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.
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.
One purpose of this paper is to present new studies on the effect of bandwidth and the coherence properties of the excitation source on the decay and the dephasing of isolated large molecules. A detailed study of the system pentacene in a p-terphenyl matrix is presented utilizing three different excitation sources, a single mode dye laser (60 KHz–6 MHz bandwidth depending on the time scale of the experiment) a multimode dye laser (240 GHz bandwidth), and an incoherent N2 flash lamp. Optical T1 (the longitudinal relaxation time) and T2 (the transverse relaxation time) are measured from the coherent and incoherent transients observed either in the forward direction of the laser or at right angles to the exciting beam. At 1.8 °K, the optical transition (1A1g→1B2u) of pentacene in p-terphenyl exhibits four sites, the lowest of which at 16 887 cm−1 has the following parameters: T2=44±2 nsec; T1=24.9±2 nsec, and μ=0.7±0.1 D. The transition moment μ, is obtained directly from the optical nutation, which exhibits a Rabi nutation time (h//μ⋅ε) of 27.3 nsec, and is corrected for the effect of the Lorentz local field inside the terphenyl crystal. The experiments presented here are categorized into two time regimes for theoretical analysis; a transient coherence regime where the observed decay is comparable with (h//μ⋅ε) and T2, and a steady-state coherence regime where transient dephasing is complete and the off-diagonal elements of the density matrix have decayed to their steady-state values in the presence of the field of amplitude ε. Using the Wilcox–Lamb method, rate equations (with T2 expressions) describing the population flow in the ’’complete’’ level structure of pentacene (ground‖0〉, singlet ‖p〉, and triplet {‖l}) are derived from the density matrix equations of motion. When these equations are averaged over the inhomogeneous width of the optical transition and the measured Gaussian transverse profile of our laser we obtain T1p0=24.9±2 nsec and T1pl=15.7 μsec, the time constants by which pentacene spontaneously decays to ‖0〉 or crosses over into l, as well as the averaged population at time t. In an effort to be complete, attention is placed upon the relationship between theory and the experimental findings. First, expressions for the OFID and nutation in the solid are presented for the pentacene case in order to relate T1, T2, and μ to the level structure. Second, at low temperatures (1.8 °K), the origin of dephasing is identified as spontaneous emission from p→0 since experimentally T2?2T1, in agreement with other work. At higher temperature, however, a strongly temperature dependent dephasing process with an onset at 3.7 °K takes place. Armed with these observations we present a theoretical treatment of these distinct dephasing channels and their temperature dependences. A discussion regarding the influence of ’’accepting’’ phonon modes (either optical or acoustic) on optical dephasing is also given. The results indicate that the treatment of Jones et al. can (1) explain the observed temperature dependence of T2 in pentacene; (2) distinguish dephasing as a result of scattering by acoustic phonons from that due to resonance or quasilocalized phonons with clear connections to gas and liquid state theories, but without invoking more than two approximation levels. (3) explain both the level shift and line width changes as a result of ’’conventional’’ dephasing or dephasing by exchange mechanisms; and (4) relate the pure dephasing term to an anisotropy in the scattering amplitudes (between the ground and excited states in the system) which contribute largely to the homogeneous width of the transition. Optical site selection of these transitions is also reported and discussed in relation to vibrational relaxation and to both homogeneous and inhomogeneous broadenings. The studies of the homogeneous broadening of the vibronic origin (267 cm−1) indicate that vibrational relaxation is fast (psec) in the excited singlet manifold of pentacene. Finally from more than ten independent experiments including single and multimode excitation, on- and off-resonance scattering, Zeeman effect and the transient decay as a function of excess energy in the molecule, a more complete picture of the pentacene level structure {‖l〉} is given. With this in mind, the influence of the laser bandwidth and coherence properties on state preparation and subsequent dephasing and decay is concluded. It is proposed that the slow decay, (∼15 μsec) observed during narrow-band excitation represents intersystem crossing to nearby triple manifolds after the transient coherence of the 0↔p subsystem is decayed. In addition, the decay of the primary state prepared in these experiments is not sensitive to the bandwidth or the correlation time of our excitation sources.
