118 Nonradiative PPTs View free & download

Nonradiative recombination mechanisms in 2.37 m InGaAsSb GaSb PowerPoint PPT Presentation

Nonradiative recombination mechanisms in 2.37 m InGaAsSb GaSb - Kevin O'Brien, Stephen Sweeney, Alf Adams, Shirong Jin, Nasir Ahmad and Ben Murdin. Advanced Technology Institute , University of Surrey, Guildford, GU2 7XH, UK ...

Kevin O'Brien, Stephen Sweeney, Alf Adams, Shirong Jin, Nasir Ahmad and Ben Murdin. Advanced Technology Institute , University of Surrey, Guildford, GU2 7XH, UK ...

| PowerPoint PPT presentation | free to view

Semiconductor lasers PowerPoint PPT Presentation

Semiconductor lasers - Number of states that have transition. Energies in the. Interval from ... Auger: Radiative. Nonradiative (defects) Surface. lecture_15. 9. Materials. lecture_15 ...

Number of states that have transition. Energies in the. Interval from ... Auger: Radiative. Nonradiative (defects) Surface. lecture_15. 9. Materials. lecture_15 ...

| PowerPoint PPT presentation | free to view

Study of Small-Molecule Thin Organic Films Deposited on Porous Silicon Substrates PowerPoint PPT Presentation

Study of Small-Molecule Thin Organic Films Deposited on Porous Silicon Substrates - Title: Fabrication and characterisation of silicon nanostructures Author: Private Last modified by: zl Created Date: 5/12/2003 5:17:03 PM Document presentation format

Title: Fabrication and characterisation of silicon nanostructures Author: Private Last modified by: zl Created Date: 5/12/2003 5:17:03 PM Document presentation format

| PowerPoint PPT presentation | free to download

Out line - Title: Slide 1 Author: Hassy Last modified by: hassy Created Date: 8/8/2004 1:30:52 AM Document presentation format: Company

Title: Slide 1 Author: Hassy Last modified by: hassy Created Date: 8/8/2004 1:30:52 AM Document presentation format: Company

| PowerPoint PPT presentation | free to view

Molecular Luminescence Spectrometry PowerPoint PPT Presentation

Molecular Luminescence Spectrometry - Phosphorescence is susceptible to O2 and collisions with solvent molecules. ... For many molecules phosphorescence can only be observed at low temperatures in ...

Phosphorescence is susceptible to O2 and collisions with solvent molecules. ... For many molecules phosphorescence can only be observed at low temperatures in ...

| PowerPoint PPT presentation | free to view

Optical studies of meso-porous siliceous PowerPoint PPT Presentation

Optical studies of meso-porous siliceous - Optical studies of meso-porous siliceous Y. J. Lee a,c J. L. Shen b,c a Department of Computer Science and Information Engineering, Tung Nan Institute of Technology ...

Optical studies of meso-porous siliceous Y. J. Lee a,c J. L. Shen b,c a Department of Computer Science and Information Engineering, Tung Nan Institute of Technology ...

| PowerPoint PPT presentation | free to download

Ch18 Fundamentals of Spectrophotometry Part I An Introduction to Optical Spectroscopy PowerPoint PPT Presentation

Ch18 Fundamentals of Spectrophotometry Part I An Introduction to Optical Spectroscopy - 2) Wave-Particle Duality. 3) Electromagnetic Spectrum. 4) Types of Quantum Transition ... 2) Wave-Particle Duality. ?: wavelength. ?: frequency. v: light speed ...

2) Wave-Particle Duality. 3) Electromagnetic Spectrum. 4) Types of Quantum Transition ... 2) Wave-Particle Duality. ?: wavelength. ?: frequency. v: light speed ...

| PowerPoint PPT presentation | free to view

Light Sources for Optical Communications PowerPoint PPT Presentation

Light Sources for Optical Communications - Light Sources for Optical Communications Optical Communication Systems Xavier Fernando RCL Lab Lenses for Coupling Improvement If the source emitting area is smaller ...

Light Sources for Optical Communications Optical Communication Systems Xavier Fernando RCL Lab Lenses for Coupling Improvement If the source emitting area is smaller ...

| PowerPoint PPT presentation | free to download

QUANTUM DOT LASER PowerPoint PPT Presentation

QUANTUM DOT LASER - * * * * * * * * * * * * * * * * * * * * * * * Quantum-dot laser tightly ... References In nanotechnology, ... Dot LASER Slide 21 QDL Application ...

