Vibrational Spectrums of HI, HBr, HF, and HCl
The fine structure in the diatomic vibrational spectra is based on various population of the rotational levels.

The best demonstration of isotopic effects in vibrational spectra is to switch hydrogen for deuterium.

The frequency of vibration (from Hooke's Law adapted to a harmonic oscillator) is given by:

$$\nu = \frac{1}{2\pi}\sqrt{\dfrac{k}{\mu}}$$

where $k$ is the force constant of the oscillator and $\mu$ is the reduced mass: 

$$\mu=\dfrac{M_1M_2}{M_1+ M_2}$$ 

Switching $\ce{HCl}$ to $\ce{DCl}$ has a much larger effect than the two chlorine isotopes (35 and 37):


$\ce{H^{35}Cl}, \ \mu\approx\frac{35}{36}=0.972$
$\ce{H^{37}Cl}, \ \mu\approx\frac{37}{38}=0.974$
$\ce{D^{35}Cl}, \ \mu\approx\frac{70}{37}=1.892$
$\ce{D^{37}Cl}, \ \mu\approx\frac{74}{39}=1.898$


The difference between $\ce{^{35}Cl}$ and $\ce{^{37}Cl}$ is in the third decimal place. 

Here is a nice Google Image search for the vibrational spectra of HCl and DCl