Orientation and alignment moments in two-color polarization spectroscopy

A theoretical analysis of two-color polarization spectroscopy (TCPS) is presented as an extension of a previous analysis of one-color PS. Three commonly used schemes in which pump and probe transitions share a common level are considered. Diagrammatic techniques are used to isolate the photon interaction sequences that can contribute to the signal. A perturbation-theory analysis expressing the signal in terms of spherical tensor moments is applied. The analysis emphasises the significance of orientation and alignment tensor moments of rotational angular momentum and their collisional evolution. The assumed context is transitions between single rotational states of gas-phase molecules that subsequently suffer discrete collisions. The time scale of the measurements is assumed to be long relative to the periods of molecular motion, as would typically be the case for signals excited by nanosecond-pulsed lasers from samples at moderate pressures. The Doppler motion of the probed species is included, as is an analytical solution to the integration over the Maxwell-Boltzmann distribution of velocities. The effects of nuclear hyperfine depolarization and velocity-changing collisions are discussed. It is shown that when pump- and probe-laser pulses are separated in time, TCPS creates and probes either orientation or alignment of rotational angular momentum in the common level shared by pump and probe transitions. Example simulations of one- and two-color polarization spectroscopies are included to demonstrate the resulting simplification of the measured signal using TCPS. TCPS is therefore a viable spectroscopic technique for the determination of rotational angular momentum orientation and alignment relaxation rates in molecular gases, of interest because they are sensitive probes of inelastic collisions.     

Electron self-exchange kinetics determined by MARY spectroscopy: theory and experiment

The electron self-exchange between a neutral molecule and its charged radical, which is part of a spin-correlated radical ion pair, gives rise to line width effects in the fluorescence-detected MARY (magnetic field effect on reaction yield) spectrum similar to those observed in EPR spectroscopy. An increasing self-exchange rate (i.e., a higher concentration of the neutral molecule) leads to broadening and subsequent narrowing of the spectrum. Along with a series of MARY spectra recorded for several systems (the fluorophores pyrene, pyrene-d(10) and N-methylcarbazole in combination with 1,2- and 1,4-dicyanobenzene) in various solvents, a theoretical model is developed that describes the spin evolution and the diffusive recombination of the radical pair under the influence of the external magnetic field and electron self-exchange, thereby allowing the simulation of MARY spectra of the systems investigated experimentally. The spin evolution of the radicals in the pair is calculated separately using spin correlation tensors, thereby allowing rigorous quantum mechanical calculations for real spin systems. It is shown that the combination of these simulations with high resolution, low noise experimental spectra makes the MARY technique a novel, quantitative method for the determination of self-exchange rate constants. In comparison to a simple analytical formula which estimates the self-exchange rate constant from the slope of the linear part of a line width vs concentration plot, the simulation method yields more reliable and accurate results. The correctness of the results obtained by the MARY method is proved by a comparison with corresponding data from the well-established EPR line broadening technique. With its less stringent restrictions on radical lifetime and stability, the MARY technique provides an alternative to the classical EPR method, in particular for systems involving short-lived and unstable radicals.     

Three-dimensional orientation of the Qy electronic transition dipole moment within the chlorophyll a molecule determined by femtosecond polarization resolved VIS pump-IR probe spectroscopy

Chlorophyll a (Chl a) is the most abundant pigment on earth. In all plants, algae, and cyanobacteria, it plays a pivotal role as an antenna and reaction center pigment in the primary steps of photosynthesis. In the past, a true three-dimensional (3D) experimental determination of the Qy electronic transition dipole moment orientation could not be obtained. With combined femtosecond polarization resolved VIS pump-IR probe experiments and theoretical calculations of the infrared transition dipole moments (tdm's) in the electronic ground state, we determined the 3D orientation of the Qy electronic tdm of Chl a within the molecular structure. Polarization resolved experiments provided angles of the Qy electronic tdm with three different infrared tdm's, whose orientations within the molecular structure were taken from our theoretical calculations. The orientation of the Qy tdm results from the intersection of all three angles and was found to have an angle of (78 +/- 3)degrees with the x-axis, (12 3)degrees with the y-axis, and (86 +/- 2)degrees with the z-axis.     

