Photon Impact Research Articles

Direct Determination of Kinetic Rates from Single-Molecule Photon Arrival Trajectories Using Hidden Markov Models.

The measurement of fluorescence from single protein molecules has become an important new tool in the study of dynamic processes, allowing for the direct visualization of the motions experienced by individual proteins and macromolecular complexes. The data from such single-molecule experiments are in the form of photon trajectories, consisting of arrival times and wavelength information on individual photons. The analysis of photon trajectories can be difficult, particularly if the motions are occurring at rates comparable to the photon arrival rate or in the presence of noise. In this paper, we introduce the use of hidden Markov models (HMMs) for the analysis of photon trajectory data that operate using the photon data directly, without the need for ensemble averaging of the data as implied by correlation function analysis. Using a simple kinetic model, we examine the relationship between the uncertainty in the estimates of the motional rate and the photon detection rate. Remarkably, we obtain relative uncertainties in the rate constants of as little as 3% even when the interconversion rate is equal to the photon detection rate, and the uncertainty increases to only 10% when the interconversion rate is 10 times the photon detection rate. This suggests that useful information can be obtained for much faster kinetic regimes than have typically been studied. We also examine the impact of background photons on the determination of the rate and demonstrate that the HMM-based approach is robust, displaying small uncertainties for background photon arrival rates approaching that of the signal. These results not only are relevant in establishing the theoretical limits on precision, but are also useful in the context of experimental design. Finally, to demonstrate how the methodology can be extended to more complex kinetic models and how it can allow one to make use of the full power of statistics for purposes of model evaluation and selection, we consider a four-state kinetic model for protein conformational transitions previously studied by Schenter et al. (J. Phys. Chem. A1999, 103, 10477). We show how an HMM can be used as an alternative to higher-order correlation function analysis for the detection of "conformational memory" and apparent non-Markovian dynamics arising from such temporally inhomogeneous kinetic schemes.

Theoretical study of ionization potentials and dissociation energies of Cnq+ fullerenes (n=50–60, q=0, 1 and 2)

We have evaluated electronic energies of neutral, singly charged and doubly charged fullerenes with sizes n=50–60 using density functional (DFT) theory. For each value of the cluster charge, we have considered around 40 possible structures. We have found that, except for C522+, the most stable isomer always has the minimum possible number of C2 units between adjacent pentagons. We have evaluated adiabatic dissociation energies corresponding to the various dissociation channels leading to the emission of carbon dimers with different charges. Our findings for dissociation leading to C2 emission are in reasonable agreement with the latest experimental values. As a byproduct of our calculations, we have also evaluated the first and second adiabatic ionization potentials. Both dissociation energies and ionization potential are useful data to interpret fragmentation of fullerenes by impact of energetic photons, electrons and ions.

Fast quantum logic by selective displacement of hot trapped ions

The "pushing gate" proposed by Cirac and Zoller in 2000 for quantum logic in ion traps is discussed, in which a force is used to give a controlled push to a pair of trapped ions and thus realize a phase gate. The original proposal had a weakness in that it involved a hidden extreme sensitivity to the size of the force. Also, the physical origin of this force was not fully addressed. Here, we discuss the sensitivity and present a way to avoid it by choosing the spatial form of the pushing force in an optimal way. We also analyse the effect of imperfections in a pair of pi pulses which are used to implement a "spin-echo" to cancel correlated errors. We present a physical model for the force, namely the dipole force, and discuss the impact of unwanted photon scattering, and of finite temperature of the ions. The main effect of the temperature is to blur the phase of the gate owing to the ions exploring a range of values of the force. When the distance scale of the force profile is smaller than the ion separation, this effect is more important than the high-order terms in the Coulomb repulsion which were originally discussed. Overall, we find that whereas the "pushing gate" is not as resistant to imperfections as was supposed, it remains a significant candidate for ion trap quantum computing since it does not require ground state cooling, and in some cases it does not require the Lamb-Dicke limit, while the gate rate is fast, close to (rather than small compared to) the trap vibrational frequency.

