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Physical Review A

Time-dependence study of radiation trapping by time-delayed two-photon absorption

William A. Molander, Michael Scott Belsley, Alan D. Streater, Keith R. Burnett

Published Mar 1, 1984• 10 citations
Publisher

Abstract

The transport of resonance radiation through an optically thick vapor of Sr atoms is studied. A pulsed dye laser tuned to the 461-nm resonance line excites a narrow (\ensuremath{\sim}250-\ensuremath{\mu}m-diameter) column of Sr atoms along the axis of a cylindrical oven containing Sr vapor and Ar buffer gas. After a delay of \ensuremath{\le}80 ns, a second dye laser excites the atom from the first excited state ($5s5p$) to a higher excited state ($5s7s$). The fluorescence from this latter transition is monitored as the second laser is translated parallel to the first. Since the excited-state-excited-state fluorescence is not trapped, the result is a plot of density of atoms in the $5s5p$ state as a function of position from the originally excited volume. The results are discussed qualitatively.

DOI
10.1103/physreva.29.1548

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The effective radiative rate for Cs(6P3/2) resonance level in the presence of Ar is determined.Cs atoms were excited to the 6P3/2 state using a single-mode diode laser(pump laser).The excited-atom density and spatial distribution were mapped by monitoring the absorption of a counterpropagating single-mode diode laser beam(probe laser),tuned to the 6P3/28S1/2 transition which could be translated parallel to the pump beam.In the presence of radiation trapping,the spontaneous radiation rate is multiplied by the transmission factor T6p3/26S1/2,which describes the average probability that photons emitted within the fluorescence detection region can pass through the optically thick vapor without being absorbed.Position dependent 6P3/2 state densities had been combined with the collisional broadening rate 6P3/26S1/2 line due to both Ar and cesium perturbers to yield the T6p3/26S1/2.The effective radiative rates of the Cs D2 line as a function of the Ar pressure PAr were obtained.The fluorescence intensity I852 of the 6P3/26S1/2emission was measured simultaneously.The measured fluorescence ratios determined the ratios of the effective radiative rates at different Ar density.These ratios are in agreement with theoretical evaluation.

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Kinetic study of Sr(5 3 P J ) by time-resolved emission, 5 3 P 1 → 5 1 S 0 +hν(λ= 689.3 nm), following dye-laser excitation

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Electronically excited strontium atoms in the low-lying 5s5p(3PJ) state, 1.8 eV above the 5 s2(1S0) ground state, have been studied in the time-resolved mode by recording the forbidden emission at λ= 689.3 nm [Sr(53P1)→ Sr(51S0)+hν]. Sr(53P1) was generated by the repetitive pulsed dye-laser excitation of ground-state strontium atoms, in equilibrium with the solid at elevated temperatures, in a slow-flow system kinetically equivalent to a static system. The resulting resonance fluorescence at λ= 689.3 nm, following rapid Boltzmann equilibration within the 53PJ spin-orbit manifold, was then monitored by means of boxcar integration. Particular consideration was given to the development of a pre-trigger photomultiplier gating system which permitted decay measurements necessarily to be made on time scales considerably shorter than achieved hitherto with this type of investigation. Measurement of the fluorescence decays in the noble gases and across a range of atomic strontium concentration revealed significant effects due to radiation trapping. Extrapolation of the decay data to [Sr(51S0)]= 0 yielded the mean radiative lifetime τe= 20.1 ± 0.4 µs for the λ= 689.3 nm transition. This is compared with the range of values reported previously. The following upper limits are reported for the collisional-quenching constants of Sr(53PJ) by the noble gases (kQ/cm3 atom–1 s–1, T= 950 K): He, <5.5 × 10–15; Ne, <3.5 × 10–15; Ar, <3.9 × 10–15; Kr, <5.5 × 10–15 and Xe, <3.1 × 10–15, which are compared with data derived from the phase-shift technique. A limit is also reported of the rate constant for quenching of Sr(53PJ) by Sr(51S0) of <2 × 10–13 cm3 atom–1 s–1(T= 850–1000 K). The quenching of Sr(53PJ) by H2 and D2 is studied in detail, yielding the following results: kH2= 3.7 ± 0.4 × 10–12 cm3 molecule–1 s–1(950 K, 1 σ), kD2= 3.0 ± 0.2 × 10–12 cm3 molecule–1 s–1(950 K, 1 σ). The results for these two isotopes are compared with analogous data reported from other measurements for Ca(43PJ) and Mg(33PJ).

