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Journal of Physics B Atomic and Molecular Physics

Time-resolved fluorescence spectroscopy of two-photon-excited singlet states of mercury

Farhad H. M. Faisal, Richard E. Wallenstein, Richard E. Teets

Published May 28, 1980• 20 citations
Publisher

Abstract

The authors report on the two-photon excitation of 1 S 0 and 1 D 2 states in mercury by pulsed ultraviolet dye laser radiation. A time-resolved registration of two-photon-excited fluorescence provided an accurate determination of the radiative lifetimes of the levels 7 1 S 0 to 9 1 S 0 and 6 1 D 2 to 8 1 D 2 . In an external magnetic field Zeeman quantum beats and the zero-field level crossing signal have been observed. Furthermore the authors describe conditions which cause strong emission of amplified spontaneous radiation in transitions between highly excited mercury levels.

DOI
10.1088/0022-3700/13/10/010

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The authors report on the two-photon excitation of 1 S 0 and 1 D 2 states in mercury by pulsed ultraviolet dye laser radiation. A time-resolved registration of two-photon-excited fluorescence provided an accurate determination of the radiative lifetimes of the levels 7 1 S 0 to 9 1 S 0 and 6 1 D 2 to 8 1 D 2 . In an external magnetic field Zeeman quantum beats and the zero-field level crossing signal have been observed. Furthermore the authors describe conditions which cause strong emission of amplified spontaneous radiation in transitions between highly excited mercury levels.
We have observed fluorescence and ionization signals resulting from two-photon resonant excitation of thallium in an atomic beam. The output of a pulsed dye laser is used to excite the resonance between the ground state (6('2)P(,1/2)) and an excited n('2)P state of thallium. Both two-photon absorption and three-photon absorption occur simultaneously; the latter results in photoionization of the atom. The excited states populated by two-photon absorption decay via cascade fluorescence back to the ground state; one or more of these spectral components constitute our fluorescence signal. The lifetimes of the excited n('2)P states of thallium have been measured directly by observing the exponential decay of this fluorescence. Calculations are included to ascertain the time required for the fluorescence signal to reach within 2% of a simple exponential decay. This decay is a measure of the lifetime of the initial state, under the condition that none of the fluorescence components arise from states which are populated by cascade through longer-lived states. The lifetimes of the n('2)P(,1/2) and n('2)P(,3/2) states of thallium for n = 7 to 11 are determined in this manner and compared with the values deduced from semi-empirical calculations of oscillator strengths by various authors. For calculations employing the effects of core polarizability in the electron wavefunctions and in the transition matrix elements, the agreement is very good--to within our experimental uncertainty, which is typically(, )<5%. Our measurements are also compared to the four other known experimental values for such excited states of thallium. Some of these previous experiments and calculations were carried out in conjunction with parity non-conservation experiments in thallium. Our measurements may provide useful checks on the accuracy of some of the calculations performed in the evaluation of such experiments. The ionization and fluorescence signals are also studied under conditions of strong saturation of the two-photon resonance with the 7('2)P states. The manifestation of saturation in the ionization signal is a broadening of the excitation spectrum, which is found to greatly exceed that of the fluorescence signal. Furthermore, the ratio of the ionization signal to the fluorescence signal at the peak of the resonance is found to vary linearly at intensities below about 10('9) W/cm('2); this too saturates at higher intensities. Various models are employed to simulate the observed behavior, encompassing both rate equations as well as density matrix formulations. The numerical solutions to these models are compared with the observed behavior; the agreement is found to depend strongly on the assumed experi- mental conditions to which the ensemble of atoms is subjected.
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We have observed the oscillatory magnetic-field and time dependence of the fluorescence from the 1P1^{1}P_{1}1P1​ levels of Ba and Ca excited by a pulsed tunable dye laser. We have observed level-crossing signals narrowed by the technique of time-delayed observation.
The effects of resonant laser excitation of an atomic transition whose lower state is not a ground state are reported. The system studied is the optical excitation to the 61D2 state from the electron-excited 61P1 state of Hg. Both the total fluorescence intensity and the time-resolved electron-photon coincidence fluorescence signal from the 61D2–63P1 relaxation channel are observed as the laser intensity is varied. In the coincidence signals, a manifestation of Rabi cycling in the optically excited transition is evident. The experimental data are in generally good agreement with predictions from a quantum-electrodynamic model.