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\[G(\omega) = {1 \over 2\pi}\int_{-\infty}^{\infty}\;dt e^{i\omega t} \langle {1 \over 2}[B(0),B(t)]_+\rangle\]

which is just the frequency spectrum corresponding to the autocorrelation function of \(B\). For different choices of \(B\), \(G(\omega)\) corresponds to different experimental measurements. Consider the example of a molecule with a transition dipole moment vector \(\mu\). If an electric field \(\textbf{E}(t)\) is applied, then the Hamiltonian \(H'\) becomes

\[H' =-\mu \cdot \textbf{E}(t)\]

If we take \(\textbf{E}(t)=E(t) \hat{\text{z}} \), then

\[H'=-\mu_z E(t)\]

Identifying \( B=\mu_z \), the spectrum becomes

\[G(\omega) = {1 \over 2\pi}\int_{-\infty}^{\infty}\;dt\;e^{i\omega t}\langle {1 \over 2}[\mu_z(0),\mu_z(t)]_+\rangle\]

or for a general electric field, the result becomes

\[G(\omega) = {1 \over 2\pi}\int_{-\infty}^{\infty}\;dt\; e^{i \omega t} \langle {1 \over 2} (\mu (0) \cdot \mu (t) + \mu (t) \cdot \mu(0) )\rangle\]

These spectra are the infrared spectra.

As another example, consider a block of material placed in a magnetic field \({\cal H}(t) \) in the \(z\) direction. The spin \(S_z\)$S_z$ of each particle will couple to the magnetic field giving a Hamiltonian \(H' \)

\[H' = -\sum_{i=1}^N S_{i,z}{\cal H}(t)\]

The net magnetization created by the field \(\underline {m_z}\) is given by

\[ m_z = {1 \over N}\sum_{i=1}^N S_{i,z} \]

so that

\[ H' = -Nm_z{\cal H}(t) \]

Identify \(B = m_z \) (the extra factor of  \(N\) just expresses the fact that \(H' \) is extensive). Then the spectrum is

\[ G(\omega) = {1 \over 2\pi}\int_{-\infty}^{\infty}\;dt\;e^{i\omega t}\langle {1 \over 2}[m_z(0),m_z(t)]_+\rangle \]

which is just the NMR spectrum. In general for each correlation function there is a corresponding experiment that measures its frequency spectrum.

To see what some specific lineshapes look like, consider as an ansatz a pure exponential decay for the correlation function \( C_{BB} (t) \):

\[ C_{BB}(t) = \langle B^2\rangle e^{-\Gamma \vert t\vert} \]

The spectrum corresponding to this time correlation function is

\[ G(\omega) = {1 \over 2\pi}\int_{-\infty}^{\infty}\;dt e^{i\omega t}C_{BB}(t) \]

and doing the integral gives

\[ G(\omega) = {\langle B^2 \rangle \over \pi}{\Gamma \over \omega^2 + \Gamma^2} \]

which is shown in the figure below:



We see that the lineshape is a Lorentzian with a width \( \Gamma \). As a further example, suppose \(C_{BB} (t) \) is a decaying oscillatory function:

\[C_{BB}(t) = \langle B^2 \rangle e^{-\Gamma\vert t\vert}\cos\omega_0t\]

which describes well the behavior of a harmonic diatomic coupled to a bath. The spectrum can be shown to be

\[ G(\omega) = {\langle B^2 \rangle \Gamma \over \pi}\left[{\Gamma ^2 + \omega ^2 + \omega ^2_0 \over \left ( \Gamma ^2 + (\omega - \omega _0 )^2 \right)\left(\Gamma^2 + (\omega+\omega_0)^2\right)}\right] \]

which contains two peaks at \( \omega = \pm \sqrt{\omega_0^2 - \Gamma^2} \) as shown in the figure below:

Figure 3: