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Journal of Physics B Atomic Molecular and Optical Physics

Collisional recycling of excited atomic populations in Ca in a thermionic diode following pulsed-laser excitation of a metastable state

Muhammad Arshad Khan, M.A. Gondal, M. H. Rais

Published Jul 28, 1994• 12 citations
Publisher

Abstract

Experimental results are reported where the rates of collisional energy transfer in optically prepared atomic metastable states are considerable compared with the rate of normal radiative decay. A remarkable consequence is a large increase in the effective lifetime of some excited states lying above the optically prepared state. The specific case of calcium is presented here as an example where over a 1000-fold increase in the lifetime of 4s3d 3 D states has been experimentally measured following pulsed-laser pumping of 4s4p 3 P 1 metastable state. This can have important applications in the development of quasi-CW lasers. The possibility of populating 4s4p 1 P 1 and 4s5s 3 S 1 states through energy pooling collisions is also investigated. The rate coefficient for pooling to the 4s5s 3 S 1 state is estimated to be 10 -8 -10 -9 cm 3 s -1 .

DOI
10.1088/0953-4075/27/14/024

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Energy pooling from Ca[4s4p(3 P J )] and Ca[4s3d(1 D 2)] studied by time-resolved atomic emission following pulsed dye-laser excitation

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Two separate series of investigations have been carried out on the optically metastable states Ca[4s4p(3PJ)] and Ca[4s3d(1D2)], generated by pulsed dye-laser excitation at λ= 657.3 nm {Ca[4s4p(3P1)]â†� Ca[4s2(1S0)]} and λ= 457.5 nm {Ca[4s3d(1D2)]â†� Ca[4s2(1S0)]}, with particular emphasis on energy pooling. The kinetic behaviour of these two states has been characterised in detail across a wide range of temperature and pressure by time-resolved atomic emission measurements at the resonance wavelengths. In the case of Ca[4s3d(1D2)] these represent the first time-resolved measurements on this atomic state, yielding a mean radiative lifetime for emission to all lower states of τe= 1.71 ± 0.03 ms, in accord with time-of-flight measurements in atomic beams. The measured temperature dependence of the diffusion coefficient D12[Ca(4 1D2)–He], when expressed in the form D12∝Tn, yielded n= 1.77 ± 0.08. Further, the rate constant for the collisional quenching of Ca(4 1D2) by Ca(4 1S0) itself is estimated to be kCa=(4 ± 3)× 10–14 cm3 atom–1 s–1, in sensible accord with similar data for the quenching of Ca(4 3PJ) by the ground-state calcium atom. Energy pooling between Ca(4 3PJ)+ Ca(4 3PJ) is shown to give rise directly to Ca[4s4p(1P1)] and Ca[4s3d(1D2)]. Energy pooling between Ca(4 1D2)+ Ca(4 1D2) is shown to yield 22 electronic states, each of whose kinetic behaviour was characterised quantitatively from the relevant atomic emission profiles, including measurement of relative atomic yields.

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Collective effects in the ionization of calcium atoms following resonant laser pumping of 4s4p3P1metastable state

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An experiment is described in which large numbers ( approximately 10 16 ) of calcium atoms are placed in the 4s4p 3 P metastable level by optical pumping with a tunable dye laser and then studied spectroscopically. Equations governing the degree of excitation by the laser are derived. Predictions of the theory are shown to be in good agreement with experimental results. In particular, at low optical depths produced by 10 15 ground state atoms cm -2 the dye laser raises approximately 50% of the calcium atoms into the metastable state. At higher optical depths associated with 10 17 ground state atoms cm -2 the number of excited states is limited only by the laser energy and line width. For the dye laser used the largest number of metastables produced was approximately 10 16 . The absorption spectra of the 4s4p 2 P level is seen out to 4s4p 3 P to 4s22d 3 D. The relaxation rate between the excited state fine structure is studied directly with an optically delayed dye laser continuum. Other applications of this technique of producing large numbers of excited states are briefly discussed.