* * * * * * * * * * * * * * * * * * * * * * * Quantum-dot laser tightly ... References In nanotechnology, ... Dot LASER Slide 21 QDL Application ...

| PowerPoint PPT presentation | free to download

Studies of Band Alignment, Two-dimensional Electron Gas and Ordering Effects in InGaPN/GaAs Heterostructures - Studies of Band Alignment, Two-dimensional Electron Gas and Ordering Effects in InGaPN/GaAs Heterostructures ...

Studies of Band Alignment, Two-dimensional Electron Gas and Ordering Effects in InGaPN/GaAs Heterostructures ...

| PowerPoint PPT presentation | free to download

West SESAPS 2006 poster PowerPoint PPT Presentation

West SESAPS 2006 poster - Band Gap Energy = 1.65 eV In the 1.43 eV structure an increase in radiative efficiency is observed from 77K-120K before the expected decrease in efficiency ensues.

Band Gap Energy = 1.65 eV In the 1.43 eV structure an increase in radiative efficiency is observed from 77K-120K before the expected decrease in efficiency ensues.

| PowerPoint PPT presentation | free to download

Complex Reaction Mechanism PowerPoint PPT Presentation

Complex Reaction Mechanism - 37.8 Photochemistry. 37.8.1 Photophysical Processes Figure ... 37.8 Photochemistry Figure 37.15. Figure 37.15. Kinetics description of photo-physical processes. ...

37.8 Photochemistry. 37.8.1 Photophysical Processes Figure ... 37.8 Photochemistry Figure 37.15. Figure 37.15. Kinetics description of photo-physical processes. ...

| PowerPoint PPT presentation | free to download

Photochromism in Organic Molecules PowerPoint PPT Presentation

Photochromism in Organic Molecules - Katsuki Okuno Miyasaka Laboratory * * * Introduction Definition Example of Photochromic Molecules History Recent research Photochromism in single crystal Shape change ...

Katsuki Okuno Miyasaka Laboratory * * * Introduction Definition Example of Photochromic Molecules History Recent research Photochromism in single crystal Shape change ...

| PowerPoint PPT presentation | free to download

Introduction to Spectrochemical Methods PowerPoint PPT Presentation

Introduction to Spectrochemical Methods - Chapter 24 Introduction to Spectrochemical Methods

Chapter 24 Introduction to Spectrochemical Methods

| PowerPoint PPT presentation | free to view

Fluorescence PowerPoint PPT Presentation

Fluorescence - Theodore Lyman discovered the first spectral line of the series whose members all lie in the UV ... spectra have brought ... QM is applied for interpretation. ...

Theodore Lyman discovered the first spectral line of the series whose members all lie in the UV ... spectra have brought ... QM is applied for interpretation. ...

| PowerPoint PPT presentation | free to view

Luminescence - . ...

. ...

| PowerPoint PPT presentation | free to view

XRay Spectral Morphologies of Young Supernova Remnants John P. Hughes Rutgers University PowerPoint PPT Presentation

XRay Spectral Morphologies of Young Supernova Remnants John P. Hughes Rutgers University - COSPAR Houston, TX. 3. Overturning Our View of Cas A ... COSPAR Houston, TX. 8. PSR J1124-5916 in G292.0 1.8. Discovered at Parkes (Camilo et al 2002) ...

COSPAR Houston, TX. 3. Overturning Our View of Cas A ... COSPAR Houston, TX. 8. PSR J1124-5916 in G292.0 1.8. Discovered at Parkes (Camilo et al 2002) ...

| PowerPoint PPT presentation | free to download

Ultrafast Carrier Dynamics in Single-Walled Carbon Nanotubes PowerPoint PPT Presentation

Ultrafast Carrier Dynamics in Single-Walled Carbon Nanotubes - Title: Slide 1 Author: Hassy Last modified by: hassy Created Date: 8/8/2004 1:30:52 AM Document presentation format: Company

Title: Slide 1 Author: Hassy Last modified by: hassy Created Date: 8/8/2004 1:30:52 AM Document presentation format: Company

| PowerPoint PPT presentation | free to download

A Heino-Centric View of the History of Galactic Center Theory PowerPoint PPT Presentation

A Heino-Centric View of the History of Galactic Center Theory - A Heino-Centric View of the History of Galactic Center Theory ... Spin, precession and variability of Sgr A*. Linear Polarization images near the event horizon ...

