New Techniques in Coherent Optical Spectroscopy: Optical Dephasing and Radiationless Processes in Molecules

Citation

Orlowski, Thomas Edward (1979) New Techniques in Coherent Optical Spectroscopy: Optical Dephasing and Radiationless Processes in Molecules. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1KE1-7557. https://resolver.caltech.edu/CaltechETD:etd-04082008-084228

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The objectives of this thesis are to develop new high-resolution (± 3 MHz) coherent nonlinear optical spectroscopic techniques and to utilize them to examine the nature of line broadening processes in optical transitions of isolated large and small molecules. By measuring the coherent and incoherent transients observed either in the forward direction along the laser beam (absorption) or at right-angles to the exciting beam (emission), these techniques allow one to determine optical [...] (the longitudinal relaxation time) and optical [...] (the transverse relaxation time). From [...] and [...] one can obtain under certain circumstances, both the radiative and nonradiative [...] contributions as well as the pure dephasing [...] contribution to the total homogeneous linewidth [...] of the optical transition. Two molecules are examined in detail. Iodine was chosen to demonstrate the new techniques because its conventional spectroscopy is well-known and to examine dephasing and decay processes important for small molecules. It was studied in the gas phase and in a molecular beam. Condensed phase experiments were performed on single crystals of pentacene isolated in a p-terphenyl host to obtain information about the nature of radiationless transitions and dephasing processes important in this large molecule. The optical transition that was studied in iodine is [...]. Using LADS (Laser Acoustic Diffraction Spectroscopy) for obtaining nanosecond time resolution with a narrowband single-mode dye laser and the IRD (Incoherent Resonance Decay) method whereby population dynamics and the coherence in the system can be obtained by monitoring the spontaneous emission, the following Stern-Volmer relationship was obtained for iodine: [...] = (0.783 ± 0.032) + (0.0143 ± 0.0005) P(mtorr). This relationship provides a zero-pressure radiative lifetime of 1.28 ± 0.05 [...]sec and a nonradiative collisional quenching cross section of [...] = 70 ± 2 [...]. These results agree with those obtained in previous studies with broadband excitation indicating that excitation bandwidth is unimportant for [...] processes in small molecules. Using the electrooptic method of laser frequency switching, an optical nutation was observed in iodine whose Rabi oscillation frequency [...] provided the dipole moment for the transition ([mu] = 0.05 Debye) after determining the laser field amplitude [...]. The Rabi frequency was also obtained from a coherent oscillation observed on top of the IRD signal in iodine at low pressure. This oscillation decayed by [...] and the overall IRD signal provided [...] such that both the coherence and decay of the system were obtained simply by monitoring the spontaneous emission. The homogeneous linewidth of the transition was measured using the 3-pulse photon echo method. At 10 mtorr pressure it was found to be 579 kHz. This is almost three orders of magnitude less than the inhomogeneous (Doppler) linewidth of ca. 400 MHz. Molecular beam experiments provided the radiative lifetime (1.24 ± 0.02 [...]sec) of iodine in a collisionless environment from which the radiative contribution (22%, 128 kHz) to the total homogeneous linewidth was obtained. Assuming that radiative losses from the lower level of the system can be neglected it was also determined that 38 kHz (7%) of the homogeneous linewidth is due to non-radiative relaxation (inelastic scattering) and a dramatic 413 kHz (71%) is due to pure dephasing [...] as a result of elastic scattering processes. Finally an OFID (optical free induction decay) was observed in the molecular beam whose decay indicated that [...]. Therefore, in a collisionless environment, the only dephasing process for iodine is spontaneous emission (i.e., no intramolecular dephasing processes exist). The optical transition that was studied in pentacene [...] exhibits four sites in a p-terphenyl host. The lowest energy site at 16,887 cm[...] has the following characteristics at 1.8[...]K: [...] = 24.9 ± 2 nsec; [...] = 44 ± 2 nsec, and [mu] = 0.7 ± 0.1 Debye. Experiments in this system are categorized into two time regimes for theoretical analysis: a transient coherence regime where the observed decay is comparable with [...] and [...], 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 laser field. The Wilcox-Lamb method is used to derive rate equations ([...] dependent) from the density matrix equations of motion. These equations describe the population dynamics in the pentacene level structure ([...] = ground state, [...] = excited singlet state and [...] = triplet manifold) and upon averaging them over the inhomogeneous linewidth of the transition and then using them to fit experimental decay curves one obtains: [...] = 24.9 ± 2 nsec and [...] = 15.7 [...]sec, the time constants for spontaneous emission to [...] and crossing into [...], respectively. OFID and nutation expressions are presented for the pentacene level structure and when appropriate averaging is done over both the inhomogeneous linewidth and the laser beam spatial profile, good fits can be obtained for the experimental transients. At 1.8[...]K it was determined that [...] = [...]. Therefore, as in iodine at zero pressure, spontaneous emission is the only dephasing process in pentacene at low temperature. At higher temperature, however, a strongly temperature dependent dephasing process takes place with an onset at 3.7[...]K. A theoretical treatment of the various dephasing channels is presented that explains the observed temperature dependence of [...] in pentacene and attributes pure dephasing to an anisotropy in the scattering amplitudes between the ground and excited states. Optical site selection of the pentacene transitions has been observed and is related to vibrational relaxation and homogeneous and inhomogeneous broadening in this system. Studies of the homogeneous broadening of the vibronic line 267 cm[...] above the lowest energy site indicate that vibrational relaxation is fast (psec) in the excited singlet manifold of pentacene. Finally from more than ten independent experiments including narrow and broad band excitation, on- and off-resonance scattering, Zeeman effect measurements and the transient decay as a function of excess energy in the molecule, a more complete picture of the pentacene level structure is given. It is proposed that the slow decay [...] observed during narrowband excitation represents intersystem crossing to nearby triplet manifolds after the transient coherence of the "two-level" system [...] has decayed. Furthermore, the decay of the primary state prepared in these experiments is not sensitive to the bandwidth or the coherence properties of the excitation source.

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