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More on chapter 9 plus some error fixing.
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b-butler committed Jun 4, 2019
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4 changes: 2 additions & 2 deletions chapters/ch6.tex
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Expand Up @@ -61,7 +61,7 @@ \section{Rigid Rotor Harmonic Oscillator}%
approximation. The approximation allows vibrational-rotational decoupling.

\section{Vibrational Partition Function}%
\label{sec:vib}
\label{sec:diatomicvib}
\subsection{Energy Levels and Spectra}
For a quantum harmonic oscillator, the energies are given by
\begin{equation*}
Expand Down Expand Up @@ -98,7 +98,7 @@ \subsection{Energy Levels and Spectra}
\includegraphics[width=0.7\linewidth]{electronic_states.png}
\caption{Electronic potentials for the ground and first excited state with
relevant quantities marked.}
\label{fig:elecronic_states}
\label{fig:diatomicelectronic}
\end{figure}

\subsection{Deriving the Partition Function}
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2 changes: 1 addition & 1 deletion chapters/ch8.tex
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Expand Up @@ -161,7 +161,7 @@ \subsubsection{Spherical Tops}
\end{equation*}
In addition, at large values of $J$ the $(J + 1) \approx J$, so
\begin{equation*}
q_{rot} &= \frac{1}{\sigma}
q_{rot} = \frac{1}{\sigma}
\int_{0}^{\infty}{4J^2 e^{-J^2 \hbar^{2}/2IkT}\d{J}}\\
\end{equation*}
Let,
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50 changes: 47 additions & 3 deletions chapters/ch9.tex
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Expand Up @@ -72,13 +72,13 @@ \section{Chemical Equilibrium in Terms of Partition Functions}%
\begin{align*}
\sigma_j \nu_j \mu_j &= -kT \sigma_j \nu_j \ln{\left[ \frac{q_j}{N_j}
\right]} = \ln{\left[ \frac{q_j}{N_j} \right]}^{\sigma_j \nu_{j}}\\
\sum_{j}{ \sigma_j \nu_j \mu_j} &= \nu_D \mu_D + \nu_C \mu_C - \nu_A \mu_A -
\sum_{j}{\sigma_j \nu_j \mu_j} &= \nu_D \mu_D + \nu_C \mu_C - \nu_A \mu_A -
\nu_B \mu_B =0 \\
&= \sum_{j}{\sigma_j \nu_j \ln{\left[ \frac{q_j}{N_j} \right]}}
= \sum_{j}{\ln{\left[ \frac{q_j}{N_j} \right]}^{\sigma_j \nu_{j}}}\\
&= \ln{\left[ {\left(\frac{q_D}{N_{D}}\right)}^{\nu_D}
{\left(\frac{q_C}{N_{C}}\right)}^{\nu_C}
{\left(\frac{N_B}{q_{B}\right)}}^{\nu_B}
{\left(\frac{N_B}{q_{B}}\right)}^{\nu_B}
{\left(\frac{N_A}{q_{A}}\right)}^{\nu_A} \right]} = 0\\
&= \ln{\left[
\frac{q_{D}^{\nu_D}q_{C}^{\nu_C}}{q_{B}^{\nu_B}q_{A}^{\nu_A}}
Expand Down Expand Up @@ -136,8 +136,52 @@ \section{Thermodynamic Tables}%
\end{align*}
Here one should note that the logarithm in the last line has units of pressure.
\subsection{Zero of Energy}
One important consideration when using thermodyanmic tables is the zero of
One important consideration when using thermodynamic tables is the zero of
energy. Previously we have set the translational and rotational ground states to
0 energy, the vibrational zero point energy to be the bottom of the potential
well, and the electronic zero to be the energy of complete atomic separation.
Experimentally, the complete ground state is taken to be zero energy. This
requires changing the electronic and vibrational partition functions from what
we have previously derived.

We first take the electronic partition function,
\begin{equation*}
q_{elec} = \omega_{e1} e^{-\varepsilon_{e1}/kT} + \omega_{e2}
e^{-\varepsilon_{e2}/kT} + \cdots
\end{equation*}
Using the same convention that the zero energy is the electrically neutral
separated atoms then,
\begin{align*}
\varepsilon_{e_2} &= -D_e \\
\varepsilon_{e_2} &= -D_e + \Delta\varepsilon_{12} \\
q_{elec} &= \omega_{e1} e^{D_e/kT} + \omega_{e2}
e^{D_e/kT}e^{-\Delta\varepsilon_{12}/kT} + \cdots \\
&= e^{D_{e}/kT} (\omega_{e_{1}} + \omega_{e_2}
e^{-\Delta\varepsilon_{12}/kT} + \cdots) \\
&= e^{D_e/kT} q_{elec}^0.
\end{align*}
By factoring out the $e^{D_{o}/kT}$, the partition function becomes a constant
times the partition function with zero of energy the ground state as
$\omega_{e_{1}}$ is multiplied by $1$ or $e^0$. We can similarly take the
vibrational partition function,
\begin{equation*}
q_{vib} = \prod_{j}{\frac{e^{-\Theta_{\nu_{j}}/2T}}
{(1 - e^{-\Theta_{\nu_{j}}/2T})}}, where \Theta_{\nu_j} = \frac{h\nu}{k}
\end{equation*}
and remove the part from the non-zero ground state. Here it is important to note
from the derivation in Section~\ref{sec:diatomicvib} that the numerator comes
from the $1 /2$ term in the energy levels which is a result of having a nonzero
ground state. Thus,
\begin{equation*}
q_{vib} = e^{-\Theta_{\nu_{j}}/2T}\prod_{j}{(1 - e^{-\Theta_{\nu_{j}}/2T})}
= e^{-\Theta_{\nu_{j}}/2T} q_{vib}^0.
\end{equation*}

From these two equations we can redefine the molecular partition function as,
\begin{align*}
q &= q_{trans}^0 q_{rot}^0 q_{vib}^0 q_{elec}^0 e^{(D_e - 1/2
\sum_{j}{h\nu_{j}})/kT} = q_{trans}^0 q_{rot}^0 q_{vib}^0 q_{elec}^0
e^{D_o /kT}.
\end{align*}
The substitution to $D_o$ is graphically explained in
Figure~\ref{fig:diatomicelectronic}.
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