A Heino-Centric View of the History of Galactic Center Theory ... Spin, precession and variability of Sgr A*. Linear Polarization images near the event horizon ...

| PowerPoint PPT presentation | free to download

Semiconductor Sources for Optical Communications PowerPoint PPT Presentation

Semiconductor Sources for Optical Communications - Title: Semiconductor Sources for Optical Communications Last modified by: gaurav Document presentation format: On-screen Show (4:3) Other titles

Title: Semiconductor Sources for Optical Communications Last modified by: gaurav Document presentation format: On-screen Show (4:3) Other titles

| PowerPoint PPT presentation | free to view

Latest development of InGaN and Short-Wavelength LD/LED/VCSEL PowerPoint PPT Presentation

Latest development of InGaN and Short-Wavelength LD/LED/VCSEL - Latest development of InGaN and Short-Wavelength LD/LED/VCSEL Man-lin Tu Introduction Using shorter wavelength blue lasers would decrease the spot size on ...

Latest development of InGaN and Short-Wavelength LD/LED/VCSEL Man-lin Tu Introduction Using shorter wavelength blue lasers would decrease the spot size on ...

| PowerPoint PPT presentation | free to download

Research Status--- High-Efficiency Organic Light-Emitting Diode PowerPoint PPT Presentation

Research Status--- High-Efficiency Organic Light-Emitting Diode - Luminescence efficiency. Optical output coupling. 2004/04/06. ???? ... q : the luminescence efficiency coupling : the coefficient of output ... Luminescence ...

Luminescence efficiency. Optical output coupling. 2004/04/06. ???? ... q : the luminescence efficiency coupling : the coefficient of output ... Luminescence ...

| PowerPoint PPT presentation | free to download

DRUG DISCOVERY PowerPoint PPT Presentation

DRUG DISCOVERY - How new drugs are discovered using high throughput screening of potential drug candidates (which are generated using a combinatorial library).....

How new drugs are discovered using high throughput screening of potential drug candidates (which are generated using a combinatorial library).....

| PowerPoint PPT presentation | free to download

Drug Discovery (1) PowerPoint PPT Presentation

Drug Discovery (1) - Discovery of drugs using high throughput screening (done using a combinatorial library)

Discovery of drugs using high throughput screening (done using a combinatorial library)

| PowerPoint PPT presentation | free to download

Molecular Spectroscopy including Absorbance, Fluorescence and working in the IR range PowerPoint PPT Presentation

Molecular Spectroscopy including Absorbance, Fluorescence and working in the IR range - Chromophore: unsaturated organic functional groups that absorb in the UV and VIS ... 1. It is used in ophthalmology, hydrologic studies and in automotive antifreeze. ...

Chromophore: unsaturated organic functional groups that absorb in the UV and VIS ... 1. It is used in ophthalmology, hydrologic studies and in automotive antifreeze. ...

| PowerPoint PPT presentation | free to view

VUV spectroscopy of rare earth ions in solids: recent studies and possible applications PowerPoint PPT Presentation

VUV spectroscopy of rare earth ions in solids: recent studies and possible applications - ascribed to ZPLs are marked. by symbol ' ', and to. dominating vibronic lines. by symbol ' ... due to zero-phonon and vibronic lines along with wide side bands ...

ascribed to ZPLs are marked. by symbol ' ', and to. dominating vibronic lines. by symbol ' ... due to zero-phonon and vibronic lines along with wide side bands ...

| PowerPoint PPT presentation | free to download

Waves and Optics PowerPoint PPT Presentation

Waves and Optics - ... contained all the colors of the rainbow, with wavelengths from about 390 nm to 780 nm. He actually labeled this rainbow a 'spectrum' ...

