def li_ein_lotz(E, I):
    """
    This function calculates electron impact ionization cross sections (in cm2) of 
    Li 4s to 4f.
    param E: requested electron-impact energy in eV
    type E: float, np.ndarray
    param I: ionization energy in eV
    type I: float
    """
    sigma = 1e-14* ((1*4*np.log(E/I)/(E*I))*(1-0.7*np.exp(-2.4*(E/I -1))) + 
                       (2*4.2*np.log(E/58)/(E*58))*(1-0.6*np.exp(-0.6*(E/58 -1))))
    return sigma