Fifth-order Raman spectroscopy of liquid benzene: experiment and theory

The heterodyned fifth-order Raman response of liquid benzene has been measured and characterized by exploiting the passive-phase stabilization of diffractive optics. This result builds on our previous work with liquid carbon disulfide and extends the spectroscopy to a new liquid for the first time. The all-parallel and Dutch Cross polarization tensor elements are presented for both the experimental results and a finite-field molecular dynamics simulation. The overall response characteristics are similar to those of liquid carbon disulfide: a complete lack of signal along the pump delay, an elongated signal along the probe delay, and a short-lived signal along the time diagonal. Of particular interest is the change in phase between the nuclear and electronic response along the probe delay and diagonal which is not seen in CS2. Good agreement is achieved between the experiment and the finite-field molecular dynamics simulation. The measurement of the low-frequency Raman two-time delay correlation function indicates the intermolecular modes of liquid benzene to be primarily homogeneously broadened and that the liquid loses its nuclear rephasing ability within 300 fs. This rapid loss of nuclear correlations indicates a lack of modal character in the low-frequency motions of liquid benzene. This result is a validation of the general nature of the technique and represents an important step forward with respect to the use of nonlinear spectroscopy to directly access information on the anharmonic motions of liquids.     

Understanding polymers by vibrational spectroscopy: Some recent advances in theory and experiment

A rapid-scan version of 2D-IR spectroscopy is introduced and applied to study the phases of an acrylonitrile-butadienestyrene polymer system. Then, as the main part of this paper and in particular to stimulate further application in this field, some recent advances in the application of computational chemistry to the evaluation of vibrational spectra are presented. Some specific applications are discussed.     

Transition moment orientation and rotational bias of three carbonyl groups in large polarization FLCs observed by polarized FTIR

Polarized FTIR measurements have been made on four ferroelectric liquid crystal compounds which have a large spontaneous polarization; two of them have a hydroxyl group that can form a hydrogen bond. Particular attention was given to three C-O stretching peaks: keto, ester, and lactic acid. We found that the polar absorbance pattern and peak positions of the keto C-O group stretching vibration are clearly different in compounds with and without the hydroxyl group. We have succeeded in interpreting these measurements by calculating IR transition moment directions for the vibrational modes of several model compounds using quantum chemical methods, including HF/6-311G SCF and B3LYP/6-31G(d) and B3LYP/6-31G(d,f) DFT calculations. Rotational bias was clearly observed even in the ester C-O group in the molecular core in the SmC* phase of the compound with a hydrogen bond.     

Single-molecule surface-enhanced Raman spectroscopy of crystal violet isotopologues: theory and experiment

Single-molecule surface-enhanced Raman spectroscopy (SMSERS) of crystal violet (CV) has been reported since 1997, yet others have offered alternative explanations that do not necessarily imply SMSERS. Recently, the isotopologue approach, a statistically significant method to establish SMSERS, has been implemented for members of the rhodamine dye family. We provide the first demonstration of SMSERS of a triphenylmethane dye using the isotopologue approach. Two isotopologues of CV are employed to create chemically identical yet vibrationally distinct probe molecules. Experimental spectra were compared extensively with computational simulations to assign changes in mode frequencies upon deuteration. More than 90 silver nanoparticle clusters dosed with a 50:50 mixture of CV isotopologues were spectroscopically characterized, and the vibrational signature of only deuterated or undeuterated CV was observed 79 times, demonstrating that the isotopologue approach for proving SMSERS is applicable to both the CV and the rhodamine systems. The use of CV, a minimally fluorescent dye, allowed direct evaluation of enhancement factors (EF), which are reported herein. Through experiment and theory, we show that molecular electronic resonance Raman (RR) and surface-enhanced Raman effects combine synergistically in SMSERS. Excluding RR effects, the EF(SERS) is ～10(9). Variations and relationships between substrate morphology and optical properties are further characterized by correlated SMSERS-localized surface plasmon resonance (LSPR)-high-resolution transmission electron microscopy (HRTEM) studies. We did not observe SMSERS from individual nanoparticles; further, SMSERS-supporting dimers are heterodimers of two disparately sized particles, with no subnanometer gaps. We present the largest collection to date of HRTEM images of SMSERS-supporting nanoparticle assemblies.     

Vibrational relaxation and line widths in liquids: a comparison of theory and experiment

Published experimental data on the Raman line widths of some a₁ bands of methyl iodide and benzene are compared with theoretical predictions of relaxation theories which assume either rapid or slow time scales for intermolecular fluctuations. The observed variations with density and with mole fraction are found to fit the rapid motion theory with an additional temperature independent core contribution.     

