Spectral Jumps Research Articles

Subnanosecond spectral diffusion measurement using photon correlation

Spectral diffusion is a result of random spectral jumps of a narrow line as a result of a fluctuating environment. It is an important issue in spectroscopy, because the observed spectral broadening prevents access to the intrinsic line properties. However, its characteristic parameters provide local information on the environment of a light emitter embedded in a solid matrix, or moving within a fluid, leading to numerous applications in physics and biology. We present a new experimental technique for measuring spectral diffusion based on photon correlations within a spectral line. Autocorrelation on half of the line and cross-correlation between the two halves give a quantitative value of the spectral diffusion time, with a resolution only limited by the correlation set-up. We have measured spectral diffusion of the photoluminescence of a single light emitter with a time resolution of 90 ps, exceeding by four orders of magnitude the best resolution reported to date.

Strong field line shapes and photon statistics from a single molecule under anomalous noise

We revisit the line-shape theory of a single molecule with anomalous stochastic spectral diffusion. Waiting time profiles for bath induced spectral jumps in the ground and excited states become different when a molecule, probed by continuous-wave laser field, reaches the steady state. This effect is studied for the stationary dichotomic continuous-time-random-walk spectral diffusion of a single two-level chromophore with power-law distributions of waiting times. Correlated waiting time distributions, line shapes, two-point fluorescence correlation function, and Mandel Q parameter are calculated for arbitrary magnitude of laser field. We extended previous weak field results and examined the breakdown of the central limit theorem in photon statistics, indicated by asymptotic power-law growth of Mandel Q parameter. Frequency profile of the Mandel Q parameter identifies the peaks of spectrum, which are related to anomalous spectral diffusion dynamics.

Low-Temperature Spectral Dynamics of Single TDI Molecules in n-Alkane Matrixes

We report on studies of the influence of the matrix on the spectral dynamics of the zero-phonon-line (ZPL) emission by means of single molecule spectroscopy at low temperature. The host-guest system combinations consist of terrylenediimide (TDI) molecules embedded in four n-alkane matrixes of hexane, heptane, pentadecane, and hexadecane. Excitations into the broad vibronic absorption band and spectrally dispersed detection allows us to monitor fluorescence of single TDI molecules as a function of time. In the case of long-chain n-alkanes (pentadecane and hexadecane), the ZPL line is quite stable, showing spectral jumps of moderate frequency of less than 10 cm(-1) with an average time between the jumps of 10 s. In a clear contrast, the spectral dynamics of TDI molecules embedded within the short-length n-alkane matrixes (heptane and hexane) feature much more frequent spectral jumps that occur on a broader energy scale. The results suggest that matrixes composed of short-chain molecules are more susceptible to translations and/or rotations, which influence the fluorescence of single guest chromophores.

2-methylterrylene in hexadecane: Do we see single rotational quantum jumps of methyl groups?

We performed comparative low temperature (2-30 K) hole-burning and single molecule experiments with 2-methylterrylene with the goal to detect single rotational tunneling jumps of methyl groups. The hole-burned spectrum with its sharply structured side features which are perfectly symmetrically arranged with respect to the central hole supports the assignment to rotational tunneling transitions. However, instead of one, three clearly distinguishable methyl groups show up in the spectrum. Based on molecular mechanics simulations we attribute them to different, nearly degenerate orientations of guest molecules in one specific site of the hexadecane lattice. The frequency distribution of spontaneous jumps of single molecules reflects the features of the hole-burned spectra, although the distribution in the single molecule experiments is significantly broader. The photoinduced frequency transformation of single molecules ("single molecule photobleaching experiments") fits to the features of the hole-burned spectra, except that, surprisingly, no significant number of spectral jumps could be generated in the frequency range where the prominent narrow antiholes are observed in the hole-burned spectra.

Multistate conformational model of a single LH2 complex: Quantitative picture of time-dependent spectral fluctuations