RefiningChandra/ACIS Subpixel Event Repositioning Using a Backside‐illuminated CCD Model

Subpixel event repositioning (SER) techniques have been demonstrated to significantly improve the already unprecedented spatial resolution of Chandra X-ray imaging with the Advanced CCD Imaging Spectrometer (ACIS). Chandra CCD SER techniques are based on the premise that the impact position of events can be refined, based on the distribution of charge among affected CCD pixels. ACIS SER models proposed thus far are restricted to corner split (3 and 4 pixel) events and assume that such events take place at the split-pixel corners. To improve the event-counting statistics, we modified the ACIS SER algorithms to include 2 pixel split events and single-pixel events, using refined estimates for photon impact locations. Furthermore, simulations that make use of a high-fidelity backside-illuminated (BI) CCD model demonstrate that mean photon impact positions for split events are energy dependent, leading to further modification of subpixel event locations according to event type and energy, for BI ACIS devices. Testing on Chandra CCD X-ray observations of the Orion Nebula Cluster indicates that these modified SER algorithms further improve the spatial resolution of Chandra/ACIS, to the extent that the spreading in the spatial distribution of photons is dominated by the high-resolution mirror assembly, rather than by ACIS pixelization.

NLO corrections to the photon impact factor

We review the program of the calculation of next-to-leading order corrections to the virtual photon impact factor. Following a brief introduction we present some technical aspects for the various contributions. Recently obtained results for transversely polarised virtual photons are discussed and an outline of how infrared divergences are cancelled is given. Implications of the subtraction of leading energy logarithms are discussed.

Doubly differential measurements for multiple ionization of argon by electron impact: Comparison with positron impact and photoionization

Doubly differential cross sections for single and multiple ionization of Ar have been measured for 500, 750, and 1000 eV electron impact. The cross sections were measured as a function of projectile energy loss and scattering angle. The energy loss range was 0--85 % of the initial projectile energy and scattering angles were between $\ifmmode\pm\else\textpm\fi{}22\ifmmode^\circ\else\textdegree\fi{}.$ The data were put on an absolute scale by normalizing to total ionization cross sections available in the literature and found to be in good agreement with the absolute electron impact cross sections from DuBois and Rudd. For 750 eV impact, a comparison was made between the present electron impact data and positron impact data obtained using the same experimental conditions. The same energy dependence and yields for single ionization were found for both electron and positron impact. On the other hand, the double- and triple-ionization yields are smaller for positron impact as compared to electron impact. Comparisons with photoionization data showed that for outer shell ionization the fractions of double and triple ionization of argon by photon impact are in quite good agreement with the present electron impact data.

Measurement of vacuum-assisted photoionization at 1 GeV for Au and Ag targets.

We report a measurement of photon impact ionization of K and L shell of Au and K shell of Ag targets in the 1-GeV energy range. We show that the cross section is dominated by a contribution from a new channel called vacuum-assisted photoionization. In this process the energy-momentum balance associated with the removal of the innershell electron is obtained by conversion of a high-energy photon into an electron-positron pair. This measurement is consistent with the theoretical prediction that vacuum-assisted photoionization is the most probable ionization mechanism at very high energies.

On the calculation of the NLO virtual photon impact factor

The definition of the virtual photon impact factor involves the integration of the s-channel discontinuity of the photon–Reggeon scattering amplitude over the right cut. It permits to formulate a new approach for the calculation of the impact factor based on analytical properties of the amplitude in question. In the next-to-leading order it may give a possibility for considerable simplification of the calculation. We have shown that a part of the diagrams contributing to the impact factor can be treated without their real calculation.

The photon impact factor in next-to leading order

This talk describes the current status of the NLO calculation of the photon impact factor.

Investigation of 4H-SiC Schottky diodes by ion and X-ray micro beam induced charge collection techniques

Silicon carbide has recently emerged as an attractive material for ionisation radiation detection. The high band gap and high radiation damage resistance should allow the fabrication of detectors capable to operate at high temperature and in high radiation fields. The development of SiC radiation detectors in the field of spectroscopy imposes severe constraints in the electronic quality and homogeneity of the material. In this work we present an investigation of the charge collection properties of ‘detector grade’ 4H-SiC Schottky diodes performed by means of the X-ray and ion beam induced charge collection (XBICC and IBIC) techniques. Such techniques allow the minority carrier diffusion length of the material to be evaluated and mapping of the transport properties to be performed with a spatial resolution of the order of 1 μm. The investigated detectors are formed by Schottky contact (Au) on the epitaxial layer and an ohmic contact on the back side of 4H-SiC substrates. IBIC measurements were performed using protons of energy 0.7–1.7 MeV. The IBIC spectra show a complete charge collection generated by ionisation in the depletion region. Similar analysis was also performed in steady state conditions using data from photocurrent measurements carried out at European Synchrotron Radiation Facility using 3 keV photons. IBIC and XBICC maps were obtained by recording the mean pulse height and the mean photocurrent as a function of the photon or ion impact co-ordinates. The analysis of such maps allowed us to individuate the spatial distribution of defects and contact imperfections.