... contained all the colors of the rainbow, with wavelengths from about 390 nm to 780 nm. He actually labeled this rainbow a 'spectrum' ...

| PowerPoint PPT presentation | free to view

Types of Laser PowerPoint PPT Presentation

Types of Laser - ... Three level lasers (ii) Four level lasers Based on state of active medium used (i) Gas Laser (ii) Solid state Laser (iii) Semiconductor Laser (iv) Tunable dye Laser

... Three level lasers (ii) Four level lasers Based on state of active medium used (i) Gas Laser (ii) Solid state Laser (iii) Semiconductor Laser (iv) Tunable dye Laser

| PowerPoint PPT presentation | free to download

APS 2001 March Meeting Poster PowerPoint PPT Presentation

APS 2001 March Meeting Poster - Eg(x) x y m n Excitation-dependent transition between defect-related and radiative recombination in lattice-mismatched InGaAs/InAsP heterostructures

Eg(x) x y m n Excitation-dependent transition between defect-related and radiative recombination in lattice-mismatched InGaAs/InAsP heterostructures

| PowerPoint PPT presentation | free to download

1????????????? ????????? ??????????? ???????????????? ????????????, 119992, ?????? ???-2, ????????? ????, natalmurav@yandex.ru (495)939-0289 PowerPoint PPT Presentation

1????????????? ????????? ??????????? ???????????????? ????????????, 119992, ?????? ???-2, ????????? ????, natalmurav@yandex.ru (495)939-0289 - ... Alexander A. Bulychev Action potential in a plant cell lowers the light requirement for non-photochemical energy-dependent quenching of chlorophyll fluorescence.

... Alexander A. Bulychev Action potential in a plant cell lowers the light requirement for non-photochemical energy-dependent quenching of chlorophyll fluorescence.

| PowerPoint PPT presentation | free to download

Chapter 6 Characteristics of Atoms PowerPoint PPT Presentation

Chapter 6 Characteristics of Atoms - Chapter 6 Characteristics of Atoms Department of Chemistry and Biochemistry Seton Hall University

Chapter 6 Characteristics of Atoms Department of Chemistry and Biochemistry Seton Hall University

| PowerPoint PPT presentation | free to download

LASER FLASH PHOTOLYSIS PowerPoint PPT Presentation

LASER FLASH PHOTOLYSIS - xenon flash lamps - pulse width s time scale. ... 3) Gilbert, A,; Baggot, J, Essentials of Molecular Photochemistry, CRS Press,1991, 115. ...

xenon flash lamps - pulse width s time scale. ... 3) Gilbert, A,; Baggot, J, Essentials of Molecular Photochemistry, CRS Press,1991, 115. ...

| PowerPoint PPT presentation | free to view

Human Impacts on Weather and Climate - Recent Research Results Require That We Adopt A Broader Assessment - Human Impacts on Weather and Climate Recent Research Results Require That We Adopt A Broader Assessm

Human Impacts on Weather and Climate Recent Research Results Require That We Adopt A Broader Assessm

| PowerPoint PPT presentation | free to view

The Interior of Single Molecules Wilson Ho University of California, Irvine - The Interior of Single Molecules Wilson Ho University of California, Irvine

The Interior of Single Molecules Wilson Ho University of California, Irvine

| PowerPoint PPT presentation | free to view

Bez nadpisu - 1. The lifetime of the upper amplifying level (3F4) is cca 10 times shorter than ... evanescent field fibre sensors. surface plasmon resonance sensors ...

1. The lifetime of the upper amplifying level (3F4) is cca 10 times shorter than ... evanescent field fibre sensors. surface plasmon resonance sensors ...

| PowerPoint PPT presentation | free to view

THE EFFECT OF POLYELECTROLYTES ON THE AGGREGATION OF CYANINE DYES IN LANGMUIRBLODGETT FILMS AND IN A PowerPoint PPT Presentation

THE EFFECT OF POLYELECTROLYTES ON THE AGGREGATION OF CYANINE DYES IN LANGMUIRBLODGETT FILMS AND IN A - Pressure-Area ( A) isotherm of the dye on the surface of PVS aqueous ... The pressure-area isotherm of LB film of the dye on the ... isotherm of ...

Pressure-Area ( A) isotherm of the dye on the surface of PVS aqueous ... The pressure-area isotherm of LB film of the dye on the ... isotherm of ...

| PowerPoint PPT presentation | free to view

APh9a - APh9a

APh9a

| PowerPoint PPT presentation | free to view

Viktor Malyarchuk PowerPoint PPT Presentation

Viktor Malyarchuk - Square Brillouin Zone Classic Crystal. Square Brilloin Zone Sealed Holes ... Nanoimprinted vs. Classic (Cuts) 09/20/2005 Plasmonic Crystal Sensor ...

