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Physical Review A

Perturbation in I2 levels studied by nonlinear coherent spectroscopy

Chen Tai

Published Oct 1, 1984• 2 citations
Publisher

Abstract

The technique of harmonic generation with circularly polarized light is applied to study the Land\'e $g$ factor of the two-quantum excited states of ${\mathrm{I}}_{2}$ molecules. Systematic and abrupt changes in the $g$ factor are observed. The large change in the $g$ factor is interpreted in terms of perturbation by a level that has no bound state. Enhancement of harmonic generation in a high magnetic field with linearly polarized light is also observed in some resonances that have a small $g$ factor in the two-quantum state. The enhancement is interpreted in terms of constructive interference of resonances originating from the ground state and the first thermally excited vibrational state.

DOI
10.1103/physreva.30.1812

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The technique of harmonic generation with circularly polarized light is applied to study the Land'e ggg factor of the two-quantum excited states of I2{\mathrm{I}}_{2}I2​ molecules. Systematic and abrupt changes in the ggg factor are observed. The large change in the ggg factor is interpreted in terms of perturbation by a level that has no bound state. Enhancement of harmonic generation in a high magnetic field with linearly polarized light is also observed in some resonances that have a small ggg factor in the two-quantum state. The enhancement is interpreted in terms of constructive interference of resonances originating from the ground state and the first thermally excited vibrational state.
Magnetic-field-induced resonant third-harmonic generation in I2{\mathrm{I}}_{2}I2​ with circularly polarized light is studied. With a magnetic field perpendicular to the propagating direction of the light, the axial symmetry is broken and the angular momentum is no longer conserved in such a system, and the third harmonic may be generated under such conditions. Quantum beats in photon-echo-like third-harmonic generation with nonsynchronous laser pulses are also studied. The intensity of the harmonic generated may be interpreted in terms of the interference between a molecule and itself as well as the interference between different molecules. The gyromagnetic ratio of the resonant states is measured.
The gyromagnetic ratio of several states in I2{\mathrm{I}}_{2}I2​ is studied with the technique of polarization rotation and level crossing in doubly resonant third-harmonic generation. The level-crossing curve has a linewidth of approximately the Doppler linewidth of the corresponding non-coherent process divided by the angular momentum quantum number. In the polarization-rotation work, a rotated angle 80\ifmmode^\circ\else\textdegree\fi{} is observed with little depolarization. The ratio of the angle rotated to the magnetic field of some 20 resonances can be divided into two groups: large ($\frac{\ensuremath{\Delta}\ensuremath{\theta}}{\ensuremath{\Delta}H}\ensuremath{\sim}10$\ifmmode^\circ\else\textdegree\fi{}/kG) and small ($\frac{\ensuremath{\Delta}\ensuremath{\theta}}{\ensuremath{\Delta}H}<2$\ifmmode^\circ\else\textdegree\fi{}/kG). A theory based on the interference between the different molecules is presented to explain the experimental data. The observed differences in $\frac{\ensuremath{\Delta}\ensuremath{\theta}}{\ensuremath{\Delta}H}$ is interpreted in terms of the difference in the transition branch in the one- and two-quantum transition. The gyromagnetic ratio of the ground state as determined by this technique is approximately -6 and that of the two-quantum excited state is approximately -20.
The intensity of the resonant third-harmonic generation in I2{\mathrm{I}}_{2}I2​ vapor is found to decrease with increasing strength of an external magnetic field even when the exciting light is polarized parallel to the magnetic field. This effect is explained with a theory based on the interference between different molecules. The gyromagnetic ratio of the excited state is determined in this work. This technique may also be applied to study the mechanism of the various processes of saturation in resonant wave mixing.
Subnatural-width resonances due to coherent effects in optical pumping at hyperfine (hf) transitions of Rb are investigated using a linearly or circularly polarized single-frequency diode laser beam. The sign and amplitude of the resonances in the fluorescence are measured by scanning the laser frequency over the hf transitions of the Rb D 1 and D 2 lines. The fluorescence perpendicular to the laser beam is detected as a function of magnetic field parallel to the beam propagation and scanned around zero value. In the case of linear polarization of the field, dark resonances are observed for hf transitions with F g → F e = F g -1 and F g → F e = F g , and bright resonances are observed for transitions with F g → F e = F g + 1. This is in agreement with the recently developed theories. For all (except F g = 2→ F e = 3, 85 Rb, D 1 line) hf transitions the sign of the resonances reverses when the polarization of the field changes to circular. The amplitudes of the resonances correspond to the probability of the hf transitions and to the calculated loss rate of the population to the ground hf level not excited by the laser field. The observed narrow resonances (FWHM: few tens of kHz) and their sensitivity to weak magnetic fields are interesting for high-resolution spectroscopy and for development of simple and precise magnetometers.
The existence of a novel resonance in the modulation of the fluorescence light emitted by an atom or a molecule when coherently excited by a rapidly rotating linearly polarized light beam is predicted. Such resonances are experimentally observed due to a transfer of coherence from light to atoms or molecules, via either stimulated emission or absorption of light, without having to shift the positions of the levels by an external perturbation. These resonances directly permit the measurement of energy level lifetimes, even in the case where usual magnetic resonance methods would require the use of large magnetic fields.
A technique of third-harmonic generation (THG) with nonsynchronous pulses is applied to study the dynamics of resonant wave mixing. A circularly polarized laser pulse is focused into a cell containing I2{\mathrm{I}}_{2}I2​ vapor to create the coherent dipole moments. An oppositely circularly polarized pulse is then focused into the same region to generate the third harmonic. Since no THG is produced by any individual pulse, correlated THG can be easily measured without using any fast detection scheme. A theory is presented to explain the measured coherent lifetime of approximately 1 ns. The technique of wave mixing with two linearly polarized pulse is applied to study saturation in resonant THG and to measure the homogeneous relaxation rate of the molecular states. This technique is also applied to study the photon echo generating process.