On the saturated fluorescence of iodine molecules

Andrei Goncharov, Mikhail N Skvortsov, Veniamin P. Chebotayev

Atsuo Morinaga

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.

Saadia Kemal

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.

Richard G. Brewer, Satoru S. Kano

HERBERT WALTHER

Louis A. Turner, Karl Taylor Compton

Dissociated iodine vapor at 1100\ifmmode^\circ\else\textdegree\fi{} to 1500\ifmmode^\circ\else\textdegree\fi{}C in a quartz tube failed to show any resonance fluorescence of the 2062 line in a 78 minute exposure in the same apparatus in which mercury vapor gave a detectable fluorescence of the 2537 line in a 1 second exposure. This fact and the narrowness of the 2062 line in absorption, seem to indicate that the lower level involved in the transition producing this line is not the normal state of the atom but is a slightly higher one. A calculation based on the observed line width shows that this level is probably at least 0.545 volt above the normal level and suggests that there may be a resonance line of atomic iodine at about 1800A.