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We have measured the time-dependent fluorescence of the sodium $D$ lines following pulsed excitation of one $D$ line, in the presence of radiation trapping with optical depths ${k}_{0}L$ of \ensuremath{\sim}10 to 2000. When collisional coupling of the $3{P}_{\frac{1}{2}}$ and $3{P}_{\frac{3}{2}}$ levels and different radiative-escape probabilities for each $D$ line are taken into account, we obtain excellent agreement with Holstein's theory for the effective radiative decay rates in the Doppler region (${k}_{0}L\ensuremath{\sim}10\ensuremath{-}300$) and in the redistributed Lorentzian region (${k}_{0}L>1000$). For ${k}_{0}L$ between these two regions, we observe an abrupt transition between the two limiting formulas. The buildup rate of the sensitized fluorescence signal also yields a $3{P}_{\frac{3}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ excitation transfer cross section in agreement with our previously reported cw measurement. Additionally, we have measured the dependence of the $3P$ fluorescence on laser power and beam diameter, and we explain the observed approach to saturation. We suggest that the previously reported "anomalous" approaches to saturation may be explained in terms of the laser beam burning through the optically thick vapor. Laser-beam spatial intensity variations and self-focusing also contribute to fluorescence signals that deviate from the usual single-atom saturation behavior.

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Single and double ionization of strontium in the vicinity of four-photon excitation of the 5p2 1S0doubly excited state

Ioannis Liontos, S. Cohen, Agisilaos Bolovinos

We report on the single and double multiphoton ionization of ground state Sr atoms observed in an atomic beam experiment with laser pulses of ∼5 ns duration, maximum intensity ∼4 × 10 11 W cm −2 and within the 710–740 nm wavelength range. The Sr + spectrum consists of two strong lines originating from three-photon resonant four-photon ionization of bound states, a number of weak autoionizing resonances and a broad line due to four-photon excitation of the doubly excited 5p 2 1 S 0 state. The latter, along with a strong, broad and structured spectral feature, is also evident in the wavelength dependence of the doubly charged Sr 2+ ion. A weakly evident but reproducible inflection point ('knee' structure) appears in the intensity dependence of the Sr 2+ yield at the location of the 5p 2 1 S 0 resonance. A complementary fluorescence experiment revealed the accumulation of population in the 5p 1/2 , 3/2 , 6s 1/2 and 5d 3/2 , 5/2 excited Sr + states during the laser pulse. All fluorescence signals depend on laser wavelength in a manner similar to the recorded Sr + spectrum. The population accumulation in the 5p 1/2 , 3/2 ionic states unambiguously proves the absorption of two photons above the first 5s 1/2 atomic threshold while that of the 5d 3/2 , 5/2 and 6s 1/2 ones suggest the absorption of at least two more. Since under our laser pulse duration and intensity the absorption of such a number of photons in an unstructured continuum is highly improbable, it may be concluded that the process is mediated by dense manifolds of near- resonant doubly excited states, their role in the dynamics of laser–atom interaction under our conditions being far more crucial than in studies conducted using intense ultrashort pulses.

