# Vibrational /rotational temperature?

1. Feb 9, 2014

### hymeme

I learned that Raman scattering can measure the vibrational / rotational temperature of certain species in a reacting flow. But couldn't figure out the physical meansing of these temperature. Why do we need such things? Can anyone help me? Thank you!

2. Feb 9, 2014

### jfizzix

Temperature is the rate at which energy changes with respect to entropy, at constant volume, and particle number.

If you can have a system with different degrees of freedom what don't interact with one another, then each degree of freedom can be treated as an independent thermodynamic system, with its own energy, entropy, and temperature.

As an example, it is possible for the spins of atoms in a crystal to have independent (and sometimes even negative) temperatures from the vibrational degrees of freedom of the atom.

3. Feb 10, 2014

### hymeme

Thank jfizzix for the reply! It helped some.
From some tables, we know that the rotational temperature of H2 is 88K. What does that imply? (Does it mean that to get the rotational bands, the temperature should be higher than 88K.) If H2 is at room temperature, or if it is at 1500K, will the rotational bands be significantly different? This is the area I am not familiar with.

4. Feb 10, 2014

### jfizzix

Ah, the characteristic rotational temperature (the thing looked up in tables) is the temperature at which thermodynamic energy fluctuations (kbT) are on par with the transitions in the rotational part of the energy spectrum.

If you are above 88K, then the thermal fluctuations are on par with the rotational energy levels, so beyond that temperature, you probably need to consider more than just the translational degree of freedom in calculating things like the partition function, entropy, heat capacity, etc.