Square Brillouin Zone Classic Crystal. Square Brilloin Zone Sealed Holes ... Nanoimprinted vs. Classic (Cuts) 09/20/2005 Plasmonic Crystal Sensor ...

| PowerPoint PPT presentation | free to view

Introduction to Optical Electronics PowerPoint PPT Presentation

Introduction to Optical Electronics - 1. Introduction to Optical Electronics. Quantum (Photon) ... Saturable Absorbers. 25. Gain Coefficient. Inhomogeneously Broadened Medium. Gain Coefficient ...

1. Introduction to Optical Electronics. Quantum (Photon) ... Saturable Absorbers. 25. Gain Coefficient. Inhomogeneously Broadened Medium. Gain Coefficient ...

| PowerPoint PPT presentation | free to view

PowerPoint-Pr PowerPoint PPT Presentation

PowerPoint-Pr - This project requires electrochemical techniques and will thus be carried out in close cooperation with:

This project requires electrochemical techniques and will thus be carried out in close cooperation with:

| PowerPoint PPT presentation | free to download

Energy Transfer - Energy Transfer

Energy Transfer

| PowerPoint PPT presentation | free to view

Introduction to Dielectric Guides PowerPoint PPT Presentation

Introduction to Dielectric Guides - Introduction to Dielectric Guides Historical Perspective 1910 : Hondros and Debye Propagation of EM waves along cylindrical dielectric guides. 1930s : Waveguiding ...

Introduction to Dielectric Guides Historical Perspective 1910 : Hondros and Debye Propagation of EM waves along cylindrical dielectric guides. 1930s : Waveguiding ...

| PowerPoint PPT presentation | free to download

Semiconductor Sources for Optical Communications - Semiconductor Sources for Optical Communications Dr Manoj Kumar Professor & Head Department of ECE DAVIET,Jalandhar 1 2 3 Laser Digital Modulation Current (I) I(t ...

Semiconductor Sources for Optical Communications Dr Manoj Kumar Professor & Head Department of ECE DAVIET,Jalandhar 1 2 3 Laser Digital Modulation Current (I) I(t ...

| PowerPoint PPT presentation | free to view

Probing very long-lived excited electronic states of molecular cations by mass spectrometry PowerPoint PPT Presentation

Probing very long-lived excited electronic states of molecular cations by mass spectrometry - Probing very long-lived excited electronic states of molecular ... The inset shows the details of the electrode assembly. J. Chem. Phys. 113, 9532 (2000) ...

Probing very long-lived excited electronic states of molecular ... The inset shows the details of the electrode assembly. J. Chem. Phys. 113, 9532 (2000) ...

| PowerPoint PPT presentation | free to view

Photonic Crystals PowerPoint PPT Presentation

Photonic Crystals - Example II: dielectric nano-sphere. Eu ions in 100 nm 1 mm polystyrene ... Dielectric nanosphere. AFM. Confocal. AFM to check individual particle diameters ...

Example II: dielectric nano-sphere. Eu ions in 100 nm 1 mm polystyrene ... Dielectric nanosphere. AFM. Confocal. AFM to check individual particle diameters ...

| PowerPoint PPT presentation | free to view

Green Fluorescent Proteins and Color Variants PowerPoint PPT Presentation

Green Fluorescent Proteins and Color Variants - Green Fluorescent Proteins and Color Variants Structure and Applications Michael Kirberger Department of Chemistry, Georgia State University, Atlanta, GA, 30303

Green Fluorescent Proteins and Color Variants Structure and Applications Michael Kirberger Department of Chemistry, Georgia State University, Atlanta, GA, 30303

| PowerPoint PPT presentation | free to download

Diapositiva 1 PowerPoint PPT Presentation

Diapositiva 1 - The hunt for 3D global or localized structures in a semiconductor resonator Ph.D student: Lorenzo Columbo Supervisor: Prof. Luigi Lugiato

The hunt for 3D global or localized structures in a semiconductor resonator Ph.D student: Lorenzo Columbo Supervisor: Prof. Luigi Lugiato

| PowerPoint PPT presentation | free to download

Figure 151 Partial energy diagram for a photoluminescent system. PowerPoint PPT Presentation

Figure 151 Partial energy diagram for a photoluminescent system. - Singlet: all electron spins are paired; no ... Better selectivity than absorption spectroscopy; ... better selectivity; poorer precision; lower temperature; ...

