What intermolecular force does liquefying helium have?
Van der waals attraction
In an ideal gas, molecules move at high velocity such that the attraction forces between the molecules are insignificant.
When you decrease the temperature (heat energy), you are removing the kinetic energyof these molecules thereby reducing its velocity.
Alternatively, you can increase the pressure by reducing the volume of the chamber. This will reduce the intermolecular distance which will also increase the influence of attraction forces.
Van der Waals had to modify ideal gas equation from
where he incorporated average attraction between particles (designated by symbol 'a') and volume excluded by a mole of particles (designated by symbol 'b')