Introduction of spectra of diatomic molecules

A diatomic molecule has , like an atom ,an electron cloud and so a displacement of the most loosely bound electron from one possible orbit to another will correspond to electronic transition leading to emission or absorption of radiation. In this perticular article ,Introduction of spectra of diatomic molecules we are going to discuss about the basic information about spectra of diatomic molecule, regions of spectrum,  etc.

For single atom there are two more types of energies which do exist . That is rotational energy and vibrational energy. And These energies remains quantized. A vibration energy generates in a molecule due to elerctronic transition. And If Vibrational energy of a molecule is decreased during electronic transition then it shows emission process . And if vibrational energy of a molecule during electronic transition increases then it shows absorption process.

Furthermore , all rotating molecule have quantized rotational energy. Consequently when a electronic plus rotational spectra takes place simultaneously then consequently ,rotational quantum number may also change . If rotational quantum number decreases that means it shows emission energy, or if it increases then it shows absorption energy.

Theory and example

Some molecules, like HCl (hydrogen chloride) has permanent dipole moment. Where hydrogen atom carries partially positive charge and chloride atom carries partially negative charge. But in homogeneous molecules like  H₂ and Cl₂ there is no such saoeration or charges and hence its dipole moment is zero. If we talk about heterogeneous molecules like CO,HBr,HgCl Etc always carries some electric  dipole moment. Again, for N₂,O₂ etc teh dipole moment is zero.

Introduction of spectra of diatomic molecules

When a molecule having dipole moment ,rotates the plus and minus charges places periodically, and the component of dipole moment in a given direction fluctuates regularly. According to classical electrodynamics, when dipole moment changes regularly then it leads to radiation. Thus it may be possible that an interaction takes place with incident electromagnetic radiation, due to which emission or absorption of radiation may take place and rotation give rise to a spectrum. Rotational spectroscopy is observe in microwave region.all the molecules having a permanent dipole moment are said to be “microwave active” .

Types of stretching

The vibration of molecules can also give rise to a dipole change. The infrared region is the region of vibrational spectroscopy. Let us Consider the carbon dioxide molecule as an example. In which three atoms are arranged linearly. Carbon atom have parllally positive charge and oxygen atom acquires partially negative charge on it. There are many types of stretching vibration present in a molecule . The first among all of them is symmetric stretching , vibration of the carbon dioxide molecule, the molecule is alternatively stretched or compressed, both C-O bonds changing simultaneously. Initially the dipole moment of molecule remains zero . and this perticular vibration is “infrared inactive” .

The second type of stretching vibration is anti-symmetric or asymmetric stretching. In this type of stretching one bond is stretched while the other one is compressed at the same time. There is a periodic alteration in the dipole moment, and hence the vibration are termed as “infrared active” .

The third type of stretching vibration is “bending mode” is also allowed and is infrared active. From the study of vibration spectrum we obtain the vibrational frequency and the force constant of molecule.

Regions of spectrum

The regions into which electromagnetic radiation has been divided are (in increasing frequency)

  1. Radio-frequency region: frequency ≈ 3×10⁶−3×10¹⁰Hz ; wavelength is 10m to 1cm , photon energy hv is 10⁻⁸ eV to 10⁻⁴ eV. Region of nuclear magnetic resonance (n.m.r) and electron spin resonance (e.s.r) spectroscopy. The energy change involved is that arising from the reversal of spin of nucleus or electron and is of the order 0.001-10 joule/mole or of order 10⁻⁶eV.
  2. Microwave region : frequency ≈ 3×10¹⁰ −3×10¹²Hz, wavelength is approx 1cm to 0.1mm and photon energy hv is 10⁻⁴eV -10⁻²eV . region of rotational spectroscopy. Separation between the rotational levels of molecules are of order of hundreds of joule per mole or of the order 10⁻³eV.
  3. Infrared region : frequency ≈ 3×10¹²−3×10¹⁴ Hz, and wavelength is approx 0.1mm to 780nm ; photon energy hv is 10⁻²eV -1eV. Region of vibrational spectroscopy also the Separation between the levels are some 10⁴ joule/mole or of the order 10⁻¹ eV.
  4. Visible and ultraviolet regions: frequency ≈ 3×10¹⁴−3×10¹⁶ Hz; wavelength is approx 780nm-10nm; photon energy hv is 1eV to 100 eV . And It lies in the region of electronic spectroscopy. The separation between the energies of valence electrons are some hundreds of kilojoules per mole or of the order of 5-10 eV.
  5.  X-ray region: frequency ≈3×10¹⁶-3×10¹⁸ Hz ; wavelength is approx 10 nm -100pm also, photon energy hv is 10⁻²eV to 10 ⁻⁴eV . Energy changes involving the inner electrons of an atom or a molecule and it may be of the order of 10³ keV.
  6. γ- ray region : frequency ≈ 3×10¹⁸ − 3×10²⁰ Hz ; wavelength is approx 100pm -1pm ; photon energy hv about 10 ⁴eV – 10⁶eV . Energy changes involve the rearrangement of nucleons and having energies of the order of MeV.

In this particular article , Introduction of spectra of diatomic molecules we have discussed about the basic information of spectra of diatomic molecule, stretching vibration present in a molecule, regions of spectrum, etc.



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