Journal of the Chemical Society Faraday Transactions 2 Molecular and Chemical Physics
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
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).
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).
The kinetics of Ca(4 3PJ), Sr(5 3PJ) and Sr(6 3S1) have been studied by time-resolved emission following pulsed dye-laser excitation. Ground-state calcium atoms, Ca[4s2(1S0)], in the presence of Sr[5s2(1S0)], were optically excited to the first electronically excited state, Ca[4s4p(3P1)], by means of a dye-laser operating at λ= 657.3 nm [Ca(4 3P1)â†� Ca(4 1S0)]. Following rapid Boltzmann equilibration within the Ca(43PJ) manifold, fast, near-resonant energy transfer took place with Sr(5 1S0) to yield Sr(5 3PJ), together with energy pooling to yield Sr(6 3S1). The following transitions were monitored in the time-resolved mode using pre-trigger photomultiplier gating and boxcar integration with repetitive pulsing on a slow-flow system, kinetically equivalent to a static system: Ca(4 3P1)→ Ca(4 1S0)+hν(λ= 657.3 nm), Sr(5 3P1)→ Sr(5 1S0)+hν(λ= 689.3 nm) and Sr(6 3S1)→ Sr(5 3P0)+hν(λ= 679.1 nm). Detailed measurements of the time dependence of the emission profiles at λ= 657.3 and 689.3 nm demonstrated the rapid establishment of the equilibrium Ca(4 3PJ)+ Sr(5 1S0)⇌ Ca(4 1S0)+ Sr(5 3PJ) and its sustenance throughout kinetic decays. A further comparison of the first-order decay coefficients for the decays of Ca(4 3PJ) and Sr(5 3PJ) with those for Sr(6 3S1) demonstrated energy pooling between Ca(4 3PJ)+ Sr(5 3PJ) and Sr(5 3PJ)+ Sr(5 3PJ). Finally, the measured first-order decay coefficients, k′, for Ca(4 3PJ) and Sr(5 3PJ) in chemical equilibrium with one another are seperated quantitatively into their component parts of spontaneous emission, diffusion, the equilibrium constant connecting the 3PJ states and collisional quenching rate constants. Rate constants for quenching of Ca(4 3PJ) and Sr(5 3PJ) by Kr, Xe, H2 and D2 derived from earlier measurements on the individual atoms are found to be quantitatively consistent with the decay profiles under equilibrium conditions.
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).
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 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.
A kinetic investigation is presented of energy pooling arising from self-annihilation of the optically metastable state, Sr[5s5p(3PJ)], generated by pulsed dye-laser excitation of strontium vapour at λ= 689.3 nm {Sr[5s5p(3P1)]â†� Sr[5s2(1S0)]} at elevated temperature in the presence of excess helium buffer gas. Time-resolved emission was observed for a number of energy-pooled states, namely, Sr[5s5p(1P1)], Sr[5s6s(3S1)], Sr[4d5p(3F2, 3, 4)], Sr[4d5p(1D2)], Sr[5s6p(3P1, 2)], Sr[5s5d(3D2)] and Sr[5p2(3P2)]. This includes characterisation of the time profiles from specific spin–orbit states and significantly extends the range of atomic pooled states reported hitherto derived from the 5s5p(3PJ) energy store. Following rapid energy pooling, the profiles of the ‘pool’ and ‘store’ states are shown to be exponential in character and the first-order decay coefficients are found to be in the ratio of 2 ∶ 1 within experimental error. Further, using the appropriate sensitivity calibration of the optical system, the variation of the ratio of the integrated atomic emission intensities of the energy pooled states to that of the energy store itself were found to be linear and in accord with a mechanism for energy pooling in each case arising from 3PJ+3PJ self-annihilation. The normalised, integrated emission intensities of the energy-pooled states which involved population via endothermic processes from Sr(3PJ)+ Sr(3PJ) were determined and used to calculate the relative distributions of the energy-pooled states on collision. These were plotted semi-logarithmically as a function of the electronic state energy of the appropriate state and yielded a Boltzmann temperature higher than the ambient temperature employed, reflecting the role of atomic collisions with high kinetic energies on energy pooling.
Collisional deactivation of the 5d7p (3)D1 state of Ba by noble gases is studied by time- and wavelength-resolved fluorescence techniques. A pulsed, frequency-doubled dye laser at 273.9 nm excites the 5d7p (3)D1 level from the ground state, and fluorescence at 364.1 and 366.6 nm from the 5d7p (3)D1 --> 6s5d (3)D1 and 5d7p (3)D1 --> 6s5d (3)D2 transitions, respectively, is monitored in real time to obtain the deactivation rate constants. At 835 K these rate constants are as follows: He, (1.69 +/- 0.08) x 10(-9) cm(3) s(-1); Ne, (3.93 +/- 0.14) x 10(-10) cm(3) s(-1); Ar, (4.53 +/- 0.15) x 10(-10) cm(3) s(-1); Kr, (4.64 +/- 0.13) x 10(-10) cm(3) s(-1); Xe, (5.59 +/- 0.22) x 10(-10) cm(3) s(-1). From time-resolved 5d7p (3)D1 emission in the absence of noble gas and from the intercepts of the quenching plots, the lifetime of this state is determined to be 100 +/- 1 ns. Using time- and wavelength-resolved Ba emission with a low background pressure of noble gas, radiative lifetimes of several near-resonant states are determined from the exponential rise of the fluorescence signals. These results are as follows: 5d6d (3)D3, 28 +/- 3 ns; 5d7p (3)P1, 46 +/- 2 ns; 5d6d (3)G3, 21.5 +/- 0.8 ns; 5d7p (3)F3, 48 +/- 1 ns. Integrated fluorescence signals are used to infer the relative rate constants for population transfer from the 5d7p (3)D1 state to eleven near-resonant fine structure states.