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Ch 42: Molecules and Condensed Matter
Young & Freedman Calc - University Physics 14th Edition
Young & Freedman Calc14th EditionUniversity PhysicsISBN: 9780321973610Non è quello che usi tu?Cambia libro di testo
Capitolo 42, Problema 6

The H2 molecule has a moment of inertia of 4.6×10−484.6\(\times\)10^{-48} kg-m2. What is the wavelength ll of the photon absorbed when H2 makes a transition from the l=3l = 3 to the l=4l = 4 rotational level?

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1
Step 1: Understand the rotational energy levels of a diatomic molecule. The energy of a rotational level is given by the formula: El = ll+1 h2π I, where l is the rotational quantum number, h is Planck's constant, and I is the moment of inertia.
Step 2: Calculate the energy difference between the rotational levels l = 3 and l = 4. Use the formula for the energy difference: ΔE = E4 - E3. Substitute the values of l into the energy formula for each level.
Step 3: Express the energy difference in terms of frequency using the relationship E = hν, where ν is the frequency of the photon absorbed. Rearrange the formula to solve for ν: ν = ΔEh.
Step 4: Relate the frequency of the photon to its wavelength using the speed of light equation: c = λν, where c is the speed of light, λ is the wavelength, and ν is the frequency. Rearrange the formula to solve for λ: λ = cν.
Step 5: Substitute the known values for the moment of inertia I = 4.6 × 10-48 kg·m2, Planck's constant h = 6.626 × 10-34 J·s, and the speed of light c = 3 × 108 m/s into the equations to calculate the wavelength λ.

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Moment of Inertia

Moment of inertia is a measure of an object's resistance to changes in its rotational motion. For a molecule like H2, it quantifies how mass is distributed relative to the axis of rotation. The moment of inertia affects the energy levels of rotational states, which are crucial for understanding transitions between these states.
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Rotational Energy Levels

In quantum mechanics, molecules can occupy discrete rotational energy levels, denoted by quantum numbers (l). The energy associated with these levels is given by the formula E_l = (h^2 * l(l + 1)) / (2 * I), where h is Planck's constant and I is the moment of inertia. Transitions between these levels involve the absorption or emission of photons.
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Photon Wavelength

The wavelength of a photon is related to its energy by the equation E = h * c / λ, where E is the energy of the photon, h is Planck's constant, c is the speed of light, and λ is the wavelength. When a molecule transitions between rotational levels, the energy difference corresponds to the wavelength of the absorbed or emitted photon, allowing us to calculate λ based on the energy change.
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