Eta(z)/kappa: a transverse relaxation optimized spectroscopy NMR experiment measuring longitudinal relaxation interference

NMR spin relaxation experiments provide a powerful tool for the measurement of global and local biomolecular rotational dynamics at subnanosecond time scales. Technical limitations restrict most spin relaxation studies to biomolecules weighing less than 10 kDa, considerably smaller than the average protein molecular weight of 30 kDa. In particular, experiments measuring eta(z), the longitudinal (1)H(N)-(15)N dipole-dipole (DD)/(15)N chemical shift anisotropy (CSA) cross-correlated relaxation rate, are among those least suitable for use with larger biosystems. This is unfortunate because these experiments yield valuable insight into the variability of the (15)N CSA tensor over the polypeptide backbone, and this knowledge is critical to the correct interpretation of most (15)N-NMR backbone relaxation experiments, including R(2) and R(1). In order to remedy this situation, we present a new (1)H(N)-(15)N transverse relaxation optimized spectroscopy experiment measuring eta(z) suitable for applications with larger proteins (up to at least 30 kDa). The presented experiment also yields kappa, the site-specific rate of longitudinal (1)H(N)-(1)H(') DD cross relaxation. We describe the eta(z)/kappa experiment's performance in protonated human ubiquitin at 30.0 degrees C and in protonated calcium-saturated calmodulin/peptide complex at 20.0 degrees C, and demonstrate preliminary experimental results for a deuterated E. coli DnaK ATPase domain construct at 34 degrees C.     

Molecular dynamics and macroscopic alignment properties of thermotropic liquid-crystalline side chain polymers as studied by dielectric relaxation spectroscopy

Abstract

It is shown that dielectric relaxation spectroscopy provides a convenient means of studying the anisotropic reorientational dynamics of the mesogenic head groups in thermotropic liquid-crystalline side chain polymers. Their alignment behaviour in directing a.c. electric fields of different amplitudes and frequencies is examined, and samples having a macroscopic alignment which is fully homeotropic, fully planar or any desired intermediate alignment have been prepared. The nature and extent of alignment in such samples has been determined by dielectric spectroscopy. In addition both the temperature and pressure variations of the average dielectric relaxation times for certain relaxation processes have been determined and a bulk alignment phenomenon in the absence of a directing electric field is reported.     

Measurement of Br photofragment orientation and alignment from HBr photodissociation: production of highly spin-polarized hydrogen atoms

The orientation and alignment of the (2)P(3/2) and (2)P(1/2) Br photofragments from the photodissociation of HBr is measured at 193 nm in terms of a(q) ((k))(p) parameters, using slice imaging. The A (1)Pi state is excited almost exclusively, and the measured a(q) ((k))(p) parameters and the spin-orbit branching ratio show that the dissociation proceeds predominantly via nonadiabatic transitions to the a (3)Pi and 1 (3)Sigma(+) states. Conservation of angular momentum shows that the electrons of the nascent H atom cofragments (recoiling parallel to the photolysis polarization) are highly spin polarized: about 100% for the Br((2)P(1/2)) channel, and 86% for the Br((2)P(3/2)) channel. A similar analysis is demonstrated for the photodissociation of HCl.     

Investigation of MWS polarization and dc conductivity in polyamide 610 using dielectric relaxation spectroscopy

Dielectric relaxation spectroscopy technique was employed to study the Maxwell–Wagner–Sillars (MWS) polarization and dc conductivity in polyamide 610. The experimental dielectric data were analyzed within the formalisms of complex permittivity and electric modulus. The results were discussed in terms of ac conductivity, MWS polarization, electrode polarization and dc conductivity. In the frequency spectra of polyamide 610, charge carriers movement resulted in high values of the dielectric permittivity. The results revealed that the motion of the polymer chains governs the charge carrier transport. Two different mechanisms for charge carrier movement showed a transition temperature located between 110 and 120 °C. The change of charge carrier movement mechanisms was resulted from the onset of the polymeric chains in the interphase between amorphous and crystalline phases.     

Multiple-quantum relaxation dispersion NMR spectroscopy probing millisecond time-scale dynamics in proteins: theory and application

New relaxation dispersion experiments are presented that probe millisecond time-scale dynamical processes in proteins. The experiments measure the relaxation of (1)H-(15)N multiple-quantum coherence as a function of the rate of application of either (1)H or (15)N refocusing pulses during a constant time relaxation interval. In contrast to the dispersion profiles generated from more conventional (15)N((1)H) single-quantum relaxation experiments that depend on changes in (15)N((1)H) chemical shifts between exchanging states, (1)H-(15)N multiple-quantum dispersions are sensitive to changes in the chemical environments of both (1)H and (15)N spins. The resulting multiple-quantum relaxation dispersion profiles can, therefore, be quite different from those generated by single-quantum experiments, so that an analysis of both single- and multiple-quantum profiles together provides a powerful approach for obtaining robust measures of exchange parameters. This is particularly the case in applications to protonated proteins where other methods for studying exchange involving amide proton spins are negatively influenced by contributions from neighboring protons. The methodology is demonstrated on protonated and perdeuterated samples of a G48M mutant of the Fyn SH3 domain that exchanges between folded and unfolded states in solution.     

