Spectroscopy: Chapter 15

Question 1

What is Luminescence?

Answer 1

Re-emission of absorbed energy

Question 2

At what wavelength does fluorescence cease to occur because it breaks bonds?

Answer 2

<250 nm

Question 3

What are the three spectroscopic methods used for quanatative elemental anaylsis?

Answer 3

1. Mass Spectrometry
2. Optical Spectrometry
3. X-Ray Spectrometry

Question 4

What is resonance radiation or resonance fluorescence ?

Answer 4

a type of fluorescence where absorbed radiation is reemitted without a change in frequency.

Question 5

what is Pauli exclusion principle?

Answer 5

No more than two electrons can occupy an orbital and they must have opposed spin states.

Question 6

what is a singlet state?

Answer 6

a molecular electronic state in which all electron spins are paired.

Question 7

what is the average lifetime range of an excited triplet state?

Answer 7

from 10^-4to several seconds

Question 8

what is the average lifetime of an excited of an excited singlet state?

Answer 8

~10^-8 seconds

Question 9

describe internal conversion.

Answer 9

when a molecule passes to a lower-energy electronic state without emission of radiation.

Question 10

what is internal conversion?

Answer 10

a crossover between two states of the same multiplicity (singlet to singlet of triplet to triplet)

Question 11

Define Intersystem Crossing.

Answer 11

when there is a crossover between electronic states of different multiplicty.

Question 12

List the different deactivation processes

Answer 12

• Fluorescence/Phosphorescence
• Vibrational, rotational
• Internal Conversion
• External Conversion
• Intersystem Crossing

Question 13

How do electon donating substitutents affect fluorescence?

Answer 13

it increases fluorescence

Question 14

how do electron withdrawing substituents affect fluorescence?

Answer 14

It decreases fluorescence.

Question 15

Why doesn’t fluorescence occur at wavelengths LESS THAN 250nm?

Answer 15

because it breaks bonds

Question 16

What is an example of the heavy-atom effect?

Answer 16

the decrease of fluorescence with increasing molar mass of halogens.

Question 17

When is intersystem crossing most common?

Answer 17

in molecules that contain heavy atoms and in paramagnetic species such as molecular oxygens

Question 18

What is external conversion?

Answer 18

A process in which molecules in electronically excited states pass to a lower electronic state by colliding with other molecules.

**In this process the electronic energy is eventually converted into heat**

Question 19

How does tempurature effect fluorescence?

Answer 19

increasing temperature causes a decrease in fluorescence which increases the chance of external conversion

Question 20

how does viscosity effect fluorescence?

Answer 20

decrease in viscosity or zero viscosity results in a decrease in fluorescence

**increases external conversion**

Question 21

excessive absorption larger than ~0.05 is referred to as ________ ________.

Answer 21

primary absorption

Question 22

Emission wavelengths overlap with an absorption band is referred to as _______ ________.

Answer 22

Secondary Absorption

Question 23

Primary and secondary absorption are reffered to as ______ ______ effects.

Answer 23

inner filter

Question 24

Define *Dynamic (collisional) Quenching. *

Answer 24

nonradiative energy transfer between an excited species and the queching agent

Question 25

Define Static Quenching

Answer 25

when the quencher and ground state fluorophore form a dark complex

Question 26

Define Long Range (Förster) quenching

Answer 26

when an energy trasfer occurs without collisons between molecules.

Question 27

What sources can be used for Fluorescence Spectroscopy

Answer 27

Xe lamps or lasers

Question 28

What are advantages of using a laser as a source

Answer 28

1. can used very little sample (microliter or less) 2. can used for highly monochromatic excitation in order to minimize fluorescing interferences

Question 29

What is the advantage of using a lamp as a source in fluorescence spectrometers

Answer 29

1. inexpensive ## Footnote 2. determines multiple anayltes with different excitation wavelengths.

Question 30

What is the difference between an **fluorescence _emission_ spectrum** and a **fluorescence _excitation_ spectrum**

Answer 30

in the emission spectrum, the excitation wavelength is held constant and emission instensity is measured as a funcion of emission wavelength while in the excitation spectrum, the emission is measured at one wavelength while excitation wavelengths are scanned.

Question 31

Define Fluorescence.

Answer 31

when an molecule excited by absorption emits a photon from an excited singlet electonic state to a lower state with the same spin multiplicity. (singlet to singlet)

Question 32

Define Phosphorescence.

Answer 32

when an molecule excited by absorption emits a photon from an excited triplet electonic state to a lower state with a different spin multiplicity. (triplet to singlet)

Question 33

Define reasonance fluorescence.

Answer 33

when excitation and emission frequencies are the same

Question 34

What is a triplet state?

Answer 34

paramagnetic electronic states in which all eletron spins are unpaired

Question 35

define vibrational relaxation.

Answer 35

when a molecule losses vibrational energy without emitting radiation

Question 36

when does predissociation occur?

Answer 36

when a molecule changes from a higher electronic state to a upper level vibrational level of a lower electronic state that is strong enough to break bonds

Question 37

when does dissociation occur?

Answer 37

when radiation promotes a molecule directly to a state with enough vibrational energy to break bonds

Question 38

why do some compounds fluoresce while others don't?

Answer 38

if a compound relaxes too fast it will not fluoresce but if it relaxes slowly there is time for flourescence to occur.

Question 39

Fluroescence involves what type of multiplicity electron transitions?

Answer 39

singlet to singlet

Question 40

Phosphorescence involves what type of multiplicity electron transitions?

Answer 40

Triplet to singlet

Question 41

why do more compounds show fluorescence rather than phosphorescence?

Answer 41

because with fluorescence you don't have to flip spin states.