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A density matrix treatment of plasmon-enhanced (PE) stimulated Raman spectroscopies is developed. Specifically, PE stimulated Raman Gain/Loss (PE-SRG/L) and coherent anti-Stokes Raman scattering (PE-CARS) due to monochromatic excitation and PE femtosecond stimulated Raman spectroscopy (PE-FSRS) are considered. A Lorentz oscillator model is used to explicitly describe the time dependence of plasmon-enhanced optical fields. These temporal characteristics are required for a density matrix based description of all plasmon-enhanced nonlinear molecular spectroscopies. Dispersive vibrational line shapes in PE-SRG/L and PE-FSRS spectra are shown to result primarily from terms proportional to the square of the complex optical field enhancement factor. The dependence on the plasmon resonance, picosecond and femtosecond pulse characteristics, and molecular vibrational properties are evident in the density matrix derived PE-FSRS intensity expression. The difference in signal detection mechanisms accounts for the lack of dispersive line shapes in PE spontaneous Raman spectroscopy. This density matrix treatment of PE-FSRS line shapes is compared with prior coupled wave results. [ABSTRACT FROM AUTHOR]

An optimized procedure is described for the acquisition of 785 nm excited SERS spectra of dried bloodstains and shown to offer great potential for rapid, portable, highly sensitive and specific, confirmatory identification for forensic applications. Following extraction in 1 μL of 50% acetic acid, a robust, highly reproducible SERS spectrum is observed from dried bloodstains resulting from a hematin-like heme moiety (ferric, high spin). As anticipated, this blood signature can be classified with 100% specificity and sensitivity with respect to the SERS spectra of other body fluids. High quality SERS spectra can be observed from stains of blood diluted by as much as 105. Dried blood spectra acquired on Au and Ag SERS active substrates exhibit very different relative intensities at this electronically, non-resonant excitation wavelength (785 nm) indicating that a strong chemical effect contributes to the SERS enhancement of this body fluid. DFT calculations further confirm the vibrational band assignments of the features seen in these SERS spectra of dried blood. [ABSTRACT FROM AUTHOR]

The optical heterodyne detected anisotropic rotational Raman responses of H2 and D2 (22 mol %) in a near critical CO2 (ρ*=ρ/ρc=0.8, T=308 K) solution are reported. J-specific rotational Raman correlation functions (RCFs) for the S(J) transitions of H2 (J=0,1,2) and D2 (J=0,1,2,3) in this CO2 solution are determined from these measurements. A mixed classical-quantum simulation methodology results in RCFs that are in excellent agreement with the experimentally derived J-specific responses. The observed S(J) coherence decay time scales, J-dependence, rotor mass dependence, and solvent-induced transition frequency shifts are well captured by these simulations. Pure dephasing of these rotational Raman transitions is shown to be close to the homogeneous limit of the standard Kubo line shape analysis and attributable to the rotor center-of-mass translation in an anisotropic solvent cage. Rotor translational motion in the vicinity of a single CO2 appears to dominate this dephasing mechanism. Mixed classical-quantum simulations, incorporating the effects of solution fluctuation driven nonadiabatic coupling of instantaneous adiabatic states, including full J-mixing, are required for the agreement between theory and experiment obtained here. Simulations of the classically excited angular kinetic energy of D2 rotors are used as an estimate of T1 relaxation rates and are found to be negligible compared to the D2 rotational Raman coherence time scale. These results are discussed in the context of previous mixed classical-quantum and rotational friction calculations of the dephasing and energy relaxation contributions to H2 rotational Raman coherence decays. Advantages of time domain acquisition of these rotational Raman responses as compared to spontaneous Raman measurements are illustrated here. [ABSTRACT FROM AUTHOR]

Ultrafast 2DIR spectra and pump–probe responses of the N2O ν3 asymmetric stretch in SF6 as a function of density from the gas to supercritical phase and liquid are reported. 2DIR spectra unequivocally reveal free rotor character at all densities studied in the gas and supercritical region. Analysis of the 2DIR spectra determines that J-scrambling or rotational relaxation in N2O is highly efficient, occurring in ∼1.5 to ∼2 collisions with SF6 at all non-liquid densities. In contrast, N2O ν3 vibrational energy relaxation requires ∼15 collisions, and complete vibrational equilibrium occurs on the ∼ns scale at all densities. An independent binary collision model is sufficient to describe these supercritical state point dynamics. The N2O ν3 in liquid SF6 2DIR spectrum shows no evidence of free rotor character or spectral diffusion. Using these 2DIR results, hindered rotor or liquid-like character is found in gas and all supercritical solutions for SF6 densities ≥ρ* = 0.3, and increases with SF6 density. 2DIR spectral analysis offers direct time domain evidence of critical slowing for SF6 solutions closest to the critical point density. Applications of 2DIR to other high density and supercritical solution dynamics and descriptions are discussed. [ABSTRACT FROM AUTHOR]

