IGNOU MCH 20 SOLVED ASSIGNMENT

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MCH 020 (January 2026 - July 2026) - ENGLISH

Tutor Marked Assignment
Atomic and Molecular Spectroscopy (MCH-020)

Course Code: MCH-020
Assignment Code: MCH-020/TMA/2026
Maximum Marks: 100

Note: Attempt all questions. The marks for each question are indicated against it.

You may use the following whenever required:
; ;

1. Answer any five of the following questions in brief: (2X5)
(a) Explain the origin of atomic spectrum.
(b) What is meant by anisotropy of chemical bond? Give an example.
(c) What makes TMS a molecule of choice to act as a reference in the NMR spectrometry?
(d) What is meant by polarizability of a molecule?
(e) In what way is NMR spectrometry different from other molecular spectroscopic methods?
f) What is the importance of spectroscopy in structure determination?
(g) What is the requirement of the vibrational mode of a molecule to be Raman active?
(h) What is the photoelectric effect? Write the equation of Einstein’s photoelectric law.

2. (a) (i) 5G services in India are provided at a frequency of . Calculate the wavelength of the radiation. 
(ii) Explain why the atomic spectra are line spectra whereas the molecular spectra are band spectra. 
(b) A photon of visible light has a wavelength of . Calculate its 
(i) wavenumber
(ii) frequency
(iii) energy per mol

3. (a) What is meant by the Balmer series of hydrogen? What will be the wave number of the longest wavelength in the Balmer series?
(b) A photoelectron spectrum of a compound containing nitrogen (N), aluminium (Al), and fluorine (F) shows three strong peaks at the following binding energies:
Peak X: ~686 eV
Peak Y: ~399 eV
Peak Z: ~73 eV

Find the best option from the choices given below that best explains the relative positioning of these peaks in terms of binding energy.

(i) Peak X corresponds to F 1s electrons, which have the highest binding energy because fluorine’s high electronegativity and small size give its 1s electrons a large effective nuclear attraction. Peak Y is N 1s and Peak Z is Al 2p.

(ii) Nitrogen has more valence electrons than fluorine or aluminium, therefore its peak appears at the highest binding energy.

(iii) The spectrum shows more aluminium atoms than nitrogen or fluorine, so its peak appears at the lowest binding energy due to dilution.

(iv) Aluminium has the highest nuclear charge, so its 2p peak should have the largest binding energy, but shielding effects reverse this trend.
Justify your answer. 

4. (a) (i) What information about the molecule can be obtained from a rotation spectrum? 

(ii) What is meant by a selection rule? Give the selection rule for a rotating diatomic molecule modelled as a rigid rotator. 

(b) Assuming that the force constants of H-Cl and D-Cl are the same, calculate the fundamental vibration frequency of D-Cl if that of H-Cl is . 

5. (a) In Raman spectra, the Stokes line are more intense than the anti-Stokes lines. Why? 

(b) The rotational Raman spectrum of a diatomic molecule has the first line of a series of Stokes and anti-Stokes lines at , relative of the of incident light. What would be its bond length. 

(c) has bond length . Calculate the wavenumbers of the first four lines in its microwave spectrum. 
[Given: mass of C=12.000 u and mass of N=14.007 u]

6. (a) (i) Calculate the number of vibrational modes for a molecule of methane.

(ii) The frequencies of the first and second overtones respectively are exactly double and triple of the fundamental vibration frequency. Comment. 

(b) Raman rotational spectra of was observed with Hg light with Rayleigh line at . Stokes lines were observed at . Draw the rotational energy level diagram with Stokes and anti Stokes lines. 

7. (a) (i) Electronic transitions are called vertical transitions. Comment. 

(ii) A light source of wavelength illuminates a metal surface and electron are ejected with the maximum kinetic energy 2 eV. If the same surface is illuminated by a light source of wavelength , then calculate the maximum kinetic energy of the ejected electron. (Work function of metal is 1 eV) 

(b) (i) How does the rule of Mutual Exclusion help in predicting the symmetry in case of and molecules? 

(ii) The molecule has 3 fundamental vibrational frequencies. One is Raman active and two are IR active. Identify the structure of the molecule. 


8. (a) (i) Explain why the spin splitting is not observed in spectrum? 

(ii) Predict the number of signal and multiplicity of peaks in the spectrum of the following compounds: 
a) 2,5-Dimethylhexane
b) Cyclohexanol

(b) (i) What is meant by AX spin system in NMR? Draw a schematic energy level diagram for this system and show the allowed transitions. 


9. (a) (i) Calculate the ESR frequency of an unpaired electron in a magnetic field of 3000 G (0.3 T). 
[Given: ; ; and ]

(ii) Which of the following systems will show electron spin resonance (ESR) spectrum? (i) H (ii) (iii) (iv) (v) (vi)  

(b) What are different mechanisms that contribute to the relaxation of nuclear spins in the excited state? Explain any one of them.


10. (a) (i) What is Mossbauer effect? 