NLO corrections to the photon impact factor: Combining real and virtual corrections

In this third part of our calculation of the QCD NLO corrections to the photon impact factor we combine our previous results for the real corrections with the singular pieces of the virtual corrections and present finite analytic expressions for the quark-antiquark-gluon intermediate state inside the photon impact factor. We begin with a list of the infrared singular pieces of the virtual correction, obtained in the first step of our program. We then list the complete results for the real corrections (longitudinal and transverse photon polarization). In the next step we define, for the real corrections, the collinear and soft singular regions and calculate their contributions to the impact factor. We then subtract the contribution due to the central region. Finally, we combine the real corrections with the singular pieces of the virtual corrections and obtain our finite results.

Electron Interactions With BCl3

In this paper we review and assess the cross sections for collisions of low-energy electrons with boron trichloride (BCl3). The only available experimental cross section data are for partial and total ionization and electron attachment, and the electron attachment cross sections are uncertain. Calculated values are available for the total elastic, differential elastic, and momentum transfer cross sections, and derived cross sections have been published for vibrational excitation and dissociation. Other than some rather uncertain data on electron attachment rate constants and some measurements of electron drift velocities in BCl3/Ar and BCl3/He mixtures, there are no measurements of the electron attachment, ionization, or transport coefficients for this gas. Analysis of the experimental data on the electron affinity, electron attachment, and electron scattering, enabled identification of negative ion states of BCl3 at about −0.3, 1.0, 2.8, 5.2, 7.6, and 9.0 eV. Because the existing electron collision data are few and uncertain, relevant data are provided for photon impact on BCl3..

Quantum computation with ions in microscopic traps

We discuss a possible experimental realization of fast quantum gates with high fidelity with ions confined in microscopic traps. The original proposal of this physical system for quantum computation comes from Cirac and Zoller (Nature 404, 579 (2000)). In this paper we analyse a sensitivity of the ion-trap quantum gate on various experimental parameters which was omitted in the original proposal. We address imprecision of laser pulses, impact of photon scattering, nonzero temperature effects and influence of laser intensity fluctuations on the total fidelity of the two-qubit phase gate.

Simulation of single-event energy-deposition spreading in a hybrid pixellated detector for γ imaging

In the framework of the Medipix2 Collaboration, a new photon-counting chip is being developed made of a 256×256 array of 55 μm-side square pixels. Although the chip was primarily developed for semiconductor X-ray imagers, we think that this type of device could be used in applications such as decommissioning of nuclear facilities where typical sources have γ-ray energies in the range of a few hundred keV. In order to enhance the detection efficiency in this energy range, we envisage connecting the Medipix2 chip to a CdTe or CdZnTe substrate (at least 1 mm thick). The small pixel size, the thickness of the Cd(Zn)Te substrate and the high photon energy motivate us to estimate first the spatial energy spreading following a photon interaction inside the detector. Estimations were made using the MCNP Monte Carlo package by simulating the individual energy distribution for each primary photon interaction. As an illustration of our results, simulating a 660 keV γ source, we found that there are two pixels on average, for each primary interaction, on which the deposited energy exceeds 50 keV. We have also made more accurate simulations using sub-pixels of side 11 μm, in order to evaluate the distance between the barycentre of the deposited energy and the photon impact point. As an example, with a 660 keV γ source, we found that the average of this distance reaches 67 μm when restricted to the events depositing more than 400 keV. If all events are taken into account, the mean distance is 26 μm, even though there is a small proportion of interactions where the scattered photon interacts somewhere else in the detector. Results are presented for different photon energies (60 keV, 660 keV, 1.25 MeV) and different materials (CdTe, GaAs).

Ionization cross section ratios of rare-gas atoms(Ne, Ar, Kr and Xe) by electron impact from thresholdto 1 keV

Electron impact multiple ionization relative cross sections ofrare gas atoms (Ne, Ar, Kr and Xe) have been measured using apulsed electron beam and a pulsed ion extraction combined witha time-of-flight analysis of the charge. The measurements covera larger range of energies and charges than in earlierexperiments, from threshold to 1 keV and from charge n = 1 to 4 (Ne),5 (Ar), 7 (Kr) and 6 (Xe). The average number(γ) of electrons ejected from the atom per ionizationevent is calculated and used to evaluate the contributions ofthe inner-shell processes. The results are also compared withthose in the case of photon impact. As regards the majorions produced, relative uncertainty as low as 1.3% for theionization cross section ratios is achieved for all the samplegases, except near threshold. As for the γ-values,relative uncertainty is estimated to be as low as 1.2%.

