what is a crystlline structure?

Question 1

what is a crystallien structure?

Answer 1

asic pattern of atoms repeated over and over

Question 2

braggs law

Answer 2

n (interner)lamda = 2d*sin(tetha)

Question 3

why does bragg peaks occur

Answer 3

due to constructive interference

Question 4

bigger atoms leads to more or less scattering

Answer 4

more due to more electrons that can ossilate which creates scattering

Question 5

what is PDF

Answer 5

provides probability of finding pair of atoms at a distance r in the material

Question 6

information in PDFs

Answer 6

peak position, peak intensity (bigger atoms = more intense peak), particle size

Question 7

what is scattering

Answer 7

change in the direction of the beam

Question 8

relation of Q and particle size in SAXS

Answer 8

inversilty related - larger Q = smaller particles

Question 9

what is contrast in SAXS

Answer 9

the difference in electron density from the object to surroundings

Question 10

Ephoton equals

Answer 10

binding + e kinetic

Question 11

photo electric effect

Answer 11

photons in x ray beam has a certain amount of energy - if this energy is equal to or higher than the binding energy of electrons - electrons will absorb energy and leave surface of material - photoelectrons

Question 12

XPS

Answer 12

surface sensitive. element specific. oxidation states. vacuum since electrons are easily trapped when interactig with e.g. air

Question 13

why do we see 2 peaks for 2p data in XPS

Answer 13

P1/2, P3/2, peak splitting due to spin orbit coupling.

Question 14

intensity of photoelectron

Answer 14

I = I_0 * e^d/lapda. lampda = inelastic mean free path, d = depth below surface.

Question 15

what does µ, absorption coefficient, provide details about

Answer 15

the probability of absorption

Question 16

bigger atoms in XPS. what does it do to binding energy

Answer 16

bigger nucleus = electrons more bound because of more proteins - more energy needed for unbinding the electron.

Question 17

what is scattering

Answer 17

photons in X ray has specific energy - when interacting with electrons in atom, the energy is reemitted by the electrons - sent out as scattered X ray

Question 18

when do we observe bragg peaks

Answer 18

when having speicifc wavelegnth that fits the distance between the atoms so that the scattered waves make construcutive interference

Question 19

what is XANES + pre edge

Answer 19

looks at transistions to unoccupied states. pre-edge privides knowledge about allowed and forbidden transistions.

Question 20

oxidations state and XANES

Answer 20

higher oxidation = less shielding = electrons are tighter bound why more energy is needed to unbind electrons

Question 21

µ is dependent on

Answer 21

the atoms in material.
distirbution of atoms
their bonding
lampda of X ray

Question 22

lambert beers law (XAS)1

Answer 22

I = I_0 * e^-µd
d = thickness of sample
I_0 = incoming intensity
I = outgoing intensity

Question 23

why is XPS element specific

Answer 23

looking at core electrons which are characteristic to the element.

Question 24

what is an auger electron (XAS)

Answer 24

when core electron is excited and leaves atom - creates vacancy inwhich a higher-lying electron can fall into. this electron can either emit flourescens or the excess energy can lead to an electron in outer shell leaving the atom. that outer electrin = auger electron

Question 25

EXAFS

Answer 25

oscillations in µ. yields short range structure of materials. photoelectron has wavelike properties and can interact with the electrons in neighboring atoms which can be seen as point scatters.

Question 26

compare gas and solid in how they would look in EXAFS

Answer 26

the closer structure, the more intereference, more oscialltions willbe observed in EXAFS. so solid = many oscillations. Gas = almost no oscillations liquid = in between

Question 27

PDF vs EXAFS (similarities)

Answer 27

- fourier transforms - provides info on distance between atoms - bulk techniques (no surface sensitivity) - can measure solids, liquids, gasses

Question 28

PDF vs EXAFS (differences)

Answer 28

- PDF = scattering, EXAFS. = absorption + scattering - EXAFS is more local - PDF can determine amorphous vs crystal. EXAFS cannot - EXAFS is element specific (since looking at core electrons). PDF is not

Question 29

AFM vs SEM (conductivity)

Answer 29

AFM does not require conductive surface (therefore usefull no matter the sample). SEM require conductive surfaces to prevent build up of charge

Question 30

contact mode vs tapping mode

Answer 30

contact mode: - constant interaction. Tip is dragged across the surface - rough interaction between sample + tip tapping mode': - good for biological / soft samples vibrating cantilever. - gentle interaction.

Question 31

forces at the surface in AFM

Answer 31

electrostatic forces van der WALL forces columbic forces (repulsive. repulsion by electrons)

Question 32

mica for AFM measurements

Answer 32

mica = a mineral has an even surface optically flat transparent free from contamination (when freshly cleaved)

Question 33

SEM

Answer 33

Scanning electron microscopy. electron beam. vacuum, since we are detecting electrons surface sensitive

Question 34

secondary electrons

Answer 34

inelastic electrons. sits at surface (top 10 nm) low energy electrons compared to the energy of the incoming beam. - when measured by detector, measured as current. more e- = more current at high point = appears brighter due to more electrons leaving

Question 35

backscattered electron

Answer 35

elastic electrons. high energy electrons. able to see surface topography. The higher the atomic number of the investigated sample, the higher the amount of backscattered electrons

Question 36

TEM

Answer 36

transmission electron microscopy. - trnasmitted electrons are measured. - thin samples - contrast in image comes from different electron density in sample. - vacuum

Question 37

Characterisitc X-ray (SEM and TEM)

Answer 37

high energy electron from primary electron beam can kick a core electron out of the atom. To stabilize the atom, a higher-lying electron will drop down and fill the vacancy. This drop down will emit a X-ray photon. this photon will be specific for the element, why it is called charateristic X rays.

Question 38

force curves AFM

Answer 38

force as a function of distance. Tell you about the forces between tip and surface of sample. tell furthermore about sample properties (stickyness)