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Journal of the Optical Society of America B

Cross-relaxation effects on the saturation of visible absorption lines of the iodine molecule

Atsuo Morinaga

Published Jun 1, 1987• 6 citations
Publisher

Abstract

The cross-relaxation effects that cause Doppler saturation in a saturated absorption signal were examined for several rotational and vibrational bands in the electronic transition of the iodine molecule. It was found that the cross-relaxation time is of the order of a millisecond and that the effect increases with pressure. It was also found that the effect decreases with an increment in the rotational quantum number of lower level. The mechanism for this effect could be explained by two inelastic rotational-level-changing collisions between molecules populated in the lower state.

DOI
10.1364/josab.4.000906

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The cross-relaxation effects that cause Doppler saturation in a saturated absorption signal were examined for several rotational and vibrational bands in the electronic transition of the iodine molecule. It was found that the cross-relaxation time is of the order of a millisecond and that the effect increases with pressure. It was also found that the effect decreases with an increment in the rotational quantum number of lower level. The mechanism for this effect could be explained by two inelastic rotational-level-changing collisions between molecules populated in the lower state.
This thesis looks into the aspects of molecular rot-vibrational energy level relaxation of iodine when it is excited by a resonant laser. In addition, the effects of Doppler detuning of molecular levels are explored. The proposed Active Hyperspectral Imaging Project [1] will detect in the atmosphere, low concentrations of a specific element's molecules by using remote sensing optical techniques. In order to improve the signal to noise ratio, a 'high field' coherent excitation technique is proposed. However such coherent excitation should take less time than the typical relaxation time of the system to avoid de-coherence effects. The purpose of the present study is to assess the values of the de-population and de-phasing relaxation times in iodine. The result of this experiment will be used for the design of an intra-cavity coherent excitation experiment in which the power needed for excitation would be obtained by a resonant enhancement inside a low-loss optical storage cavity. We can estimate the minimum power required for coherent excitation based on our findings of the relaxation times. This high field experiment would gain information regarding the coupling between the molecule and the laser field. In the present 'low field' experiment, output from a low-powered tunable diode laser was sent through a Pockels cell to create a top hat pulse which was used to excite gaseous iodine, located in an evacuated cell. The time dependence of a spectrally resolved fluorescence signal was used to observe the relaxation behavior of some targeted excited state and the behavior was fitted to a model resulting in estimates of de-phasing and depopulation relaxation parameters. The experiment was also set-up to observe the effects of Doppler broadening due to molecular velocities. Fluorescence was collected from a narrow strip inside the iodine cell (see figure 3.3) whose short axis was parallel to the laser beam and its length was altered to assess the Doppler contribution. The information of de-phasing and de-population collected from the low field experiment would be useful for the high field experiment in which enough intensity will be created to induce Rabi oscillations [2] by the placement of a custom iodine cell inside a short near-confocal cavity. These oscillations would aid in the estimation of the dipole matrix element (which quantifies the coupling between the molecule and the laser field). The time duration of de-phasing and de-population will determine the minimum field intensity required to produce a Rabi oscillation whose period is less than the de-phasing and de-population times. The situation for the low field experiment was modeled by a two-level energy system interacting with a single mode field. In chapter 2, equation of motion is developed which includes the detuning effect. An output intensity model designed to fit the data is presented that would be used to calculate the rise and decay times helping us to ascertain the relaxation effects. The practical setup is presented in chapter 3. Details regarding the beam line, stabilization of the diode laser, selection of the iodine absorption line by tuning the laser and selection of a fluorescence line of iodine are all described in that chapter. The procedure of placement of the Pockels cell, production of a 1 µs laser pulse and data collection are also given. The chapter concludes with a representative data-set. For the Analysis, the model from the second chapter is used to fit to the experimental data. Values of rise and fall times are presented and the relationship between these values is explored. The average value of de-phasing due to molecular collision is calculated and the power requirements for the Rabi oscillations are discussed in this chapter. The thesis concludes with a summary and suggestions for further work.
Stimulated absorption of spontaneous emission of a gas-discharge plasma can cause a significant increase in the population of the emitting states, compared to their population determined by inelastic collisions and spontaneous decay from upper levels. By imposing a magnetic field, this self-saturation is reduced and its contribution to the level population can be identified. The magnetic-field-dependent changes in Doppler profiles due to radiative transitions caused by saturating spontaneous emission are analyzed in the case of absorption of a weak monochromatic wave.
The possibility of the resonant variation in the total number of particles on the excitation level with the transmission of the field prequency through the center of a Doppler-widened absorption line is examined. The article examines the effect of the formation of the resonance of the total number of particles in the simplest mode of the moving two-layer particles with a varied degree of saturation of absorption, and also the effect of rotational relaxation on the value of the resonance in the case of molecular transitions.
Previously derived equations describing recoil effects in saturated absorption by a gas of two-level atoms or molecules are solved numerically for the case of standing plane-wave excitation and large Doppler broadening. The numerical techniques and accuracy estimates are discussed briefly. The lineshapes obtained are well approximated by the superposition of two Lorentzian functions whose width, heights, and separation parametrise the lineshape. The dependence of these parameters on saturation, recoil splitting, and relaxation rates is described and compared with corresponding results in the rate equation approximation and in the weak-probe-beam approximation. Saturation broadening depends strongly on these latter approximations, but very little on recoil splitting. When multiphoton processes are retained in the equations, increasing saturation causes a slight decrease in the fitted separation of the peaks. The ratio of the heights of the two Lorentzians is closely related to the ratio of relaxation rates for the level populations.
Time characteristics of the secondary emission from iodine molecules excited with light pulses from a cavity-dumped, single-frequency Ar + ion laser at 514.5 nm have been compared with existing theories. It has been found that the experimental data are well explained quantitatively by the theories, provided the 21 hyperfine lines as well as collisional effects and Doppler broadening are taken into account. The cross sections for the phase-changing and population-changing collisions have also been determined. The contributions of coherent and incoherent secondary emission processes are discussed.