Questions and answers for exam Preparation

MCQs

Total Questions : 9

Question 1.
What is the relationship between wavelength and wave number?

Wavenumber = 1 / wavelength in centimeters
Wavenumber - wavelength in nanometers = 1
Wavelength in nanometers x wavenumber = 1
None of the above

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Answer: Option A. -> Wavenumber = 1 / wavelength in centimeters

Question 2. Why do fluorescence spectrometers often use double-beam optics?

So a reference solution can be used
To compensate for beam attenuation by the monochromator
To compensate for power fluctuations in the radiation source
All of the above

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Answer: Option C. -> To compensate for power fluctuations in the radiation source

Question 3. In the intersystem crossing

the spin of an excited electron reverses, changing the state of the molecule (from singlet state to triplet state or vice versa)
a molecule converts its excess energy to light, and emits a photon
a molecule converts excess electronic energy to vibrational energy
all of the above

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Answer: Option A. -> the spin of an excited electron reverses, changing the state of the molecule (from singlet state to triplet state or vice versa)

Question 4. Internal conversion is where

the spin of an excited electron reverses, changing the state of the molecule (from singlet state to triplet state or vice versa)
A molecule converts its excess energy to light, and emits a photon
A molecule converts excess electronic energy to vibrational energy
none of the above

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Answer: Option C. -> A molecule converts excess electronic energy to vibrational energy

Question 5.
For a molecule to absorb IR, why must the molecule's vibrations cause fluctuations in the dipole moment of the molecule?

Because a change in dipole moment lowers the energy required for electronic transitions
Because for absorption to occur, the radiation must interact with the electric field caused by changing dipole moment
Because fluctuations in the dipole moment allow the molecule to deform by bending and stretching
All of the above

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Answer: Option B. -> Because for absorption to occur, the radiation must interact with the electric field caused by changing dipole moment

Question 6. Why must the radiation source for fluorescence spectrometry be more powerful than for absorption spectroscopy?

Because the magnitude of the output signal is proportional to the power of the incident radiation
Because the sample won't fluoresce if the incident radiation is of low power
To allow for scattering by the sample
None of the above

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Answer: Option A. -> Because the magnitude of the output signal is proportional to the power of the incident radiation

Question 7. Why phosphorescence measurements are usually made at a low temperature?

To prevent thermal degradation of the phosphorescent species
To promote phosphorescence by slowing the rate of radiationless transfer processes
To increase the efficiency of the detector
To decease the efficiency of detector

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Answer: Option B. -> To promote phosphorescence by slowing the rate of radiationless transfer processes

Question 8. Fluorescence occurs when

a molecule returns to the electronic ground state from an excited triplet state by losing it's excess energy as a photon
a molecule returns to the electronic ground state from an excited singlet state by losing it's excess energy as a photon
a molecule lowers its vibrational energy by losing it's excess energy as a photon
none of the above

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Answer: Option B. -> a molecule returns to the electronic ground state from an excited singlet state by losing it's excess energy as a photon

Question 9. . Fluorescence occurs when

a molecule returns to the electronic ground state from an excited triplet state by losing it's excess energy as a photon
a molecule returns to the electronic ground state from an excited singlet state by losing it's excess energy as a photon
a molecule lowers its vibrational energy by losing it's excess energy as a photon
none of the above

Discuss Question

Answer: Option B. -> a molecule returns to the electronic ground state from an excited singlet state by losing it's excess energy as a photon