Two-dimensional Optical Spectroscopy Research Articles

Two-dimensional optical three-pulse photon echo spectroscopy. II. Signatures of coherent electronic motion and exciton population transfer in dimer two-dimensional spectra

Using the nonperturbative approach to the calculation of nonlinear optical spectra developed in a foregoing paper [Mancal et al., J. Chem. Phys. 124, 234504 (2006), preceding paper], calculations of two-dimensional electronic spectra of an excitonically coupled dimer model system are presented. The dissipative exciton transfer dynamics is treated within the Redfield theory and energetic disorder within the molecular ensemble is taken into account. The manner in which the two-dimensional spectra reveal electronic couplings in the aggregate system and the evolution of the spectra in time is studied in detail. Changes in the intensity and shape of the peaks in the two-dimensional relaxation spectra are related to the coherent and dissipative dynamics of the system. It is shown that coherent electronic motion, an electronic analog of a vibrational wave packet, can manifest itself in two-dimensional optical spectra of molecular aggregate systems as a periodic modulation of both the diagonal and off-diagonal peaks.

Neurovascular coupling investigated with two‐dimensional optical imaging spectroscopy in rat whisker barrel cortex

Optical imaging slit spectroscopy is a powerful method for estimating quantitative changes in cerebral haemodynamics, such as deoxyhaemoglobin, oxyhaemoglobin and blood volume (Hbr, HbO2 and Hbt, respectively). Its disadvantage is that there is a large loss of spatial data as one image dimension is used to encode spectral wavelength information. Single wavelength optical imaging, on the other hand, produces high-resolution spatiotemporal maps of brain activity, but yields only indirect measures of Hbr, HbO2 and Hbt. In this study we perform two-dimensional optical imaging spectroscopy (2D-OIS) in rat barrel cortex during contralateral whisker stimulation to obtain two-dimensional maps over time of Hbr, HbO2 and Hbt. The 2D-OIS was performed by illuminating the cortex with four wavelengths of light (575, 559, 495 and 587 nm), which were presented sequentially at a high frame rate (32 Hz). The contralateral whisker pad was stimulated using two different durations: 1 and 16 s (5 Hz, 1.2 mA). Control experiments used a hypercapnic (5% CO2) challenge to manipulate baseline blood flow and volume in the absence of corresponding neural activation. The 2D-OIS method allowed separation of artery, vein and parenchyma regions. The magnitude of the haemodynamic response elicited varied considerably between different vascular compartments; the largest responses in Hbt were in the arteries and the smallest in the veins. Phase lags in the HbO2 response between arteries and veins suggest that a process of upstream signalling maybe responsible for dilating the arteries. There was also a consistent increase in Hbr from arterial regions after whisker stimulation.

Diagnostics of Quasar Broad Absorption Line Geometry: X-Ray Observations and Two-dimensional Optical Spectroscopy

A new generation of sensitive X-ray measurements are indicating that the existence of X-ray attenuation column densities, NH > 1024 cm-2, is quite common among broad absorption line quasars (BALQSOs). This is significant to the geometry of the broad absorption line (BAL) outflow. In particular, such an X-ray shield also shields equatorial accretion disk winds from the UV, thereby preventing high-velocity equatorial outflows from being launched. By contrast, bipolar winds initiated by continuum radiation pressure from the funnel of a slim accretion disk flare outward (like a trumpet) and offer vastly different absorbing columns to the X-ray and UV emission that are emitted from distinct regions of the disk, ~6M and ~10M-40M, respectively (where M is the radius of the black hole). Recent numerical work indicates that it is also possible to launch bipolar outflows from the inner regions of a thin disk. The recent discovery with VLBI that the Galactic analog of a BALQSO, the X-ray binary Circinus X-1 (with high-velocity P Cygni X-ray absorption lines), is viewed virtually along the radio jet axis (and therefore along the spin axis of the black hole and the normal to the accretion disk) has rekindled interest in the bipolar models of BALQSOs. We explore this possibility by studying the nearest BALQSO, Mrk 231. High-resolution two-dimensional optical spectroscopy and VLBI mappings of the radio jet axis indicate that the BAL outflow is parallel to the parsec-scale radio jet.

Rapid phase-cycled two-dimensional optical spectroscopy in fluorescence and transmission mode

Two dimensional magnetic and optical spectra contain information about structure and dynamics inaccessible to the linear spectroscopist. Recently, phase cycling techniques in optical spectroscopy have extended the capabilities of two-dimensional electronic spectroscopy. Here, we present a method to generate collinear pump/probe pulses at high update rates for two-dimensional electronic spectroscopy. Both fluorescence mode and transmission mode photon echo data from rubidium vapor is presented.

