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Journal of Physics B Atomic and Molecular Physics

Collisional effects and two-photon absorption in calcium vapour

Alan Corney, J V M McGinley

Published Sep 28, 1982• 2 citations
Publisher

Abstract

Experiments on collisionally assisted two-photon absorption (TPA) in calcium vapour buffered by argon gas have been performed using two tunable dye lasers. Both direct TPA and stepwise transitions through the collisionally populated resonance level. 4s4p 1 P 1 , have been observed. Detailed study of the lineshapes of the stepwise transitions to the 4p 2 1 S 0 , 4p 2 1 D 2 and 4s6s 1 S 0 levels reveals large differences in the strengths of the Ca-Ar interatomic potentials for these levels. These differences probably arise from configuration mixing.

DOI
10.1088/0022-3700/15/18/003

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Journal of Physics B Atomic and Molecular Physics•Oct 28, 1987•12 citations
Experiments on collisionally assisted two-photon absorption (TPA) in calcium vapour buffered by argon gas have been performed using two tunable dye lasers. Both direct TPA and stepwise transitions through the collisionally populated resonance level. 4s4p 1 P 1 , have been observed. Detailed study of the lineshapes of the stepwise transitions to the 4p 2 1 S 0 , 4p 2 1 D 2 and 4s6s 1 S 0 levels reveals large differences in the strengths of the Ca-Ar interatomic potentials for these levels. These differences probably arise from configuration mixing.
By tuning a pulsed dye laser through the resonance line of calcium at 422.7 nm the authors have investigated the spectral redistribution of laser radiation by calcium vapour. Two main components were resolved in the side emission from an oven containing calcium vapour and argon buffer gas. These are produced by Rayleigh scattering and by collision-induced fluorescence. The intensity of the Rayleigh component was found to vary as Delta -2 where Delta = omega 21 - omega L is the laser detuning from resonance. In contrast the intensity of the collision-induced fluorescence showed a markedly asymmetric dependence on detuning. Detailed studies of the collision-induced fluorescence have enabled the complete line shape of the calcium resonance line perturbed by argon to be determined. The interpretation of this data in terms of a frequency-dependent collision rate, gamma E ( Delta ), is discussed. This paper also reports calculations of theoretical line profiles using the unified Franck-Condon theory and trial interatomic difference potentials. The comparison of the experimental and theoretical line profiles enables the range of validity of the unified Franck-Condon theory to be assessed and the form of the optimum Ca-Ar difference potential to be selected.
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 .
Using a pulsed tunable dye laser pumped by a nitrogen laser, the 4s4p 1 P 1 0 level in Ca I was selectively populated from the 4s 2 1 S 0 ground state. Transitions from the 4s4p 1 P 1 0 level to 4sns 1 S 0 and 4snd 1 D 2 and, for n approximately=20, also to 4snp 1 P 1 0 and 4snf 1 F 3 0 were observed. The latter transitions arise because of Stark mixing of Rydberg states in the field of laser-produced ions and electrons. By applying a voltage to electrodes in the calcium vapour, Stark shifts were measured for levels belonging to the 4snf 1 F 3 0 and 4s(n+1)d 1 D 2 series in a region where the series cross. Diagonalisation of the energy matrix for interacting states with hydrogenic dipole matrix elements gave theoretical Stark shifts in reasonable agreement with those measured. The 4snf 1 F 3 0 series has been extended from n=12 to n=28 by absorption from 4s3d 1 D 2 . The latter level was found to be populated by stimulated emission from 4s4p 1 P 1 0 .
In the present study we have investigated three-photon ionization in Ca in which autoionizing states are engaged. The two-photon resonant process (from the Ca ground state 4s 2 1 S 0 ) occurred through or at least in the vicinity of one of the following states: 4s4d 1 D 2 , 4p 2 3 P 2 , 4s6s 1 S 0 , 4p 2 1 D 2 and 4p 2 1 S 0 , with the third photon either reaching the continuum directly or one of the autoionizing states. The three-photon resonant transitions to 3d m p, m f: 1 P 1 , 3 P 1 and 3 D 1 autoionizing states for m up to 21 have been observed. Some of the autoionizing resonances which we have found had not been observed before in a high-resolution one-photon absorption experiment (for J = 1) and in multiphoton experiments (for J = 3). We have compared the ionization signal as a function of the laser detuning and the laser intensity with theoretical curves obtained within a simple model (three-level atom + one-mode laser field). This gives information about the order of magnitude of the three-photon ionization probability through autoionizing states.
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.
Saturation and power broadening of the calcium singlet resonance line (422.7 nm) was found to show strong pressure dependence for helium, argon and xenon. This was ascribed to transfer of excitation from the (4s 4p) 1 P 1 level through the sequence of levels 1 D 2 , 3 D J , 3 P J by predominantly collisional processes. The first step of this cascade, 1 P 1 - 1 D 2 , was identified as the slowest process and cross sections are presented. The temperature independence of the process for xenon indicates that the large cross section for this gas may be due to a favourable potential curve crossing. Quenching of the 3 P J levels is also detected and cross sections are given for argon and xenon.