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Energy pooling in an equilibrium-coupled system between Ca[4s3d(1D2)] and Mg[3s3p(3P j )] atoms following pulsed dye-laser excitation

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We present a kinetic study of energy pooling in the equilibrium-coupled system: Ca[4s3d(1D2)]+ Mg[3s2(1S0)]⇌ Ca[4s2(1S0)]+ Mg[3s3p(3PJ)] following pulsed dye-laser excitation of both Mg(3PJ) and Ca(1D2) atoms at λ= 457.1 nm. [Mg(3P1)â†� Mg(1S0)] and λ= 457.5 nm [Ca(1D2)â†� Ca(1S0)] in the presence of helium across the temperature range 800–1100 K. Quantitative characterisation of decay profiles for Mg(3PJ) and Ca(1D2) by time-resolved emission measurements at the above wavelengths using boxcar integration indicates the effectively instantaneous establishment and maintenance of the near-resonant equilibrium under all conditions of temperature and pressure via the observed equality in the first-order decay coefficients for removal of both states. Energy pooling into some 16 higher-lying states of magnesium and calcium, including atomic levels close to the energetically accessible limit, were observed and their temporal behaviour characterised. With the exceptions of Mg[3s4s(3S1)] and Ca[4s4p(1P1)] atoms, all these pooled states exhibited first-order kinetic behaviour. Measured first-order decay coefficients for these pooled states were found to be always twice those of the energy store states, indicating their production via collisions involving the metastable states Mg(3PJ), Ca(1D2) and possibly Ca(3PJ). Initial relative populations of energy-pooled states were estimated in all cases and found to lie in the range 10–4–10–6, thus constituting a small ‘bleed-off’ from the and Ca(1D2) energy stores without disturbing the observed first-order decay of these two states in the equilibrium-coupled system.

Journal of the Chemical Society Faraday Transactions 2 Molecular and Chemical Physics•Jan 1, 1988•2 citations

Energy-pooling collisions in barium

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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 A•Aug 1, 1994•20 citations

Role of some selected intermediate states of Ca I in the collisional excitation of 4p2P states of Ca II in a laser-pumped Ca vapour

Muhammad Arshad Khan, M.A. Gondal, M. H. Rais

The role of some intermediate states, e.g. 4s5p 3 P, 3d4p( 1 D, 3 F) of Ca I in the collisional excitation of the 4p 2 P 3/2,1/2 states of Ca II (located at 74720.35 and 74497.46 cm -1 above the ground state of Ca I) following pulsed-laser pumping of the 4s4p 3 P 1 state of Ca I (E ex =15210.063 cm -1 ), is investigated. A 'pump' and 'probe' technique is employed together with the study of fluorescent emission. The 'probe' laser selectively prepares a substantially higher population of a specific intermediate state in Ca I and the increase in the fluorescent intensity of the 4p 2 P 3/2 - 4s 2 S 1/2 transition of Ca II is read as the relative strength of the particular collision channel utilizing that intermediate state. It is found that the 3d4p 1 D 2 state has the strongest influence.