(ii) The lifetime of (excited state) is . If the excited state is above the ground state determine the linewidth and ratio . 

(b) Write short notes on the following. 
(a) Isomer shift (b) Spin-Spin splitting

MCH 020 (July 2026 - January 2027) - ENGLISH

Tutor Marked Assignment
MCH-020: ATOMIC AND MOLECULAR SPECTROSCOPY
Course Code: MCH-020
Assignment Code: MCH-020/TMA/2026-27
Maximum Marks: 100
Note: Answer all the questions given below.
You may use the following whenever required:
ℎ =6.626×10-34 ??; ?? = 9.11×10−31 ??; ? = 3.0×108 ??−1
1. a) A photon of violet light has a wavelength of 500 nm. Calculate its energy. (5)
 b) State the Lambert’s Law and translate it into a mathematical expression. (5)
2. a) Explain the vector nature of orbital angular momentum. Predict the values of
magnetic quantum number. With the help of a suitable diagram explain the concept
of total angular momentum.
(5)
 b) What is Stark Effect? In which condition do we get linear stark effect and quadratic
stark effect?
(5)
3. a) Following spectra gives the relative positioning and intensity of the 2s
peaks? Which of the following statements explain it best?

(5)
 i) Be has a greater nuclear charge than Li and more electrons in the 2s orbital.
 ii) Be electrons experience greater electron-electron repulsions than Li electrons.
 iii) Li has a greater nuclear attraction on the 2s electrons, so it is harder to remove
those
 iv) Li has greater electron shielding by the 1s orbital, so the 2s electrons can be
easily removed.
 b) With the help of suitable diagrams explain the rotation of BF3 molecule as an
example of an oblate symmetric top molecule.
(5)
4. a) The first line in the rotational spectrum of HF molecule is observed to be at
60.22 cm1
. Calculate the location of the band in DF molecule obtained by isotopic
substitution of H atom in HF by D. Assume that there is no change in the bond
length of the molecule.
(5)
 b) The oscillation frequency of 1H
35Cl is determined to be 3660 cm1
. Calculate the
zero point energy and the force constant for the molecule.
(Given: Atomic mass of Hydrogen: 1.008 u; atomic mass of 35Cl=34.969 u)
(5)
5. a) What is meant by anharmonic oscillator? Apart from mechanical anharmonicity
may arise? Why does it arise? With the help of a diagram show the allowed
absorption transitions for an anharmonic oscillator according to the selection rule.
(5)
 b) What is meant by Fermi resonance? When are hot bands observed in transitions? (

Derive the expression for the frequency of a hot band under anharmonic
conditions.
6. a) The Raman line for a substance was observed at 546.4 nm on irradiating with
322.4 nm radiation. Calculate the wave numbers of Stokes and anti-Stokes lines for
the substance if the exciting line has a wavelength of 328.5 nm.
(5)
 b) Explain the coherent anti-Stokes Raman Spectroscopy (CARS) technique. Give
suitable illustrations.
(5)
7. a) Illustrate the general procedure for determining the term symbols for (g)
1
 and
(u)
3
.
(5)
 b) The vibrational frequency and anharmonicity constant of HF molecule in an
excited electronic state are found to be 3456.25 cm1
 and 0.030 respectively.
Calculate the dissociation energies, (De) and (D0) of the HF molecule in this
excited state.
(5)
8. a) Explain the different mechanisms in fluorescence quenching. (5)
 b) (i) Describe the Nuclear Overhauser Effect (NOE) and its significance in structural
elucidation using NMR.
(2)
(ii) Predict the number of signals in 13C-NMR spectrum in the following
compounds.
I. 2,3-Dimethylbuta-1,3-diene
II. Cyclohexanol
III. p-Xylene
IV. Ethylbenzene
V. But-1-ene
VI. Pyrrole
(3)
9. a) (i) Free Mössbauer nucleus ¹¹⁹Sn emits a γ-radiation of frequency 5.76 × 10¹⁸ Hz.
Calculate therecoil velocity and energy of the nucleus. [Given: Avogadro number =
6.02 × 10²³ mol⁻¹].
(2)
 (ii) For a system with an axially symmetric electric field gradient, derive the
energy level scheme and determine the resulting quadrupole splitting for a
transition from a ground state with spin ??? = ?/? to an excited state with spin
??? = ?/?.
(3)
 b) (i) The ESR spectrum of an organic radical containing two carbon atoms consisted
of 12 lines having relative intensities of 1,2,3,1,6,3,3,6,1,3,2 and 1. Identify the
radical.
(3)
 (ii) Why does the g value in ESR spectroscopy become anisotropic in solid
samples or molecules with low symmetry?
(2)
10. a) What is Nuclear Overhauser Effect (NOE)? How does it arise. (5)
 b) In the ESR spectrum of K3Mo (CN)8, 25% isotopes of Mo show in spin I= 5/2.
Predict the pattern of lines around a central line from 75% non-magnetic isotope. 

 

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