This model demonstrates the attosecond transient absorption spectra computed using the following formula:
$$
\sigma (\omega,\tau) = \frac{4\pi\omega}{c}
\text{Im} \sum_I \sum_J
c_I c_J
\exp\left( i (E_I - E_J) \tau \right)
\sum_F \langle \Phi_I | \hat{\mu} | \Phi_F \rangle
\langle \Phi_F | \hat{\mu} | \Phi_J \rangle
\left(
\frac{1}{\tilde{E}_F - E_I - \omega}
+ \frac{1}{\tilde{E}^*_F - E_J + \omega}
\right)
$$
See the references listed below for a detailed discussion:
$$\langle \Phi_1 | \hat{\mu} | \Phi_{F_1} \rangle =$$
$$\langle \Phi_1 | \hat{\mu} | \Phi_{F_2} \rangle =$$
$$\langle \Phi_2 | \hat{\mu} | \Phi_{F_1} \rangle =$$
$$\langle \Phi_2 | \hat{\mu} | \Phi_{F_2} \rangle =$$
$$c^2_1 =$$