The fluorescence (FL) spectrum of single LH2 complexes fluctuates on a time-scale of seconds in wavelength, spectral shape and intensity [D. Rutkauskas, V. Novoderezhkin, R.J. Cogdell, R. van Grondelle, Biophys. J. 88 (2005) 422]. Here we propose a model capable to explain the statistics of these time-dependent FL-fluctuations. In this model this evolution of the spectra is described in terms of slow conformational motion of the pigment–protein complex inducing random shifts of the site energies. These shifts manifest themselves as inhomogeneous broadening of the bulk spectra and can be directly observed as spectral fluctuations in single molecule experiments. Our model assumes a finite number of conformational sub-states for each pigment of the complex and allows a calculation of the conformational dynamics by introducing phenomenological transfer rates between these sub-states. The simplest version of the model with two conformations for each pigment in enough to explain the fast small-size (±10 nm) fluctuations within the 860–880 nm spectral region. Larger spectral jumps that are observed in the experiment (10–20 nm jumps to the blue and 10–40 nm to the red) can only be reproduced by including additional conformational states. The simplest model includes one more pair of states (producing big red and big blue shifts of the site energies). This four-state model enables us to reproduce quantitatively (i) the distribution of the FL peak positions, (ii) the changes of the width and asymmetry of the FL profiles as a function of the FL peak position, and (iii) distribution of the amplitudes and times of spectral jumps as a function of the initial FL peak position. We classify and describe the dynamic patterns that appear most often in the measured FL traces and in the calculated dynamics. We demonstrate a similarity between the measured and modelled patterns and show how these main types of the dynamics are related to specific changes in the conformational state of the 18 pigments of the LH2-B850 antenna.

Optical Fourier Transform Spectroscopy of Single‐Walled Carbon Nanotubes and Single Molecules

We use two-beam interferometry in combination with confocal microscopy for Raman and fluorescence studies of spatially isolated single-walled carbon nanotubes and single dye molecules. We investigate the potential of optical Fourier transform spectroscopy for the spectral characterization of single molecules and molecular nanostructures. We demonstrate that it is possible to obtain the temporal coherence characteristics as well as reliable spectroscopic data of the single photon fluorescence emission of an isolated molecule from one measured interferogram, even though the molecule exhibits intensity fluctuations and spectral jumps.

Simultaneous Measurement of Orientational and Spectral Dynamics of Single Molecules in Nanostructured Host−Guest Materials

Nanostructured host-guest materials are important for various applications in nanoscience, and therefore, a thorough understanding of the dynamics of the guest molecules within the host matrix is needed. To this aim we used single-molecule fluorescence techniques to simultaneously examine the spectral and the orientational behavior of single molecules in nanostructured porous host materials. Two types of host-guest systems have been investigated. First, oxazine-1 dye molecules were fixed rigidly in the channels of microporous AlPO4-5 crystals. Second, it was shown that terrylenediimide (TDI) dye molecules move in the mesoporous network of an uncalcined M41S thin film. In the first sample both spectral fluctuations ( approximately 5 nm) and rare spectral jumps (>10 nm) of the emission maximum were observed. However, the orientation of the emission dipole of the dye molecules remained constant. In contrast, the second system showed orientational dynamics as well as substantially more spectral dynamics. In this system the molecules were found to move between different regions in the host. The typical motion of the TDI molecules in the pores of M41S was not continuous but characterized by jumps between specific sites. Moreover, the spectral and orientational dynamics were correlated and arose directly from the different environments that were being explored by the mobile molecule.

Protein dynamics revealed in the excitonic spectra of single LH2 complexes

The fluorescence emission spectrum of single peripheral light-harvesting (LH2) complexes of the photosynthetic purple bacterium Rhodopseudomonas acidophila exhibits remarkable dynamics on a time scale of several minutes. Often the spectral properties are quasi-stable; sometimes large spectral jumps to the blue or to the red are observed. To explain the dynamics, every pigment is proposed to be in two conformational substates with different excitation energies, which originate from the conformational state of the protein as a result of pigment–protein interaction. Due to the excitonic coupling in the ring of 18 pigments, the two-state assumption generates a substantial amount of distinct spectroscopic states, which reflect part of the inhomogeneous distributed spectral properties of LH2. To describe the observed dynamics, spontaneous and light-induced transitions are introduced between the two states. For each ‘realization of the disorder’, the spectral properties are calculated using a disordered exciton model combined with the modified Redfield theory to obtain realistic spectral line shapes. The single-molecule fluorescence peak (FLP) distribution, the distribution dependence on the excitation intensity, and the FLP time traces are well described within the framework of this model.

Probing Anomalous Relaxation by Coherent Multidimensional Optical Spectroscopy

We propose to study the origin of algebraic decay of two-point correlation functions observed in glasses, proteins, and quantum dots by their nonlinear response to sequences of ultrafast laser pulses. Power-law spectral singularities and temporal relaxation in two-dimensional correlation spectroscopy signals are predicted for a continuous time random walk model of stochastic spectral jumps in a two-level system with a power-law distribution of waiting times psi(t) approximately t;{-alpha-1}. Spectroscopic signatures of stationary ensembles for 1<alpha<2 and aging effects in nonstationary ensembles with 0<alpha<1 are identified.