Following the solvent directly during ultrafast excited state proton transfer.

Excited state intramolecular proton transfer in 1-chloroacetylaminoanthraquinone is investigated from the perspective of the solvent. Using a new two-dimensional nonlinear optical spectroscopy the solvent response is probed directly as the proton transfer takes place. The measurements indicate that solvent reorganization controls the proton transfer in acetonitrile by dynamically shifting the position of equilibrium in the excited state, even on subpicosecond time scales.

Two-dimensional spectroscopy using diffractive optics based phased-locked photon echoes

A novel technique has been developed to passively phase-lock heterodyne-detected three-pulse photon echo experiments for two-dimensional optical spectroscopy. By using a diffractive optic to generate the pulses required, and with careful introduction of the time delays between the pulses, we achieve excellent passive phase-locking, approaching λ/100 at a wavelength of 540 nm. The ability to generate phase-locked pulse pairs with independent time delays solves a long standing impediment for stable phase sensitive detection in true optical analogues of multi-dimensional nuclear magnetic resonance (NMR) techniques, and should be equally valuable in executing related multi-dimensional spectroscopies in the infrared.

Rapid two-dimensional optical spectroscopy through acousto-optic pulse shaping

Phase-matching techniques are widely used to retrieve nonlinear optical signals of electronic and vibrational transitions. Here, collinear, phase cycled pulses are used to collect the same nonlinear signals in direct analogy to nuclear magnetic resonance studies. An acousto-optic pulse shaper is used to create suitable sequences of ultrashort pulses with arbitrary relative delays and phases. The rapid update rate of the acousto-optic modulator allows for impressive data rates.

Propagation effects on the peak profile in two-dimensional optical photon echo spectroscopy

The effect of pulse propagation on the lineshape in a two-dimensional photon echo spectrum is investigated using the coupled Maxwell Bloch equations. Reshaping of excitation and signal pulses due to propagation results in broadening of peak profiles. The amount of broadening is not symmetric along the two dimensions and depends on pulse intensity and relaxation mechanism. It is found that relaxation mechanisms that give the same lineshape at low optical density (OD) can give different lineshapes at high OD. Even at relatively low OD the reshaping can be quite severe, and might result in misinterpretation of the obtained spectrum.

Energy coupling efficiency of a hydrogen microwave plasma reactor

Zero-dimensional and two-dimensional plasma models and optical emission spectroscopy are used in tandem to investigate the power coupling efficiency for a pure hydrogen microwave plasma. The zero-dimensional model accounts for the vibrational kinetics of H2, the chemistry of H2 and H excited states, and the kinetics of ground-state species. The set of species conservation equations are then coupled to the electron Boltzmann equation (to account for the non-Maxwellian electron energy distribution function) and the total energy equation for solution. The two-dimensional model makes use of a simpler thermochemical description of the plasma. The chemistry is described with nine species and thirty chemical reactions. Three energy modes are considered to describe the plasma’s thermal nonequilibrium, and Maxwellian distribution functions for kinetic and vibrational modes are assumed. The non-Maxwellian nature of the electron energy distribution function is separately accounted for. Experimentally, the absolute line emission intensity is utilized to obtain number densities of up to five hydrogen excited states using the following transitions: Hα (6563 Å), Hβ (4861 Å), Hγ (4340 Å), Hδ (4102 Å), and Hε (3970 Å). The first three transitions were used for a 38 Torr, 1000 W hydrogen discharge, and all five transitions were used for a 121 Torr, 4000 W hydrogen discharge. The absolute continuum emission from the plasma was compared to numerical predictions. The comparison of the numerical and experimental data indicates that 90%–100% of the input power is deposited in the plasma and that both the line and continuum emission match within a factor of 3, with the exception of the high energy excited states for the 4000 W plasma. A control volume heat transfer analysis validates the energy coupling.

Demonstration of differential backscatter absorption gas imaging

Backscatter absorption gas imaging (BAGI) is a technique that uses infrared active imaging to generate real-time video imagery of gas plumes. We describe a method that employs imaging at two wavelengths (absorbed and not absorbed by the gas to be detected) to allow wavelength-differential BAGI. From the frames collected at each wavelength, an absorbance image is created that displays the differential absorbance of the atmosphere between the imager and the backscatter surface. This is analogous to a two-dimensional topographic differential absorption lidar or differential optical absorption spectroscopy measurement. Gas plumes are displayed, but the topographic scene image is removed. This allows a more effective display of the plume image, thus ensuring detection under a wide variety of conditions. The instrument used to generate differential BAGI is described. Data generated by the instrument are presented and analyzed to estimate sensitivity.