Singlet: all electron spins are paired; no ... Better selectivity than absorption spectroscopy; ... better selectivity; poorer precision; lower temperature; ...

| PowerPoint PPT presentation | free to view

Occurs on the order of femtoseconds ... Friday, Octobe PowerPoint PPT Presentation

Occurs on the order of femtoseconds ... Friday, Octobe - Occurs on the order of femtoseconds ... Friday, October 13th: No seminar ... 'Fluorescence Lifetime Imaging Microscopy (FILM)' Friday, November 3rd from 1:00 ...

Occurs on the order of femtoseconds ... Friday, October 13th: No seminar ... 'Fluorescence Lifetime Imaging Microscopy (FILM)' Friday, November 3rd from 1:00 ...

| PowerPoint PPT presentation | free to view

A comparison of two fluorescence techniques FRET vs' Anisotropy PowerPoint PPT Presentation

A comparison of two fluorescence techniques FRET vs' Anisotropy - ... intensity and the steady-state spectrum. Steady-state anisotropy. Time-resolved intensity decay ... Time resolved used for intensity or anisotropy decay. ...

... intensity and the steady-state spectrum. Steady-state anisotropy. Time-resolved intensity decay ... Time resolved used for intensity or anisotropy decay. ...

| PowerPoint PPT presentation | free to view

Carlos E. CrespoHernndez PowerPoint PPT Presentation

Carlos E. CrespoHernndez - Prof. Bern Kohler and Group Members. National Institute of Health (R01-GM64563) ... Crespo-Hern ndez, C.E.; Cohen, B.; Kohler, B. Nature 2005, 436, 1141. ...

Prof. Bern Kohler and Group Members. National Institute of Health (R01-GM64563) ... Crespo-Hern ndez, C.E.; Cohen, B.; Kohler, B. Nature 2005, 436, 1141. ...

| PowerPoint PPT presentation | free to view

Optical Spectroscopy of Single-Walled Carbon Nanotubes PowerPoint PPT Presentation

Optical Spectroscopy of Single-Walled Carbon Nanotubes - ... Rolled-up Graphene Sheet Electronic Structure Electrical Transport Properties of SWNTs Optical Properties Light Emission from SWNT Slide 10 ...

... Rolled-up Graphene Sheet Electronic Structure Electrical Transport Properties of SWNTs Optical Properties Light Emission from SWNT Slide 10 ...

| PowerPoint PPT presentation | free to view

Fluorescence and filters PowerPoint PPT Presentation

Fluorescence and filters - Absorption of photon elevates fluorophore to excited singlet state S1' ... Bradwell A R, Hughes R G, Harden E L (2003) Atlas of HEp-2 patterns, 2nd edition ...

Absorption of photon elevates fluorophore to excited singlet state S1' ... Bradwell A R, Hughes R G, Harden E L (2003) Atlas of HEp-2 patterns, 2nd edition ...

| PowerPoint PPT presentation | free to view

Fluorescence and Fluorescence Probes Confocal Microscopy and Image Analysis PowerPoint PPT Presentation

Fluorescence and Fluorescence Probes Confocal Microscopy and Image Analysis - Intensity. related to the probability of the event. Wavelength ... Reduce excitation intensity. Use 'antifade' reagents (not compatible with viable cells) ...

Intensity. related to the probability of the event. Wavelength ... Reduce excitation intensity. Use 'antifade' reagents (not compatible with viable cells) ...

| PowerPoint PPT presentation | free to view

CH162 Kinetics PowerPoint PPT Presentation

CH162 Kinetics - Beer-Lambert law if x

Beer-Lambert law if x

| PowerPoint PPT presentation | free to download

Lecture 5: Charge carrier dynamics PowerPoint PPT Presentation

Lecture 5: Charge carrier dynamics - Whenever there is a gradient in the concentration of a species of mobile ... This non-radiative process competes with radiative recombination. ...

Whenever there is a gradient in the concentration of a species of mobile ... This non-radiative process competes with radiative recombination. ...

| PowerPoint PPT presentation | free to view

Nonradiative PowerPoint PPT Presentations