docs: def doppler_width(transition, Temperature):
      r"""Return the Doppler width of a transition at a given temperature
      (in angular frequency).
  
      The usual Doppler FWHM of the rubidium D2 line (in MHz).
  
      >>> g = State("Rb", 87, 5, 0, 1/Integer(2), 2)
      >>> e = State("Rb", 87, 5, 1, 3/Integer(2))
      >>> t = Transition(e, g)
      >>> omega = doppler_width(t, 273.15 + 22)
      >>> "{:2.3f}".format(omega/2/np.pi*1e-6)
      '522.477'
  
      """
      atom = Atom(transition.e1.element, transition.e1.isotope)
      m = atom.mass
      omega = transition.omega
      return omega*np.log(8*np.sqrt(2))*np.sqrt(k_B*Temperature/m/c**2)