The spontaneous resonance emission of CH3I in high pressures (800–1600 psi) of CH4 and Ar excited in the region of the Rydberg B-state origin (∼201 nm) are reported. These emission spectra consist of narrow Raman-like (RL) and broad fluorescence-like (FL) spectral features. The observed ratio of the Raman/fluorescence intensity in these high pressure solutions is a function of the excitation wavelength as the incident radiation is tuned through the pressure broadened electronic origin band. Molecular dynamics simulations are implemented for the analysis of the observed emission spectral shapes and their excitation frequency dependence. The four-time dipole correlation functions required for the calculation of this nonlinear polarization derived signal are successfully approximated by a product of two two-time dipole correlation functions for these chromophore-bath systems (factorization approximation). The complex emission band shapes and their excitation frequency dependence are captured by this approach. The dispersion in the RL/FL emission redistribution is due to the multiple time scales inherent to the decay of the resonant optical coherence of these pressure-broadened absorptions. The wavelength dependent pure-dephasing rate is determined by the nonlinear shape of the solute–solvent difference potential. The observational time scale dependence of the spectroscopic homogeneous and inhomogeneous line broadening labels is clearly demonstrated and contrasted here for absorption and Raman scattering.
The spontaneous resonance emission of CH3I in high pressures (800–1600 psi) of CH4 and Ar excited in the region of the Rydberg B-state origin (∼201 nm) are reported. These emission spectra consist of narrow Raman-like (RL) and broad fluorescence-like (FL) spectral features. The observed ratio of the Raman/fluorescence intensity in these high pressure solutions is a function of the excitation wavelength as the incident radiation is tuned through the pressure broadened electronic origin band. Molecular dynamics simulations are implemented for the analysis of the observed emission spectral shapes and their excitation frequency dependence. The four-time dipole correlation functions required for the calculation of this nonlinear polarization derived signal are successfully approximated by a product of two two-time dipole correlation functions for these chromophore-bath systems (factorization approximation). The complex emission band shapes and their excitation frequency dependence are captured by this approach. The dispersion in the RL/FL emission redistribution is due to the multiple time scales inherent to the decay of the resonant optical coherence of these pressure-broadened absorptions. The wavelength dependent pure-dephasing rate is determined by the nonlinear shape of the solute–solvent difference potential. The observational time scale dependence of the spectroscopic homogeneous and inhomogeneous line broadening labels is clearly demonstrated and contrasted here for absorption and Raman scattering.
Abstract This chapter focuses on spectroscopic applications of time dependent quantum mechanics focusing on molecular spectroscopy in condensed phases. (1) Molecular spectroscopy in the dressed state picture. (2) Resonance Raman scattering. (3) Resonance energy transfer. (4) Collective excitation and emission. (5) Thermal relaxation and dephasing (the Bloch equations, relaxation of a prepared state, dephasing (decoherence), the absorption lineshape, homogeneous and inhomogeneous broadening, motional narrowing, thermal effects in resonance Raman scattering). (6) A case study: Resonance Raman scattering and fluorescence from Azulene in a Naphthalene matrix. (7) Probing inhomogeneous bands (hole-burning spectroscopy, photon echoes, single-molecule spectroscopy. (8) Optical response functions.
We calculate the signal in a time-resolved, coherent light scattering experiment on a dilute solution of a nonpolar solute dissolved in a nonpolar solvent. We model the solute and solvent molecules as electronic two level systems, and assume that only the solute interacts with radiation. The electronic degree of freedom is treated perturbatively within the Born–Oppenheimer approximation. The nuclear degrees of freedom are treated classically, and their effects are modeled by a molecular dynamics simulation. We assume the electronic absorption spectrum is dominated by inhomogeneous broadening. The validity of this assumption for our model is verified through the calculation of the absorption line shape with and without the inhomogeneous broadening assumption. A quantitative measure of the homogeneous dephasing time is obtained from a calculation of the photon echo signal for our model. The time dependence of the light scattering signal is shown to reflect both dynamics of the fluid in its electronic ground state and in its electronic excited state. We analyze the relative importance of contributions to the signal from these two types of dynamics as a function of experimental time scale and laser frequency.
