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Journal of the Physical Society of Japan

Time Behavior of Secondary Emission from I2Molecules Excited by Nearly-Resonant Light Pulses

Willem Overlaet, Masahiro Kimura, Shuichi Kinoshita, Takashi Kushida

Published Jan 15, 1987• 5 citations
Publisher

Abstract

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.

DOI
10.1143/jpsj.56.60

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Thermalization of fast cesium 5D3∕2 atoms in collisions with ground-state cesium atoms

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

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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.

Journal of the Optical Society of America B•Jun 1, 1987•6 citations
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.
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.
Conventional coherent pulse sequences such as photon echoes measure only highly averaged relaxation rates in complex multilevel systems, such as molecules undergoing state‐changing collisions. Pulse frequency, phase, and shape control lets us generate sequences which give a more detailed understanding of the dynamics. Results of dual frequency, crafted shape sequences on I2 are presented which show that the "coherence dephasing" time T2 is primarily due to population redistribution (energy changing collisions) in the electronically excited state, and that the electronic ground state has a much smaller cross section for such collisions. Quantitative analysis is only possible with modified laser pulse shapes which excite a single velocity component, and requires pulse sequences which correct for the hyperfine dependence of predissociation. Since this correction is rarely made in previously reported coherent transient measurements, literature values of T2 and T1 may not be reliable in the zero pressure limit.
The response of emission in I2 vapor to intensity modulated light at radial frequency ω is measured and described in terms of the lifetime of the excited state, T1, the rate of phase interrupting collisions γ, and the inhomogeneously broadened absorption line. When the laser is tuned to resonance, the variation of amplitude of the modulated emission with ω corresponds to a single exponential decay with lifetime T1. Off resonance, the modulation is described as the sum of the above ‘‘delayed’’ component and a ‘‘prompt’’ in‐phase component which is independent of modulation frequency. The ratio of the delayed to prompt component for off resonant excitation is shown to be 2γT1. This relation is used to determine the rate of phase interrupting collisions in I2 vapor.
The resonance secondary emission (RSE) in femtosecond laser excitation is discussed in reference to the motion of the created wave packet moving on the excited state potential surface. The density matrix of emitted light for the multi-intermediate-level system is outlined, from which the emission correlation function is derived. The correlation function is put into the theoretical expression of the time-dependent ‘‘physical spectrum’’ for the Fabry–Perot interferometer (which is used in order to consider temporal and energetic resolution inherent in detection). The compact and practical expressions obtained connect the time- and frequency-resolved spectrum with the time evolution of the wave packet. Numerical results for a displaced harmonic oscillator model indicate that the time- and frequency-resolved spectrum can reveal how the wave packet created by a fs laser pulse travels on the excited potential surface if the response time 1/Γd of the photodetector satisfies the relation that Ω<Γd <∼ the Stokes shift (where Ω is the vibrational frequency). It is shown that the excited state wave function can be split into two terms, the one that adiabatically follows the temporal change in incident light (the adiabatic term) and the one that represents the effect of spectral broadening of light (the Fourier broadening term). It is only the Fourier broadening term that survives after the termination of incident light and reflects the motion of the created wave packet on the excited potential surface. In off-resonance excitation, the adiabatic term produces Raman-like emission and the Fourier broadening one produces fluorescence-like emission. In resonance excitation, these two terms are indistinguishable from each other with respect to emission frequency: for the duration of incident light, the adiabatic term offsets the Fourier broadening one, leading to a slow buildup of intensity in the time- and frequency-resolved spectrum (which is slower than the initial rise of the incident pulse profile).
We have investigated collisions involving fast, excited Cs atoms produced by photodissociating Cs2{\mathrm{Cs}}_{2}Cs2​ molecules with a pulsed dye laser. The velocities of the atoms in the 5D5D5D state formed by the process ${\mathrm{Cs}}_{2}(X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+})+\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{pump}}\ensuremath{\rightarrow}{\mathrm{Cs}}_{2}^{*}\ensuremath{\rightarrow}\mathrm{Cs}(5D)+\mathrm{Cs}(6S)$ are much greater than typical thermal velocities associated with the cell temperature. Using a narrow-band cw probe laser to observe the increased Doppler broadening of the $5{D}_{3∕2}\ensuremath{\rightarrow}5{F}_{5∕2}$ excitation line shape, we are able to monitor the time evolution of the velocity distribution of these 5D5D5D atoms. We analyze the data using a model that predicts the time-dependent excitation line shape of the fast atoms. Because the photons used to dissociate the molecules have a well-defined energy, the velocity distribution of the excited atoms in the early time after they are produced can be fairly well determined. Over time, velocity-changing collisions with ground-state Cs atoms cause the velocity distribution of excited atoms to approach the thermal limit. An analysis based on the strong-collision model leads to a prediction that the observed line shape at intermediate times will be a linear combination of contributions from distinct ``fast'' and ``thermalized'' atomic populations. By fitting our data to this model, a rate coefficient for velocity-changing collisions of fast Cs(5D3∕2)\mathrm{Cs}(5{D}_{3∕2})Cs(5D3∕2​) atoms with ground-state Cs atoms has been determined. The result ${k}_{\mathrm{VCC}}=(6.1\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ corresponds to an effective velocity-changing collision cross section of ${\ensuremath{\sigma}}_{\mathrm{VCC}}^{\mathrm{Cs},\mathrm{eff}}=(1.2\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$.
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.