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'problem and solution'
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DPotoyan authored Nov 6, 2024
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Expand Up @@ -317,39 +317,15 @@ When the centrifugal distortion is taken into account, the rotational transition

$${\tilde{\nu} = \tilde{E}_r(J+1) - \tilde{E}_r(J) = 2\tilde{B}(J+1) - 4\tilde{D}(J+1)^3\textnormal{ where }J=0,1,2,...}$$

:::{admonition} **Example**
:class: note

Measurement of pure rotational spectrum of H$^{35}$Cl molecule gave the following positions for the absorption lines:

$$\tilde{\nu} = \left(20.794\textnormal{cm}^{-1}\right)\left(J+1\right) - \left(0.000164\textnormal{cm}^{-1}\right)\left(J+1\right)^3$$

What is the equilibrium bond length and what is the value of the centrifugal distortion constant?
:::

:::{admonition} **Solution**
:class: note, dropdown

We first write the expression for $\tilde{B}$ and then use the definition of the moment of inertia $I$:

$$\tilde{B} = \frac{h}{8\pi^2cI} = \frac{h}{8\pi^2c\mu R_0^2}$$

where $\mu$ is the reduced mass for the molecule and $R_0$ is the equilibrium bond length. Solving for $R_0$ gives:

$$R_0 = \sqrt{\frac{h}{8\pi^2c\mu\tilde{B}}} = 129\textnormal{ pm}$$


The centrifugal distortion constant can obtained by comparing the above equation with the equation for rovibronic coupling

$$\tilde{D} = 4.1\times 10^{-5}\textnormal{ cm}^{-1}$$
:::




### Problems

#### Problem -1
#### Problem 1

Consider a diatomic molecule with the following constants:

Expand Down Expand Up @@ -420,4 +396,30 @@ In rotational-vibrational spectroscopy:
- The **$R$-branch** consists of transitions where the rotational quantum number increases by 1 ($\Delta J = +1$). These transitions appear at wavenumbers higher than $\omega_e$, resulting in a series of lines at progressively higher energies as $J$ increases.

In a spectrum, the $P$-branch lines appear on the lower wavenumber side of the fundamental vibrational frequency, while the $R$-branch lines appear on the higher wavenumber side. These branches provide a characteristic double-sided pattern centered around $\omega_e$, reflecting the rotational structure superimposed on the vibrational transition.
:::

#### Problem 2

Measurement of pure rotational spectrum of H$^{35}$Cl molecule gave the following positions for the absorption lines:

$$\tilde{\nu} = \left(20.794\textnormal{cm}^{-1}\right)\left(J+1\right) - \left(0.000164\textnormal{cm}^{-1}\right)\left(J+1\right)^3$$

What is the equilibrium bond length and what is the value of the centrifugal distortion constant?


:::{admonition} **Solution**
:class: dropdown

We first write the expression for $\tilde{B}$ and then use the definition of the moment of inertia $I$:

$$\tilde{B} = \frac{h}{8\pi^2cI} = \frac{h}{8\pi^2c\mu R_0^2}$$

where $\mu$ is the reduced mass for the molecule and $R_0$ is the equilibrium bond length. Solving for $R_0$ gives:

$$R_0 = \sqrt{\frac{h}{8\pi^2c\mu\tilde{B}}} = 129\textnormal{ pm}$$


The centrifugal distortion constant can obtained by comparing the above equation with the equation for rovibronic coupling

$$\tilde{D} = 4.1\times 10^{-5}\textnormal{ cm}^{-1}$$
:::

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