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Journal of Physics D Applied Physics

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

Muhammad Arshad Khan, E.E. Khawaja, Mohammad F. Al-Kuhaili

Published Oct 14, 1993• 11 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3727/26/10/012

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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.

Journal of the Chemical Society Faraday Transactions 2 Molecular and Chemical Physics•Jan 1, 1986•37 citations

Kinetic study of Ca(4 3 PJ) by time-resolved emission, 4 3 P 1 → 4 1 S 0 +hν(λ= 657.3 nm), following dye-laser excitation. Spontaneous emission, diffusion and collisional quenching

David Husain, José Schifino

We present a kinetic study of the low-lying electronically excited state of atomic calcium, Ca[4s4p(3PJ)], 1.885 eV above the 4s2(1S0) ground state. Ca(4 3P1) was generated by dye-laser pulsed excitation at λ= 657.3 nm of calcium vapour [Ca(4 3P1)â†� Ca(4 1S0)] at elevated temperatures (1000 K), in equilibrium with solid calcium, in a slow-flow system, kinetically equivalent to a static system. Following rapid Boltzmann equilibration within Ca(4 3PJ) through collisions, the forbidden time-resolved atomic resonance fluorescence at the same wavelength was monitored using boxcar integration. The decay of Ca(4 3PJ) in the presence of all the noble gases, He, Ne, Ar, Kr and Xe, was studied, yielding measurements of relative diffusion coefficients. Extrapolation of the diffusional component of the decay in each noble gas to infinite pressure yields an independent value of the mean radiative lifetime for Ca(4 3P1)→ Ca(4 1S0)+hν(λ= 657.3 nm), the average of which from the measurements in all the noble gases yields τe= 0.34 ± 0.02 ms (error 2σ). This result is compared with values reported from previous measurements, particularly atomic beams and time-resolved measurements. The kinetic analysis also gives rise to data for the collisional quenching of Ca(4 3PJ), which is found to be insignificant for most of the noble gases [kHe,Ne,Ar,Kr⩽ 4 × 10–15 cm3 molecule–1 s–1(1000 K)], in contrast to measurements from earlier work using phase-shift techniques. Xenon, however, exhibited measurable rates of collisional quenching of Ca(4 3PJ) characterised by kXe= 2.4 ± 0.3 × 10–14 cm3 molecule–1 s–1(1000 K). The time-resolved emission technique is further applied to the study of the removal of Ca(4 3PJ) by added gases, for which we report (errors 1σ)kN2=(8.9 ± 0.9)× 10–13 cm3 molecule–1 s–1(1000 K), kH2=(6.0 ± 0.6)× 10–14 cm3 molecule–1 s–1(1000 K), kD2=(2.7 ± 0.3)× 10–14 cm3 molecule–1 s–1(1000 K).

Journal of the Chemical Society Faraday Transactions 2 Molecular and Chemical Physics•Jan 1, 1983•25 citations
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.
Experimental results on the ionization of calcium atoms following resonant pumping of the 4s 2 1 S 0 -4s4p 3 P 1 transition with a CW ring dye laser are reported. Absorption profiles of the 4s 2 1 S 0 -4s4p 3 P 1 transition around 6572.78 AA are measured under different conditions of temperature (T) and buffer gas pressure (p) where ionization was observed. Voigt profile fits are seen to adequately describe the observed absorption profiles. The results on collisional widths are in agreement with some previously published measurements. At low buffer gas pressures, collisional widths are found to scale as T 0.5 .
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 .
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.
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.
We present a kinetic study of the low-lying electronically excited state of atomic calcium, Ca[4s4p(3PJ)], 1.885 eV above the 4s2(1S0) ground state. Ca(4 3P1) was generated by dye-laser pulsed excitation at λ= 657.3 nm of calcium vapour [Ca(4 3P1)â†� Ca(4 1S0)] at elevated temperatures (1000 K), in equilibrium with solid calcium, in a slow-flow system, kinetically equivalent to a static system. Following rapid Boltzmann equilibration within Ca(4 3PJ) through collisions, the forbidden time-resolved atomic resonance fluorescence at the same wavelength was monitored using boxcar integration. The decay of Ca(4 3PJ) in the presence of all the noble gases, He, Ne, Ar, Kr and Xe, was studied, yielding measurements of relative diffusion coefficients. Extrapolation of the diffusional component of the decay in each noble gas to infinite pressure yields an independent value of the mean radiative lifetime for Ca(4 3P1)→ Ca(4 1S0)+hν(λ= 657.3 nm), the average of which from the measurements in all the noble gases yields τe= 0.34 ± 0.02 ms (error 2σ). This result is compared with values reported from previous measurements, particularly atomic beams and time-resolved measurements. The kinetic analysis also gives rise to data for the collisional quenching of Ca(4 3PJ), which is found to be insignificant for most of the noble gases [kHe,Ne,Ar,Kr⩽ 4 × 10–15 cm3 molecule–1 s–1(1000 K)], in contrast to measurements from earlier work using phase-shift techniques. Xenon, however, exhibited measurable rates of collisional quenching of Ca(4 3PJ) characterised by kXe= 2.4 ± 0.3 × 10–14 cm3 molecule–1 s–1(1000 K). The time-resolved emission technique is further applied to the study of the removal of Ca(4 3PJ) by added gases, for which we report (errors 1σ)kN2=(8.9 ± 0.9)× 10–13 cm3 molecule–1 s–1(1000 K), kH2=(6.0 ± 0.6)× 10–14 cm3 molecule–1 s–1(1000 K), kD2=(2.7 ± 0.3)× 10–14 cm3 molecule–1 s–1(1000 K).