions are studied by comparing their fluorescence intensity to that of the laser-excited level. The rate coefficients are on the order of gas-kinetic rates, and they are not strongly dependent on energy defect, spin changes, or angular-momentum changes.

Physical Review AAug 1, 199420 citations

The following energy-transfer reactions have been observed in a Sr vapor following pulsed excitation of the 5

^{1}$${\mathit{P}}_{1}

state:

{\mathrm{Sr}}^{\mathrm{*}}

^{1}$${\mathit{P}}_{1}

{\mathrm{Sr}}^{\mathrm{*}}

^{1}$${\mathit{P}}_{1}

)\ensuremath{\rightarrow}Sr(5

^{1}$${\mathit{S}}_{0}

{\mathrm{Sr}}^{\mathrm{*}\mathrm{*}}

(

{\mathit{n}}^{2\mathit{S}+1}

L)-\ensuremath{\Delta}E,

{\mathrm{Sr}}^{\mathrm{*}}

^{1}$${\mathit{D}}_{2}

) +

{\mathrm{Sr}}^{\mathrm{*}}

^{1}$${\mathit{D}}_{2}

)\ensuremath{\rightarrow}Sr(5

^{1}$${\mathit{S}}_{0}

{\mathrm{Sr}}^{\mathrm{*}\mathrm{*}}

(

{\mathit{n}}^{2\mathit{S}+1}

L)-\ensuremath{\Delta}E. The 4

^{1}$${\mathit{D}}_{2}

state was populated by a nonlinear stimulated process. By measuring the rate coefficients for these reactions for over 40 final states

{\mathit{n}}^{2\mathit{S}+1}

Energy pooling (EP) has been observed in Sr vapor following pulsed optical excitation to the 5

{}^{3}

{P}_{1}

state. From the time-dependent radiative decay of 6

{}^{3}

{S}_{1}

and of

{6}^{1}

{\mathit{S}}_{0}

via the cascade resonance line, the EP rate coefficients have been investigated for the following: Sr(5

^{3}P_{J}

)+Sr(5

{}^{3}

{\mathit{P}}_{\mathit{J}\mathcal{'}}

)

{(5}^{1}

{S}_{0}

) +Sr(6

^{3}S_{1}

{6}^{1}

{S}_{0}

). The values of

{k}_{11}

have been measured, as well as

{k}_{\mathrm{JJ}\mathcal{'}}

averaged over a thermal distribution of 5

^{3}P_{J}

populations. The time-dependent Sr(

{5}^{3}

{P}_{J}

) densities were measured by absorption of lines from a Sr lamp crossing the excitation region. The rate coefficients are surprisingly large; in spite of the small statistical weights of these product states, they are nearly gas-kinetic. The dependence of the rates upon spin, J, and energy defect \ensuremath{\Delta}E has also been examined.

Inert-gas-induced collisional transfer between the first four excited states of Sr has been measured by determining excited-state densities following pulsed laser excitation of the 5

^{1}

P state. We observe 5

^{1}

P collisional transfer exclusively to the 4

^{3}

D state, with (\ensuremath{\sim}800 K) rate coefficients ranging from 0.24 to 2.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$

{\mathrm{cm}}^{3}

${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for the various gases. We also measure 4

^{1}

{\mathit{D}}_{2}

transfer primarily to the 4

^{3}

D state, with rate coefficients of 0.65 to 6.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$

{\mathrm{cm}}^{3}

${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. The 4

^{3}

D state radiative lifetime is observed to be 2.9\ifmmode\pm\else\textpm\fi{}0.2 \ensuremath{\mu}s, collisional mixing between the 4

^{3}

{\mathit{D}}_{\mathit{j}}

fine-structure levels is essentially gas kinetic (k\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$

{\mathrm{cm}}^{3}

${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$), and k${10}^{\mathrm{\ensuremath{-}}13}$

{\mathrm{cm}}^{3}

${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for 4

^{3}

D\ensuremath{\rightarrow}5

^{3}

P collisional transfer.

5s5p{}^{3}{P}_{1}

level. Excited-state energy pooling collision products from the endothermic $5s5p{}^{3}{P}_{1}+5s5p{}^{3}{P}_{1}\ensuremath{\rightarrow}5s6s{}^{1}{S}_{0}{+5s}^{2}{}^{1}{S}_{0}$ reaction and the exothermic $5s5p{}^{3}{P}_{1}+5s5p{}^{3}{P}_{1}\ensuremath{\rightarrow}5s5p{}^{1}{P}_{1}{+5s}^{2}{}^{1}{S}_{0}$ reaction are studied. Alignment effects are studied as a function of collision energy, using pulsed laser excitation followed by delayed fluorescence detection to lower the transverse collision velocity and laser detuning to vary the collision velocity in the laser-beam direction. The rate coefficient for energy transfer to the

6{}^{1}{S}_{0}

state is enhanced by $38\ifmmode\pm\else\textpm\fi{}13%$ when the

{M}_{J}

alignment of the collision partners is perpendicular to the collision axis and parallel to each other compared to when there is no alignment of the collision partners. No alignment dependence is observed for energy transfer to the

5{}^{1}{P}_{1}

state. This experiment offers the unique capability of simultaneously and independently controlling collision velocity and angular momentum alignment.

{5s5p}^{3}{P}_{1}

level, and laser detuning establishes the collision velocity in the direction of counterpropagating laser beams. A probe laser detects the excited-state collision product of the endothermic ${5s5p}^{3}{P}_{1}{+5s5p}^{3}{P}_{1}\ensuremath{\rightarrow}{5s6s}^{1}{S}_{0}{+5s}^{21}{S}_{0}$ reaction. The cross section rises steeply at threshold and is nearly flat for 1.2--2 times the threshold energy. This behavior can be partly explained by the shape of the molecular potential curve of the collisionally produced state. From the measured $1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}{\mathrm{cm}}^{3}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ thermally averaged rate coefficient, the above-threshold cross section is found to be approximately $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}{\mathrm{cm}}^{2}.$ The thermally averaged rate coefficient for the highly exothermic energy pooling reaction ${5s5p}^{3}{P}_{1}{+5s5p}^{3}{P}_{1}\ensuremath{\rightarrow}{5s5p}^{1}{P}_{1}{+5s}^{21}{S}_{0}$ is also reported and measured to be $1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1}.$ The cross section for this exothermic reaction is found to be nearly independent of collision velocity.

Rate coefficients for energy-pooling collisions between low-lying excited states of Ba have been measured. The 6s6p

{(}^{3}

{\mathit{P}}_{1}

) level in Ba is pumped by a cw diode laser, and radiative decay and collisions with buffer-gas atoms also populate the 5d6s

{(}^{1}

D) and 5d6s

{(}^{3}

D) metastable levels. The densities of these low-lying excited states are measured by the absorption of lines from a Ba hollow-cathode lamp, and the states populated by energy-pooling collisions are studied by comparing their fluorescence intensity to that of the laser-excited level. The rate coefficients are on the order of gas-kinetic rates, and they are not strongly dependent on energy defect, spin changes, or angular-momentum changes.