This thesis is concerned with coherent effects in high resolution mid-infrared gas phase spectroscopy using quantum cascade lasers (QCLs). An introductory chapter explains the importance of QCLs as radiation sources in the mid-infrared region of the spectrum and goes on to detail their development and structure. A discussion of coherent effects in spectroscopy follows, leading into the second chapter which discusses the theories relevant to the experimental sections of the thesis. In chapter 2 the theory underpinning direct and velocity selective, Doppler-free spectroscopy is discussed and a density matrix formalism is followed to derive the equations of motion that govern coherent excitation effects in two-level systems. In the final part of the chapter this treatment is extended to three-level systems. The equations derived in this chapter form the basis of quantitative interpretations of the phenomena observed in experimental data and presented in the remainder of the thesis. In chapter 3 the characterisation of a high power, narrow linewidth QCL is carried out. This laser is then used to perform direct and sub-Doppler resolution spectroscopy on NO, demonstrating non-linear absorption at high laser intensities and providing a measurement of the laser linewidth in the limit of slow frequency tuning. As the slow tuning rate increases, evidence of coherent transient effects is presented and density matrix theory used to model this behaviour. The data presented include the first observations of asymmetric Lamb dips and the onset of rapid passage oscillations from a Lamb dip. Pump-probe experiments on NO, utilising two cw QCLs are presented in chapter 4. The high level of velocity selection afforded by QCL excitation leads to coherent transient signals at far lower probe scan rates than previously reported. The effect of altering both the scan rate and the gas pressure and the importance of hyperfine structure are presented. A radio frequency noise source applied to one of the lasers is shown to broaden the laser linewidth, leading to rapid dephasing. A two-colour polarisation spectroscopy experiment is also presented which allows the measurement of both the absorption and the Doppler-free dispersion signals and the three-level density matrix formalism presented at the end of chapter 2 used to model the non-linear response of the system. The final chapter details the use of an acousto-optic modulator to create a pulse of mid-IR light using a cw QCL and the application of this to time resolved pump-probe spectroscopy. This capability suggests the prospect of achieving coherent population transfer by stimulated Raman adiabatic passage (STIRAP) using two such pulses. Simulations based on a simple three-level model and including Zeeman coherences are presented, which take the measured properties of the lasers used in this thesis as inputs to predict the potential population transfer achievable in NO as well as providing useful information about the angular momentum polarisation of the excited molecules. An experimental realisation of STIRAP would require the lasers to be stabilised, and so the final part of the chapter details experimental attempts to achieve stabilisation of an external cavity QCL, and suggests future avenues for improved implementation.
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.
Using a semiclassical, effective-state approach to laser pulse propagation in a dilute gas of polyatomic molecules, a numerical model of multiphoton excitation which includes quasi- continuum transitions is used to calculate the effects of coherent propagation on multi-photon absorption. The initial laser field is assumed to be a quasimonochromatic Gaussian envelope which is nearly resonant with a portion of the lower vibrational states of the molecule. The molecular model consists of a single ground state and several bands of discrete states which are successively connected via electric dipole transitions. This model correctly mimics some of the major characteristics of multiphoton absorption, such as power threshold, fluence dependent absorption, and complete population inversion at high fluences. The model also allows for the calculation of the polarization induced in the medium by the electric field, and the subsequent influence that the polarization has on the field. As the pulse propagates, the Rabi oscillations induced in the lower level states become encoded onto the pulse, creating a series of peaks and troughs similar to the behavior of pulse propagation in two-level systems. Another coherent propagation effect which can change the excitation process is coherent sideband generation. The temporal distortions and the generation of new frequencies results in changes in the population distribution among the states, as well as changes in the total population transferred to the quasi-continuum. The pulse reshaping occurs while the total fluence of the pulse is decreasing due to energy loss to the quasi-continuum. The effective increase in total population transfer due to propagation effects is offset by the decrease in fluence. The resulting transfer of energy from the pulse to the molecule depends strongly on the initial characteristic of the pulse. These processes play an important role in any system where the efficient transfer of laser energy to the medium is required, such as laser-induced chemistry and laser isotope separation.
Pulse propagations in an amplifier in the case of the photodissociation iodine laser are investigated with the use of semiclassical treatments, especially when the input pulse fed into the amplifier has a width comparable to or shorter than the transverse relaxation time of the amplifier medium. In our semiclassical equations for pulse propagation in an iodine amplifier, the populations of all hyperfine levels [2P1/2(F= 3, 2),2P3/2(F = 4, 3, 2, 1)], the polarizations of optically allowed transitions (\DeltaF = 0, \pm 1), and the Doppler detuning effects were taken into consideration. Based on the numerical integrations of the semiclassical equations, the propagated pulse shape and the population distributions of the hyperfine levels are discussed. From our calculations, it has been made clear that bandwidth limitations of pulses in the range of pulsewidths of 0.1 ns and below can be avoided by making use of the nonlinear amplification that results from the coherent interaction between the propagated pulse field and the amplifier medium.
The influence of molecular collisions on the production of the degenerate four-wave mixing signal in I(2) is presented. Measurements were performed on gaseous molecular iodine, I(2), contained in a glass cell in which pressure, temperature, and species concentration are easily and independently varied. Frequencydoubled outputs from a seeded Nd:YAG laser and an excimer-pumped dye laser were used as excitation sources. We have studied the dependence of signal strength versus buffer gas pressure, with pump intensity as a third parameter. It is evident from our results that, for pump intensities of less than 1 MW/cm(2), the pressure dependence of the signal follows that given by a simple two-level model in the homogeneously broadened regime. In this regime collisional deexcitation becomes significant, leading to changes in saturation intensity. This is evidenced by a reduction in the signal with an increase in buffer gas pressure. This behavior is similar to that seen in laser-induced fluorescence. At higher pump intensities, the signal is seen to increase with pressure; this behavior cannot be described by the simple two-level model.