Spectral diffusion and drift: Single chromophore and en masse

We develop a systematic description of spectral diffusion of ideal chromophores interacting with incoherently relaxing two-state, localized environmental degrees of freedom ("spins") for general initial environment configurations. We remedy the existing, incomplete treatments by formulating the problem in terms of the proper correlation function and by obtaining an accurate solution for generic aperiodic arrangements of environmental spins, nearly free of the customary simplifying assumptions on the multiparticle spin coordinate distribution. We report and estimate, for the first time, the effects of the drift and distortion of a narrow spectral line that arise when the line is not in the center of the inhomogeneous band. While the drift turns out to be modest in most ensemble measurements, accounting for its effects is imperative in analyzing single chromophore spectral jumps, to which end the authors propose a novel experiment. Further, we argue that by employing a sufficiently large chromophore one can decouple the concentration of the fluctuating centers from the strength of their interaction with the chromophore. Finally, the additional line broadening, owing to a distribution of the central chromophore frequencies, is evaluated. Upper estimates for an analogous broadening stemming from a nonequilibrium environment are made.

Assignment of Red Antenna States in Photosystem I from Thermosynechoccocus elongatus by Single-Molecule Spectroscopy

Single-molecule spectroscopy at cryogenic temperatures was used to elucidate spectral properties, heterogeneities, and dynamics of the chlorophyll a (Chla) molecules responsible for the fluorescence in photosystem I (PSI) from the cyanobacteria Thermosynechococcus elongatus. Absorption and hole burning data suggest the presence of three pools absorbing at wavelengths greater than 700 nm with their absorption maxima at 708, 715, and 719 nm. The responsible Chla molecules are termed C708, C715, and C719. In the emission spectra of single PSI complexes, zero-phonon lines (ZPLs) were observed over the whole red emission range of PSI. The spectral region of the C708 pool is dominated by intense ZPLs; on the other hand, the broad emission of C715/C719 is unstructured and ZPLs are seen in this region much less frequently. Spectral jumps of ZPLs were observed. The dynamics as well as the spectral range covered by such jumps differ for C708 and C715/C719. This heterogeneity is likely caused by differences in the close environment of the chromophores. A tentative assignment of C708 and C715/C719 to Chla dimers and a Chla trimer is discussed, which is based on the remarkable structural differences in the environment of the most probable candidates for the red-most fluorescence.

Conformational Relaxation of Single Bacterial Light-Harvesting Complexes

We have employed the technique of single-molecule fluorescence microspectroscopy to investigate the spontaneous conformational evolution of individual peripheral LH2 complexes from the purple bacterium Rhodopseudomonas acidophila. Fluorescence microscopy is a sensitive tool, which allows the spectral changes of single complexes to be monitored on a time scale from 0.1 s to many minutes. Here we have investigated "natural" (occurring in the absence of excitation) spectral diffusion after a spectral jump has occurred. In a quarter of all the observed spectral jumps recorded with the LH2 complexes, a further spontaneous evolution occurs, in the absence of illumination, that results in the formation of a different spectroscopic state. We suggest that this is due to a natural conformational development of the pigment-protein complex, which so far has not been observed for this type of complex at the single-molecule level. The functional significance of such structural rearrangements is not yet clear but may be associated with the necessity for the light-harvesting complexes to adjust their shape in the densely packed photosynthetic membrane.

Electric Field Induced Switching of the Fluorescence of Single Semiconductor Quantum Rods

The exceptional fluorescence properties of single CdSe quantum rods (QRs) arising from internal and external electric fields are studied. Reversible external field induced switching of the emission in single QRs is reported for the first time. This effect was correlated with local field induced emission intensity reduction and newly observed darkening mechanism. Bimodal spectral jumps under a zero field were also observed and assigned to charged exciton emission, a phenomenon that was likewise directly controlled through an external field. These phenomena point to the use of single QRs as spectrally tunable charge sensitive fluorophores with polarized emission in fluorescence tagging and optical switching applications.

Two-state analysis of single-molecule Raman spectra of crystal violet

Surface-enhanced Raman spectra of individual crystal violet molecules adsorbed on silver colloids were measured and analyzed. It was found that, because of the small number of hot spots on the surface of the colloids, the single-molecule limit is attained already at a molecule to colloid ratio of 10 3. Series of time-dependent spectra measured from individual molecules showed significant spectral jumps, involving essentially only the low-frequency part of the spectrum. Each spectrum could therefore be analyzed as belonging to one of two groups, depending on the ratio between its two parts. Spectral trajectories were converted into simplified two-state (high/low) trajectories. Probability distributions of the dwell times in the two states were constructed from the trajectories, and statistical analysis showed that the dynamic process underlying the spectral jumps is likely to be Markovian. We suggest that the jumps originate in fluctuations of molecule-surface charge transfer interactions, although our studies did not provide evidence for lateral motion of the molecules, as found previously with rhodamine 6G.

