Recently I had studied vibrational spectroscopy using the quantum harmonic oscillator model.
After stating the gross selection rule [that vibrational oscillation demands a change in dipole moment for polar molecule] and showing that transitions are possible only between adjacent energy states ($\Delta v = \pm 1$), my professor's notes state that:
Most of the molecules will populate the lowest energy states, following Maxwell–Boltzmann distribution, the dominant contribution to IR absorption spectra comes from $v = 0 \to v = 1$ transition and is called fundamental transition.
I do not understand the point that 'Most of the molecules will populate the lowest energy states...'. What does it mean and how does it lead to the conclusion that most of the times a transition of $v = 0 \to v = 1$ will take place?