An instantaneous normal mode (INM) analysis of the short-time solvation dynamics of the B-state (200 nm) Rydberg excitation of methyl iodide in high pressures of Ar (ρ*=0.08, 0.3, and 0.8) is presented. Solute–solvent interaction potentials for this system have been determined by previous absorption and resonance scattering studies. The B-state transition energy correlation function (ECF), also known as the solvation correlation function, calculated by the linear coupling INM theory is in good agreement with the ECF given by molecular dynamics simulation at short times (≤150 fs) that are well beyond the so-called inertial regime (≤100 fs). The shape and peak frequency of the solvation spectra are relatively constant over the wide range of bath densities considered here in contrast to the INM total density of states. This is attributed to the relative density independence of the first peak in the solute–solvent pair distribution function. Similarly, the ECFs are also only modestly dependent on solvent density. A cancellation of the density dependence of the solvation spectrum area and the second moment of the absorption spectrum line shape, and the nearly constant solvation spectrum shape, accounts for the relatively weak density dependence of the ECF decay. A computationally fast, semianalytical method for calculating the weighted density of states incorporating both two- and three-body correlations is shown to be in reasonable agreement with the total INM weighted density of states. A participation ratio analysis of the eigenvectors contributing to the solvation spectrum reveals that single solvent–solute interactions are responsible for the solvation response of the ρ*=0.08 and 0.3 solutions. More collective, totally symmetric solvent motions involving just a few solvent particles, in addition to single solvent interactions, contribute to the solvation response at the liquidlike density of ρ*=0.8. The effects... [ABSTRACT FROM AUTHOR]

The polarization of the resonance fluorescence of symmetric top rigid rotors is described by a third-order density matrix treatment of resonance emission and a sum-over-all-rovibronic states scattering-tensor invariant framework. Within this theoretical approach the resonance fluorescence depolarization is a function of the excited electronic state population and rovibronic coherence decay rates, as well as the electronic absorption/emission line shapes. This description of the depolarization of resonance fluorescence is contrasted with that of resonance Raman in terms of angular momentum selection rules and dependence on material relaxation parameters. In contrast to resonance Raman emission in solution, the accompanying resonance fluorescence polarization is found to be most sensitive to the resonant excited state lifetime when this population decay time is of the order or less than rotational periods. These effects are demonstrated for excitation resonant with the B-state origin of CH3I vapor in high pressures of CH4 and CO2. The solute–solvent interaction responsible for the pure dephasing of the resonant optical coherence does not appear to cause orientational redistribution of the excited chromophore, at least on the time scale of the CH3I B-state origin lifetime. The influence of excited electronic B-state rovibrational pure-dephasing effects on the resonance fluorescence polarization measurements are discussed. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A density matrix treatment of the time evolution of the third order polarization response describing the optical heterodyne detected (OHD) transient birefringence and dichroism excited by ultrafast pulses is given. The relationship between frequency domain (Raman scattering) and time domain (pump–probe) spectroscopies is revealed by this pathway explicit description. Constructive and destructive interferences between time evolution density matrix pathways account for the respective strong birefringent and weak dichroic ground state nuclear response when the pulses are electronically nonresonant. However, for electronically resonant chromophores, the dichroic response is larger than the corresponding birefringent response due to constructive and destructive interferences respectively between density matrix time evolution histories. No such interferences contribute to spontaneous Raman scattering. The relative magnitude of the resonant dichroic and birefringent responses is pulse width dependent in the fast pulse limit and dependent on the relative rates of optical dephasing and ground state nuclear motion in the rapid optical dephasing limit. The spatial interpretation of the ground and excited state OHD polarization responses is given within the context of this polarization approach and the familiar Maker–Terhune notation. These relationships between time and frequency domain spectroscopies are illustrated by the observed OHD birefringence and dichroism and the spontaneous Raman spectra of both a nonresonant liquid (chloroform) and a resonant solution (I2 in n-hexane). [ABSTRACT FROM AUTHOR]