Role of Pulse Phase and Direction in Two-Dimensional Optical Spectroscopy

This paper examines the parallels between magnetic resonance and optical spectroscopy, with the goal of determining to what extent the benefits of 2DNMR might be extended into the optical regime. Precise optical analogues of the simplest 2DNMR sequences (collinear pulse sequences with phased laser pulse generation, phase sensitive detection, and phase cycling) are now feasible, and we demonstrate that they do generate cross-peaks which reveal common energy levels, even when averaged over the distribution of pulse flip angles expected in most optical experiments. One enormous difference between laser and NMR experimentsthe use of pulses in different directions in opticscan be exploited to eliminate much of the phase cycling required in NMR. Phase control does permit rotating-frame detection, which is likely to be a substantial practical advantage. Finally, we point out optical analogues of the simplest 2DNMR sequences (COSY and NOESY) will likely add little to our understanding of ultrafast dynamics. Optic...

Two-dimensional spatial distributions of sputtered particles produced in a planar magnetron discharge of indium-tin-oxide target

Spatial distributions of sputtered particles during the dc planar magnetron sputtering of indium-tin-oxide (ITO) target in 5 mTorr Ar/O 2 were measured by two-dimensional optical emission spectroscopy (2D-OES) and laser induced fluorescence spectroscopy (2D-LIF). Absolute number density of sputtered In atoms was determined by calibrating the LIF signal with Rayleigh scattering signal from Ar. The flux of atomic species onto the substrate were estimated from the experimental In density and the assumed average velocity of sputtered particles. Comparison between the total flux evaluated from experimental deposition rate and the atomic flux estimated above indicated that the flux of molecular species should exist according to the fractional ratio of introduced O 2 to Ar. Spatial inhomogeneity in film properties such as deposition rate and resistivity and the influence of oxygen partial pressure on them were discussed based on the 2D spatial profiles of sputtered species.

Two-dimensional spatial profiles of plasma parameters in DC reactive magnetron sputtering of indium-tin-oxide

Two-dimensional spatial profiles of plasma parameters during the DC magnetron sputtering process of an indium-tin-oxide (ITO) target in Ar/O 2 are studied. Two-dimensional spatial profiles of sputtered indium atoms in the ground state and of argon and indium atoms in their excited states are measured by two-dimensional laser induced fluorescence (2D-LIF) and two-dimensional optical emission spectroscopy (2D-OES), respectively. The 2D-OES profiles of the excited species have strong radial inhomogeneity due to magnetron plasma ring formation. On the other hand, the 2D-LIF profile of sputtered In atoms has the maximum at the discharge center axis except for the proximity of the cathode and has a smooth decrease from the cathode to the anode. Effects of the preset partial-pressure of introduced O 2 on these parameters is also reported.

Two‐dimensional Spectroscopy in the Circumnuclear Region of the Seyfert 1 Ring Galaxy NGC 985

We present two-dimensional optical spectroscopy of the central region (94×122) of the Seyfert 1 ring galaxy NGC 985 obtained with an optical fiber system (two-dimensional Fiber ISIS System). The 95 spectra presented here include the Hβ-[O III] λλ4959, 5007 emission lines and the Mg I b absorption lines, which permit the study of the distribution and kinematics of the ionized gas and the stars in two dimensions. In agreement with the results of other authors, the continuum maps show the presence of two maxima: the bright Seyfert 1 nucleus and a second nucleus located about 37 to the northwest. These observations confirm that this second nucleus is an extragalactic object at the same redshift as that of NGC 985. These observations indicate an anisotropic distribution of the ionized gas around the Seyfert nucleus. Despite this, the velocity field of the ionized gas shows a rather regular pattern, its general kinematic properties being similar to those found in other unperturbed spiral Seyfert galaxies. Many of the spectra in the region between the Seyfert and the secondary nucleus have asymmetric and relatively broad [O III] emission-line profiles. The kinematic center of the stellar velocity field is located in the neighborhood of the Seyfert nucleus, suggesting that this nucleus is related to the main stellar component. Therefore, these observations support the hypothesis that NGC 985 is the result of a two-galaxy collision in which the intruder (elliptical or spheroidal) galaxy would be what is now the secondary nucleus, while the Seyfert activity is associated with the nucleus of the primary disk galaxy. In spite of the strong disruption that defines the large-scale morphology of NGC 985, the stellar and ionized gas kinematics do not appear significantly perturbed in the circumnuclear region.

Two-dimensional optical spectroscopy of ULIRG's: comparison with HST imaging

We present preliminary results of our programme aimed at the study of asample of ULIRGs on the basis of two-dimensional (integral field)optical spectroscopy and multi-frequency high spatial resolution HSTimaging. In particular, we show here results on Mrk 273 and IRAS 12112+0305recently obtained with the INTEGRAL system at the 4.2 m WHT. Abrief comparison with the HST images is also made.

Ignition phase of a pulsed microwave-excited oxygen plasma

A pure oxygen plasma has been investigated for continuous mode as well as for pulsed power operation. Microwave power is fed to the plasma by an annular waveguide with 10 slotted line radiators on the inner side (SLAN principle). Plasma diagnostics were performed using (i) electrostatic probe methods for electron diagnostics, (ii) two-dimensional optical emission spectroscopy for excited oxygen mapping, and (iii) two-photon laser-induced fluorescence LIF for the quantitative measurement of atomic oxygen. Time-resolved emission spectroscopy was possible with a fast gated CCD camera system equipped with suitable optical filters. Of special interest was the ignition phase of the discharge. The temporal and spatial density variation of excited atomic oxygen was monitored. A first signal is visible after 7 μs after power application followed by an emission burst localized at a position nearby the quartz wall at about 15 μs after gas breakdown. End-on images from the camera prove that the plasma then expands mainly in the azimuthal direction. After 150 μs a symmetrical excitation zone with 10 emission maxima has formed and a homogeneous power coupling from all slot antennae is achieved. Electron densities measured by probes and atomic oxygen ground state densities determined by LIF show much slower temporal changes than the emission intensity. The experimental results are interpreted by considering the relevant channels leading to the production of atomic oxygen.

The STIS Parallel Survey: Introduction and First Results

The installation of the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) allows for the first time two-dimensional optical and ultraviolet slitless spectroscopy of faint objects from space. The STIS Parallel Survey (SPS) routinely obtains broadband images and slitless spectra of random fields in parallel with HST observations using other instruments. The SPS is designed to study a wide variety of astrophysical phenomena, including the rate of star formation in galaxies at intermediate to high redshift through the detection of emission-line galaxies. We present the first results of the SPS, which demonstrate the capability of STIS slitless spectroscopy to detect and identify high-redshift galaxies.

Imaging of atomic oxygen in a microwave excited oxygen plasma with two-dimensional optical emission spectroscopy

The density distribution of atomic oxygen was investigated in a pure oxygen microwave discharge. Plasma is excited by use of a circular waveguide with slot antenna. Two-dimensional maps of the O(3p 3P) to O(3s 3S 0) emission line intensity have been obtained using planar optical emission spectroscopy imaging (POES). The dependence on operating parameters of the plasma source has been studied. With increasing pressure the atomic oxygen homogeneity degrades. At pressures below 20 Pa the oxygen density is nearly homogeneous due to the greater diffusion length. Absolute ground state concentration of atomic oxygen has been measured at a downstream position by two-photon laser-induced fluorescence (LIF). Values in the range of 10 13–10 14 cm −3 are obtained. Comparing POES to LIF results the POES signals must be corrected for line-of-sight path integration. If this is taken into account then LIF and POES measurements show a good correlation between atomic ground state and excited oxygen for pressures up to 50 Pa.

Electric-field effects on hole spectra: method for studying dye–matrix interactions in doped polymers

The magnitude of matrix-induced electric dipole moments of centrosymmetric dye molecules increases from the blue to the red edge of the inhomogeneous absorption band in a number of dye–matrix systems. For the polymer polyvinyl butyral doped with a symmetrically substituted zinc tetrabenzoporphin this behavior is analyzed with a microscopic stochastic theory of dye–matrix interaction. Moreover, it is shown how electric-field measurements on hole spectra performed at different optical frequencies can be interpreted as a sort of two-dimensional optical spectroscopy. The two frequency scales are given by the inhomogeneous and the homogeneous linewidths. The technique is best suited for investigating amorphous systems, in which the two scales differ most strongly.

Two-dimensional optical spectroscopy by periodic excitation of sublevel coherence with sub-Doppler resolution.

The response of ground-state spin coherence to narrow-band periodic optical excitation is studied theoretically and experimentally. The dependence of the induced oscillating magnetization on the transverse magnetic field and on the frequency of the laser radiation is calculated for a four-level model system from a Bloch-type equation, yielding a two-dimensional resonance line. The application of this two-dimensional high-resolution Zeeman and optical spectroscopy is demonstrated experimentally in the case of the ${\mathit{D}}_{1}$ line of atomic rubidium vapor. Resolution in the kilohertz range is achieved, which allows clear spectral separation of the two isotopes $^{85}\mathrm{Rb}$ and $^{87}\mathrm{Rb}$, showing overlapping lines in the conventional optical absorption spectrum.