Journal of Physics B Atomic Molecular and Optical Physics•Feb 11, 2008•13 citations
The transport of resonance radiation through an optically thick vapor of Sr atoms is studied. A pulsed dye laser tuned to the 461-nm resonance line excites a narrow (\ensuremath{\sim}250-\ensuremath{\mu}m-diameter) column of Sr atoms along the axis of a cylindrical oven containing Sr vapor and Ar buffer gas. After a delay of \ensuremath{\le}80 ns, a second dye laser excites the atom from the first excited state (5s5p5s5p5s5p) to a higher excited state (5s7s5s7s5s7s). The fluorescence from this latter transition is monitored as the second laser is translated parallel to the first. Since the excited-state-excited-state fluorescence is not trapped, the result is a plot of density of atoms in the 5s5p5s5p5s5p state as a function of position from the originally excited volume. The results are discussed qualitatively.
The effective radiative rate for Cs(6P3/2) resonance level in the presence of Ar is determined.Cs atoms were excited to the 6P3/2 state using a single-mode diode laser(pump laser).The excited-atom density and spatial distribution were mapped by monitoring the absorption of a counterpropagating single-mode diode laser beam(probe laser),tuned to the 6P3/28S1/2 transition which could be translated parallel to the pump beam.In the presence of radiation trapping,the spontaneous radiation rate is multiplied by the transmission factor T6p3/26S1/2,which describes the average probability that photons emitted within the fluorescence detection region can pass through the optically thick vapor without being absorbed.Position dependent 6P3/2 state densities had been combined with the collisional broadening rate 6P3/26S1/2 line due to both Ar and cesium perturbers to yield the T6p3/26S1/2.The effective radiative rates of the Cs D2 line as a function of the Ar pressure PAr were obtained.The fluorescence intensity I852 of the 6P3/26S1/2emission was measured simultaneously.The measured fluorescence ratios determined the ratios of the effective radiative rates at different Ar density.These ratios are in agreement with theoretical evaluation.
Electronically excited strontium atoms in the low-lying 5s5p(3PJ) state, 1.8 eV above the 5 s2(1S0) ground state, have been studied in the time-resolved mode by recording the forbidden emission at λ= 689.3 nm [Sr(53P1)→ Sr(51S0)+hν]. Sr(53P1) was generated by the repetitive pulsed dye-laser excitation of ground-state strontium atoms, in equilibrium with the solid at elevated temperatures, in a slow-flow system kinetically equivalent to a static system. The resulting resonance fluorescence at λ= 689.3 nm, following rapid Boltzmann equilibration within the 53PJ spin-orbit manifold, was then monitored by means of boxcar integration. Particular consideration was given to the development of a pre-trigger photomultiplier gating system which permitted decay measurements necessarily to be made on time scales considerably shorter than achieved hitherto with this type of investigation. Measurement of the fluorescence decays in the noble gases and across a range of atomic strontium concentration revealed significant effects due to radiation trapping. Extrapolation of the decay data to [Sr(51S0)]= 0 yielded the mean radiative lifetime τe= 20.1 ± 0.4 µs for the λ= 689.3 nm transition. This is compared with the range of values reported previously. The following upper limits are reported for the collisional-quenching constants of Sr(53PJ) by the noble gases (kQ/cm3 atom–1 s–1, T= 950 K): He, <5.5 × 10–15; Ne, <3.5 × 10–15; Ar, <3.9 × 10–15; Kr, <5.5 × 10–15 and Xe, <3.1 × 10–15, which are compared with data derived from the phase-shift technique. A limit is also reported of the rate constant for quenching of Sr(53PJ) by Sr(51S0) of <2 × 10–13 cm3 atom–1 s–1(T= 850–1000 K). The quenching of Sr(53PJ) by H2 and D2 is studied in detail, yielding the following results: kH2= 3.7 ± 0.4 × 10–12 cm3 molecule–1 s–1(950 K, 1 σ), kD2= 3.0 ± 0.2 × 10–12 cm3 molecule–1 s–1(950 K, 1 σ). The results for these two isotopes are compared with analogous data reported from other measurements for Ca(43PJ) and Mg(33PJ).
We present experiments of a laser with variable pulse energy and beam width saturating sodium vapor near the D1{D}_{1}D1​-line transition. At high optical depth and in a buffer gas atmosphere of argon, the effect of nonlinear radiation trapping is shown to drastically change the spatial distribution and temporal evolution of the excited-state population and of the fluorescence light trapped by the nonlinear medium. Besides observing the transient behavior of the fluorescence light, a pump-probe beam experiment enables us to gain a tomographic view inside the atomic vapor. Nitrogen as a buffer gas is used to compare with the situation where radiation trapping effects are negligible. Finally, numerical simulations based upon equations of radiative transfer for a two-level atomic system elucidate the role of nonlinear radiation trapping and support given interpretations of the experiments.
We have measured the time-dependent fluorescence of the sodium DDD lines following pulsed excitation of one DDD line, in the presence of radiation trapping with optical depths k0L{k}_{0}Lk0​L of \ensuremath{\sim}10 to 2000. When collisional coupling of the 3P123{P}_{\frac{1}{2}}3P21​​ and 3P323{P}_{\frac{3}{2}}3P23​​ levels and different radiative-escape probabilities for each DDD line are taken into account, we obtain excellent agreement with Holstein's theory for the effective radiative decay rates in the Doppler region (${k}_{0}L\ensuremath{\sim}10\ensuremath{-}300$) and in the redistributed Lorentzian region (k0L>1000{k}_{0}L>1000k0​L>1000). For k0L{k}_{0}Lk0​L between these two regions, we observe an abrupt transition between the two limiting formulas. The buildup rate of the sensitized fluorescence signal also yields a $3{P}_{\frac{3}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ excitation transfer cross section in agreement with our previously reported cw measurement. Additionally, we have measured the dependence of the 3P3P3P fluorescence on laser power and beam diameter, and we explain the observed approach to saturation. We suggest that the previously reported "anomalous" approaches to saturation may be explained in terms of the laser beam burning through the optically thick vapor. Laser-beam spatial intensity variations and self-focusing also contribute to fluorescence signals that deviate from the usual single-atom saturation behavior.
We report on the single and double multiphoton ionization of ground state Sr atoms observed in an atomic beam experiment with laser pulses of ∼5 ns duration, maximum intensity ∼4 × 10 11 W cm −2 and within the 710–740 nm wavelength range. The Sr + spectrum consists of two strong lines originating from three-photon resonant four-photon ionization of bound states, a number of weak autoionizing resonances and a broad line due to four-photon excitation of the doubly excited 5p 2 1 S 0 state. The latter, along with a strong, broad and structured spectral feature, is also evident in the wavelength dependence of the doubly charged Sr 2+ ion. A weakly evident but reproducible inflection point ('knee' structure) appears in the intensity dependence of the Sr 2+ yield at the location of the 5p 2 1 S 0 resonance. A complementary fluorescence experiment revealed the accumulation of population in the 5p 1/2 , 3/2 , 6s 1/2 and 5d 3/2 , 5/2 excited Sr + states during the laser pulse. All fluorescence signals depend on laser wavelength in a manner similar to the recorded Sr + spectrum. The population accumulation in the 5p 1/2 , 3/2 ionic states unambiguously proves the absorption of two photons above the first 5s 1/2 atomic threshold while that of the 5d 3/2 , 5/2 and 6s 1/2 ones suggest the absorption of at least two more. Since under our laser pulse duration and intensity the absorption of such a number of photons in an unstructured continuum is highly improbable, it may be concluded that the process is mediated by dense manifolds of near- resonant doubly excited states, their role in the dynamics of laser–atom interaction under our conditions being far more crucial than in studies conducted using intense ultrashort pulses.