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Spectral energies!

  1. Jan 20, 2006 #1
    Rotational energies are in microwave region and vibrational are in infrered region, then how can the both be excited symultaneously? To excite vibrations we need infrared radiation then how can rotations be excited which are in microwave region?
  2. jcsd
  3. Jan 20, 2006 #2


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    The excitation source is rarely a laser. It produces a spread of wavelengths peaked at the desired (selected) wavelength.
  4. Jan 20, 2006 #3
    thanks gokul, you mean to say its because of incapability of the source that cannot produce exact wavelength? (if i got your point!!)
  5. Jan 20, 2006 #4
    Or is it true that rotational levels can easily be excited bacause this involves just the change in orientation of the molecular dipole (torque produced by electric field can rotate the molecule), unlike configurational change in vibration or charge distributional change in electronic excitations?
    Hence no matter what radiation is there rotations are excited(as torque is a macroscopic property!),,,,,, (donno how far itis true!!)
    Last edited: Jan 20, 2006
  6. Jan 22, 2006 #5
    In a gas at room temperature, molecules will be in many states with different rotational states with rotational angelar momentum larger than zero.

    Now, if they absorb an IR-photon, the moment of inertia will change. Angular momentum is conserved, så the rotational frequency must change.

    That is as far as my understand goes. I do not quite remember the details of the p- and the q-branches (or the Q- and the R-branches), and I will leave that for you to look up.

    Just as an analogy that I am more familiar with: in electronic optical or x-ray transitions, also molecular vibrations are excited, with intensities given by Frank-Condon factors.
  7. Jan 22, 2006 #6

    Claude Bile

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    Rotational and Vibrational states are usually excited simultaneously, which is why the term rovibrational pops up frequently in IR spectroscopy. In fact, in many molecules, selection rules require that a jump in vibrational energy be accompanied by a jump (or drop) in rotational energy (This is where P and R branches come from).

    To Gokul; Lasers frequenctly used in Raman spectroscopy where high intensities are required, and high precision spectroscopy where small linewidths are required. In fact, the Nobel prize for physics was shared in 2005 by two men who pioneered the Optical frequency comb technique (which uses femtosecond-pulse lasers). Add that to the emergence of broadband coherent sources via supercontinuum generation and tunable coherent sources, and I think you'll find lasers are rather prevalent in spectroscopy :biggrin: .

  8. Jan 22, 2006 #7


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    The lab next door to mine is a laser spectroscopy lab ! :biggrin: While I've seen a bunch of laser spectroscopy (particularly Raman & Brillouin) studying the solid state, I've seen them much less often used in analytical chemistry labs.
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