Theory of diffractive structure functions

We discuss the connection between the dipole approach and the DIS structure functions. Particular emphasis is given to DIS diffraction. We report on current work on the NLO photon impact factor. As an application of the dipole approach in DIS diffraction, we discuss the diffractive production of jets and of charm.

Spontaneous and electron-induced adsorption of oxygen on Au([formula omitted])-(1×2)

The interaction of dioxygen with a gold(110)-(1×2) surface has been investigated between 28 and 700 K by means of thermal desorption spectroscopy (TDS), UV photoelectron spectroscopy (UPS), work function measurements (ΔΦ), low-energy electron diffraction (LEED), and near-edge X-ray absorption spectroscopy (NEXAFS). It proves that Au––unlike most of the other metals including its congeners Cu and Ag––does not spontaneously dissociate physisorbed molecular oxygen. Rather, additional activation of the physisorbed O2 either by electron or by photon impact is required to make the oxygen chemisorb. Below 50 K, dioxygen adsorbs readily with a binding energy <12 kJ/mol, causing a work function decrease of ΔΦ=0.22 eV at Θ=1.0 ML. TDS reveals three molecular desorption states with first-order kinetics around 51 and 45 K (first layer), and at 37 K (second layer). A zeroth-order peak around 34 K corresponds to multilayer desorption. Both UPS and NEXAFS exhibit signals typical for physisorbed O2. Irradiation of physisorbed O2 layers with low-energy electrons or UV photons produces chemisorbed oxygen, providing a convenient possibility for the preparation of chemisorbed oxygen adlayers. The chemisorbed oxygen species is characterized by a single desorption state above 500 K, with second-order kinetics at low coverages (Θ<0.25 ML) suggesting adsorbed oxygen atoms. A desorption energy of 140±3 kJ/mol was determined. Another desorption peak around 490 K and at coverages >1.0 ML is associated with the decomposition of an oxidic species. LEED observations reveal that chemisorbed oxygen destroys the long-range order of the Au substrate surface. Our results provide possible explanations for the `beam damage' often observed in UPS/LEED experiments with adsorbed dioxygen.

Erratum:(γ*→qq¯)-Reggeon vertex in next-to-leading order QCD [Phys. Rev. D63, 056014 (2001)

As a first step towards the computation of the NLO corrections to the photon impact factor in the $\gamma^*\gamma^* \to \gamma^* \gamma^*$ scattering process, we calculate the one loop corrections to the coupling of the reggeized gluon to the $\gamma^*\to q\bar{q}$ vertex. We list the results for the Feynman diagrams which contribute: all loop integrations are carried out, and the results are presented in the helicity basis of photon, quark, and antiquark.

Differential ionization measurements using positron impact

This paper describes techniques being used at the University of Missouri-Rolla for the purpose of obtaining differential ionization information for positron impact. Data are presented for single and multiple ionization of argon as a function of energy loss by positrons scattered in the forward direction. The impact energy is 750 eV. The data and their ratios are used to provide insights into which ionization channels are active. For large energy losses the double ionization ratios are found to be very similar to those obtained for photon impact energies where only outer shell ionization is possible

Chemistry of O( 1D) atoms in the coma: implications for cometary missions

The forbidden red oxygen lines at 6300 and 6364 Å, which results due to 1D→ 3P transition, provide an important diagnostic tool in the study of comets. These lines cannot be produced by resonance fluorescence excitation of the ground-state oxygen atom, and therefore are mainly produced due to dissociation of H 2O and other O-containing species in comets (e.g., OH, CO, CO 2, H 2CO etc.) by photon and electron impact and in other collision reactions. Since the lifetime of 1D state is quite long (∼110 sec) collisional de-excitation processes are important. We have used a coupled chemistry-transport model in conjunction with an efficient emission production code to study the chemistry of O( 1D) atoms and the production of OI 6300 Å emission in comets. The model calculations are made for comet 46P/Wirtanen: the target of the ROSETTA mission. It is found that in the inner coma the density profile of O( 1D) is controlled dominantly by the H 2O. The model predicts ∼300 R of OI 6300 A brightness on comet Wirtanen.