Journal of Physics B Atomic Molecular and Optical Physics•Mar 14, 1995•2 citations
Experimental results are reported where the rates of collisional energy transfer in optically prepared atomic metastable states are considerable compared with the rate of normal radiative decay. A remarkable consequence is a large increase in the effective lifetime of some excited states lying above the optically prepared state. The specific case of calcium is presented here as an example where over a 1000-fold increase in the lifetime of 4s3d 3 D states has been experimentally measured following pulsed-laser pumping of 4s4p 3 P 1 metastable state. This can have important applications in the development of quasi-CW lasers. The possibility of populating 4s4p 1 P 1 and 4s5s 3 S 1 states through energy pooling collisions is also investigated. The rate coefficient for pooling to the 4s5s 3 S 1 state is estimated to be 10 -8 -10 -9 cm 3 s -1 .
Two separate series of investigations have been carried out on the optically metastable states Ca[4s4p(3PJ)] and Ca[4s3d(1D2)], generated by pulsed dye-laser excitation at λ= 657.3 nm {Ca[4s4p(3P1)]â†� Ca[4s2(1S0)]} and λ= 457.5 nm {Ca[4s3d(1D2)]â†� Ca[4s2(1S0)]}, with particular emphasis on energy pooling. The kinetic behaviour of these two states has been characterised in detail across a wide range of temperature and pressure by time-resolved atomic emission measurements at the resonance wavelengths. In the case of Ca[4s3d(1D2)] these represent the first time-resolved measurements on this atomic state, yielding a mean radiative lifetime for emission to all lower states of τe= 1.71 ± 0.03 ms, in accord with time-of-flight measurements in atomic beams. The measured temperature dependence of the diffusion coefficient D12[Ca(4 1D2)–He], when expressed in the form D12∝Tn, yielded n= 1.77 ± 0.08. Further, the rate constant for the collisional quenching of Ca(4 1D2) by Ca(4 1S0) itself is estimated to be kCa=(4 ± 3)× 10–14 cm3 atom–1 s–1, in sensible accord with similar data for the quenching of Ca(4 3PJ) by the ground-state calcium atom. Energy pooling between Ca(4 3PJ)+ Ca(4 3PJ) is shown to give rise directly to Ca[4s4p(1P1)] and Ca[4s3d(1D2)]. Energy pooling between Ca(4 1D2)+ Ca(4 1D2) is shown to yield 22 electronic states, each of whose kinetic behaviour was characterised quantitatively from the relevant atomic emission profiles, including measurement of relative atomic yields.
The ionization of calcium atoms resonantly excited to the 4s4p 3 P 1 metastable state is investigated. In particular, the influence of various experimental parameters on the total ionization yield and the rate of its growth are reported. Maximum ionization yield was obtained with the buffer gas (Ar) pressure in the range 100-650 mbar. Furthermore, the time history of depopulation of the metastable state 4s4p 3 P 1 monitored in a 'pump' and 'probe' experiment using a second dye laser tuned to the 4s4p 3 P 1 -4s5s 3 S 1 transition is presented. The effective lifetime of the 4s4p 3 P 1 state is shortened to 31+or-2 mu s at 850 degrees C and 800 mbar Ar pressure from the known value of about 350 mu s.
An experiment is described in which large numbers ( approximately 10 16 ) of calcium atoms are placed in the 4s4p 3 P metastable level by optical pumping with a tunable dye laser and then studied spectroscopically. Equations governing the degree of excitation by the laser are derived. Predictions of the theory are shown to be in good agreement with experimental results. In particular, at low optical depths produced by 10 15 ground state atoms cm -2 the dye laser raises approximately 50% of the calcium atoms into the metastable state. At higher optical depths associated with 10 17 ground state atoms cm -2 the number of excited states is limited only by the laser energy and line width. For the dye laser used the largest number of metastables produced was approximately 10 16 . The absorption spectra of the 4s4p 2 P level is seen out to 4s4p 3 P to 4s22d 3 D. The relaxation rate between the excited state fine structure is studied directly with an optically delayed dye laser continuum. Other applications of this technique of producing large numbers of excited states are briefly discussed.
We present a kinetic study of energy pooling in the equilibrium-coupled system: Ca[4s3d(1D2)]+ Mg[3s2(1S0)]⇌ Ca[4s2(1S0)]+ Mg[3s3p(3PJ)] following pulsed dye-laser excitation of both Mg(3PJ) and Ca(1D2) atoms at λ= 457.1 nm. [Mg(3P1)â†� Mg(1S0)] and λ= 457.5 nm [Ca(1D2)â†� Ca(1S0)] in the presence of helium across the temperature range 800–1100 K. Quantitative characterisation of decay profiles for Mg(3PJ) and Ca(1D2) by time-resolved emission measurements at the above wavelengths using boxcar integration indicates the effectively instantaneous establishment and maintenance of the near-resonant equilibrium under all conditions of temperature and pressure via the observed equality in the first-order decay coefficients for removal of both states. Energy pooling into some 16 higher-lying states of magnesium and calcium, including atomic levels close to the energetically accessible limit, were observed and their temporal behaviour characterised. With the exceptions of Mg[3s4s(3S1)] and Ca[4s4p(1P1)] atoms, all these pooled states exhibited first-order kinetic behaviour. Measured first-order decay coefficients for these pooled states were found to be always twice those of the energy store states, indicating their production via collisions involving the metastable states Mg(3PJ), Ca(1D2) and possibly Ca(3PJ). Initial relative populations of energy-pooled states were estimated in all cases and found to lie in the range 10–4–10–6, thus constituting a small ‘bleed-off’ from the and Ca(1D2) energy stores without disturbing the observed first-order decay of these two states in the equilibrium-coupled system.
Rate coefficients for energy-pooling collisions between low-lying excited states of Ba have been measured. The 6s6p(3{(}^{3}(3P1{\mathit{P}}_{1}P1​) level in Ba is pumped by a cw diode laser, and radiative decay and collisions with buffer-gas atoms also populate the 5d6s(1{(}^{1}(1D) and 5d6s(3{(}^{3}(3D) 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.
The role of some intermediate states, e.g. 4s5p 3 P, 3d4p( 1 D, 3 F) of Ca I in the collisional excitation of the 4p 2 P 3/2,1/2 states of Ca II (located at 74720.35 and 74497.46 cm -1 above the ground state of Ca I) following pulsed-laser pumping of the 4s4p 3 P 1 state of Ca I (E ex =15210.063 cm -1 ), is investigated. A 'pump' and 'probe' technique is employed together with the study of fluorescent emission. The 'probe' laser selectively prepares a substantially higher population of a specific intermediate state in Ca I and the increase in the fluorescent intensity of the 4p 2 P 3/2 - 4s 2 S 1/2 transition of Ca II is read as the relative strength of the particular collision channel utilizing that intermediate state. It is found that the 3d4p 1 D 2 state has the strongest influence.