Vibronic Excitation of Single Molecules: A New Technique for Studing Low‐Temperature Dynamics

Herein, we present vibronic excitation and detection of purely electronic zero-phonon lines (ZPL) of single molecules as a new tool for investigating dynamics at cryogenic temperatures. Applications of this technique to study crystalline and amorphous matrix materials are presented. In the crystalline environment, spectrally stable ZPLs are observed at moderate excitation powers. By contrast, investigations at higher excitation intensities reveal the opening of local degrees of freedom and spectral jumps, which we interpret as the observation of elementary steps in the melting of a crystal. We compare these results to spectral single-molecule trajectories recorded in a polymer. The way in which much more complicated spectral features can be analysed is shown. Surprisingly, pronounced spectral shifts on a previously not accessible large energy scale are observed, which are hard to reconcile with the standard two-level model system used to describe low-temperature dynamics in disordered systems.

Surface‐ and Resonance‐Enhanced Micro‐Raman Spectroscopy of Xanthene Dyes: From the Ensemble to Single Molecules

Surface-enhanced resonance Raman scattering (SERRS) spectra of various rhodamine dyes, of pyronine G and thiopyronine adsorbed on isolated silver clusters were recorded at the ensemble level and at the single-molecule level with a high-resolution confocal laser microscope equipped with a spectrograph and a CCD-detector. Comparing single-molecule spectra with ensemble spectra, various inhomogeneous spectral features, such as line splitting, spectral wandering, spectral diffusion and abrupt spectral jumps between different metastable spectral states, are revealed positions and the relative intensities of the vibronic bands. Resonance enhancement is investigated with respect to single-molecule surface-enhanced Raman scattering (SERS) spectroscopy and is found to be responsible for approximately three orders of magnitude in sensitivity. A significant influence of the substituents on the single-molecule SERRS sensitivity is found, showing that various chemical effects are responsible for surface enhancement in addition to the electromagnetic enhancement effect.

A polarization method for measuring absorption spectra of single molecules

A new method for measuring low-temperature absorption spectra of single impurity molecules using a laser confocal polarized-light transmission microscope is proposed. The sensitivity of this method is of the same order of magnitude as that of fluorescence excitation spectroscopy. Combined use of both methods makes it possible to study distributions of impurity molecules by the magnitude of fluorescence quantum yield and their correlations with the linewidth distributions, as well as to determine if spectral jumps lead to changes in the fluorescence quantum yield.

Diffraction-Limited Photogeneration and Characterization of Silver Nanoparticles

Visible-light-induced photogeneration of silver nanoparticles in a diffraction-limited focal region is demonstrated. The photochemical growth depends quadratically on illumination intensity indicative of a multiphoton generation process, with the identity of the silver nanoparticles confirmed by UV/vis absorption spectroscopy. Mie simulations of the absorption spectrum reveal a size distribution dominated by Ag particles with radii in the range of a few nanometers. Spectrally resolved laser excitation and emission studies demonstrate that the likely luminescence source is surface-enhanced Raman scattering from silver nanoparticles, with spectral jumps occurring on a time scale comparable to that of fluctuations in the total luminescence intensity. Possible routes for the photogeneration process as well as identity of the Raman-active species are discussed. Such diffraction-limited photoproduction methods for luminescent silver nanoparticles offer novel routes toward optical data storage and nanometer-scal...

Spectral dynamics in the B800 band of LH2 from Rhodospirillum molischianum: a single-molecule study

The BChl a absorptions in the B800 spectrum of individual LH2 complexes from Rhodospirillum molischianum show sudden, reversible spectral jumps between a finite number of spectral positions. From our data, we conclude that these fluctuations result from conformational changes of the protein backbone in close vicinity of the chromophores which provides a sensitive tool to monitor local modulations of the pigment–protein interaction.

Photon correlation probing of spectral diffusion in impurity glasses

A method of photon correlation probing of spectral diffusion at ultralow temperatures is suggested for polymer glasses. The method makes it possible to determine the ensemble-averaged autocorrelation function of fluctuations in the spectra of individual chromophores from the fluctuations of the fluorescence intensity of a sample, as well as to select subensembles of impurity molecules by the amplitudes of spectral jumps of their absorption lines due to the transitions of molecules between equilibrium states closely spaced in energy. The method has a number of advantages in comparison with the conventional methods of investigation of spectral diffusion: single-molecule spectroscopy, hole-burning spectroscopy, and the measurement of hole broadening with time.