QUIZ 1

What happens when a molecule absorbs UV/Vis radiation?

• It emits radiation immediately
  
  
  

• It is excited to a higher energy level

• It becomes diamagnetic

• It undergoes chemical decomposition

Answer explanation

Absorption of UV/Vis photons promotes electrons to higher electronic states (excitation). Emission only happens after relaxation.

Which state has paired electron spins and is diamagnetic?

• Triplet state
  
  
  

• Doublet state

• Singlet state

• Free radical state

Answer explanation

In a singlet state, all electrons are paired with opposite spins (↑↓). This makes the molecule diamagnetic (no net magnetic moment).

What is the main difference between fluorescence and phosphorescence?

• Fluorescence involves singlet–singlet transitions, phosphorescence involves triplet–singlet transitions

• Fluorescence has longer lifetime than phosphorescence

• Phosphorescence occurs at shorter wavelengths than fluorescence

• Both occur with the same lifetime

Answer explanation

Fluorescence is a fast (10⁻⁹–10⁻⁶ s) radiative return from excited singlet to ground singlet. Phosphorescence is slower (10⁻³–10 s) because it requires forbidden triplet–singlet transitions.

In a Jablonski diagram, the lowest horizontal line represents:

• Excited singlet state (S₁)

• Excited triplet state (T₁)

• Ground state (S₀)

• Vibrational relaxation

Answer explanation

S₀ (singlet ground state) is the lowest energy level where most molecules reside before excitation.

Which of the following is a radiationless deactivation process?

• Fluorescence

• Phosphorescence

• Internal conversion

• Emission spectrum

Answer explanation

Internal conversion transfers energy between electronic states without emitting photons (non-radiative).

Fluorescence emission is always observed at:

• Shorter wavelength than excitation

• Longer wavelength than excitation

• Equal wavelength as excitation

• Random wavelength independent of excitation

Answer explanation

Emission occurs after energy loss (relaxation), so emitted photons have less energy (longer λ) than absorbed photons — known as the Stokes shift.

The relationship F = Kc is valid at:

• High concentrations only

• Low concentrations only

• All concentrations

• Zero concentration

Answer explanation

At low concentration, fluorescence intensity is directly proportional to concentration. At high concentration, self-absorption and quenching distort the linearity.

Which factor generally reduces fluorescence efficiency?

• Low temperature

• High viscosity solvent

• Increased molecular collisions

• Addition of electron donor groups (–OH, –NH₂)

Answer explanation

Collisions favor non-radiative deactivation pathways, reducing fluorescence intensity. High viscosity and electron-donating groups usually enhance fluorescence.

What is the role of a photomultiplier tube in a fluorescence spectrometer?

• Excitation source

• Sample holder

• Detector of emitted light

• Wavelength selector

Answer explanation

The photomultiplier tube detects weak fluorescence signals and amplifies them into measurable electrical signals.

Which of the following species is commonly analyzed by fluorescence spectrometry?

• Organic enzymes and vitamins

• Nitrogen gas

• Noble gases

• All inorganic ions equally

Answer explanation

Fluorescence is strongest in organic molecules with aromatic rings (enzymes, vitamins, plant metabolites). Only a few inorganic species (e.g., UO₂²⁺) fluoresce naturally.