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Journal of Physics B Atomic Molecular and Optical Physics

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

Published Mar 14, 1995• 2 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/28/5/016

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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.
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.
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 .
Time-resolved nsec kinetic methods are used to measure state specific electronic excitation transfer for the near-resonant process: Ca(4s5p /sup 1/P/sub 1/) +Ar /sup k//sub 1/arrow-right-left/sub k//sub -/1 Ca(4s5p /sup 3/P/sub 2,1,0/)+Ar. In stimulated emission experiments it is found that the Ca(4s5p /sup 3/P/sub 2/) level is preferentially populated. The rate constant k/sub 1/ for the specific /sup 3/P/sub 2/ channel is 5.2 +- 0.5 x 10/sup -11/ cm/sup 3/ molecule/sup -1/ sec/sup -1/. These results are discussed in terms of near-resonant spin changing collision mechanisms. Radiative lifetimes for Ca(4s5p /sup 1/P/sub 1/) and Ca(4s5p /sup 3/P/sub 1/) are also determined to be 77 +- 2 nsec and 67 +- 2 nsec, respectively.
The possibility of probing the collisions of aligned Rydberg atoms by stimulated emission is assessed with studies of a polarized state and a new measurement of a collisional alignment effect in atomic Ca. The stimulated emission method uses a laser to dump the desired state to a lower level which subsequently fluoresces. The technique can be used to obtain populations and polarization dependent information. First, the method is tested by applying it to an aligned Ca(4s17d 1D2) state. Alignment curves are measured when the initial state is prepared with both parallel and perpendicular relative polarizations. The experimentally observed alignment compares well with that derived from theoretical considerations of a saturated stimulated transition. Second, a two-vector collisional alignment experiment (initial state and relative velocity vector) is performed to study the energy transfer process Ca(4s7d 1D2)+He→Ca(4s6f 1F3)+He+ΔE=17.7 cm−1, and alignment effects are measured by both stimulated emission and conventional direct fluorescence detection. A preference for the ‖m‖=1 and 2 initial states is observed in the relative cross sections. Essentially identical data are obtained with the two detection methods when elliptically polarized light is used for the stimulated emission detection method. The stimulated emission technique can provide alignment and population information of the final states, making it an excellent new tool for both three-vector correlation experiments and state-to-state Rydberg transitions.
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 .