Ch1 Spectroscopy

1. More than half

of all instrumental

methods of

analysis involve the

absorption or

emission of light.

Thus the

degree of light

absorption and the

intensity of light

emission are

the critical

measurements

2. Characterizing

Light

Particle theory of

light

Wave theory of

light

Electromagnetic

Wave3.

3. Radiant Energy

(Wave theory)

Luminance (亮度)

Φ: 光通量 Ω: 立體角θ: 給定方向與單位面積元 ds 法線方向的夾角Luminous intensity

(發光強度)

Φ: 光通量 Ω: 立體角4.Photon Energy

Wavenumber=1/

(𝒄𝒎)5.AbsorptionExcitationEmission

5.Beer’s Law For liquids For gases𝑇 : Transmission (or transmissivity) 𝛼 : Absorption coefficient of the substance 𝑙 : Path length 𝜀 : Molar

absorptivity (extinction coefficient) of the absorber 𝑐 : Molar concentration of

absorbing species in the material 𝜎 : Absorption cross section 𝑁 : Number density of absorbers 𝛼 = ′ 𝛼ln 10 = 2.303𝛼6. Cuvettemade of plastic,

glass, or fused quartz (for UV light) Beer’s Law PlotWavelength of Maximum Absorbance• The larger the molar absorptivity, the more sensitive the method. The greatest analytical sensitivity occurs at the wavelength at which the absorptivity is a maximum.Ch2Ultravioletvis

ibleSpectrometry

This technique is

complementary 螢光光譜某些物質經某波長入射光照射後，分子被激發從基態到激發態，並在很短時間內去激發從激發態返回基態，發出波長長於入射光的螢光。因此入射光的

能量(hν)需大於螢光能量。Light Sources

Visible light(鎢絲燈UV light 氘燈Ultraviolet-Visible

light(氙弧燈

Xenon Arc LampA

continuous

ultraviolet and

visible emission

contains xenon at a

fairly high pressure

and the light is

formed via a

discharge across a

pair of electrodes

BandwidthandRe

solution

The width of the

band that is

allowed to pass is

called the

bandwidthThe

narrowness of the

band that is

allowed to pass

varies from one

design to another

and is called the

resolution.

Absorption

Filters

Monochromators

An entrance slit, a

dispersing element,

and an exit slit.

Luminescence

Phosphorescence

(燐光):

Luminescence

resulting from a

jump back to

the ground state

after routing

through other

electronic states

(triplet

state,三重激發態) is

called

phosphorescence.

HOMO / LUMO

and Bandgap

HOMO: Highest

occupied molecular

orbital or called

“Valence band”.

LUMO: Lowest

unoccupied

molecular orbital,

or called

“Conduction band”

Bandgap: The

difference of the

energies of the

HOMO and LUMO

The smaller the

energy, the more

easily it will be

excited.

Conjugation

The spectrum on

the below

illustrates that

conjugation of

double and triple

bonds also

shifts the

absorption

maximum to longer

wavelengths. It is clear that each

additional double

bond in the

conjugated π-

electron system

shifts the

absorption

maximum about 30

nm in the same

direction. Increased

conjugation brings

the HOMO and

LUMO orbitals

closer together.

When two double

bonds are

conjugated, the

four π-atomic

orbitals combine to

generate four π-

molecular orbitals

(two are bonding

and two are

antibonding).In a

similar manner, the

three double bonds

of a conjugated

polyene create six

π-molecular

orbitals, half

bonding and half

anti-bonding.

Limitations of

UV-VIS

Spectroscopy

Sample:

works well on

liquids and

solutions solid will

scatter the light

Calibrationand

reference:

Blank reference will

be needed(濃度需夠精準Choice of solvent:

Every solvent has a

UV-vis absorbance

cutoff wavelength

which the solvent

itself absorbs all of

the light

Choice of

container:

also have a UV-vis

absorbance cutoff

Concentration of

solution:

想要得到精準的數據，材料的吸收波峰至少要大於背景值三倍(濃度要夠高才行材料的大小受限制(相

比容器不能太小4.Photoluminesc

ence(PL)

probing the

electronic structure

of materialsOne

way this excess

energy can be

dissipated by

the sample is

through the

emission of light, or

luminescence The

excitation

wavelength should

be lower than the

PL wavelength of

the measured

material.

Relationship to

UV-VIS and PL

在紫外 可見光譜的吸光度與波長的曲線圖是測量從基態躍遷到激發態，而光致發光譜是從激發態躍遷到基態。excitation

spectrum is a

graph of emission

intensity

versus excitation

wavelength

PL在半導體的應用Bandgap

determination

雜質與缺陷量測Recombination

emchanisms

表面結構和excited states

The simplest

photoluminescent

processes are

resonant

radiations, in which

a photon of a

particular

wavelength is

absorbed and an

equivalent photon

is immediately

emitted.

