say a system has 3 modes. under free vibration does it vibrate in all the modes all just one mode? why. also if we apply a either a impulse or pulsating force to the system, in which modes the system is going to vibrate and why? thanks.
atoms in a molecule normally vibrate.
One molecule cannot vibrate in all modes perfectly (but it can vibrate in ground state in all modes, something random).
Each mode of vibration has its own energy, i.e. in order to make the atom to vibrate in a particular mode one have to excite the by molecule supplying photon of specific energy.
Normally one varies this photon energy from 0 to 1000 cm-1 (0 to 123 meV).
Why you do like this? vibrational properties are fundamental and are important for predicting many properties of the molecule, eg., symmetry properties, bonding, force constant, etc.
Moreover, vibration is not dependent on temperature and even at 0 K they will vibrate.
I agree you generalized the reply..
Why dont you say the motion of atom as vibration (like to know)?
Each mode has its own energy, what is wrong in this?
Just think you have a molecule and that molecule vibrates in a specific mode...and that mode has a energy (E). One cannot change this E..that means only with that E you can excite/deexcite that specific mode..
I understood that in Raman/IR you scan the energy range from 7 to 500 meV..
But there are some techniques where people usually scan from 0 to 100 meV (metal-oxide vibrations occurs in this region) and in this techniques the important part is 0 to 10 meV (boson peak appear in this region), moreover some important properties like Debye-Waller factor is decided in this small region.
what i mean by temperature dependent is..
provided the molecule doesn't breaks, then for any temperature each mode has its own energy E..of course peak amplitude of the vibrational peak may change.
If there is something wrong please reply.