Enhanced alignment and orientation of polar molecules by vibrational resonant adiabatic passage

The authors show that polar molecules can be adiabatically aligned and oriented by laser pulses more efficiently when the laser frequencies are vibrationally resonant. The aligned molecules are found in a superposition of vibrational pendular states, each associated with the alignment of the rotor in one vibrational state. The authors construct the dressed potential associated with this mechanism. Values of detunings and field amplitudes are given to optimize the degree of alignment and orientation for the CO molecule.     

Dielectric relaxation spectroscopy and alignment behavior of a polymer‐dispersed liquid crystal and its component materials

The dielectric properties of a polymer‐dispersed liquid crystal (PDLC), a liquid‐crystal (LC) mixture (BL036), and three polymer matrices of PN314 containing different amounts of BLO36 were determined over a range of frequencies and temperatures and, for the LC and PDLC, over a range of voltages leading to homeotropic alignment of the LC. The overall dielectric relaxation process was a weighted sum of contributions from (1) the primary (δ) process in the LC arising from the motions of the dipoles about the short molecular axis and (2) dipole motions in the polymer matrix. The dielectric spectra were determined as a function of frequency, temperature, and, when appropriate, applied voltage. An equivalent electrical circuit was used as a working model to describe the dielectric behavior of the PDLC in the absence and presence of applied voltages. Agreement between the dielectric data and this model was achieved if a portion of the LC phase at the interface was assumed to be immobile. The director order parameter for the LC component in the PDLC was determined from dielectric measurements as the material was aligned homeotropically in an applied electric field. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1173–1194, 2001     

Molecular alignment of ammonia studied by electron-ion-ion coincidence spectroscopy

Electron-ion-ion coincidence measurements carried out at discrete resonances near the N 1s threshold in ammonia are reported. The measured coincidence spectra show clear alignment of the molecule upon resonant core-electron excitation. The coincidence data are analyzed to extract information about the molecule in the excited state by simulating the alignment and the dissociation processes. Dynamic changes in molecular geometry are found as the photon energy is scanned through the N 1s-->4a(1) resonance, whereas for the N 1s-->2e state the geometry and kinetic energy released upon dissociation remain unchanged. The alignment of the core-excited molecules is found to be preserved even in two-step dissociation processes.     

Picosecond polarization spectroscopy as a probe of intramolecular dynamics: rovibronic relaxation in the S1 state of trans-stilbene

The first application of picosecond polarization spectroscopy to probe gas phase intramolecular relaxation under collision free conditions is reported. This technique is a sensitive probe of molecular rotation and when applied to trans-stilbene vapor at 200°C an exponential decay in the anisotropy with a lifetime of 48 ± 8 ps is observed. The results are discussed in the context of recent work on IVR and rovibrational intramolecular processes.     

Coherent and incoherent orientation and alignment of ICN photoproducts

We report extended measurements of the rotational polarization and correlated angular distribution of CN photofragments from ICN photodissociation, with a particular emphasis on the creation and detection of molecular orientation with circularly-polarized light. Doppler profiles of the nascent photoproducts are measured by Frequency-Modulated (FM) transient absorption, and the resulting high signal-to-noise data are valuable for verifying the form of the angular correlations between the recoil velocity, the photofragment rotational angular momentum, and the space-fixed frame defined by the dissociation polarization. A space-fixed bipolar moment notation can be used for an unambiguous characterization of the maximal set of polarization properties that can be created with one-photon excitation and detected with one-photon Doppler-resolved absorption spectroscopy. Relating the observed polarization moments to the various coherent and incoherent, adiabatic and non-adiabatic mechanisms, that have been derived and verified extensively in the case of diatomic photodissociation to polarized atomic fragments, is not unambiguous in the case of diatomic fragments from triatomic precursors. Constraints among various polarization moments confirmed in the case of diatomic dissociation are not confirmed in this triatomic case, where the perpendicular transitions to non-degenerate A' and A' components of a linear Omega = 1 state are qualitatively different from excitation to degenerate Omega = +/-1 states in a diatomic molecule.