PL在有機的應用Light emission

patterns

Absorption and

Emission

ACQ, TICT and

AIE

Aggregation-

caused quenching

(ACQ):

fluorescence is

weakened with an

increase in its

solution

concentration

Aggregation-

induced emission

(AIE)

exactly opposite to

the ACQ effect

twisted

intramolecular

charge

transfer(TICT)

Excitation

Wavelength

Dependent

入射能量越高，因照射時間越長所造成的紅移現象越明顯，且時間越久，強度越弱

CH4X-Ray

Photoelectron

Spectroscopy

(XPS)

measures the

elemental

composition,

empirical formula,

chemical state and

electronic state of

the elements by

irradiating a

material with a

beam ofX-rays

while

simultaneously

measuring the

kinetic energy and

number of

electrons that

escape from the

top 1 to 10 nm of

the material being

analyzed. XPS

requires ultra-high

vacuum (UHV)

conditions (10 -8

Torr).

Function and

Application

XPS detects all

elements with an

atomic number

(Z) of 3 (lithium)

and above

Detection limits for

most of the

elements are in

the parts per

thousand

range(ppm) Alias Names of

XPS

ESCA: Electron

spectroscopy for

chemical analysis

UPS: Ultraviolet

photoelectron

spectroscopy

PES:

Photoemission

spectroscopy

Electron

Configurations

Fundamental

Theory of XPS𝒉𝝂= 𝑬𝒌 + 𝑬𝒃 + 𝝓𝒔𝒑𝒆𝒄 𝒉𝝂 :

Photon energy from

X-ray source 𝑬𝒌 :

Kinetic energy of

the electron 𝑬𝑩 :

Binding energy of

the sample 𝝓𝒔𝒑𝒆𝒄 : Work

function of

spectrometer Work

function: Energies

required to remove

electrons from the

Fermi to

vacuum levels

可畫出跟 binding

energy 有關的能偕圖XPS Spectrum

XPS spectrum is to

measure intensities

of

photoelectrons vs.

𝑬𝒃Spin Orbital

Splitting

For p, d and f

peaks, two peaks

are observed.The

separation

between the two

peaks are named

spin

orbital splitting,

which is

attributed to L-S

coupling (耦合作用).

L-S coupling

Peak notations

L-S coupling ( 𝑗= 𝑙+ 𝑠 ) 𝑗 : Total

angular

momentum 𝑙 : Orbital angular

momentum 𝑠 : Spin

angular

momentum

Peak Area

Ratios

2p 1/2 electrons

have a smaller

kinetic energy in

comparison to the

2p 3/2 . The

integrated intensity

of the 2p 1/2signal

is half of that of 2p

3/2 due to the

difference of the

energetic

degeneration of

both terms ( 𝟐𝒋+ 𝟏 ). Peak separation is

increased with

heavier atoms.

X-ray Induced

Auger Electrons

Satellite Peak &

Shake-up Peak

During the

emission of a

photoelectron one

may get an

additional

excitation of a

second

electron.The

related distribution

of the energy 𝒉𝝂

to both processes

causes a shift of

the photoelectron

to larger binding

energy, which

yields a satellite

peak. If the second

electron is also

ejected the

satelliteis called a

shake-off peak, if it

is only excited to a

higher electronic

level it is called a

shake-

uppeak.Shake-up

satellites are

observed for Cu(II)-

ions. They have

only partially filled

d-levels,which may

accept an excited

electron from

anions of an

appropriate

compound.

Effect of

Bandgap

If the sample is an

insulator or a

semiconductor with

a large bandgap

∆𝑬𝒈 the intensity

of the XPS signal

goes back to the

level at the leading

edge with a smaller

binding energy for

several eV and

then increases.

This feature mirrors

the bandgap of the

material. For

metals, electrons

may be excitedwith

any energy and no

gap is observed. background

subtraction

by Shirley

method

assumes that the

background

increases with the

binding

energy

proportionally to

the part of the

signal immediately

before a given

energy. For the

background

subtraction, it must

include or correct

for X-ray satellites

chemically shifted

specie Shake-up

peaks Plasmon or

other losses

Quantitative XPS

積分面積 of a peak 𝑰𝒊𝝆𝒊: Average atomic

concentration of

element 𝒊 in the

surface𝑲𝒊: Relative

sensitivity factors

(RSF)