A method is introduced to calculate for a model set of molecular levels the spectral line shapes expected for a variety of conventional laser experiments including absorption, hole burning, fluorescence line narrowing, and Raman scattering. The method allows the incident laser field to have arbitrary intensity. Furthermore, the effects of model Gaussian or Lorenzian inhomogeneous distributions are readily incorporated. Earlier results for a two-level system are easily obtained and new results are presented for inhomogeneously broadened two and three level systems, and for the effects of pure dephasing on the strong field spectra. In particular we have described the differences between fluorescence and Raman in strong fields, and the effect of strong fields on the spontaneous emission of inhomogeneously broadened transitions. Some predictions are made regarding line narrowing experiments in the strong-field limit.
The recently demonstrated technique of optically phase-locked pulse-pair (PLPP) excited spontaneous emission is described by a third-order perturbative density matrix approach. A nonlinear polarization description shows how PLPP spectroscopy depends on all the relevant material dephasing time scales. The time and frequency integrated resonance spontaneous emission consists entirely of resonance fluorescence, and is derived exclusively from excited-state population decay terms, i.e., diagonal second-order density-matrix elements. These third-order polarization results are proportional to the previously derived linear polarization expressions found to describe the observed PLPP I2 vapor emission. The nonlinear treatment allows a comparison of this technique to other forms of ultrafast pump–probe spectroscopies such as transient absorption and photon echo techniques. The role of impulsively prepared coherences is clearly described by this analysis. The effect of pulse duration, relative to material dephasing times, is explored for relaxation given by the optical Bloch equations. The most significant differences between a linear and nonlinear polarization treatment occur for pulse durations greater than the optical dephasing time or when excited state population and coherence decays are of the order of rovibrational periods. A fluorescence line narrowing effect, due to short pulse excitation, is predicted.
The effect of collisions on the resonance fluorescence spectrum of two-level atoms excited by monochromatic resonance radiation is studied. Analysis is performed for systems where the Doppler broadening is small compared to the collision frequency (high buffer gas pressures) and for the general case where collisions arbitrarily change the phase of the radiation-induced dipole moment (from completely interrupted to completely unaffected phase of the dipole moment). Both at a relatively low and at a high excitation radiation intensity, the resonance fluorescence spectrum is shown to depend on whether the two-level system is closed or open. This is especially true for the narrow unshifted Rayleigh scattering line. It is shown that, although the absorption line is homogeneously broadened, the resonance fluorescence spectrum exhibits a clearly pronounced anisotropy. In a direction close to the direction of propagation of the excitation radiation, the Rayleigh scattering line is maximally narrowed. Under certain conditions (that can easily be created in experiments), the width of this line is proportional to the diffusion coefficient of atoms interacting with the radiation.
The absorption spectrum of I2 in solid Xe shows resolved zero-phonon lines and phonon side bands near the origin of the B←X transition (550-625 nm). The long-lived |B⟩⟨X| coherence in this energy range (T2 = 600 fs on average) emerges as vibrationally unrelaxed fluorescence in resonance Raman (RR) spectra. Upon excitation in the structureless continuum at 532 nm, the oscillatory RR progression exhibits electronic dephasing time of T2 = 150 fs. Two RR progressions with markedly different vibrational coherence on the X-state are observed. The main progression of sharp overtones (T2 > 21 ps) is assigned to molecules trapped in double-substitution sites. The minor progression, which shows dephasing times T2 = 6-0.6 ps for v = 1-8, is assigned to molecules in triple-substitution sites. The line progressions allow a detailed characterization of the solvated B- and X-state potentials. Time-resolved coherent anti-Stokes Raman scattering is used to probe selected vibrational coherences on the X-state. Assignments are obtained through molecular dynamics simulations, which reproduce the relative dephasing rates between the two sites, clarify the role of rotation-translation dynamics, and enable quantum dynamics simulations of the spectra by the potentials of mean force that accurately describe the molecule-surrounding interactions.